PreAmp volume location

[wormdirt]

What is the deal with where the pre-amp volume is placed in a circuit?
I've been working extensively on two Sovtek Migs, a 60 and a 100, both have different pre amp volume placements which got me thinking about the theory behind it.
The Mig 60 places it right after the second gain stage, while the 100 places it right at the plate of V1a.
Any articles or wisdom in this matter?

[Enzo]

I typed up a nice reply last night, then the forum crashed before I could post it. On the internet, someone will rationalize how any circuit you can find is somehow "better" than another. But if the amp sounds good to its owner, it doesn;t matter how "better" some other way would be.

The 100 places it after the input jack, not after the plate directly. The other after the next stage. Doesn;t really matter. I myself prefer the later position as I don;t like a volume control directly from the input jack. I just went through one of my MIG60s to put on sale recently, and it sounded great to me. I did a MIG30 at the same time, sounded good too.

[Dave H]

I don;t like a volume control directly from the input jack.

Neither do I. The output of most guitar pickups isn't high enough to overdrive the first stage so why attenuate it? If the volume is after the first stage the noise is less at low vol pot settings because both the guitar signal and first stage noise are attenuated by the vol pot. When the vol pot is before the first stage only the guitar signal is attenuated, the first stage noise receives full amplification.

[R.G.]

I read a good sendup of where to place volume controls in one of Doug Self's articles or books. The bottom line is that from the theoretical point of view, there isn't any good place, only degrees of bad. The following is a quick overview of the quandary I picked up from that.

A volume control exists to destroy gain, and does it with a circuit element that introduces noise and differential capacitive loading. This is especially critical with high impedance signal sources (like guitar pickups) that produce small signals (like guitar pickups), or with amplifiers that have high output impedances (like tubes).

The best of the bad is to get your signal amplified up out of the low-signal muck with a single stage that also has a low output impedance. That lets you put a volume control after it that is low-resistance for less thermal noise and sensitivity to stray capacitance. The "single stage" is to minimize amplifying the front-end noise of the amplifier by additional stages.

So getting a signal up to a few volts rms in a single stage and following that with a low-ish resistance volume control is about as good as you can do. Well, you could use a pot to change the stage gain, not attenuate the signal, but that generally needs multiple amplifying stages to get enough gain to waste as feedback to modify gain.

Having a first tube stage with a gain of 20-30 as on most amps isn't the worst possible, so that's probably why it's used. Using a tone stack and losing 20db to it, as most amps do, then amplifying it back up isn't good, as it just introduces more noise because you have to now introduce another noisy gain stage to get the level back. Feedback tone controls are good because they don't require that the signal be greatly attenuated then re-amplified.

There are issues with headroom on each one of these, but with tubes typically having 50+ volts of signal headroom, they mostly don't apply.

I guess the bottom line is that placement of a volume control (and tone controls) is a fight between amplification noise and headroom, as well as the side effects of needing to use high impedances for the controls. Each amplifying stage has an input dynamic range and an output dynamic range, and you're juggling those to keep the signal being amplified without introducing gross noise.

[Malcolm Irving]

.... but with tubes typically having 50+ volts of signal headroom, they mostly don't apply. ...

Sorry, didn't quite get this bit. I normally think of the headroom of a stage as being the maximum input signal swing before output clipping. A typical fully-bypassed 12ax7 stage can only take an input swing of about 5V.

[wormdirt]

So really, what it boils down to, is where I want to throttle the gain at. Simple as hell.

[R.G.]

Sorry, didn't quite get this bit. I normally think of the headroom of a stage as being the maximum input signal swing before output clipping. A typical fully-bypassed 12ax7 stage can only take an input swing of about 5V.

I should have said "output headroom". Sorry for not being clear. A lot of the headroom considerations in Self were about output clipping, which is very limited in low-voltage SS stuff. Not so for tubes, where a 12AX7 can swing upwards of 50-70V on the plate before gross distortion. If you have a 50V plate signal, attenuating it by more than 20db still leaves a signal where the noise contribution of the controls themselves will be unnoticeable.

A 12AX7 can take less than 5V. Most 12AX7s are biased at about -1 to -1.5, so the same voltage will shut it off or clip it. You're right, leaving an unbypassed cathode resistor increases that at the cost of lower gain.

[Mike Sulzer]

I should have said "output headroom". Sorry for not being clear. A lot of the headroom considerations in Self were about output clipping, which is very limited in low-voltage SS stuff. Not so for tubes, where a 12AX7 can swing upwards of 50-70V on the plate before gross distortion. If you have a 50V plate signal, attenuating it by more than 20db still leaves a signal where the noise contribution of the controls themselves will be unnoticeable.

A 12AX7 can take less than 5V. Most 12AX7s are biased at about -1 to -1.5, so the same voltage will shut it off or clip it. You're right, leaving an unbypassed cathode resistor increases that at the cost of lower gain.

Yes, I think it is pretty much a non-issue in a guitar amp where the noise resistance of the first stage is generally more than 10K. The noise voltage from a resistor increases only with the square root of the resistance, and so a gain go twenty with a 10K input resistance would require a 4 Meg resistor to match it. Losing 20 db in the TS with configuration tube==> pot ==> tube ==> TS is not a problem either.

[Malcolm Irving]

So really, what it boils down to, is where I want to throttle the gain at. Simple as hell.

The Mig 60 and 100 both have high and low sensitivity inputs. The ‘high’ input brings in an extra gain stage. The Mig 60 puts the volume control after the second stage for the ‘high’ input (which is after the first stage for the ‘low’ input). The Mig 100 puts the vol. control after the first stage for the ‘high’ input (and before the first stage for the ‘low’ input).

With a strong guitar input signal, I think the Mig 60 version could give a distorted sound even with the volume turned right down, as the first stage would give enough output to cause the second stage to clip. So I would prefer the Mig 100 version.

Edit: I think similar Marshall amps did it the Mig 100 way.

[Mike Sulzer]

With a strong guitar input signal, I think the Mig 60 version could give a distorted sound even with the volume turned right down, as the first stage would give enough output to cause the second stage to clip. So I would prefer the Mig 100 version.

Yep. Some guitar pickups put out almost 2V p-2-p and can somewhat overload the first stage. Even a Fender single coil does not qualify as a low output device and so two stages with no attenuation in between grossly overloads for almost any pickup, exactly what some guitarists want. But if you want maximum flexibility you need gain controls after both of the first two stages in a high gain amp. You still need another one before the PI, what is usually called, for silly historical reasons even in a single channel amp, the master volume. Therefore you need three total. But guitar amp design is a strange combination of trying to make it work while paying homage to designs from before it was understood how they would be used.