4 ms·

To clarify what I meant about headroom, I was specifically talking about headroom before the circuit starts to clip/break up/etc. e.g. If you have a diode clip

To clarify what I meant about headroom, I was specifically talking about headroom before the circuit starts to clip/break up/etc. e.g. If you have a diode clipping OD/Distortion circuit made out of some diodes with a Vf of 5mV as opposed to the 700mV of a normal diode, you would need to have a smaller input signal for an equivalent amount of distortion. That smaller signal has a lower SNR given some constant noise floor from thermal noise etc, and the SNR can only stay the same or get worse after you start amplifying it.

kazinator 12y ago
That is well and good, but pointless. People use such a circuit to get a heavier distortion with the same input signal, not to get light distortion with a smaller signal. Let's put it this way: if you have high gain metal distortion circuit gained up to a 5mV clipping threshold, and then you roll down your guitar volume knob to play the blues, you're kinda using it wrong.
- hamiltonkibbe 12y ago
  It's pointless in the case that you mention, where you're going for a hard clipped metal distortion tone. I was envisioning more of an overdrive sort of application where hitting the rails is NOT the desired effect. The 5mV hard clipper was an extreme example, but you could conceivably use it for soft overdrive/compression with a small enough input signal which was what I had in mind.
  - kazinator 12y ago
    If hitting the rails is not desired, then don't configure the effect to have so much gain. The point still stands that you can get the distortion effect, at the desired amount, at various places in the signal chain. The problem you're talking about is about using too weak a signal and then using make-up gain. That can also happen anywhere in the chain. I can reproduce the problem without distortion at all, using a clean signal chain. Repro 1: turn down the guitar volume to simulate an extremely weak pickup. Then turn up the input gain to compensate. Result: noticeable increase in noise. Repro 2: turn down pre-amp volume, and crank up power amplifier volume to compensate. Result: noticeable increase in noise. Gains/levels have to be evenly distributed in the chain. Sure, you will lose headroom if your intent is to have some soft distortion from the back-end of the amplifier, but you put a heavier distortion earlier in the signal chain. The heavy distortion masks the later one; it evens out the level so the headroom from the back is not available. There is a fix for that: you can punch that distortion in and out with your foot as needed.
grandalf 12y ago
are SNR issues like that and the headroom an issue for a guitar amp? I mean why couldn't the solid state part be designed conservatively with extra headroom? Or do the desirable distortion products make that difficult even with a conservative design in later stages?
- hamiltonkibbe 12y ago
  Generally you want to avoid noise early on in the gain chain just because it gets amplified later on. Headroom is entirely relative, you just need to design your gain stages such that the power amp isn't railing when you turn your guitar up halfway.
  - grandalf 12y ago
    I see. Intersting. How do you think about the desirable distortion characteristics vs bad "noise"? Is there an industry standard definition for guitar amps? Seems like the bad noise in the case we're discussing would be what the tube circuit generated when it was not being intentionally driven to distortion... is that a correct interpretation?
    - hamiltonkibbe 12y ago
      Noise would be thermal noise generated in circuit components, or mains hum, anything that is there when you aren't playing any notes. I wasn't spraking exclusively about guitar amps, all this applies to analog electronics in general.
    - kazinator 12y ago
      Noise is anything that is still present when you take away the desired input signal, excluding deliberate time-based effects like reverbs and delays. Frequency components that are not in the original signal, but are only present when the signal is there are usually distortion. (Why "usually": radio-frequency interference is noise. In audio, you may not notice RF when there is no signal. But when signal is present, it may mix with RF in a nonlinear way, causing additional distortion products attributable to the RF. I.e. the RF noise is "descends" into the signal's frequency range via intermodulation.)

kazinator 12y ago

That is well and good, but pointless. People use such a circuit to get a heavier distortion with the same input signal, not to get light distortion with a smaller signal. Let's put it this way: if you have high gain metal distortion circuit gained up to a 5mV clipping threshold, and then you roll down your guitar volume knob to play the blues, you're kinda using it wrong.

hamiltonkibbe 12y ago
It's pointless in the case that you mention, where you're going for a hard clipped metal distortion tone. I was envisioning more of an overdrive sort of application where hitting the rails is NOT the desired effect. The 5mV hard clipper was an extreme example, but you could conceivably use it for soft overdrive/compression with a small enough input signal which was what I had in mind.
- kazinator 12y ago
  If hitting the rails is not desired, then don't configure the effect to have so much gain. The point still stands that you can get the distortion effect, at the desired amount, at various places in the signal chain. The problem you're talking about is about using too weak a signal and then using make-up gain. That can also happen anywhere in the chain. I can reproduce the problem without distortion at all, using a clean signal chain. Repro 1: turn down the guitar volume to simulate an extremely weak pickup. Then turn up the input gain to compensate. Result: noticeable increase in noise. Repro 2: turn down pre-amp volume, and crank up power amplifier volume to compensate. Result: noticeable increase in noise. Gains/levels have to be evenly distributed in the chain. Sure, you will lose headroom if your intent is to have some soft distortion from the back-end of the amplifier, but you put a heavier distortion earlier in the signal chain. The heavy distortion masks the later one; it evens out the level so the headroom from the back is not available. There is a fix for that: you can punch that distortion in and out with your foot as needed.

grandalf 12y ago

are SNR issues like that and the headroom an issue for a guitar amp? I mean why couldn't the solid state part be designed conservatively with extra headroom? Or do the desirable distortion products make that difficult even with a conservative design in later stages?

hamiltonkibbe 12y ago
Generally you want to avoid noise early on in the gain chain just because it gets amplified later on. Headroom is entirely relative, you just need to design your gain stages such that the power amp isn't railing when you turn your guitar up halfway.
- grandalf 12y ago
  I see. Intersting. How do you think about the desirable distortion characteristics vs bad "noise"? Is there an industry standard definition for guitar amps? Seems like the bad noise in the case we're discussing would be what the tube circuit generated when it was not being intentionally driven to distortion... is that a correct interpretation?
  - hamiltonkibbe 12y ago
    Noise would be thermal noise generated in circuit components, or mains hum, anything that is there when you aren't playing any notes. I wasn't spraking exclusively about guitar amps, all this applies to analog electronics in general.
  - kazinator 12y ago
    Noise is anything that is still present when you take away the desired input signal, excluding deliberate time-based effects like reverbs and delays. Frequency components that are not in the original signal, but are only present when the signal is there are usually distortion. (Why "usually": radio-frequency interference is noise. In audio, you may not notice RF when there is no signal. But when signal is present, it may mix with RF in a nonlinear way, causing additional distortion products attributable to the RF. I.e. the RF noise is "descends" into the signal's frequency range via intermodulation.)