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Baxandall’s Original Tone Control Article from Wireless World 1952
This article was published in October of 1952, when Baxandall was 31 years old, the design having already won him a prize 2 years earlier in a competition. His design basically relegated other tone control circuits to the scrap heap, although cheap passive, or non-inverting circuits still persisted well into the 1980’s, especially in low cost mass market products from Japan. Of course, now tone controls can be implemented digitally, but for the analog type, this remains the gold standard.
Stan Curtis 60W Class A Amplifier from ETI
Stan Curtis was involved in both engineering and management roles in companies like Quad, Cambridge, Rotel (a family owned Japanese brand) Mission and Lecson to name a few. He is primarily an audio business consultant and remains highly respected and a noted product designer within the sector in his own right.
This is his 60 Watt DIY class A amplifier published in ETI in 1985 which has been referenced on numerous web sites and publications over the last 25 or so years. It’s a rather complex design, but, 60W of class A power translates to a BIG amplifier with significant standing dissipation.
Low Cost Isolation Amplifier for Industrial Applications
A Design Idea of mine that appeared in EDN around 2000. The article appeared with the title ‘Lost Cost . . . ‘ – seems someone got a bit mixed up. The idea has been referenced in a few papers on the web – always good to see!
Low_cost_isolation_amplifier_suits_industrial_applications
Here’s another one using a delta modulator that also works very well. I submitted this to EDN but it was rebuffed. I guess whoever reviewed the idea wasn’t used to seeing an opto operated in photo-diode mode.
Another Low Cost Isolation Amplifier for Industrial Instrumentation Applications
Hifisonix UBx Universal JFET Buffer
A while back, one of the diyAudio forum members who was building the nx-Amp (here’s the thread on DIYaudio.com) enquired about either reducing the amplifier’s overall gain, or providing a volume control facility. Since both the 15 W class A sx-Amplifier and its bigger 100W sibling the nx-Amplifier, are Current Feedback Amplifiers, performance is quite carefully optimized for a specific gain – they are not as tolerant as voltage feedback amplifiers in this regard. Although not specifically raised in the thread, a consensus seem to develop that what was actually needed was a general purpose buffer that would accommodate the input from a pot, and be able to comfortably drive a power amplifier with typical input impedance of 10 k Ω.
No originality is claimed – Nelson Pass’s B1 buffer has been around for years and is well regarded – the UBx ‘Universal Buffer’, as this will I hope come to be known, also uses this classic configuration (but without the cascode), which I believe was first published by either Siliconix or National Semiconductor way back in the late 1960’s.
You can download the PDF file here UBx
OPEX-1: A Discrete OPAMP for Audio
This design showcases a discrete opamp for audio applications. It features very low noise, low distortion and a class A output stage that will deliver 1ppm distortion into a 600 Ohm load at 12 V peak.
Solid State Relay with PCB Layout
A complete relay that is smaller than a Tyco RT series EMR Simple_solid_state_relay_Updated . This design uses a small double sided PCB and some SMD components. Here are the Gerber files SSLR. The recomended mosfets for this design are the PSMN4R3-100PS[1] for supply rails of up to +-50 V absolute maximum, and for supply rails of up to +-75 V, the Fairchild FDB075N15A[1]
Augmented Feedback Error Correction (AFEC)
AFEC is a simple technique that augments the feedback of an amplifier to dramatically improve the Large Signal Non-linearity (LSN) distortion performance by up to 20 dB across the audio band. Additionally, AFEC acts to remove any DC offsets and also improve PSRR significantly.
Augmenting the feedback in a manner similar to that described in this document has been tried previously – the first example is from the early 1980’s at Hitachi. However, the technique never entered the mainstream – one can only postulate that lack of very high performance opamps (bandwidth, SR and distortion performance), along with the difficultly of being able to characterize such a system for stability modeling purposes (as we can do now on LTspice for example) were contributing factors to its early demise. However, as of 2012, these problems are now solved, so perhaps we shall now see the return of AFEC as a viable distortion reduction technique.
You can download the article here AFEC-V3.0
Excerpt
One of the advantages of CFA power amplifiers is their very high slew rates (200+ V/us is not uncommon), but loop gains tend not to be as high as VFA topology designs. This is fine at low to medium signal levels, where the very good front end linearity and wide loop gain bandwidths of CFAs’ manifest as low distortion; however, at higher power output levels where the output stage and TAS/TIS LSN starts make itself felt, CFA distortion is generally worse, and this is as a direct result of the lower loop gains – note, I am talking here about minimalist CFA topology designs. Cross over is the major type of distortion in class AB amplifiers and there have been various schemes invented to deal with this over the years – most relying on feed forward techniques (see Peter Walker’s Quad Current Dumping for example, or Michael Renardson’s MJR7 design). AFEC does a good job of reducing cross over distortion artifacts, and in particular, those arising from OPS bias current shifts, but it will not compensate for an under biased OPS where there is a clear discontinuity in the crossover region. CFA amplifier PSRR is lower than VFA. This usually necessitates a front-end stage regulator – but even then, the best designs are still usually 20~30 dB below that of VFA exemplars. It will be shown that AFEC can improve CFA PSRR such that it matches or exceeds VFA PSRR, whilst at the same time removing any output offsets as a result of the servo action of the AFEC control amplifier.
Output Coupling Inductors
Inductors on the output of an amplifer are used to isolate the amplifer from capacitive loads at HF and ensure stability. There has been quite some debate on DIYaudio.com as to whether they are audible, necessary, or just the result of designers being overly cautious. Some very well known designers in the field have decided against using output inductors. In this short presentation, I explore the pros and cons.
Oscillation Sniffer
No ‘scope? This simple circuit can help you quickly see if you have any oscillation problems on your opamp or amplifer circuit.
Note, its NOT designed to be connected to high impedance nodes.
H.S. Black: Feedback
Feedback, or its application by humans as control theory, despite the protestations of audio designers that eschew its use, is an entirely natural phenomena. Every time you reach out to pick up an object, get up and walk around, or write something on a piece of paper, feedback data from multiple body sensors are being processed by your brain so that your immediate objective can be executed flawlessly and without error. And, here we have not even begun considering the complex bio-chemical metabolic pathways within the human body – or every single other life form on the planet for that matter – that rely on multi-loop, self-regulating (i.e. feedback control) behaviour. Nature abounds with examples of the miracle of feedback – a bird in flight, the earths weather system (human CO2 emissions notwithstanding), the thermonuclear reaction taking place in the core of a stable main sequence star, and even planet wide climatic control as exemplified by James Lovelock’s Gaia Hypothesis and so forth. It may therefore be more correct to say that humans discovered feedback but did not invent it.
H.S. Black published this article about feedback in the Bell Labs in house magazine ‘The Bell Laboratories Record’ in 1934, based on work he had done on the subject starting as early as 1927. In it he explains how feedback can be used to improve the performance of repeater amplifiers. Feedback has been described as the most important development in electrical engineering in the 20th century.
Feedback has been thoroughly and completely explored and codified over the last 80 years resulting in a huge body of published scholarly work across many hundreds of application fields ranging from aerodynamics, power generation, consumer electronics and onto process control in chemical production and self-regulating metabolic processes in the life sciences. It is no exaggeration to say that our modern world would not function without it, and neither would mother nature.
Next time you board an airplane, rest assured you will only be arriving safely at you destination because there are hundreds of inter-linking feedback loops all operating flawlessly – from the airplane ailerons, tail fins, pressure sensors to the jet engine turbine control systems and on to navigation and guidance and so forth. Audio amplifiers in this context, and within the natural world, are therefore an exceedingly simple application of the science of control theory aka feedback.