I have a cheap bluetooth audio module (with no documentation) that outputs a signal greater than line-level. It was probably designed to be connected directly to small speakers, since the ground on the output signal is floating.

I'm trying to connect it (as line-level) to the input of a more powerful amplifier. I assume that a potentiometer is a reasonable way to adjust the signal level, but should I put it before or after the isolation?

In other words, should I do it like this:


simulate this circuit – Schematic created using CircuitLab

Or like this:


simulate this circuit

Or some other way entirely? Assuming that I'm on the right track, what value of potentiometer should I be using?

  • \$\begingroup\$ Lose the transformer and use a 50k logarithmic pot. \$\endgroup\$
    – user207421
    Mar 28, 2015 at 0:22
  • \$\begingroup\$ How does that guard against shorting the floating ground to actual ground? \$\endgroup\$
    – Ian
    Mar 28, 2015 at 0:54

1 Answer 1


Drive the transformer from a low impedance.

The source impedance and the transformer's primary inductance form an R-L high pass filter which determines the LF rolloff of the circuit.

So with the attenuator ahead of the transformer it'll be all about the treble ... no bass.


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