High Gain - Cold Bias

[Mike Sulzer]

An amplifier can and should be designed so that the later stages do not contribute any significant level of noise when set up reasonably. There is no inherent reason why they have to as you are saying. For example, consider this design: http://music-electronics-forum.com/t41822/. It has more total gain than you can use, but no audible noise except from the first stage unless you purposely misadjust the three level controls to make it happen.

I'd say the general principle is that the more total gain the amp has, the more the critical noise from the first stage is amplified. In addition, more stages mean more noise. For example, adding 30dB of gain and then attenuating by 30dB adds more noise than not amplifying the signal at all.

The topic concerns high-gain amps such as the Soldano SLO or Peavey 6505+. So, we don't have to consider non-master amps that need to blast the PI and poweramp to get distortion. On the contrary, poweramp distortion is usually not part of the desired sound for modern high-gain amps.

Many of these amps attenuate the signal from the high-gain channel at the end of the preamp, for various resaons. For example, to match the level with the clean channel or to interface with reverb circuits, effect loops, etc. In addition, there is a master volume that attenuates the signal futher (maximum master volume is often not the "sweet spot" for these amps). Therefore, we can get away with a diode clipped signal without having to compensate with extra gain (depending on the type of diodes used).

Let's say we have a typical 3-stage preamp. To turn it into a high-gain monster (more clipping/distortion/sustain) we can either: Increase the amount gain of the existing stages, add more gain stages or use clipping diodes at the end. Increasing the gain or increasing the number of stages also increases noise. Adding clipping diodes adds no noise.

I have built a 3-stage amp with switchable clipping diodes á la Jose. If you engage the diodes and turn the gain down to get the same level of distortion/clipping/saturation, you can hear that the level of noise is lower. However, I usually prefer the sound with gain up and diodes off. I can live with the added noise

[Mike Sulzer]

Yes, thanks, that is a good example.

Let’s compare two simple cases which just reach clipping in the pre-amp.

Take the guitar input signal to be 100mV peak-to-peak, and assume that the PI and power stage run clean and require 5V peak-to-peak into the PI to achieve full output.

Case A (with diode clipper): gain required before diodes to reach clip 1.4/0.1 = 14, gain of clipper stage = 1, gain required after clipper 5/1.4 = 3.57. Overall gain of pre-amp = 14 x 1 x 3.57 = 50

Case B (with triode stage clipper): gain required before clipper 5/0.1 = 50, gain produced by clipper = 40, gain required after clipper = 5/200 = 0.025. Overall gain = 50 x 40 x 0.025 = 50

Same in both cases! This supports Mike’s point of view (I think).

The resistive attenuation required after the triode clipper in case B will add a little noise but this will be negligible compared to input stage noise.

(I’ve assumed that the diodes clip at 1.4V p-p and that the triode clipper requires 5V p-p input to clip, has a gain of 40 and produces 200V p-p output.)

[PeanutNore]

I’m just wondering if this might be part of the reason why some guitarists prefer a clean running tube amp (to get the slight distortion, sag, etc. that we call ‘tube sound’) plus solid-state pedals for the heavily distorted sounds?

From my experiments with all sorts of solid state distortions as well as tube amps, solid state pedals for heavy distortion aren't really any quieter than a high gain tube preamp, with the exception being circuits which have a gated signal response and won't pass any output below a certain signal threshold - an example being the Devi Ever Soda Meiser fuzz pedal that I've got.
Guitarists I know fall into one of two camps: either don't care about the noise because the signal is so loud you never actually hear it while playing, or use a noise gate in the effects loop with the 4 cable method (guitar -> noise gate input; noise gate send -> preamp input; amp effects send -> noise gate return; noise gate output -> amp effects return).