Using a variable resistor in the feedback loop.

[darren01]

Hello,

I am trying to make a stereo electric mic preamp and am thinking of using a variable restor in the gain feedback loop for volume control instead of placing a volume knob at the end of the signal path, as shown in the few DIY preamp pages.

Is this a good idea? If yes, please explain. If no please explain.

What are the pros and cons?

My thoughts are that I would put a 1k resistor at the begining of the signal path and a modified 100K pont in the feedback loop.

The 100k pont would act as a variable 100k resistor for volume control insead of putting a volume knob at the end of the circut.

Am I on the right path or is this a bad idea? I want the sound of the preamp to be as transparent as possible.

Thanks in advance.


Darren

 

[GregGC]

Hello,

I am trying to make a stereo electric mic preamp and am thinking of using a variable restor in the gain feedback loop for volume control instead of placing a volume knob at the end of the signal path, as shown in the few DIY preamp pages.

Is this a good idea? If yes, please explain. If no please explain.

What are the pros and cons?

My thoughts are that I would put a 1k resistor at the begining of the signal path and a modified 100K pont in the feedback loop.

The 100k pont would act as a variable 100k resistor for volume control insead of putting a volume knob at the end of the circut.

Am I on the right path or is this a bad idea? I want the sound of the preamp to be as transparent as possible.

Thanks in advance.


Darren

Problems I can see:
- You’d never get to 0 volume. The gain is min 1.
- It better be a good pot. It’ll also be a DC NF component, so you may get some really big spikes/noise (+/-V) if it doesn’t make a good contact at all times. May be a good idea to put a 200k resistor in parallel to the pot. I won’t be surprised if you hear noise every time you turn the pot.
I wouldn’t do it that way.

Greg

 

[mrfeedback]

Gain control variable NFB resistors are standard equipment in audio consoles/mixers.
You will also need a volume control after the preamp stage if you want to mute the outputs.
http://www.sound.au.com is a good place to start learning this stuff.

Eric.

 

[phase_accurate]

I once did this on a preamp for bass-guitar and it worked well for that application.

Regards

Charles

 

[Richard C]

This arrangement was used by Musical Fidelity on their A1 amplifier and the A1 was plagued by noise every time the voume control was adjusted, replacing the pot was only a temprorary solution.

 

[phase_accurate]

Pot scratching can indeed be a problem.

But

- there are applications where it doesen't matter because you don't turn on the pot very often

- scratching can be minimised by minimising DC-current through the pot

Below is what the input stage of my aforementioned bass-amp looked like. C3 is there in order to minimise DC current through the pot (in addition to the use of a FET input opamp). You now trade scratching against some variation in DC offset, which may or may not be a problem, depending on the application and type of op-amp used.

Regards

Charles

 

[StratoFan]

I do same on my guitar preamps without R5 and no problem, no scratching effect with TL072s. I do same with simple jfet preamp too and nothing problem. I think about scratching effect, the good quality output condensers or for example a mixer/line driver/tone control after preamps solve the problem.

 

[Shaun Perez]

Possibly using a conductive plastic pot available from digikey would eliminate this crackling problem as they are infinite in varable resistance and very smooth. i would try for a sealed pot too so as to keep any dust out, which is a common cause of crackling with age.

 

[StratoFan]

I'm not sure how changing the scratching effect because never tested if all pots and inputs soldered to the PCB like in the picture:

An externally hosted image should be here but it was not working when we last tested it.

This is 6 channel mixer with 6 preamps with gain and volume pots. Parallel R5 is really would be good later

 

[StratoFan]

I have one more thing, this is two (switchable) several stereo preamps (jfet and opamp) with two gain pot and one volume, and no scratching effect. I hope never vill

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.