Gain & volume controls on circuits

[Acoustic Addict]

Hi Guys,
I just stumbled on your excellent forum & thought I'd post a couple of questions for which I need your expertise & advice.

Fig:1
This is charge amplifier circuit driving a small piezo disc.At first I wired a 100k pot in series with the 100 ohm Re. on the output of the buffer stage.It works well until I turn the "pot" nearly to maximum when there's a noticeable change in the tone. Further this wiring does not cut the gain completely either.

I then wired the pot on the output as shown in the diagram. This too works fine until I turn it down the fully when there's a very loud hiss. I also notice that there's a slight variation in the overall tonal characteristics. Replacing the values of the pot to 47k/100k only made the noise/gain either less or more!

Q:
Which configuration is the better or preferable of these two? Am I overlooking something? Why is this behaving this way?

Fig:2

This is a simple two stage summing/mixer circuit.
Q:
Is the value of the pot -47K- is an ok?

Originally, the volume pot was connected in the main the output, but I rewired it this way to reduce noise in the circuit.

I'd be very grateful anyone of you out there can tell me of any design error(s) I might have made & help me to further improve on these circuits.

Cheers!

 

[Nigel Goodwin]

You might try looking how to wire a volume control to an opamp, fig 1 is completely wrong - there's no input resistor to set the gain of the stage.

 

[Acoustic Addict]

Thank you indeed.
I actually forgot all about the series Re,since this charge amp looked like the voltage follower circuit without the the usual divider network between in the non invert/out put! Also,since the input impedance in non -inverting configuration would be very high(in mega ohm region) i assumed that it would be safe to ignore the series Re.in this circuit. I usually use the following formula to calculate/obtain the value of the series Re. for non-inverting op-amps.

Rs= R1.R2/R1+R2 to calculate the app.value of resistor where Rs= input Re,R2=feedback,R1= divider Re.

So, would any value in the M ohm region say, 1M-4M7 suffice as an input Resistor? I really do not know how to calculate for this particular circuit! Any further help or suggestions will be much appreciated.
Thanks & cheers.

 

[Nigel Goodwin]

How much gain do you want in the stage?, using a 100K input resistor would give you a gain of 1 - the value also depends on the value of the volume control.

 

[Acoustic Addict]

Hi,
Sorry,Did you mean an 100k input resistor on the in the inverting input of the second stage after the C=0-33uf cap? Ahhhh! now I see where I've gone wrong!! Yes, it does make sense! Thank you so much for pointing this out! I think I'd keep the gain as low as possible,say to 3dB at the most in the second stage.So by using a 47k input resistor in series in the feedback loop should give me gain of 2 then? So, what value should I use for the VR1 pot? Would a lower value ie:10k preferable to,say 100k?

Thanks!

 

[Nigel Goodwin]

Hi,
Sorry,Did you mean an 100k input resistor on the in the inverting input of the second stage after the C=0-33uf cap? Ahhhh! now I see where I've gone wrong!! Yes, it does make sense! Thank you so much for pointing this out! I think I'd keep the gain as low as possible,say to 3dB at the most in the second stage.So by using a 47k input resistor in series in the feedback loop should give me gain of 2 then? So, what value should I use for the VR1 pot? Would a lower value ie:10k preferable to,say 100k?

It needs to be no higher than 1/5th of the input resistor, and a 1/10th would be even better.

 

[Acoustic Addict]

I will then wire a 10K pot..as this should be just right with the 47K i/p Re. I'll keep the forum posted once I rewire everything. Really appreciate your invaluable help! Learned a thing or two in the last couple of hrs!

 

[Acoustic Addict]

The circuit is working properly now as it should. Thanks!