How to optimal change voltage gain in PA

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Hello,

I would like to ask the question ..., how to optimal change / reduce voltage gain in power amp in att.

Is correct my calculating?

Au = cca R120/R118 = 4300/82 = 52,439
Gain = 20*log Au = 34,39 dB

Input sense is cca 270mV for max. rate power.

I need help for reduce Gain from 34,39 to 28 dB && increase input sense from 270mV to cca 1,2V. Which resistor is better change - R120 or R118 or both? Must I change also value of R512 in bridge mode for same value like R120?

In the next step I would like to reduce the input impedance. Is Zin defined only like value of resistors R100 /ch1 or R200/ch2 ?

Thank you for recommendations and advice.
Regards
Milan
 

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This looks like an automotive (car) amplifier!
In any case, do not attempt to change the gain of a power amplifier. The frequency compensation network in amplifiers only works safely with the specific voltage gain it has been designed with. You will create instability, a poor frequency response and most likely a smoking disaster by adjusting its gain ratio without first redesigning the whole amplifier(s).

Sensitivity though, is easily reduced by the addition of an adjustable level control which has similar output impedance to the source, at the input of each channel. This could be in the form of an attenuator consisting of 2 trimpots, a stereo pot. or the equivalent in fixed resistors. Use an L or T pad circuit to implement this properly. Google is your friend and there's dozens of websites with articles explaining the circuits, calcs etc.
Incidentally, the input impedance of the amplifier(s) is not determined by R100,200 and is probably in the normal range of 10-50k - check the manufacturer's spec sheets.

On the other hand, a high sensitivity input is not necessarily a real problem unless you have idiots messing with your audio system who turn volume controls up full to clipping or protection levels. If you are just looking at and comparing the specs of your gear, you may find that simply fitting it as a standard device with a direct connection is not the problem you imagine.
 
If you change the gain you are likely to change the stability margins.
Lowering the gain results in an increase of feedback. That can go even further than just altering the margins. It could turn the amplifier into an oscillator.

If you contemplate changing the gain, then you must be able to redesign the compensation to ensure the stability margins are maintained.
 
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