Why not use a voltage divider volume control?

[Taylor]

I've been studying lots of effects to figure out why people do certain things, and I've noticed some different ways to do an output volume control: Some have an opamp gain stage, with feedback resistor replaced by a pot. Varying this feedback controls volume. Then a lot of other effects have the output go to lug 1 of a pot, with lug 3 grounded and the wiper is the output, AKA a voltage divider.

The advantages of the latter seem to be that it's simpler, uses fewer components, and actually allows you to bring the volume to zero. I can't see any advantages of the former, except maybe if the circuit involves so much signal loss that it needs that gain stage to even get back to unity. Even if this is the case, couldn't there be a fixed gain stage that outputs more than unity, then a voltage divider? I find it very irritating when an effect puts out a ton of volume and can't be attenuated completely (the Ring Stinger's the only one I've come across so far).

What's the rationale behind this, and what (I'm sure it's something) did I get wrong in the above?

[cpm]

a volume pot at the end is simpler, but it will mess with output impedance, wich may or not be significant to you.
here is some info on this issue with volume pots: http://www.muzique.com/news/boosters-are-not-buffers/

some circuits may use a buffer after volume pot to avoid that, or if a buffered output is needed, or recovery gain, it may be worth inserting a volume control there, like that opamp stage you mention.

also, look at how different boosters are implemented, some has a full fixed amplification, and then its output attenuated. This is the idea you suggest, which may produce clipping when headroom is not enough, and it cant be fixed just turning down the volume pot.
Others use a gain pot wich sets the amplificaction variable, up to the desired level, wich is also the output volume, so clipping would occur only when gain is set too high.

[brett]

Hi

Varying this feedback controls volume.

Sort of...  It controls the gain of the circuit.  If unclipped, it does affect the volume.  But you'll usually see it used in distortion boxes (e.g. Tubescreamer), where the signal volume increases by only a small amount, but the signal gets clipped "harder" and the waveform becomes more "square-ish".

As you note, in 99% of cases a pot used as a voltage divider just before the output is a fine volume control.
cheers  

[newfish]

V.Divider is much simpler - and in this case, I'd say better (subjective I realise).

As far as Booster volume controls, my metric 2-cents is to have the Booster max-ed out at all times with a divider volume control on the output (as I favour dirty boost).

Should I want a cleaner sound (and possibly some medical attention), I simply roll back the Guitar's master Volume - which cleans up the resulting boost.

When it comes to volume, I divide - and concur!

[MikeH]

There are a lot of ways to control volume, some better than others- some so lousy they're not worth trying.

[Taylor]

Hi

Varying this feedback controls volume.

Sort of...  It controls the gain of the circuit.  If unclipped, it does affect the volume.  But you'll usually see it used in distortion boxes (e.g. Tubescreamer), where the signal volume increases by only a small amount, but the signal gets clipped "harder" and the waveform becomes more "square-ish".

As you note, in 99% of cases a pot used as a voltage divider just before the output is a fine volume control.
cheers  

I guess what I'm talking about is the String Ringer - schem here:

http://homepage.ntlworld.com/paul.nelson666/StringRingerv12r.pdf (large PDF)

The volume seems to be this kind of control, right? It doesn't seem to clip (though it's hard to tell since there is clipping elsewhere in the circuit) but it's very loud and it's not possible to attenuate the volume completely, or even mostly. I find that irritating. Apparently this control wasn't in the original, so I'm wondering why it was decided to do this sort of control razther than a voltage divider.

[MikeH]

Yeah, the volume there is just adjusting the gain of the last stage, right before the output.  This will cause an increase and decrease in volume.  Although, the drawback is that usually this type of control won't go to zero output.  Of course, is that really a drawback?  How often do you want a pedal to be able to have no output at all?

[Taylor]

Well, it's often too loud even at the lowest volume setting - sometimes it distorts effects after it that shouldn't be distorting. I'm just happy that I'm getting to be able to not just read schematics, but actually undesrstand what's going on in them.