I'm testing a preamp design I hacked together from various sources and mostly its working quite well. But I'm seeing an unexpected (for me at least) drop in signal in my tone circuit. I borrowed this design for a Baxandall 2-band active tone circuit from Elliott Sound Products. It has a center frequency at about 640 Hz, and in theory it should not exhibit any boost/cut at the center frequency. My LTSpice simulation supports that expectation.
But when I supply the circuit with a 640 Hz sine wave at 1.0 volts (measured at TP3 & TP4), I'm getting 760 mV on the output of both channels. The treble and bass pots are centered but it seems like that shouldn't matter either way. Since the signal loss is the same for both I assume it's a design-related issue that I don't about. I have enough head room in the input to compensate for the loss in gain but I would really like to know if this is typical for this kind of circuit.