unexpected gain loss in tone circuit - PROBLEM SOLVED

[hummusdude]

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.

[Andy Watson]

Can you confirm that pins 3 and 4 of the bass control are not connected together.

[hummusdude]

Yes, continuity test confirms they are not connected.

But I have to apologize for overlooking the obvious...the preamp was connected to my power amp input when I made the measurement. When I disconnect the power amp there is no signal loss so the power amp is obviously loading down the preamp. I didn't think that would happen because it's an LM3886 op amp and so I assumed a very high impedance input but obviously something else is going on.

Below is the schematic of the LM3886 circuit for reference. It works very well although my scope measurements suggest the gain is closer to 23 than 21 (the designer says R4 and R3 set the gain for the circuit) so perhaps a clue there. The input low pass filter (R1 & C2) cutoff is 234 kHz and the input high pass filter (C1 & R2) cutoff is 1.7 Hz.

For the low pass filter calculated reduction in gain, my calculation is Log₁₀(234k/640) = 2.56 decades. At 20 dB/dec that gives 51.26 dB. So the reduction in gain would be a factor of 10^(-51.26/20) = 0.00274. That gives about 999 mV.

For the high pass filter the math works out to almost exactly the same result. So on the face the input filtering doesn't appear to be the issue. I'm stumped.

 

[hummusdude]

Problem solved.

The issue turned out to be an unreliable connection from one of the pot leads to the board.

Thanks to all for your input!