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So, to pair with my LM1875 power amplifier, I'm building a preamp to take the output of my acoustic guitar's pickup and play it into the power amp. I've fleshed out a nice design so far, two gain stages and a tilt tone control with a mid scoop option (useful at lower volumes to compensate for psychoacoustic effects). Circuit diagram is attached. Please note this is an unfinished and untested circuit.
A few questions:
1) Do I need a DC blocking capacitor and biasing resistor somewhere between the two stages? If so, would it be better off before or after the tone controls (my guess is after, if it's needed).
2) Does anyone see any obvious problems with the circuit? I think the input bandwidth should be fine - the output impedance of the guitar pickup amp is about 1.5 KOhms, and the master volume pot for the power amp has a DC blocking capacitor and biasing resistor after it before the LM1875 amp.
The dipswitch block S4 allows the second stage to have variable gain, going from +0.82 dB all the way to +10.37 dB, in 16 reasonably divided steps, using the dip switches like they are binary increases, all off is +0.82 dB, 1 on is +1.74 dB, 2 on 1 off goes to +2.94 dB, 1 and 2 on is +3.66 dB, and so forth.
The tone control is a simple tilt tone control, shifting the emphasis from bass to treble. The switch s3 introduces some mid cut or mid scoop. I plan on using shielded wire for the input circuits, grounded to the chassis, and there's a ground isolator between the chassis and circuit grounds (resistor, reversed diode pair, and a RF blocking cap, all in parallel). S1 is in place because the tone control loses 6 dB, so you can get it back using that option. If there still isn't enough gain, it's even easier to use a SPST switch to have the circuit provide something like +4/+10 db gain (or whatever values you need) by paralleling the ground resistor with another switched resistor. I also want to leave open the possibility of adding a microphone input and giving some control over levels, so I might need a volume control before an output and another op-amp configured to sum inputs, then putting the master after that op-amp. (It's a good thing I have a lot of open space on the circuit board, and a 10 pack of OPA2134 shipping to me.)
Preamp power supply is a standard split supply +/-15 V, using LM317 and LM337 regulators (for their better accuracy and stability vs. 7815 and 7915 regulators). FWIW, LM1875 amp runs on an unregulated +/-22 V supply, the 15 V supplies are powered by that.
Any comments appreciated, thanks in advance.
Martin
A few questions:
1) Do I need a DC blocking capacitor and biasing resistor somewhere between the two stages? If so, would it be better off before or after the tone controls (my guess is after, if it's needed).
2) Does anyone see any obvious problems with the circuit? I think the input bandwidth should be fine - the output impedance of the guitar pickup amp is about 1.5 KOhms, and the master volume pot for the power amp has a DC blocking capacitor and biasing resistor after it before the LM1875 amp.
The dipswitch block S4 allows the second stage to have variable gain, going from +0.82 dB all the way to +10.37 dB, in 16 reasonably divided steps, using the dip switches like they are binary increases, all off is +0.82 dB, 1 on is +1.74 dB, 2 on 1 off goes to +2.94 dB, 1 and 2 on is +3.66 dB, and so forth.
The tone control is a simple tilt tone control, shifting the emphasis from bass to treble. The switch s3 introduces some mid cut or mid scoop. I plan on using shielded wire for the input circuits, grounded to the chassis, and there's a ground isolator between the chassis and circuit grounds (resistor, reversed diode pair, and a RF blocking cap, all in parallel). S1 is in place because the tone control loses 6 dB, so you can get it back using that option. If there still isn't enough gain, it's even easier to use a SPST switch to have the circuit provide something like +4/+10 db gain (or whatever values you need) by paralleling the ground resistor with another switched resistor. I also want to leave open the possibility of adding a microphone input and giving some control over levels, so I might need a volume control before an output and another op-amp configured to sum inputs, then putting the master after that op-amp. (It's a good thing I have a lot of open space on the circuit board, and a 10 pack of OPA2134 shipping to me.)
Preamp power supply is a standard split supply +/-15 V, using LM317 and LM337 regulators (for their better accuracy and stability vs. 7815 and 7915 regulators). FWIW, LM1875 amp runs on an unregulated +/-22 V supply, the 15 V supplies are powered by that.
Any comments appreciated, thanks in advance.
Martin
