Using a variable resistor in the feedback loop. - diyAudio
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Old 28th December 2003, 08:52 PM   #1
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Default Using a variable resistor in the feedback loop.

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
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Old 29th December 2003, 06:42 AM   #2
GregGC is offline GregGC  Canada
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Default Re: Using a variable resistor in the feedback loop.

Quote:
Originally posted by 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

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
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Old 29th December 2003, 06:52 AM   #3
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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.
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Old 29th December 2003, 08:21 AM   #4
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I once did this on a preamp for bass-guitar and it worked well for that application.

Regards

Charles
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Old 29th December 2003, 09:23 AM   #5
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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.
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Old 29th December 2003, 10:20 AM   #6
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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
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Old 29th December 2003, 10:42 AM   #7
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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.
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Old 30th December 2003, 03:35 AM   #8
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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.
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Old 30th December 2003, 10:54 AM   #9
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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:

Click the image to open in full size.

This is 6 channel mixer with 6 preamps with gain and volume pots. Parallel R5 is really would be good later
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Old 30th December 2003, 11:05 AM   #10
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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

Click the image to open in full size.

Click the image to open in full size.
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