I have two different power amplifiers (on PCBs) and I want them to produce the same output volume. Both already have the same gain (26db) but their input impedance is different. One has 100k and other 20k.

Does adding a single 80k resistor to the line in of the 20k amplifier increase its input impedance or it has consequences to the design? I don't have its datasheet because I bought it assembled, but it's basically a Gainclone.

If the resistor doesn't solve it, what should I do to match the impedances?

  • \$\begingroup\$ What frequency are you operating at? \$\endgroup\$ – The Photon Nov 5 '15 at 0:00
  • \$\begingroup\$ @ThePhoton it's for audio, so basically all frequencies below 20khz. \$\endgroup\$ – Carlos Eduardo da Fonseca Nov 5 '15 at 0:05

Don't try to match input impedances. Drive each with a low-impedance source such as an op amp.

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  • \$\begingroup\$ Can you elaborate a bit more on this? Using a low impedance source will make this 80k input impedance difference irrelevant? \$\endgroup\$ – Carlos Eduardo da Fonseca Nov 5 '15 at 0:04
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    \$\begingroup\$ @CarlosEduardodaFonseca By taking your single input signal and putting it into the (high impedance) input of an op-amp buffer (actually a pair of buffers) and driving the amplifier inputs with the (separate) low-impedance outputs of the (separate) buffers the interaction between the different input impedances goes away. \$\endgroup\$ – Ecnerwal Nov 5 '15 at 0:12
  • \$\begingroup\$ @Ecnerwal Right. In this case I'm processing my audio signal with a unit that has a 560 ohm output impedance. Will I still need the op-amp buffers or this low source impedance should do it? \$\endgroup\$ – Carlos Eduardo da Fonseca Nov 5 '15 at 0:21
  • \$\begingroup\$ @CarlosEduardodaFonseca You missed the point that the buffers isolate the input of the power amps. You need the buffers to do that. Without the buffers, one will get 5x the input of the other. With the buffers, they get the same input. Buffers are kinda magic that way ;-) (but engineering magic, not mumbo-jumbo magic.) \$\endgroup\$ – Ecnerwal Nov 5 '15 at 0:27
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    \$\begingroup\$ The buffers are not needed. If the drive signal has a 560 ohm output impedance then the signal will be attenuated by 100000/100560 or 0.9944 by the 100k amplifier and by 20000/20560 or 0.9728 by the 20k amplifier. The difference is negligible. Both amplifiers will produce the same output when driven by your source. You don't need to do anything. \$\endgroup\$ – Barry Nov 5 '15 at 0:32

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