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RE: Differential Opamp

Posted by VoltSecond (M) on July 16, 2024 at 13:27:21

In Reply to: Differential Opamp posted by Triode_Kingdom on July 13, 2024 at 17:00:11:

I like the idea of using two high CMMR op-amps to pick out differential left and right and convert to mono.

If on a tight budget, discrete 0.1% 25ppm/C resistors could be used with a less expensive op-amp.

I'd also consider breaking the ground between the sub and the pre-amp with a 100 ohm resistor. i.e. RCA shell to Sub grounded to pre-amp ground through ~100 ohm 1W.
- Bypass the 100 ohm with a 0.1uF >24V NPO ceramic to control RF interference.
- Reference the INA's power and MONO ground to RCA Shell.
- You may be able to get away with using two phone chargers for a +/- 5V supply if the main power amp delivers full power to the speakers at 1-2V rms.
- The 100 ohm could go up to 1K and the cap to a 0.01u if you run the circuit from +/- 15V.

You could possibly even add a 3rd amp to make a "Differential send" for higher rejection of the ground noise between the pre-amp and sub. The input signal reference to the INA goes to the mono-signal ground and the output reference sense resistor goes to "RCA shell." I've done something similar to this 3 decades ago and it worked well.

Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.

. . .Oh!. . .Remember: Modifying things voids their warranty.

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