Nature of preamp distortion

[tonebrulee]

I'm sure a good explanation for this exists somewhere on the web, but I've never been able to find it:

In a guitar amp with GAIN and VOLUME controls, I understand that the level of amplication in the preamp section is controlled by the GAIN control, and the level of amplification in the power amp section is controlled by the VOLUME control. We know that increasing the GAIN beyond a certain point will introduce what is called "preamp distortion" into the signal, which is then fed into the power amp section and further amplified prior to output. My question is whether "preamp distortion" is 1) the signal being clipped within the preamp circuit as it is pushed beyond it's headroom threshold, 2) the signal being clipped at the power amp input as the preamp puts out a clean signal that is too hot or 3) both?

Also, while the overall tone/quality of the signal might differ somewhat between tube and solid-state amps, is there any practical difference in HOW "preamp distortion" works in tube vs. solid state amps?

Thanks much.

[SWRing]

Re: Nature of Pre-Amp Distortion

Quoting above:
"We know that increasing the GAIN beyond a certain point will introduce what is called "preamp distortion" into the signal, which is then fed into the power amp section and further amplified prior to output. My question is whether "preamp distortion" is
1) the signal being clipped within the preamp circuit as it is pushed beyond it's headroom threshold,
2) the signal being clipped at the power amp input as the preamp puts out a clean signal that is too hot or
3) both?"

As far as I know, the answers to your questions are:
1) Typically, in tube amplifiers, pre-amp distortion occurs when the signal is amplified beyond the "headroom" of the tubes in that particular stage. Increasing the "gain" knob increases the level of amplification, causing more distortion to occur.
2&3) Depends on the amplifier, although typically I think that "pre-amp" distortion occurs only in the pre-amp and not at the "front-end" of the power amplifier. The front end of the power amp most likely has more headroom than the pre-amp distortion stage (or the pre-amp signal is attenuated after distortion, prior to the power amp). This is especially true for multiple channel amplifiers. For there to be a "clean" channel, the front-end of the power amplifier must have lots of headroom to keep the signal clean, but when you switch channels to a distorted channel, you want distortion. This points to the fact that the distortion in the "dirty" channel is entirely within the pre-amp section.

To answer your other question:
"Also, while the overall tone/quality of the signal might differ somewhat between tube and solid-state amps, is there any practical difference in HOW "preamp distortion" works in tube vs. solid state amps?"

Yes. There is a significant difference between how solidstate and tube pre-amps cause distortion. Typically, in a tube amplifier, distortion causes by saturating the actual amplifiers (the tubes). That is to say that they drive the signal past the "headroom" or physical limits of the amplifier stage. Tubes will saturate relatively gracefully and/or asymmetrically. For example if you look at a clipped sine wave from a tube amplifier, the top will be flat, but the edges of the clipped top will not have sharp corners, they will be more rounded. Also, the top and bottom of the waveform might not saturate in the same way, creating even order harmonics, which apparently sound "better".

In solid-state amplifiers, as a rule of thumb, you never want to clip an amplifier (this is true for amplifiers in the pre-amp and power amp). When solid-state amplifiers clip, they clip very abruptly and harshly, which typically does not create the nice sounds that people have come to like from tube amps. So, in solid state amplifiers, designers attemp to mimic tube distortion by creating disortion stages which typically involve some type of voltage limiting stage such as a pair of diodes clamping at the output, or in the feedback loop, of an OPAMP. So, in solid state pre-amps, the distortion does not come from driving an OPAMP past its headroom, but rather from driving a signal past the threshold voltage of a pair of diodes. The type of diodes used depends on the design, sometimes typical Silicon signal diodes are used, sometimes LEDs are used because of their higher turn-on voltage and/or because of their more "rounded" (less abrupt) turn-on characteristic.

As a final note:

Distortion in tube amplifiers is typically created by the tubes themselves clipping. THis sounds good in both the pre-amp AND the power amp.

Distortion in solid state amplifiers is typically created by clamping the signal with diodes (simulated tube saturation), and not by saturationg the actual solid state amplifiers (i.e OPAMPS). Because solid-state amplifiers do not typically sound good when they clip, distortion in the power amp is typically avoided. Therefore, distortion in solid-state amps is typically only desired in the pre-amp section.

Hope that answers your question.