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Publisher's Letter

Valve Preamplifier A Very Big Hit

By Leo Simpson

Click for larger image

We were really surprised at the overwhelming response to the valve preamplifier featured in the November 2003 issue. It has generated more correspondence in just a month or so, than any other project that we can remember. Not all of the response has been favourable, of course. Some people have said "Hiss, Boo" for featuring a circuit using ancient technology. That is partly the response we did expect and it is generally in line with our overall attitude to valves.

Generally though, the response was very favourable and not just because older readers regarded it as a trip down memory lane. Quite the contrary actually, because people realised that we had attempted to present a very realistic and detailed description of the technology and its capabilities. We did this because we had not seen a magazine article anywhere which explained the graphical method of gain calculations.

However, some of the responses were quite negative because we had used negative feedback to improve the performance and thereby negate the distortion characteristic of valves. Shock, horror! The circuit would now not be a musical or as "warm sounding" as "true" valve circuits really are. My response to that is "what a load of garbage!"
In hindsight, we should have published the distortion curves for the first circuit we produced, which did not have any feedback. Its distortion rose to over 6%.

Sure most of that would be low-order harmonics but anybody who thinks that level of distortion is OK or even desirable clearly doesn't understand sound reproduction. Why? Because any circuit producing high harmonic distortion ALWAYS produces high intermodulation distortion as well. And while low-order harmonic distortion might be regarded as innocuous or even desirable, intermodulation (production of sum and difference frequencies between two or more input frequencies) is always unpleasant. In fact, intermodulation over a couple of per cent is just horrible.

It is also clear that some musicians think that valve amplifiers have benign "soft overload" or "soft" clipping, as opposed to the "undesirable" hard clipping typical of solid-state amplifiers with lots of negative feedback. Well, that ain't the case either, as the scope photos on page 6 of this issue clearly demonstrate.

Most push-pull valve amplifiers do use modest feedback but once they go into clipping, the weaknesses in the output transformer generally conspire to produce truly horrible distortion as you drive them seriously into overload.

We took these measurements a year or so back on a commercial valve guitar amplifier. It was quite instructive for me, as I had forgotten just how bad valve amplifiers could sound! In fact, with a nominal power output of about 50 watts, its performance could be summarised in one word: gutless.

Will we publish another valve audio circuit? Possibly. A new valve power amplifier? Maybe. But if we do, you can be sure that we will pull every trick in the book to make sure that it is as "state of the art" as possible. It would be very quiet, very low distortion and probably, very expensive. And if we couldn't make it very quiet and with low distortion, we would not publish it.

Leo Simpson

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