Comments on Selected Audio Articles, Letters, & Circuit Ideas in Electronics World.

In chronological order.

Updated: 18 Oct 2009
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COMMENTS ON EW/WW ARTICLES

Baxandall, P J "Negative-Feedback Tone-Control" Article published Wireless World Oct 1952, p402.

This is the famous article that introduced the Baxandall tone-control, which was for decades the universal choice in preamplifiers and mixing consoles, and is still by far the most popular arrangement where people feel they can permit themselves tone-controls at all. Before this article, tone-controls could be incredibly complicated, often involving switches and inductors. See Chapter 15 of The Radio Designer's Handbook by Langford-Smith; the currently reprinted 4th edition came out in 1953, and does not mention the Baxandall concept.
Apart from the payment for the article (which would not have been generous in 1952) I think I am right in saying that Peter got no financial return from this, his most famous idea.

Dinsdale, J "Transistor High-Quality Audio Amplifier" article published Wireless World Jan 1965, p2.

This two-part article is considered to have introduced the classic two-transistor RIAA input amplifier, which was widely used for many years afterwards. By modern standards, the configuration has many drawbacks, such as a lack of open-loop gain (which is particularily felt at the LF end of the audio band, where the RIAA equalisation gives the most boost) and poor load-driving capability. But you must remember that active devices were very expensive in those days, and you used as few as possible.

Turnbull & Townsend "A Feedback PWM Audio Amplifier" published Wireless World Apr 1965, p160.

This article, together with a two-parter on Class-D efficiency published in 1967, are the earliest publications on the application of Class-D to audio amplification that I am aware of. I have always assumed that they were the inspiration for the Sinclair X10 and X20 Class-D amplifers, (though the circuitry is in fact rather different) but I was dead wrong, as on looking into it, the X-10 actually appeared in December 1964, before the Wireless World articles. These Class-D amplifiers had one thing in common. The linearity was dire.

Arthur Bailey "High-Performance Transistor Amplifier" (20W) published Wireless World Nov, Dec 1966, p542

This famous amplifier is notable for using an interstage transformer between the two-transistor voltage amplifier and the output stage; this allows the output to work like a complementary circuit (I think) even though the two output devices are both PNP. Interestingly, a letter from F Butler in the December issue indicated that even in 1966 the use of such a transformer was considered old-fashioned.
The preamplifier is notable for using a three-transistor input amplifier, though the other two stages (buffer, tone-control) use simple one-transistor amplifiers.

"Low Distortion Class-B Output" published Wireless World, Apr 1968, p67

This anonymous article describes the triple output stage of the famous Quad 303 power amplifier, which was a major step forward in reducing amplifier distortion, not only because of its inherently low distortion, but also because having small and cool pre-driver transistors gave better quiescent control.
Peter Baxandall once told me that the stabilised power supply in the Quad 303, which must have added significantly to the cost, was incorporated because he and the Quad team were worried about exploiting the full collector voltage capability (Vcbo= 100V, Vceo= 70V) of the relatively new 2N3055 output transistor, and that he later concluded it was unnecessary.

"30W High Fidelity Amplifier" by Arthur Bailey, published Wireless World, May 1968, p94
Arthur Bailey produced a new design that dispensed with an output transformer. It was a single-rail rail design with output capacitor, but apart from that looked fairly modern with a fully complementary EF output stage and VI overload limiting. The input stage only had one transistor though, not a differential pair, and must have intoduced a lot of unnecessary distortion.

"Output Transistor Protection in AF Amplifiers" by Arthur Bailey, published Wireless World, Jun 1968, p154
One of the first expositions of what is now the standard approach to VI-limiting.

"Low-cost 15W Amplifier" by Hardcastle & Lane, published Wireless World, Oct 1969, p456

This design was the first amplifier in WW to have a differential pair input stage, complete with the same value of emitter degeneration resistors that I prefer myself, though the tail was only a simple resistor rather than a current-source. Despite the input pair, it was a single-rail rail design with output capacitor, (of only 800 uF) which seems to be missing the point rather.

Douglas Self "Sound Mosfet Design" published Wireless World, Sep 1990, p760

This was the first article on power amplifiers that I wrote, though I had been designing them for manufacture since 1975. This investigation into the concept of combining power FET output devices with bipolar drivers was done some years before I undertook my major investigation into the root causes of power amplifier distortion. (See Distortion in Power Amplifiers, Parts 1–8) In the throes of the design process, I realised with greater force than hitherto that the distortions in the small-signal part of a power amplifier were (a) far from negligible and (b) susceptible to analysis by a mixture of SPICE simulation and a few well-chosen experiments. I also determined that SPICE could be extremely useful in the analysis of output stages. The rest is history...

To prevent people being misled, I must point out that the final circuit falls somewhat short of the Blameless performance standards set by the Distortion in Power Amplifiers series. The input stage may look symmetrical, but in fact calculation shows it to be grotesquely unbalanced, with 16 uA flowing through the left transistor of the pair, and 580 uA through the right. This stage must have generated far more second-harmonic distortion than necessary, and with the benefit of hindsight I am not at all proud of it. This input Ic imbalance also means that the input stage transconductance is far too low, which precludes any emitter degeneration of the input pair, and this also explains why a dominant-pole capacitor as small as 15 pF is enough for stability.
The voltage-amplifier stage could be improved in linearity by adding another transistor within the Cdom local feedback loop.

Colin Wonfor "Class A to 300W" published Electronics World Mar 1999, p188

This is the letter I sent to EW at the time, which was duly published:

"I read Colin Wonfor's article on Class-A with interest. Having spent some time myself trying to make Class-A amplifiers both tractable and as linear as they should be, I was surprised he did not adopt any of the technology introduced in the Class-A (March 1994) and Trimodal (June, July 95) designs. No doubt there were good reasons, but it would be nice to know what they were. Likewise it is a great pity that no performance details were given, to confirm that the great cost and heat are really worthwhile.

My investigations showed that the use of power FETs in the Class-A output stages makes linearity worse. The Class-B problem of a horribly jagged crossover region is no longer relevant, but the low device transconductance still causes high distortion.

An automatic bias controller would have cost a few pence, and removed the need for a perilous quiescent-adjust preset that allows infinite current to flow at one end of its travel.

The instruction to trim the DC offset to less than 10mv with a preset seems rather strange when it is straightforward to make an amplifier that gives an offset of less than 25 mV with no adjustments.

It concerns me that there is no effective DC-offset protection given the large amount of power available. I note the output fuse (no value shown) but I would be interested to know how the constructor is going to select its value so it can protect loudspeakers without nuisance-blowing or introducing distortion. A 1 Amp slo-blo output fuse carrying 20W/8 Ohm generates 0.01% THD (third harmonic) at low frequencies, so fuse distortion could easily make the finer nuances of Class A linearity somewhat irrelevant.

It seems astonishing to use the constant-current (single-ended) mode for a large Class-A power amplifier as this doubles the already enormous heat dissipation. The largest version described releases 3 KW of heat, which will not be significantly reduced by playing music at full volume. Surely this will be an uneasy companion in summertime? The quiescent current for the so-called 300W/4R version is prescribed as 10.2A, so negative peaks cannot go beyond 4 x 10.2 or 40.8V below ground. This corresponds to a maximum sine output of only 208 W, so using supply rails as high as +/-75V appears to do nothing but increase the power dissipation. It allows no safety margin for loudspeakers that fall below a nominal 4 Ohm impedance. Perhaps there is a typo in the article for this huge emission of heat makes little sense to me.

At the risk of seeming discouraging, no information is given that makes me see this design as a significant advance in Class-A amplification."

Graham Maynard "Class-A Imagineering" published Electronics World 2004

Page after page after page of mind-rotting twaddle from a man who does not understand his own simulation results.

Wangeri Kaguongo "Reduce Ear Fatigue In Power Amps" published Electronics World Mar 2009, p24

This is the email I sent to EW:

"Hi Svetlana
You've probably heard more about this than you want to by now, but I couldn't let the article "Reduce Ear Fatigue In Power Amps" in the March 09 issue go by without making a protest. It really is complete twaddle from start to finish.

Since almost every sentence contains a statement that is either completely incoherent or flat-out wrong I won't waste your time by going through it in detail, but as a sample, this is what I think of Fig 5, which claims to be a "fully simulated and tested circuit that may be fun for designers". Not much fun at all, I suspect:

1) The amplifier has a bipolar input but is fed from a source impedance of not less than 4.4K, and as much as 29.4K when VR1 is set in the middle. This error alone will generate avoidable extra distortion of between 4 and 20 times that of the amplifier when it is fed from a low impedance as it should be.

2) There are two bias transistors Q18 Q22 both trying to control the output stage bias. At best, one is redundant.

3) The Zobel network R20 C8 is on the wrong side of the output inductor L4, so cannot fulfil its function. Expect HF instability with inductive loads. Loudspeakers, for example.

I'll stop there, hopefully having underlined the fact that Wangeri Kaguongo, whoever he may be, has no clue, and publishing this sort of nonsense diminishes the reputation of EW. In contrast, the preceding article on Hum by Burkhard Vogel is first-class stuff, and can only enhance EW's prestige."

Her reply was that she had had no other complaints.

However, to my surprise, the email (slightly edited to remove some of the criticism) was eventually published in the September 2009 issue.

COMMENTS ON EW/WW LETTERS

The Input Filter Distortion Debate (started Aug 2000)

I never bothered to get involved in this debate, because it was about inaudible phase-shifts at silly frequencies. What escaped me at the time (and everybody else, it would appear) is that the tiny shunt capacitor was wholly irrelevant; the crashing error was putting a 10K resistor in series with the amplifier input. Such a source resistance increases both the distortion and worsens the hum performance of the typical power amplifier with bipolar input devices. By 2002, I had realised just how harmful this effect could be, and I wrote an article on the subject that was published in EW in May 2003. I found that a input resistor of 3K9 degraded the THD of a Blameless amplifier at 1 kHz from 0.0018% to 0.0060% at 50W/8R. At 10 kHz and 50W/8R the degradation was from 0.013% to 0.042% with 3K9.

Since the effect is proportional to input resistance, I extrapolate that the wholly unnecessary 10K input resistor would have given 0.015% THD at 1 kHz, which I suggest is approaching audible proportions, and a distinctly excessive 0.10% at 10 kHz. Oh dear.

The Class-A Imagineering Debate (started Sept 2004)

Graham Maynard's interminable "Class-A Imagineering" series quite rightly received a lot of very negative comments. I never got involved myself, something I now regret. I should have stood up to be counted. However, it would have been hard to improve on Phil Denniss's merciless dissection. I particularly like the phrase "Faith-based audio".

COMMENTS ON EW/WW CIRCUIT IDEAS

150W AB amplifier by Harrison Kaguongo (Jan 2007)

This design at first looks pretty conventional, but it has an enigmatic transistor (Q17 in Fig 3) whose function is mysterious because it is never mentioned in the text. The design has been simulated but not measured so it may not work at all in reality.

Could this be the same chap that perpetrated the appalling Earache article in Mar 2009? See above. There can't be many people in the audio world called Kaguongo.

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