DC on the pot...

[Projectile]

I'm just curious about something.

I was looking at the Tubescreamer schematic here and I noticed that the ground lug of the VOLUME pot was connected to DC through the 4.5v Vref feed. Since there is a coupling cap both before and after the pot I don't understand the point of this. Wouldn't it make more sense to just connect that lug to ground? In fact, isn't connecting pots to DC generally bad practice in audio, as it can create additional noise? Now, I understand that DC isn't actually flowing through the pot, as it doesn't have anywhere to go, but wouldn't it make a lot more sense just to connect this pot to ground and avoid the trouble of introducing DC here? I just don't see the purpose. Furthermore, most of the boutique tubescreamer derivatives I looked at (fulldrive, cjod, etc.) also connected the volume pot this way. What is the point of this? Anyone care to fathom an explanation? Mysterious mojo factor? 

[aron]

Wow, I never noticed that. I think I always connected it to ground.

[R.G.]

I was looking at the Tubescreamer schematic ... and I noticed that the ground lug of the VOLUME pot was connected to DC through the 4.5v Vref feed. Since there is a coupling cap both before and after the pot I don't understand the point of this. Wouldn't it make more sense to just connect that lug to ground?

It depends on what you are trying to do. As a functional matter, Vbias is equal to ground for AC signals if you have a well made Vbias, for which there is an article at GEOFEX. In general, if you have a good, low impedance Vbias, it makes not the slightest difference.

In fact, isn't connecting pots to DC generally bad practice in audio, as it can create additional noise?

Only if the parts of the pot have some DC voltage difference across them. If they're all at the same voltage, there is no effective DC to make noise.

Now, I understand that DC isn't actually flowing through the pot, as it doesn't have anywhere to go, but wouldn't it make a lot more sense just to connect this pot to ground and avoid the trouble of introducing DC here?

One of my own self-made rules is that whenever I see something that doesn't seem to make sense, it's a reliable indicator that I don't know the rules. 

I just don't see the purpose. Furthermore, most of the boutique tubescreamer derivatives I looked at (fulldrive, cjod, etc.) also connected the volume pot this way. What is the point of this? Anyone care to fathom an explanation? Mysterious mojo factor? 

For the original, could be as simple as it made the layout easier. After that, anyone not tying it to Vbias was accused of their clone being "not original" and therefore seriously lacking in mojo. In any case, it's much like the resistor/cap from the inverting input of the first stage. It makes no difference whether this ties to Vbias or ground, and for the same reasons. But there was much wailing and gnashing of teeth about this being "not original" when I grounded it on one of my early layouts. Layout, personal idiosyncrasy on the part of the designer, there are many explanations for why one of two equivalent solutions was chosen.

[Projectile]

Thanks R.G., I was kind of figuring that it must just have just been done that way out of convenience for the layout or something like that. Interesting to know that there has been so much teeth gnashing over changing something so trivial. 

[aziltz]

Thanks R.G., I was kind of figuring that it must just have just been done that way out of convenience for the layout or something like that. Interesting to know that there has been so much teeth gnashing over changing something so trivial. 

i've wondered this myself but never got around to asking.  thanks for the info!

[earthtonesaudio]

Since capacitors are not perfect, there's a potential (but small) benefit to it.  Also, if any of the coupling caps fail shorted, the control will be scratchy but still work.