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  • Audio Power Amplifier Design – Peter J Baxandall

    In this 6 part series of articles published from January 1978 in the now defunct Wireless World magazine, Peter Baxandall takes the reader through some of the fundamentals of audio amplifier design as they were understood at the time.

    Baxandall_Audio Power Amplifier Design

    In 1978  there was still much discussion about feedback and how to apply it. Otala’s famous paper about TIM had been out for a few years and some of the assertions in that paper were beginning to be challenged. Solid state amplifier design was still very much a formative discipline with most practitioners trying to see through what appeared to be the conflicting requirements of high feedback, zero TIM, and low distortion. It would be a few more years before the rules of the game would emerge, and designing ‘blameless’ amplifiers became a reality.

  • Douglas Self’s 8 Distortions and a Few More

    (pictured above is the hifisonix 180 Watt per channel e-Amp)

    No one had codified all the major audio amplifier distortion mechanisms until  Douglas Self published the first edition of his ‘Audio Power Amplifier Design Handbook’ or APAD’ as it has become known, in 1996.  He used a standard Lin* topology amplifier and then proceeded to show in a logical and easily understood manner, how all of the major distortion mechanisms in audio amplifiers could be reduced to ‘vanishingly low levels’ i.e. parts per million levels.  In an industry awash with voodoo engineering claims, he was thus careful not claim his amplifier was the best, but merely that in terms of the whole signal chain from studio microphone to the consumers loudspeaker in a domestic environment, the power amplifier contributed the least distortion. He thus termed his resultant design the ‘blameless’ amplifier. 8 key distortion mechanisms were identified in his book and in the short article you can download below, I’ve described each one, how much distortion it can contribute, and how it can be negated with good design.

    Douglas-Selfs-8-Distortions-and-a-Few-More

    *Lin amplifier topology is named after Harry Lin, a Bell Research Labs scientist who in the late 1950’s  first proposed the now standard three stage Voltage Feedback Topology (VFA) amplifier: Input voltage to current stage, integrator and the output stage buffer. All VFA amplifiers are derived from this basic topology.

     

  • The Theory of TIM – Matti Otala

    Matti Otala  follows up on earlier research on the subject of TIM.

    The Theory of TIM Matti Otala

  • Some Ideas on Temperature Compensation for Audio Amplifier EF Triples

    Temperature compensating an EF3 audio power amplifier output stage is not a trifling task.  There are 6 Vbe junctions, running at 3 different current densities, different temperatures and neither is the thermal performance of the heatsink assembly characterized in most DIY cases. This makes deploying a conventional 2 transistor  Vbe spreader (or Vbe multiplier as it is sometimes called) a little less than straight forward if one is to secure decent temperature compensation – i.e. to within 10% over the full operating range. The article below explores a slightly different approach to the problem in which, during the development phase, the compensation is designed to intersect the ideal bias voltage at two different temperatures, ideally the first at ambient and the second up above 50 deg C – a technique I later called ‘two point temperature compensation’.  Some time after writing this in 2010, I started the design and then construction work of the e-Amp, and used what I learned writing this up to solve the problem using an NTD thermistor, which you can read about in the e-Amp article on pages 37 to 40.

    Temperature-Compensation-for-Audio-Amplifier-EF-Triples-V1.0231Download

  • Review of Bob Cordell’s Book ‘Designing Audio Power Amplifiers’

    ‘Designing Audio Power Amplifiers’ by Bob Cordell  – Reviewed by Andrew C. Russell in 2011

    Image result for bob cordell of cordell audio

    Bob Cordell (on the right) pictured with Jan Didden, publisher of ‘Linear Audio’

    After a 15 year hiatus from electronics, I returned in 2005 to linear design not as a professional but as a hobbyist. Earlier in my career, I spent six years designing industrial instrumentation – things like thermocouple amplifiers and linearizers, A-D’s, power supplies, isolation amplifiers and so forth. I’ve always been passionate about music and audio equipment, so it was only a matter of time, with children grown up and out of the home, that these interests would be rekindled.

    Audio amplifier design is a specialized branch of electronics in which the  engineer seeks to amplify and reproduce very accurately through a speaker system,  low level source signals from CD’s, tuners, turntables and the like.  As such, it is a fairly unique speciality since it combines precision (very high linearity requirements) over a wide bandwidths (200 to 300 kHz) along with high power requirements (amps of output current with voltage swings of up to +-70 V in  a high power unit). Further, with the application of feedback, there is a dash of control theory thrown into the mix as well, and this is especially challenging when one considers that the loudspeaker load is highly non-linear wrt frequency.

    Cordell is a professional engineer who has been blessed with the ability to explain complex technical concepts in a concise, understandable manner. This book starts off with the fundamentals of amplification and then goes on to show how to take a basic design, and with a few well-honed circuit approaches, evolve it to create very high performance, low distortion amplifiers. Chapter 5 is an in-depth discussion on feedback and compensation techniques which is traditionally one of the more challenging areas of amplifier design, but he covers this in a practical and succinct manner, exposing even seasoned, professional designers, to an array of advanced compensation techniques.

    Voltage amplifier (VAS) and output stage design are also covered, along with the various trade-offs between the circuit approaches and output device technologies (bipolar and mosfet), and associated protection schemes. There is an extensive section on output stage topologies, covering both bipolar and mosfet technologies, along with a very interesting chapter on Hawksford error correction as applied to mosfet output stages, which Cordell helped popularize back in the early eighties with a ground-breaking design at the time.

    The book delves into the subtleties of zero global feedback design amplifiers and Cordell diplomatically deals with the debate raging in audiophile circles about feedback (some for, and others against). What is special about this book is that it is grounded in very solid engineering theory and practice, and, rather than express opinions on why a certain design approach or philosophy is best (a temptation most writers and practitioners in the field are unfortunately unable to avoid), Cordell actually covers both sides. The reader thus comes away with an appreciation of the design challenges required in both feedback amplifiers and zero global feedback amplifiers. The same can be said of his discussion around bipolars and mosfets along with output stage protection.

    This book is enormously important for the high end audio design community, and makes the state of the art accessible to a whole new generation of practitioners. If you are a professional audio engineer, then this book is an invaluable reference, and will almost certainly help you raise your game. On the other hand, engineering students will find the practical, down to earth explanations a useful resource in helping get from classroom theory into practical designs. And, for a DIY’er, I’d say it is an absolute necessity if one is to gain a solid, clear and unbiased introduction to the art. If Douglas Self is to be credited with removing the misunderstanding around power amplifier design, then Bob Cordell must be credited for bringing state of the art design techniques to within the reach of everyone with any interest in high performance audio in a thoroughly practical and highly readable book.

    Highly recommended

  • Saturn V Rocket Performance Report

    Absolutely nothing to do with audio, but a great read for the techies.

    This performance report was pulled together in the late 1960’s by Walter Haeussermann , one of the German rocket scientists who went to the US after WWII, where he settled and went on to have a spectacularly successful career with NASA.  He was a key member of Werner von Braun’s team that made the 1969 US moon landing a reality. Just take a look at the section on the control computer they used to guide the beast . . . 

    Saturn Guidance System

    Here is a link to a very nice YouTube video about the Saturn V control and guidance computer https://youtu.be/dI-JW2UIAG0

  • CFA Diamond Buffer IPS: Does it Operate in Class B or class A?

    There is a persistent assertion by small group of amplifier design practitioners that the diamond buffer input stage of CFA audio amplifier operates in class B (or AB) mode wherein there is a hard ‘handover’ between each half of the diamond buffer input stage as the input signal passes through 0V.  The most recent claim of this kind is in the June 2017 issue of AudioXpress by M. Kiwanuka .

    The short article below explores this subject and finds that the claim is wholly incorrect: the front end of a CFA diamond buffer input stage operates well within the class A region for all known audio signals

    You can download the article here:-  CFA Front End

    Note that this article discusses discrete audio power amplifiers and not IC CFA operational amplifiers which are severely power constrained by design.

  • Barney Oliver: Crossover Distortion in Class B Audio Amplifiers

    This is the famous analysis of class B amplifier cross-over distortion by the then head of HP Reasearch Labs, Dr. Barney Oliver,  published in the February 1971 edition of the HP Journal.  The  bias current Iq for a class B emitter follower amplifier is shown to be approximately  Iq = .026/(Re+re+(rb/hFE)). In practice,  – the ideal value ending up somewhere between a 13 to 26mV drop across the output transistor emitter degeneration resistors.  This paper provides the theoretical underpinings for that relationship. So, when you hear about the ‘Oliver’ voltage, you know where it came from. You can read more about Barney Oliver here and here

    Download the PDF here:  Cross Over Distortion in Class B Amplifiers

    See the comments below. My practical experience over many amplifier builds is that indeed somewhere  13 and 26 mV  is ideal.

  • More Notes On Cascode Amplifier Oscillation

    Here is a short write up on cascode oscillation I did back in 2012 when designing and developing the e-Amp.

    Cascode Oscillation in Audio Amplifiers.pdf

    I recently (2017) had a recurrence of the problem on another high power design – some pictures are shown below.  When I went back and looked at the notes above, I realised I had not followed my original advice, and the problem had returned to plague me  – clearly a case of ‘those that fail to learn from their mistakes are condemned to repeat them’.

    This is what you get when you place the scope probe on the emitter of the cascode transistor. The probe capacitance is probably contributing to the problem and causing it to break into oscillation, or it may be increasing the level of existing oscillation. Either way, its not acceptable, and especially so if you are trying to design a circuit to deliver single digit ppm distortion performance.

    Here it is with the time scale expanded:-

    If you want to prevent or limit the probe from affecting the circuits behaviour, one trick is to look at it with a 10x probe – the input capacitance is much lower.  Another option on 1x, is to place a 50-100 Ohm resistor in series with the probe – this helps to isolate the probe capacitance although you will still have some attenuation because of the scope probe and scope input capacitance.  Use a 1206 surface mount device and solder it upright on the node you want to probe. Note that a 1x scope probe input capacitance is about 50 pF//1MEG Ohm and a 10x probe is 15 pF//10 MEG Ohm.

    Its very important to note that you can form Colpitts oscillator structures in the base, emitter and collector circuits of transistors. Small signal audio transistors  have fT’s of 100 ~ 300 MHz so all you need is some inductance (on a PCB this is often in the 40-60 nH range corresponding to 4-6 cm trace lengths) and then capacitive coupling (10-30 pF – layout dependant) from each end of the inductance to a non-inverting terminal on your amplifying device along with some gain. Below is a screen shot of three circuit examples.  They all show HF instability and oscillation to some degree with 10’s to 100’s of mV at 20 to 100 MHz frequencies, but with some value combinations, it is quite easy to get volt level HF oscillation. As you can see, the LC networks that lead to problems can arise across any two terminals.

    Although the PCB traces in a conventional analog amplifier are unlikely to be long enough to qualify as antenna’s at the frequencies mentioned, you will still couple a lot of this garbage capacitively into other small signal parts of your circuit. As noted above, if you are working on high performance audio circuits,  problems like this will quickly put paid to any ppm or sub-ppm distortion aspirations you may harbour.

    The high voltage PNP MMBT5401 and its NPN counterpart the MMBT5551 are often used for level shifters and feature an fT of 100 MHz to 300 MHz and a Cob of 6pF – they are fast and in the cascode configuration will easily oscillate if the conditions are right.

    The following preventative measures (not an exhaustive list) provide a good starting point:-

    1. Place a 470 to 1k SMD (1206 or 0805) resistor as close as possible to the base of the cascode transistor. This lowers the Q of any inductance (i.e. ‘dampens’ it) in the base circuit and swamps any -ve resistance reflected into the emitter.
    2. In some cases,  a SMD ceramic capacitor from the base of the cascode transistor to ground may help.  I’ve found values between 10nF and 100 nF work well. Do not use film or anything else exotic – XR7 dielectric rated at 3-4 times the voltage on the base is about right. The capacitor ESR is also part of the fix.
    3. Make sure the overall loop area from the cascode base reference voltage to ground and the driver transistor in the emitter is small.  If not, you will simply be adding inductance in the base circuit and will exacerbate the problem – loop areas have to be kept small to minimize inductance.
    4. Following on from (3) above, recall that the output at the cascode transistor collector is a current, so you can run fairly long traces from the cascode collector to the next part of the circuit – typically a common emitter stage referenced to the supply rails. However, you must minimize any capacitive coupling from the cascade collector circuit to its emitter – the best way to do this is through attention to layout.
    5. If the signal currents are low (1~10 mA), the propensity for the cascode circuit to break into oscillation can be further reduced by inserting a resistor of 100 Ohms to 1k in the collector of the cascode, located as close as possible to the device.  This technique also helps by the way in emitter followers or beta helpers.
    6. If your circuit currents are low enough to allow, insert a low value resistor (100~200 Ohms) in the trace close to the cascode transistor emitter – this will help reduce the Q of the trace inductance.
    7. Pay attention to layout during the design stage – keep the cascode, driver transistor and associate circuit compact and with short traces. Keep loop areas small.

    One final point about using Zener diodes as the reference voltage to the base of the cascode transistor. Zeners above  about 7 V generate a lot of broadband noise right up into 100’s of MHz.  Without filtering, damping and careful layout as described above, this noise can promote instability in cascode circuits.