OPINIONS ON VALVES （3）

Published:2011/8/19 1:52:00 Author:Phyllis From:SeekIC

The valve sound: myth or reality?

By Klaus Rohwer, PhD

That famous valve sound, does it really exist? Yes, it does — but it doesn’t have to come from a valve! Here we have to make a distinction between devices for producing music and devices for reproducing music. The first category includes music amplifiers and effect generators, while the second category includes hi-fi and high-end sound systems for the living room. The first type of equipment is optimized for high efficiency (volume) and designed to enable musicians to obtain the sound they want to produce. The second type of equipment should be optimized to reproduce the input signal in an amplified form with the least possible adulteration or distortion. However, many musicians and audio enthusiasts evidently believe that something must glow somewhere in order to generate a ’warmer’ sound. I consider this to be purely an idee fixe.

Firstly, as regards music production, what we mean here by ’valve sound’ is primarily demanded by guitar players and blues harmonica players. The sound of a jazz guitar is just as inconceivable without valve sound as the sound of Chicago blues. It consists of four components:
- soft clipping, instead of the hard clipping characteristic of transistor amplifiers;
- o compression effect, due to a power supply with high internal impedance;
- a ’fuller’ sound, due to additional even harmonics produced by a characteristic curve that is asymmetric (even in the small-signal range);
- increased coloration by loudspeaker characteristics, due to a high-impedance output.

Soft clipping results from the gradual bend in the gain characteristic [the ratio of output voltage to input voltage), in contrast to the sudden bend in the characteristic curve of a normal transistor amplifier. Symmetric clipping, whether hard or soft, always produces only odd harmonics in the output signal. However, with soft clipping the amplitude of the higher-order harmonics drops much faster than with hard clipping, with the result that the sound is less ’hard’ (Figure 2). Similar considerations apply to high-impedance drive (current drive) for the loudspeakers. Signal compression can also be obtained using transistors, although it is more difficult than with valves. In the present age of digital signal processors (DSPs), it can be achieved quite easily, and there are several commercially available effect generators and even complete amplifiers (such as those made by Line 6; see the weblink) that can emulate certain types of amplifiers with extraordinary fidelity, as the author has been assured by several users.





As regards music reproduction, if you want to depart from a straight-line characteristic, which is actually the ideal for any hi-fi or high-end amplifier, you don’t necessarily hove to use valves, since the same results can be obtained using transistors, as already mentioned.

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