Preamp question

[Jaxon]

Would it be possible to take a dist circuit such as the Eighteen over at ROG and use it for a preamp to a LM1875 based power amp? And would it also be possible to build said amp so that a different setup such as a pedal board could be inserted into the circuit at the power amp thus bypassing the builtin preamp? If not, please educate me as to why. Thanks!

[darwindeathcat]

Jaxon, take a look at active thread "preamp for LM3886" over in the preamp forum. We are currently discussing just this issue!

[teemuk]

The bypassing is possible and even very easy; all you need is a so-called switching jack. This is a jack that has an integrated N.C. ("normally closed") switch that connects to the jack's tip. The connection is broken when a plug is inserted into the jack. In schematics, an arrow connecting the jack typically depicts this switch. You can even vary the same idea and build a complete effect loop without making the circuit much more complex. Also, jack switches exist in many different forms.

I have shown the amp circuits in a simplified form but a reasonable assumption is that both inputs and outputs are tied to ground with a resistor. Usually the circuit should also be equipped with AC coupling and over-voltage protections since the source may have DC offset and the cord in between the source and amp may introduce some static electricity (resulting to voltage spikes that can damage circuitry). Some versions may have level controls. Anyway, going into details is a bit vain since you can vary this basic configuration in so many ways. Just take a time and download a couple of schematics to few amps that have effect loops, study them for a moment and you'll surely get the idea of how to build various these kinds of circuits very quickly.

[Jaxon]

Thanks teemuk. That's pretty much what I was thinking.