Volume pot to VR with signal bleed.

[JJPJ83]

Hello. I'm building an MT-10 clone and just like the Tube Screamer, the Volume pot (Lug 1) shorts to VR (-4.5v) rather than GND (0V). However, in my MT-10 clone, the Volume never shuts off completely. Could my signal be bleeding back through one of the many components also attached to VR? ie. Tone Stack, Input buffer? Is there an audible difference in changing the Volume to short to GND instead? Or do you think there's something else going on? Any help is appreciated.

[JJPJ83]

Edit. The Tube Screamer Volume shorts to GND. My bad. But the MT-10 does not, so my question still stands. Please help!

[mth5044]

Could you please post a schematic? Many people may not know what an MT-10 is.

[JJPJ83]

Ibanez MT-10 Mostortion.

Schematic:

[DIY Bass]

The level pot in that schematic connects to ground

[idy]

I thought so too, but the schematic has triangle for VB and a horizontal with three diagonals for ground. The poster might try to see what happens if the volume pot was instead attached to ground.

[willienillie]

Edit. The Tube Screamer Volume shorts to GND. My bad.

No, you were right the first time.  Volume Lug 1 in a TS connects to Vref.  It's not uncommon for a very low level of signal to get through when the volume is all the way down.

[DIY Bass]

I thought so too, but the schematic has triangle for VB and a horizontal with three diagonals for ground. The poster might try to see what happens if the volume pot was instead attached to ground.

Oh yeah.  That is very confusing

[ElectricDruid]

It's an odd schematic since mostly the reason fro taking the bottom of the pot to Vref not ground would be to avoid the need to put a DC blocking cap in before the volume control. But in this case they seem to have done that anyway, so there's no reason why they couldn't have taken the bottom of the pot to ground.

[antonis]

mostly the reason fro taking the bottom of the pot to Vref not ground would be to avoid the need to put a DC blocking cap in before the volume control.

I might get you wrong, Tom, but connecting the bottom of the pot to Vref doesn't avoid the need of DC blocking cap..
(DC flows through voltage divider lower resistor to GND - by doing that you simply "offset" bottom lug voltage..)

[ElectricDruid]

mostly the reason fro taking the bottom of the pot to Vref not ground would be to avoid the need to put a DC blocking cap in before the volume control.

I might get you wrong, Tom, but connecting the bottom of the pot to Vref doesn't avoid the need of DC blocking cap..
(DC flows through voltage divider lower resistor to GND - by doing that you simply "offset" bottom lug voltage..)

Oh? I'd have thought if the top of the pot and the bottom of the pot are biased the same, then there's no DC across the pot, so it shouldn't sound scratchy when you move it. Does that not work?

[anotherjim]

Whatever, the Level control is Vref grounded and a common cause of signal leakage at zero level is if there is too high an impedance in the Vref supply. That could be resistors in the divider too high value and or the bypass cap too small or missing/open circuit. The schematic values for those components should be perfectly adequate.
However, when the first stage is at a high gain setting, a little signal can still find its way through by stray coupling via wiring/pcb track etc.

[antonis]

I'd have thought if the top of the pot and the bottom of the pot are biased the same, then there's no DC across the pot,

So you have a dream.. 
(for a world with absolutely no input bias current and exactly unity DC gain output..)

[ElectricDruid]

Lol! Fair point I suppose! 

[JJPJ83]

Thanks to everyone that has replied.

I tried connecting Vol 1 to GND and got some scratchy sounds while moving it. I'm just going to stick with the original and live with a slight amount of signal when the Vol is at 0. I mean, if the pedal is on, why would I ever NOT want a signal!? Thanks for all the insight.

[antonis]

It's not uncommon for commercial pots wiper to not fully travel..
(i.e. not all down to zero ohms or not all up to full body resistance..)

[POTL]

Hi all. came across this old thread. many years ago I asked the same question, is there any difference to what to connect the volume potentiometer, VR vs Ground. Earlier I was told that there is no difference, although in fact the center point of the signal will drop if you connect the volume to ground. Regarding the leaking signal and the impedance in the VR voltage divider. I correctly understood that it is more efficient to connect to ground? what is the real benefit of connecting volume to VR, besides that I can remove one capacitor from the circuit?

[ElectricDruid]

As Antonis convinced me above, there *is* no real benefit. In theory, you should be able to do away with one cap, but in practice, that's not guaranteed anyway, so you're better off putting the cap in to be sure. And if you're going to do that, it makes much more sense to terminate the pot to Ground not Vref.

So: No, use the cap, bottom of the pot to Ground. It's more reliable.

[POTL]

Thank you. Another plus, you do not need to trace extra VR lines, you can use a ground polygon.

[amptramp]

The trouble with terminating a level pot to Vref in this case is the signal is amplified by around 200 so even a slight amount of feedback due to the non-zero impedance of the Vref gets fed back into the input of the clipping stage.  This would tend to cause oscillation even though it is negative feedback since the electrolytic cap from Vref to ground will shift the phase of the feedback signal.

It looks like all the controls are returned to Vref since the control board doesn't even have a ground connection to it.  You may only have a gain of 5.7 in the output stage but that is capable of suffering from feedback effects as well.  All of the controls on the control board should be connected to ground rather than Vref.