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VU Meter Buffer Gain Adjustment

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Hertzilla

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I'm looking for a simple way (minimum components) to pre-set/adjust the gain of this mixing console VU meter circuit by approx. +/-1db:

vu-meter_amp.jpg


Component values:
--------------------
R125, R128 = 470K
R129 = 10 ohms
C103, C105 = 22uf
IC26 = TL071

Thanks!
 
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FvM

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Change the buffer into a +1 dB (G=1.12) non-inverting amplifier (+ 2 resistors) and R128 into a variable divider (+ 1 pot).
 

BradtheRad

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It sounds as though you want to detect any slight change in volt level (+/-1dB is about the same as +/-10 percent). The meter deflects immediately, then takes a second or two to settle back to center. In that sense the circuit automatically pre-sets itself.

Is this your plan? Your schematic has one or both signals going through a series capacitor, which is a simple method to detect a change in voltage up or down. The capacitor is in an RC network with values selected to yield a time constant of about 1 second. Whatever is the incoming DC level, while it is constant the resulting output is 0, or centered. Any change causes a reading up or down lasting briefly.
 

betwixt

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It basically a unity gain buffer at the moment. I think my approach would be like FvM, add two resistors in the feedback to increase the gain to +1dB then add a potentiometer at the input to back it off again.

Brian.
 

Hertzilla

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Thanks for all the responses. I'm not looking to change detection level/time constant, only amplitude.

Although adjustable gain would be ideal, I only need to match the output of this to another identical circuit (for left & right VU meters), so just attenuating the signal by 1/2 to 1db or so on one of the circuits would probably work. Ideally, I would like to just replace a single component (R129?) to accomplish this without cutting traces on the mixer's existing PCB.

If that won't do the trick and I do have to cut a trace, would inserting this pot work?:

meter_amp_mod.gif
 
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Hertzilla

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I replaced R129 with a 100 ohm 1% resistor, providing the correct attenuation at just under 1dB.

Thanks.
 
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