Connecting two acoustic preamps to one output jack

[tony9081]

Hello, I've got a bit of an odd question here. I'm building a guitar that is a bit of a strange one, it has two acoustic preamps, one for an undersaddle pickup, and the other for a pickup imbedded in the guitar top, as I play with a percussive style at times. How do I wire the two preamps to the output jack, do I need a small circuit to mix the signals? Or will the preamp circuits themselves prevent the signals interacting with each other instead of just going to the output? I'm just worried about the two signals interacting with each other's tone, volume, etc.

[reTrEaD]

Jul 3, 2018 18:28:36 GMT -5 tony9081 said:

Hello, I've got a bit of an odd question here. I'm building a guitar that is a bit of a strange one, it has two acoustic preamps, one for an undersaddle pickup, and the other for a pickup imbedded in the guitar top, as I play with a percussive style at times. How do I wire the two preamps to the output jack, do I need a small circuit to mix the signals? Or will the preamp circuits themselves prevent the signals interacting with each other instead of just going to the output? I'm just worried about the two signals interacting with each other's tone, volume, etc.

Hello tony9081

It really depends on how the output stage of each preamp is configured. Most designers who work on products meant to be used by the DIY market tend to make their outputs rather 'Idiot-proof', but that's not always the case.

I would think a pair of resistors in a Y configuration might be a simple, safe, and effective method of coupling the two signals together. Each preamp output feeds one resistor, the other end of each resistor leads directly to the output jack.

Use resistors that are equal or greater than the load the preamps can drive. Use the data sheet from the preamps to determine this. If you use both preamps simultaneously and one tends to contribute more to the mix than you like when both are at maximum volume, you can experiment with increasing the value of its resistor until the right balance is achieved.

If you have a link to the data sheet(s) or instruction manual(s) we can look at this more closely.

[tony9081]

Unfortunately, I cannot find a datasheet or user manual anywhere at all. Is there any way I can find the load of the circuit myself? Can I use a multimeter to determine the size resistor to use for each preamp?

[reTrEaD]

Without a circuit diagram or spec sheet, you could measure unloaded output then compare to output under a modest load. Then calculate the voltage division, working backward from the known load. But since it's impractical to create a reproducible output without specialized equipment, this will be difficult.

I'd suggest just using a pair of resistors that will be fairly certain to be well above the acceptable load, say 10k each. Then increase the value of one of the resistors if one preamp's output is too loud when both are at maximum.

[tony9081]

Alright, I'll give that a try, thank you!