Single Pot in parallel to control 2x identical gain controls

Thread Starter

PattyT

Joined Nov 7, 2019
16
Howdy all,

I've recently been stepping away from my regular field of auto-electrics and diving into the world of DIY guitar pedals.

I've come up with a concept that I'm interested to try, and was hoping to gain some feedback as to the feasibility or the usage case for a potentiometer.

In order to control the gain of a pair of little mono audio amp modules, I am considering removing the trim pots on each and connecting them in parallel to an A1k 16mm potentiometer so both the modules can be controlled with identical levels using the same pot.

It's only tuesday but it's already been an incredibly long week so my brain is just not doing what I want it to do.

I think option 1 is to use a stereo pot (hard to find in a A1k rating affordably)

Option 2 would be to just use equal lengths of wire to connect both trim pots to a mono A1k 3 pin pot (much easier to get affordably)

Option 3 would be to add diodes to the wires for the setup of option 2.


I'm interested to hear the suggestions from everyone as to the best option to go with and why (knowing my preference is to avoid option 1 if I can (cos stereo logarithmic pots are not easy to come by)
 

Ian0

Joined Aug 7, 2020
10,031
I think option 1 is to use a stereo pot (hard to find in a A1k rating affordably)
10k log would be a more common value for a volume control. (Note that in Europe A=linear and B=log, in Japan A=log and B=linear, so better not to specify A or B)
Option 2 would be to just use equal lengths of wire to connect both trim pots to a mono A1k 3 pin pot (much easier to get affordably)
Length of wire won't make any difference, but earth connections might.
Option 3 would be to add diodes to the wires for the setup of option 2.
Perfect, if you were intending to make a distortion pedal.
 

Thread Starter

PattyT

Joined Nov 7, 2019
16
Thanks for the response Ian0

I am making a distortion pedal however, all I'm trying to do is combine the 2 trim pots from the LM386 modules (eBay specials) to be controlled simultaneously with one pot.

I reckon I'll go to 10k but the stereo pots still aren't cheap. compared to the single pots so if I can control both module gain trims with one single pot then that would be ideal. I'm trying to fit it in a 1590B casing so don't really have space for a pair of Pots for this.

The plan is to use 3x LM386 modules...
2x in series using one 5mm and one 3mm red LED on the final output for clipping diodes (I want to remove the trim pots from this pair of modules connected in series and control them with one pot)
The 3rd LM386 module it to go in parallel with the others and use BAT41s for the clipping diodes.

The reason I need 2 in series for the LED clippers is the BAT41s are much fuller sounding and result in higher output than the LEDs if each set of clipping diodes only uses one module when in parallel with each other. I'm hoping to rectify this issue that other makers have experienced by adding an extra amp in series for the red LED clipping diode set.
 

Thread Starter

PattyT

Joined Nov 7, 2019
16
You can do that if both have the same input signal. if not, you are shorting the two signals together.

Well they're gonna be in series with each other so one will get it's input from the clean audio signal and the other with get its input from the output of the first one...
 

Ian0

Joined Aug 7, 2020
10,031
Pots are rather going out of fashion. Everywhere used to sell them, but not any more. I don’t know what country you are in, so this information may be no use to you, but I get them here
https://www.switchelectronics.co.uk/collections/potentiometers

I really like 2.4V Zeners for distortion pedals. A good soft clipping characteristic. Also, 2.4V Zeners are so useless for anything else that I’m surprised that they are still made.
 

MrChips

Joined Oct 2, 2009
30,939
You can use gain controlled amplifiers and use one voltage to control the gain of both amplifiers. The variable gain element could be a FET on each amplifier.
 

BobTPH

Joined Jun 5, 2013
9,147
Well they're gonna be in series with each other so one will get it's input from the clean audio signal and the other with get its input from the output of the first one...
Now there are two pots? I thought this thread was about controlling two amps with one pot.

Post a schematic.
 

Thread Starter

PattyT

Joined Nov 7, 2019
16
Distortion pedal using LM386?
Wrong op amp IC.
They create the specific tone I'm looking for.
They aren't "wrong" they're just not commonly used for this application.
They work, and will give me the sound I'm after.
 
Last edited:

Thread Starter

PattyT

Joined Nov 7, 2019
16
1000009243.png

I want to remove this circled trim pot from both the modules connected in series and use a single pot to adjust both modules


Hopefully this clarifies
 

BobTPH

Joined Jun 5, 2013
9,147
You cannot do that. You need double ganged pot. One reason I am asking you for a schematic is that I hope you would see the problem if you draw it.
 

Alec_t

Joined Sep 17, 2013
14,376
You cannot do that. You need double ganged pot.
I'm not so sure. From the post #14 pic it looks as though the module's signal input terminal connects directly to the top of the pot, and the attenuated signal from the wiper goes to the IC input. The IC could be configured with a fixed gain.
If so, the signal at the wiper could feed a second module. Hence one mono pot could replace both trim-pots and both modules would have the same overall gain.
 

crutschow

Joined Mar 14, 2008
34,682
There is no way around not having a separate resistances for each circuit if you want two separate (stereo) outputs.
I assume that's the reason for having two amps.
 
Top