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  • Restoring a 57 Year old Receiver: The Pioneer SX-1000TA from 1966

    Restoring a 57 Year old Receiver: The Pioneer SX-1000TA from 1966

    I usually write about new designs, technical aspects of audio etc but on one previous occasion waxed lyrical about the JBL SA600, a vintage stereo amplifier from 1966 designed by Bart Locanthi, which has turned out to be one of the most popular pages on hifisonix.com – it regularly tops the monthly hits table.

    I am going to share a few thoughts about restoring a vintage Pioneer SX-1000TA stereo receiver. First, a bit of history. I’ve rooted around on the web and YouTube and from what I can tell, this is the original receiver that kicked off the Japanese audio invasion starting in 1967/8. This cemented their dominance in the home audio field for the next 20 years until the late 1980s, when widespread interest in home audio seemed to wane, probably due to multiple factors like the arrival of the Sony Walkman in July 1979 and its progeny, portable CD and MD players, but also the rise of computer gaming. ‘Personal audio’ epitomised by the Walkman seemed to shift the appreciation of music up to the 1980s from being a family activity in a living room to something individuals with headphones did while on the move. Besides, it was a good way to block out the din and hubbub of city commuter life. In January 2001, iTunes was released followed in October that year by the Apple iPod, basically finishing off the home audio components industry as we know it. Homes up and down the country no longer seemed to want those beautiful Japanese receivers of yesteryear with their fantastic blue and green tuning dials, VU meters and plethora of knobs covering tone controls and filters, tape dubbing facilities and speaker switching. Fast forward 30 or 40 years to 2025 and vintage receivers from that era are objects of desire.

    The ‘aesthetics’ war raged for about fifteen years, with two examples shown above, until manufacturers drifted towards ugly black boxes, which in my purely subjective opinion, also helped finish off the segment.

    Pioneer Corporation opened its first US sales office in 1966, but was in effect waiting for a hit product to really grab consumer attention, and that product came long in the form of the SX-1000TA, but via an unusual route. Serviceman returning from Vietnam via Japan, or having taken active duty leave in Japan whilst out there, had picked up Japanese receivers, the SX-1000TA being one of the early examples, introducing many folks back home to Japanese products that were every bit as good as US brands like Fischer, HH Scott, Marantz and McIntosh but at a much lower price points. The SX-1000TA brought Pioneer the brand attention it needed to really take off with consumers. It looked cool, and at 40 Watts per channel produced loads of power for the day (most receivers and amplifiers were in the 20 to 40 watt range). It quickly established itself as a durable, well-built amplifier – the fact that these are still being restored and repaired nearly 60 years later attests to that. In the early 1970s, the ‘college audio revolution’ took off which saw dorm rooms and campus radio stations across the US sporting Japanese audio systems, with Pioneer featuring particularly strongly.

    My particular unit, pictured below on my workbench, dates from 1966/7 as far as I can guess, and has a vacuum tube-based FM tuner. I went to a local garden shop (now defunct) in about July 2019 to buy a bag of compost and happened to wander around the back of the shop where I discovered a room full of ancient, decrepit looking audio gear and musical instruments. The SX-1000TA was lying on its side and I enquired about it and was told it was working and I could get it for £20, but not the £10 that I offered. I parted with £20 and soon discovered back home that it was barely working. One channel was completely dead, FM stereo didn’t work properly, and the side that did distorted terribly and the various pots and switches crackled and popped in use – some ‘Servisol’ switch cleaner seems to have resolved most of the problems, and the potentiometer crackle (now much reduced) is an indicator of cap leakage which is to be expected on something this ancient, and easily solvable with re-capping. The front panel was dirty (as of January 2025, I still have to tackle that) and the inside covered in some sort of fine sawdust, powder or similar that appears to have stuck to all the components. I presume this receiver sat on a shelf in a workshop or place of business somewhere for many years, getting turned on for 8-10 hours every day to provide background music. A miserable existence no doubt for something that started out really looking beautiful.

    The first job was to get the amplifier working. The left channel had a blown TO-3 output device (2SC793 100V 7A NPN) so I replaced all 4 output devices on both channels with MJ15024 (250V 16A TO-3 NPN) since the 2SC793s were no longer available. Q901, the first stage amplifier was also gone in the faulty channel, so I replaced it in both channels with BC546 which I had to hand and that sorted the amp out. Using an old iPod as the source and headphones, the sound is rich and ‘mellifluous’ (I’ll come back to this later). After the initial flurry of activity, it sat under my workbench until late 2024, when I pulled it out to continue working on it.

    Pictured below is an immaculate 57 year old SX1000TA dating from 1968 going for £599 in the UK on eBay.

    The next thing that required attention was the FM tuner bit. It seemed to tune in very well, but the one channel distorted very badly and I could not get stereo out of it. I guessed the issue was around the MPX board and found that one of the germanium diodes (D701-D704 OA79s) was leaky, so replaced all 4 of these with OA91s that I got from Cricklewood Electronics who are based in London. I removed and checked C708 and C709 (1000pF film) which measured well, but given the passage of time were extremely flimsy and the leads corroded, so these were each replaced by 2 off 470pF NP0/COG in parallel. C710 and C711, both 10uF electrolytics that feed the output amplifier/buffer stage of the MPX unit looked pretty grotty, so I replaced these with 22uF bipolar 35V devices. After this, I checked the sound and the distortion was gone – FM sounded really good. I was unsure of the tubes in the FM tuner section, so as a precaution then ordered a new set from Langrex here in the UK for about £20 excl. VAT and shipping (6HA5 and 2 nuvistor devices 6CW4 – all of them NOS). I didn’t notice any marked change compared to the old ones but am happy to have taken the precaution of replacing them. Also, it doesn’t appear to have any FM alignment issues, which in my case is especially good as I do not have any gear to be able to do that. BTW, the user manual (there’s a link to it below) has the full AM and FM alignment procedure, so back in the day any radio tech would be able to repair the unit – and Pioneer weren’t the only ones that did this as almost every Japanese manufacturer provided a schematic, parts list and service related data in the back of the user manual. You just don’t have that kind of thing today – it really was a fantastic reflection in my view of a holistic approach to product marketing and after sales support and care. Nowadays, you are more likely to read stories about manufacturers (and especially high-end ones) refusing to provide owners or second-hand purchasers with schematics, as if whatever they did was that different or important that to reveal it to a repair tech would jeopardize the commercial viability of the company. Absolute nonsense of course.

    As of January 2025, I still don’t have FM stereo reception, so there is more work to do. Once the FM stereo issue is resolved – I suspect it might have to do with the selector switch – I will take a look at the PSU. Despite the 40W RMS per channel into 8 ohms, the main supply consists of just 2 off 1000uF 100V chassis mount capacitors which do look like high quality items. I may upgrade these to more modern components at higher capacitance values, but will have to see if the bridge rectifier diodes (D901~D904) are up to the job.

    Here are the schematics and detailed user manual. The schematics are beautifully drawn in my view and a work of art in themselves.

    hfe_pioneer_sx-1000ta_schematicsDownload
    hfe_pioneer_sx-1000ta_enDownload

    Here are some videos of repairs and/or restorations on this classic receiver from around the web.

    To be continued . . .

  • My Beautiful Michell Gyro SE

    My Beautiful Michell Gyro SE

    I bought my Gyro SE (SE for ‘Spider Edition’) in 2016, having read about Michell turntables on and off for more than 20 years. While working in Taiwan between 2011 and 2015, I visited a local Audio shop in Tianmu, Taipei where a full Gyrodec was on display. It was a huge piece of machinery in its acrylic cabinet, and one could not help but admire the beauty of the thing – affordable British precision engineering at its very best. I returned to the UK at the end of 2015, after a stint in Shanghai, and finally bought a Gyrodec SE in 2016. A local audio store in Norwich assembled the beast for me and I duly got a call to let me know it was ready to be collected. I can tell you I was way more excited than I was getting a new car!

    My turntable came with a bog standard Rega arm that I used with an Ortofon Black MM cartridge with a Shibata stylus for a year or two (there’s a whole back story to this specific cartridge as well – one for another post) until I was able to find a mint SME 3009 II ‘Improved’ on eBay which I still have and use with a Denon DL103 MC cartridge. Some may criticize my SME arm + DL103 pairing, but I haven’t had reason to doubt the sound is anything but sterling. The folks at Michell were helpful in guiding me through the process of converting the standard arm mounting plate to a new one that catered for the SME arm.

    As of 2025, the Michell Gyrodec, and the various progeny (Orb, Gyro SE and associated upgrades) have been around in their current form for 30 to 40 years. The Gyrodec was launched in 1982 and, in the words of Michell ‘turned turntable design upside down’. The fact that this iconic turntable, and its various derivatives, is still available decades after launch, wowing reviewers and buyers alike, stands as testimony to the quality and robustness of the original industrial design and engineering. Over the intervening decades, the Gyrodec and Gyro SE have gradually been improved with better materials and manufacturing tolerances so that today, in 2025, you have a topflight product that can compete with turntables costing 2 or 3 times as much. The finished products are heavy, robust and the finish is superb.

    Pictured above is the Michell Orb shown without its acrylic lid.

    In 2016 when I bought my Gyro SE, they were priced at £1750 with the basic Rega arm. That same table now retails at £3500, and if you go for its bigger brother, the straight ‘Gyrodec’, you are looking at £4500 with a better arm. Part of this of course is due to increased demand for high-end turntables as audiophiles have come to recognize the special qualities vinyl brings to the listening experience, but another reason I believe, is that Michell have come to the realisation that in a world where everything is made of tacky plastic and pressed plywood, a beautifully designed and engineered product that has stood the test of time can and should demand a premium. The other great thing about the Michell turntables is you can start with a basic Gyro SE and then over time upgrade it, so you end up with a top-of-the- range Gyrodec. This would entail replacing the platter and then perhaps later adding the acrylic base and plinth, and of course whatever arm takes your fancy.

    What does a Michell Gyro SE sound like? Michael Fremer, of Stereophile fame, reviewed the Gyro SE in 2000 and then again just a few years ago, describing the turntable as producing ‘airy highs’ and in the latter review, ‘solid crisp, bass’. In my system, over KEF LS50s’ or my B&W 703s’, I’d describe the sound as lush and expansive, with a clean, extended sonic palate right across the audio spectrum. With a Michell, you really are in the company of an extraordinary sounding turntable for the money, no matter what kind of music you play – the preferences in my case lying with late 19th and 20th century classical music and jazz, although I do enjoy Sting and Shaggy’s 44/876 when the mood takes me!

    Here is a short video from Michell, talking about the product range and the company ethos https://youtu.be/KtcsXCB1Fc4

    Below, are some pictures of my Michell Gyrodec SE and the equally stunning SME arm with my X-Altra MC/MM phono preamp. In normal use, the turntable sits on an Atacama stand upon which I have placed a 40kg block of polished granite which in turn is isolated from the stand via 4 very large rubber bumper/shock absorbers. This decouples the turntable from any extraneous vibration from a very low frequency.

    My Michel Gyrodec and the X-Altra MC/MM phono preamp
  • The ax-Amplifier: A Very High-Performance 60W RMS Class A Amplifier featuring AFEC

    The ax-Amplifier: A Very High-Performance 60W RMS Class A Amplifier featuring AFEC

    Parts 1 and 2 of the ax-Amplifier were published in the April and May 2024 issues of US based audioXpress magazine.

    Part 1 – The ax-Amplifier

    Part 2 – The ax-Amplifier

    Click on the link to get the supplementary data (BOM etc) from audioXpress

    Click this link to order 2 off ax- Amplifier PCB’s https://hifisonix.com/shop/2-off-ax-amplifier-module-pcb/

    You can also order a complete ax-amplifier PCB bundle here which gives you all the PCBs used in the original amplifier as described in the audioXpress articles

    You can order copies of audioXpress here audioXpress Magazine Subscription – CC-Webshop

    Here is a short YouTube video introducing the ax-Amp  https://youtu.be/Tr4ledT5DLs

    Click here if you need lower overall gain than the specified 32 dB (40x) 

    The ax-Amp is a topologically simple 60 W RMS per channel (140W peak class A at <7 ppm distortion) class A current feedback amplifier employing Augmented Feedback Error Correction (AFEC) which I first described in 2012, to achieve ultra-low distortion, very high PSRR and low DC offset. Direct regulation of the class A output standing current results in stable bias over the full operating temperature and all three bias modes. The design uses 4 pairs of 200 Watt sustained beta output transistors per channel to manage the thermal dissipation and allow the amplifier to comfortably deliver peak load currents of >26A.  At the rated 60W RMS power, 1 kHz distortion into 8 Ohms measures 3.5ppm, almost all of that contributed by the test instrumentation, while into 2 Ohms, delivering >220 Watts, the distortion is just 12 ppm. The 19+20 kHz full power IMD 1 kHz residual is -118 dBr, attesting to the ax-Amplifiers very good HF linearity. The 8 ohm load full power mains noise floor peaks are better than -126 dBr. Using a small 3 position (ON-OFF-ON) toggle switch mounted on the chassis underside just behind the front panel, the amplifier can be switched to class A, class AAB or class AB. To save power, a fast response ‘plateau’ bias circuit switches the amplifier to class A or AAB operation (if selected) when input signal is detected and reverts it to class AB operation 40 seconds after the absence of any input signal.

    Here is a quick teaser of the Part 1 article as it appeared in audioXpress: –

    The ax-Amplifier uses the diyAudio.com deluxe 5U chassis that comes with predrilled heatsinks and drilled rear panel. US builders can order the chassis from diyaudio.com, while European builders can order the chassis directly from Modushop in Italy, specifying ‘diyAudio 5U deluxe chassis’. Here is the enclosure. When ordering the chassis from Modushop, be sure to specify ‘diyAudio pre-drilled hole specification’.

    I have done a lot of listening with the ax-Amp to a wide range of music covering jazz, classical and rock on KEFLS50s paired with a B&W sub,  B&W 703s (the big originals back from 2003/2004) and Dali Oberons. The bass is clean, and goes exceptionally deep which is great for rock and fusion jazz, but that is matched with a fantastically open and detailed mid-range and a delicate, expansive top end, particularly evident on the LS50s. I  have a particular penchant for 19th and 20th century classical music, for example Stravinsky’s Firebird Suite and Sibelius’ 3rd, 4th and 5th symphonies. This complex, multilayered music, demands clean open amplification free of IMD and harmonic distortion of any sort if every nuance in the recording is to be reproduced faithfully. Claudio Abbado’s LSO recording of the Stravinsky Firebird Suite emerges out of absolute silence, followed a few minutes later by the climactic crescendo at the end of the first movement that demands huge reserves of power, dynamic range and soundstage breadth, all of which the ax-Amp reproduces confidently and with the utmost finesse, no doubt a lot of this resting on the class A bias. My musical journey into jazz covers everything from Ella Fitzgerald, Chick Corea through to Miles Davis and Dave Brubeck.  On playback with jazz recordings, and especially small ensembles, I look for the space between the musicians which creates that 3D ‘being there’ sonic canvas, and Brubeck’s ‘Take Five’ 1959 recording on the album ‘Time Out‘ has this in bucket loads which the ax-Amp delivers superbly through the KEF LS50’s.  Many of Chick Corea’s early recordings like ‘The Mad Hatter‘ (1978) hint at over-zealous compression by the recording engineer but are nevertheless well recorded and sound at their best through the B&W 703s which are able to reproduce the ‘in your face’ musical experience.  Again, the ax-Amp is able to reproduce every nuance and instrument with clarity and when required, the power this genre demands.

    Physically, this is a BIG, imposing, amplifier weighing in at about 23 kg (50 lbs). It isn’t a cheap amplifier to build, but it will look the business in any system, and most importantly deliver the goods no matter what type of music it is asked to reproduce. The ax-Amp is best suited to medium efficiency speakers of around 86 dB sensitivity in average sized living/sound rooms which will enable it to confidently reproduce orchestral crescendos. My build used 10mm feet to ensure the amp stood well off the ground, allowing good air circulation.  After continuous use for about an hour, the heatsink temperatures will be about 56C, with the ambient at c. 21C. As discussed in the technical write-up, the plateau bias circuit means the amp will revert to class AB if no music is detected for 40 seconds or more, and switch back to class A in about 2-3 milliseconds when it detects a music signal.  It should be noted when operating in class AB, the ax-Amp will deliver 5W peak class A before transitioning to class AB – so it is a very generous class AB.

    As always, if you have any questions, I’ll be very happy to answer them and provide any technical support or other enquiries you may require should you decide to build the ax-Amplifier.

  • Imroved Soft Start + DC Blocker aka iSSDCB

    Imroved Soft Start + DC Blocker aka iSSDCB

    The original Soft Start + DC Blocker published on this website a few years ago required quite complex wiring encompassing both the mains supply to the transformer and wiring from the rectified and DC-smoothed secondary.  Additionally, the mains power switch had to be rated for the full primary AC load current which placed severe limitations on builders looking to find an attractive switch that looked good on a power amplifier front panel. These issues, rightly, put many constructors off and they looked elsewhere to solve their power ON/OFF,  in-rush current, and mains DC offset problems.

    Offering a neat, easy-to-assemble solution, the new Hifisonix Improved Soft Start + DC Blocker + Power On/Off Controller  aka iSSDCB overcomes all of those problems:-

    1. Greatly simplified wiring required only on the mains supply side of the transformer
    2. Double the full AC mains load capability of the original SSDCB – 4.6A vs. 2.3A
    3. Allows use of attractive anti-vandal power control switches – max required switch current rating is 30mA DC – for full ON/OFF power control
    4. Very low standby power draw of c. 0.2W
    5. In-rush current limiting of 10A for 2-3 seconds
    6. Small compact, easy-to-assemble PCB

    Here is a presentation describing the circuit, assembly, test, and wiring details Improved DC Blocker Soft Start

    Here is the BOM, Updated on 4th February 2024 (thank you Luigi from Italy for the feedback) iSSDCB BOM Feb 2024

    Head over to the shop to buy a PCB!

    As always, if you have any questions, please feel free to contact me on Hifisonix!

  • Hifisonix X-Altra Mini II Line Preamplifier with Remote Control (Deprecated)

    Hifisonix X-Altra Mini II Line Preamplifier with Remote Control (Deprecated)

    The X-Altra Mini II has been discontinued as a complete project. The individual PCBs are still available over in the shop however and are great line stages and power supplies. The X-Altra Mini II is a superb preamplifier and it sits at the heart of my system, but it was complex with lots of inter-module wiring and I think that put some builders off. There is a new preamplifier coming – tentatively called the X-Altra Mini III – that places everything on one PCB (PSU, headphone amplifier, phono amp, line stage etc). A single ribbon cable connects the main analog board to the controller board. It offers exactly the same functionality as the original design but is easier and much cheaper to build – stay tuned for updates!

    The rest of this post discusses the listening tests I conducted on the original X-Altra Mini II back in 2022.

    X-Altra Mini II Listening Tests

    I conducted listening tests using an Oppo BD103 CD player and an external phono source (X-Altra MC/MM Phono EQ Preamp published in audioXpress Feb and March 2021) for all three vinyl recording assessments and on one of the vinyls, I also listened to the X-Altra Mini II internal MC/MM EQ preamp board. The discrete line stage is open with a fantastically smooth sonic signature that is quite different from the AD797 commercial preamp, despite both preamplifiers measuring well under 10ppm distortion and with measured hum and noise at or below -120 dBV ie inaudible. The measured X-Altra Mini II spot noise floor is c. -140 dBV and so it is exceedingly quiet with no discernible hiss from the speaker tweeters. The -3dB bandwidth of the Model 1501 is c. 200 kHz while the X-Altra Mini II is 130 kHz. I have to point out that my hearing is not that of a young person – but HF is just one part of the overall listening experience with things like imaging, bass extension, midrange articulation etc playing equally, if not more, of an important role.

     I used my kx2-Amp (class A 15W RMS, 28-Watt peak class A ) and Dali Oberon 5, B&W 703 and KEF LS50s’ to do the listening evaluation and the smooth, open sonic signature is consistent across all three speakers. The X-Altra Mini II brought the venue and the artists into my listening room unlike any of my previous system setups. The imaging is absolutely outstanding on the X-Altra Mini II – one of my go to test CD’s for this is Fourplay’s eponymous CD that features fantastic left to right width and back to front sound stage depth. This of course has a lot to do with the speakers and the recording, but if the electronics are subpar in any way, they will not support the illusion that the sound is not coming from the speakers themselves – you really want to be able to close your eyes and not be able to point to a speaker source but rather individual instruments laid out left to right and front to back. The top end is smooth and silky with no hint of sibilance. This is the first discrete JFET line level design I have done, and I think the JFET front end plays an important part. I also listened extensively to Chick Coreas’ (RIP) ‘Super Trio’  CD recorded in 2005 by Chet Himes and Bernie Kirsh. This is a superb live recording with great bass extension and a wonderful soundstage and lots of excitement and sonic fireworks on display which was faithfully conveyed by the X-Altra Mini II.

    For the vinyl assessment, I listened to three recordings – ‘Ella Fitzgerald Sings the Irving Berlin Songbook‘, and two Chick Corea recordings from the 1970’s – ‘Mad Hatter’ and ‘The Leprechaun‘. Out of the three, the Ella Fitzgerald recording is the best (I’ve written about it here). On the Ella Fitzgerald recording, the X-Altra Mini II conveyed the sense of weight in the lower registers with an effortless, open sound in the mid-range and top octaves, especially noticeable on brass. Images were precisely placed on the sound stage, extending well beyond the edges of the speakers and a long way back behind them. On this particular recording, I also listened using the X-Altra Mini II MC/MM preamp board and whilst not quite in the same league as the stand-alone X-Altra MC/MM preamp, the overall sound was nevertheless superb and still quiet for an opamp based MC preamplifier. The Chick Corea recordings don’t really lend themselves to trying to assess sound and stage tonal quality in the same way the Ella Fitzgerald recoding does. They however are exciting recordings with a lot of propulsive bass, ethereal vocals (courtesy Gale Moran) and drum kit fireworks from the inimitable Steve Gadd. The preamp conveyed the excitement of the recording with great pace, attack and timing.

     

    A few words on the headphone amplifier are in order. The X-Altra Mini II uses the HPA-1 class A headphone amp and my iteration has the OPA1642 dual JFET opamp (although you can also use an LM4562 or NE5532 as discussed in the HPA-1 write-up) driving a MJE15032/33 push-pull output stage biased at 90mA into class A (180mA peak class A current), so it will deliver 1.75 W in class A into a 32 Ohm pair of headphones. I use 12 year old Audio Technica ATH AD900 ‘Air’ headphones I bought in Japan which are not particularly bassy but have the best imaging, midrange and top end I’ve heard bar the Stax tube driven electrostatics – they really are quite special in my view. Through the phones, the preamp retains its open, smooth sound, again without any hint of harshness or sibilance in the top registers. Since the headphone amplifier output impedance is very low, the bass is crisp and goes much deeper than is the case with large diaphragm, open back headphones driven from high impedance resistive dividers. The dedicated, low impedance output headphone amp makes a huge positive impact on the sound.

     The addition of two unbalanced (one main and one auxiliary) outputs in addition to a pair of balanced outputs means that with this preamp, I can now have both my power amps (kx2 and Model 1721 240-Watt power amplifiers) along with the sub bass permanently hooked up without recourse to cable swapping or special split cables – so it’s really convenient. The kx2 drives the Dali Oberons and the KEF LS50’s, and the Model 1721 the B&W 703 with the sub bass switched on with the KEF LS50’s.

    The picture below is of the ‘Classic’ version of the X-Altra Mini II.

  • Hifisonix kx2 Class A Current Feedback Amplifier

    Hifisonix kx2 Class A Current Feedback Amplifier

    The 15W RMS (28W peak class A) kx2-Amplifier offers outstanding noise and distortion performance and is specifically designed for driving high efficiency speakers in average to large listening spaces but also performs well driving more normal efficiency speakers in restricted listening spaces.

    Click Here for the kx2-Amplifier Build Document

    Click Here for the kx2-Amplifier BOM

    Please note the kx2-Amplifier PCBs are supplied by Jim’s Audio. You can buy a set of two kx2-amplifier boards and a basic power supply board here:-  kx2-Amplifier PCB Set

    The kx2 features very low distortion (see the build doc linked to above for a full set of measurements) and uses TMC (‘transitional miller compensation’). By opening a jumper on the PCB, the compensation can be changed to standard Miller compensation which some people prefer. On the kx2, the output operating mode can be switched between class A, class AAB or class AB in similar fashion to its larger brother, the ax-Amplifier, by means of a 3-position toggle switch located on the underside of the amplifier just behind the front panel.

    Careful attention to the PCB layout and routing of some tracks resulted in a dramatic 15 dB reduction in the measured noise floor. In class AB mode, the noise floor -120 dBr reference 15W RMS into 8 Ohms, and in class A mode -115 dBr 15 W RMS into 8 Ohms (inputs shorted to signal ground in both measurements).

    The OPS bias is controlled with a single transistor servo that maintains the DC operating conditions to within 3% over the full operating temperature range (15C through to >60C).

    The version you see in the image above was built in a housing that I originally purchased in Taipei when I lived there. I had a new top plate fabricated by Schaeffer AG in Germany in 2018. For UK builders, there is also the option in the UK of Meface Ltd, but the maximum panel thickness is 3mm.

  • Hifisonix X-Altra MC/MM RIAA Phono EQ Preamp

    Hifisonix X-Altra MC/MM RIAA Phono EQ Preamp

    The X-Altra MC/MM phono preamp, whether bought as a completed unit or DIY built, delivers the ultimate performance with respect to noise, overload, and RIAA conformance, placing it among the very best available anywhere at any price.

    In 2021, the design was featured in audioXpress, a leading audio industry publication based in the US. Part 1 and part 2 of the article can be downloaded from the audioXpress website here: – 

    You Can DIY! X-Altra MC-MM RIAA EQ Preamp – Part 1 | audioXpress

    You Can DIY! X-Altra MC-MM RIAA EQ Preamp – Part 2 | audioXpress

    Here is a 27-minute YouTube video describing the X-Altra MC/MM preamp design

    Click here for a set of very high quality THP DS SS PCBs

    Click here to link to the BOM and other supplementary materials hosted on the audioXpress website

    Note carefully: only use the BOM dated September 2024.

    Click on this link if you’d prefer to buy a fully assembled and tested X-Altra MC/MM preamplifier

    If you have any questions, or require technical support with your build, contact me via the comments section below or email me at bonsai(at)hifisonix.com (replace the (at) with @)

    There is also a diyAudio thread about this preamplifier here :-

    Bonsai’s X-Altra MC/MM Phono Preamp | diyAudio

    Filter switch wiringDownload

    Click here for the user manual for the X-Altra MC/MM

    The PCB set offered to build this preamp is designed to work with the Modushop Galaxy Maggiorato GX287 230 x 170 mm 10mm front panel housing. The main analog board with the black, gold flashed and silk-screened rear panel attached, slides into the housing from the rear and is then held in place by 4 x M4 x 10mm retaining screws on the rear panel.

    The table below allows you to get an idea of how noisy a MC preamp is given the cartridge coil resistance and the amplifier input referred noise. Once you have the number off the table, use the following formula to calculate the S/N Ratio: 20 log [(Rdg*144)/Cartoutput] where Rdg is the reading from the table and Cartoutput is the MC cartridge output in Volts.  It is clear from this table that the X-Altra MC/MM is in a league of its own in terms of thermal noise (and overload capability).

    For the X-Altra, the measured equivalent input thermal noise resistance is 1.4 Ohms, so its ABOVE the 2 Ohm row, but we will use the 2 Ohm row since it is close enough. The figures in RED show the expected X-Altra preamp S/N ratio for a 200uV output cartridge. The ‘A’ weighted figures will be about 4 dB better than those shown.



    Some photos of the completed preamplifier, along with my Michel Gyro SE are shown below.

     

     

     

  • Hifisonix nx-Amplifier: A High Performance 100 W Class AB Current Feedback Amplifier

    Hifisonix nx-Amplifier: A High Performance 100 W Class AB Current Feedback Amplifier

    Originally published in December 2012.

    This power amp design is based on the sx-Amplifier topology, but with the TIS and output stage re-configured for class AB operation, enabling it to deliver 100 W into 8 Ohms. I have kept the design very simple, retaining the CFA topology, providing a very wide -3 dB bandwidth of 570 kHz, high slew rates (140 V/us) and low distortion. Since it was first published in 2012, about 400 board sets have been sold world-wide (the sister sx and kx2 class A current feedback amplifiers have had combined sales of around 450 board sets). The nx-Amplifier is a perfect introduction to current feedback amplifiers, showcasing the topologies legendary open midrange, fantastic bass performance and delicate, silky top end.

    Amplifier write up with circuit description, wiring diagram and construction details:- The-Ovation-nx-Amplifier-V2.10  (updated November 2014 to V2.0 PCB’s incorporating all feedback and updates).  Please IGNORE the BOM in the build document and instead use this latest BOM, released in December 2021 that uses currently available semiconductors  nx-Amp BOM December 2021

    You can get a complete set of  double sided THP Gold Plated V2.0 PCB’s for the above from Jim’s Audio on ebay here:

    Hifisonix nx-Amplifier PCB Set

    (2 off nx-Amplifier boards + 1 off  PSU +Protect excl. shipping).

    Here is a link to the diyAudio forum discussing this amplifier and the sx-Amplifier.  If you have any questions, you can post them up here or on the diyAudio forum.

    1. The nx&sx-Amp PSU board is a simple unregulated PSU board with no muting, output short circuit or DC offset  protection. The only protection is the amplifier module fuses

    2. .  The nx-Amp_PSU+Protection board option  couples an unregulated PSU with speaker muting, output short circuit protection  and DC offset detection protection.

    See the nx-Amplifier article above for details.  If you have any questions, feel free to post your questions up here.

    If you are a manufacturer, kit supplier or DIY shop, and you want to use the designs (circuit or PCBs) you need to contact me.

    I’d be happy to hear your comments/feedback.

    Notes and Addendums

    1. Please note, there in no R2 on the PSU +Protect PCB or on the schematic although the BOM calls for it.

  • Hifisonix sx-Amplifier: A 15 W Class A Current Feedback Amplifier

    Hifisonix sx-Amplifier: A 15 W Class A Current Feedback Amplifier

    Originally published in August 2012.

    Click here to order a set of DS THP PCBs for the sx-Amplifier from Jim’s Audio

    About 150 sx-Amplifier PCB sets have been sold, with most of those going on to be built. As you can read about in the PDF below, my class A journey started off after listening to an ancient Musical Fidelity A1 that I repaired for a friend while living in Taiwan.  I’ve had many emails from sx-Amp builders who have loved the sound, despite this being a very simple and not particularly low distortion amplifier. If you are new to DIY audio, this is probably a good as any place to start.

    The hifisonix sx-Amplifier is a low component count current feedback topology (CFA) design that delivers  15W RMS in class A into an 8 Ohm load (about 25W  peak in class A) and around 25W RMS into 4 Ohms in class  AB mode.   The design uses readily available components, achieves very wide bandwidth, along with fast rise and fall times.  Distortion is low compared to competing designs (e.g. JLH’s 10 Watter from 1969,  or  Jean Hiraga’s 20 W class A), thanks to the availability of modern bipolar devices, and the use of LTSpice to optimize the design.

    You can down load the full pdf article here The-sx-Amplifier-V2.10

    Here is  an Excel file that contains the BOM lists, including Mouser part numbers for the sx-Amplifier (covers both the double sided THP PCB version ABOVE and the new single sided PCB version discussed ): –

    Click here for the BOM that covers all PCB versions

    I have had a lot of requests for PCB’s and/or people indicating they would like to etch their own boards. To support these requests, I’ve created a second layout that is singled sided and more amenable to home etching. (The PDF file scale is 1:1):

    SS sx-Amp Bottom Copper

    Here is the component overlay for the single sided sx-Amp  SS sx-Amp Top Component Overlay

    Here are the Gerbers for the single sided sx-Amp  Single sided sx-Amp Gerbers

    Pop Alexandru, also known as Alex MM on diyAudio.com has also kindly produced a set of single sided PDF layouts below that closely follow my DS THP version.  He has produced both mirrored and non-mirrored versions. His boards are neater than mine!  Thank you Alex!

    SX-AMPLIFIER TOP SILK SCREEN MIRRORED

    SX-AMPLIFIER SINGLE SIDED COMP OVERLAY

    SX-AMPLIFIER SINGLE SIDED BOTTOM

    SX-AMPLIFIER SINGLE SIDED BOTTOM MIRRORED

    Note: there are no single sided PSU layouts.  If you build the sx-Amp using single sided PCB’s, and your own PSU, do not forget to fit your finished amplifier with a Zobel network of 0.1uF and 10 Ohms in series from J7 to the 0V star ground on your PSU.  If using the DS THP PSU board from Jim’s Audio, the Zobel network is located on the PSU board. Just follow the wiring up instructions in the writeup.

    Note: the component numbers and BOM list are all the same for the single sided PCB but with the following changes: J2 is removed. C2 becomes a 16V 10uF through hole capacitor (C1 remains a 1206 10uF 50V MLCC SMD device); Q3 becomes a BC547C.

    If you have any questions or concerns, please post your comment up here, or on the DIYaudio forum here:  https://www.diyaudio.com/forums/solid-state/236522-sx-amp-nx-amp.html

    Here is diyAudio member Thimios from Greece showing his fantastic sounding sx-Amplifier

    https://youtu.be/Jf8HHAS6Q9s

    _____________________________________________________________________________

    The sx-Amplifier: A 15 W Current Feedback Class A Amplifier

    Jean Hiraga’s  20 W class A design became something of a cult amplifier in the 1980s in the audio community and constructors from Europe, Australia, Japan and the USA praised it’s sonic performance.  In that design,  Hiraga used a much simplified (I’d call it sparse, or ‘stripped down’) current feedback amplifier  (CFA) topology  and low feedback to create an amplifier that was described somewhere as sounding ‘liquid’  and ‘tube-like’. Hiraga has always been noted for his minimalist, idiosyncratic designs, and comments about the sonics aside,  I was attracted to the simplicity – only 8 transistors in the original and a handful of discrete components (excluding the power supply of course) for a complete class A amplifier. Distortion was by any standards very high, topping out at about 1.8% at the rated power into 8 Ω, although he seems to have indicated it was of the ‘good variety’ due to its harmonic structure.  However, Hiraga’s amplifier (rated at 20 Watts  per channel into 8 Ω) was the product of 1980’s know-how and device technology.  I found myself wondering what I could cook up with today’s components and the insights afforded by circuit simulation tools that were unavailable back in the early eighties.  I did quite a bit of research on  Geoff Moss’s excellent ‘the class A amplifer site’ (tcaas) and Rod Elliot’s ESP site and the  Death of Zen (DoZ) amplifier, modelled after JLH’s classic 10-Watt design from 1969. Compared to the 1980s, we now have some really good power transistors, and there is no shortage of cheap good quality small signal devices either, other than the ultra-low noise small signal bipolar and JFETs from Toshiba in Japan of course, which are now discontinued.

    sx-Amplifer Circuit

    Looking at the reprint of JLH’s original 1969 10 W class A design,  it is immediately apparent  that the distortion vs. frequency graph at 9 W output is flat all the way out to the measurement limit,  which was beyond 20 kHz. In a conventional Voltage Control Amplifier (VCA) class AB topology,  when driving a load that causes the output stage to exit the very narrow class A bias region (ideally set at .026/Retot where Retot is the total output transistor emitter degeneration resistance including the reflected base resistance and typically ranging from 0.1 Ω through to 0.47 Ω), distortion kinks upwards at 40 dB/decade, with the kink point a few  hundred  Hz up to a few kHz.  Distortion increases at 40 dB/decade, arising from the fact that the amplifier loop gain beyond the -3 dB point is decreasing at -20 dB/decade, while the THD contribution doubles with every octave because with every  doubling of frequency, there is a doubling of cross over events, and hence distortion.

    The shape of the distortion curve over frequency tells us something about the nature of class AB amplifiers when required to move from their class A region into class B – feedback can only do so much to reduce the distortion,  and if that feedback (loop gain) is already decreasing  at 20 dB/decade within the audio band, you simply have to accept the uptick in the distortion vs. frequency plot.  JLH’s output stage design was class A of course, and so did not have to contend with crossover distortion, and this is the fundamental advantage of class A over class AB. The major distortion component in any competently designed class AB amplifier is the crossover distortion,  and if that can be removed as a factor, there are significant performance and sound benefits to be had. At low frequencies (i.e. below about 30 Hz), the distortion on JLH’s amp rose rapidly because of the electrolytic coupling cap between the output and the speaker.  Later enhancements of his design did away with the output coupling capacitor at the expense of a split rail supply and output offset adjustment.

    The sx-Amplifier is a thoroughly modern take on the class A genre that employs CFA topology, 30 MHz Ft output devices and features a – 3dB bandwidth of 540 kHz, along with slew rates of 140 V/us.  This is a very smooth sounding amplifier, suited to jazz, classical and acoustic material in general which should be married to speakers with efficiencies of 92 dB/W or better if one is after realistic concert hall sound levels – however, I used mine whilst in Asia with my B&W 703’s, which are rated at 89 dB/W,  in a modest sized listening space and found that satisfactory volume levels were easily attainable.

    In the design you see here, I had the transformer specially wound by ‘PowerStar’ in Taiwan and it features a screen and GOSS band for the best noise performance. Careful attention to layout and wiring have resulted in an exceptionally quiet amplifier. What does it sound like? As mentioned earlier, this amplifier is best suited to classical, jazz and acoustic  material –  because of the limited power on tap, its not really suited to rock or material that demands high power.

    The photograph below is of the second build I did of the sx-Amp.

    IMGP1264

    This is the sx-Amp I ended up using up until the kx2-Amp arrived some 10 years later in 2022.  Incidentally, I built one of the nx-Amplifiers into  a similar chassis.

    I have to admit I started this project with some apprehension about what a small amp would sound like – especially since I am used to listening to a big amp (250 W per channel which you can read about here Ovation 250 Amplifier and a 180 Watter called the  e-Amp ). Why design a 15 W class A amplifier, and especially one that is decidedly minimalist, after two big, complex, powerful, high performance ones? Well, there are a few reasons. Firstly, designing single digit, or sub 1 ppm distortion amplifiers can provide a rewarding intellectual experience, if only to later be thwarted by practical execution that impacts both performance and build aesthetics. Secondly, reductions in distortion below about 0.5% offer little or no further improvements in the perceived sound quality of an amplifier in controlled testing and there is a lot of academic material in support of this contention. Anecdotally, Nelson Pass has built a name for himself with class A amplifiers that for the most part never see the south side of 0.1%, and yet are highly rated by the cognoscenti. What about power levels? Big amplifiers sound wonderfully at ease with themselves – they are unflappable and handle music dynamics well. Low power amplifiers, like the sx-Amp under discussion here, really need to offer something unique to justify the effort in construction, and exploring this territory is the 3rd reason for undertaking this project: is there a magic class A sound that makes building something like this worth it?

    Speakers are notoriously non-linear often 2 to 3 times higher than the 0.5% I quoted above. Now, none of this can serve as an excuse for badly designed, sloppily engineered, high distortion amplifiers. We know that the ear is much more sensitive to some types of distortion (crossover for example) and to higher order harmonic content in particular. If these things are taken as considered inputs into the design process, we then have some latitude in design philosophy: Challenging, all out, ultra-low distortion designs, or something a little less demanding, but that ticks all of the right boxes given our knowledge about how the ear/brain system works and therefore sounds good because we avoid the major pitfalls, and is FUN TO BUILD! This is exactly where the sx-Amp is positioned.

    Once testing and set up were completed, I was very excited to hear what this thing could do. For initial listening I chose a few classical CD’s – a Lexus classical CD (freebie), Julian Bream ‘The Ultimate Guitar Collection’, a wonderful ‘LSO Live’ sampler from Hi-Fi News, A Philips Sampler from the 1990’s ‘Introducing Mozart’, followed by two jazz CD’s: Michel Petrucciani’s ‘Both Worlds’ and ‘Time Out’ from The Dave Brubeck Quartet.

    Pictured above is the very first sx-Amplifier I build in 2012 while living in Japan

    I’ve had a Hi-Fi News LSO sampler for about seven or eight years. The tracks date from performances made between 1999 through 2002.   All tracks superbly recorded with tremendous space (holographic) and dynamic range. The sx-Amp produced a wonderful three dimensional sound stage that extended well beyond the speakers, very deep and layered front to back. If you are ever looking for a classical demo CD – this has to be it! To be sure, a big part of this is the quality of the recordings, but no doubt the class A magic also played an important role in what I was hearing. Strings have that ‘bite’ to them in their lower registers and the top end shimmers marvelously; brass has the leading edge snap followed by the tizz that you only get from a really good recording played through a sympathetic signal chain. The top end on this amplifier is very beguiling without any hint of harshness and the overriding sensation is one of smoothness and relaxed detachment. The scale on the Brahm’s piece (‘Denn Alles Fleisch Est Wie Gras’) was very well reproduced which was surprising to me given the fact that only 15 W was on hand.

    Up next was a double CD collection of Julian Bream recordings covering the four decades from 1959 through 1982. Some of the early recordings are a little noisy (tape hiss) but the sound is very spacey and the notes wonderfully rounded and resonant. My favourite is disc 2, which was recorded in 1982/83 and consists entirely of solo guitar and lute pieces. Here again, the sound staging and recording venue are beautifully captured and easily re-created by the sx-Amp.

    Philips – when they were still in the music business – released  a huge Mozart collection in about 1992 or ’93 and I’ve had the collection sampler  about 20 years. There are 19 tracks and the recordings vary from good to outstanding. One of my favourites is the horn concerto in E-flat. I think Sir Neville Mariner’s recording is one of the best – the horn really floats out above the orchestra and the reverb and scale of the recording space make for an incredibly immersive experience. The whole piece is energetically played – I have found some other recordings, because of the arrangements and the conducting no doubt, to be laborious, plodding and acoustically flat by comparison. This recording is one of the better ones on this CD – I think some of the tracks are a little bright (maybe that’s just because it’s Mozart!), but the horn concerto is beautifully balanced. The sx-Amp presented a very smooth, rounded sound with no hint of harshness. The layering front to back was very precise, and the left to right sound stage wide, though not as far beyond the edges of the speakers as the LSO CD – a wonderful listen however.

    Dave Brubeck’s ‘Time Out’ always amazes be because it was recorded in 1959 (like some of Julian Bream’s recordings mentioned above) and you can hear the tape hiss and one or two other minor imperfections, and yet the sound is absolutely palpable. This is a re-mastered re-release but is has lost none of the quality of the original. The cymbals, always a very difficult sound to reproduce accurately, are as smooth as silk and seem to hang in the air – I’ve heard more recent recordings where they sound flat and lifeless by comparison. Paul Desmond’s alto sax and the bass, played by Eugene Wright, have some wonderful space around them on ‘Strange Meadow Lark’, one of my favorite tracks on this CD. The sx-Amp is able to convey the sparseness of the music, and the recording, reproducing the very wide and deep sound stage – very three dimensional. Again, as with the other recordings, there is a sense of a very relaxed, effortless, smooth sound.

    Most of the tracks on Michael Petrucciani’s ‘Both Worlds’ are spaciously recorded and the sound staging is good. The sx-Amp again did a great job of conveying the space around the musicians. There are a few tracks where the brass is set well back in the mix and this lends great depth to the recording, although in general the sound stage is not particularly wide. I was pleased to discover the sx-Amp could give the same sensation of depth as the Ovation 250 and the e-Amp, which offer a first class listening experience in this regard.

    I have not said much about the bass performance of this amplifier. You’d expect a 15 W amplifier like this lack the scale of higher power examples, but I was pleasantly surprised at how realistic the bass reproduction was. Importantly, it had weight and the notes were well sustained. I’ve heard a lot of systems where the bass is very lumpy. No doubt the speakers and recording play a role here, but if there are any shortcomings in the amplifier’s ability to reproduce bass notes exactly as they are recorded, or drive the speakers effectively, you can bet the overall sound is going to be disappointing. Bass plays an important part in imparting space and weight to a piece of reproduced music – this is one of the reasons sub’s often seem to bring a system to life, despite the fact that they are producing little or no acoustic output above 100 Hz or so.

    My B&W 703s are moderately efficient at 89 dB/W, and they are a relatively easy load to drive, so getting reasonable SPLs out of this set up is doable. The sx-Amp output stage is hefty, so up to the limits of the power supply voltage, it has no problem delivering plenty of current when required.

    Of course, this is not an amplifier for a ‘head banger’ music set-up – the sx-Amp is better suited to jazz, classical chamber and acoustic music. If you want 3D sound staging and shimmering highs on strings, this amp does it. If you have some efficient horns or suchlike (96 dB/W and above), then 15 W is going to allow you to get realistic orchestral levels, although I never found this to be an issue on the material I tried on my speakers as described above.

    The sx-Amp achieved all of the goals I set out when starting this project: a simple design using modern, readily available components with wide bandwidth and speed (i.e. fast rise/fall times). The design goal called for wide loop gain, which was achieved through the selection of the CFA topology. I was not expecting any huge surprises sonically, but after completing a few hours of listening tests, I can say the sx-Amp is wonderfully smooth, open and has a very relaxed, non-fatiguing sound – not what I was expecting at all, and a really pleasant surprise.

    The earlier designs from JLH and Hiraga are highly regarded and as of 2025, JLH’s is over 55 years and Hiraga’s close to 45, but they have clearly stood the test of time as constructors return time and again to their simplicity, circuit elegance and sound. Nelson Pass’s mosfet based designs, some of which date back 35 or 40 years, feature simple, elegant circuits, and much of the effort is focused on the harmonic structure of the distortion – his class A amplifiers are also legendary within the DIY community and noted for their sonics.

    I hope that the sx-Amp joins this august group of DIY amplifiers, and emerges as a ‘modern take’ on what is ultimately a very specialist and esoteric audiophile segment: minimalist low power class A amplifiers that focus on listenability.

  • Hifisonix High Performance Balanced Line Stage

    Hifisonix High Performance Balanced Line Stage

    Click here to buy a stereo PCB for this line stage over in the Shop.

    Click here for the schematic

    Click here for the BOM

    Click here for some measurements of the Balanced Line Stage

    This compact OPA1641 and OPA1642 board is the perfect building block around which to build a high-performance line stage.

    Here are the specifications for the Balanced Line Stage

    Here is a block diagram of the Balanced Line Stage