You may have noticed that the wire pads are not placed where they might be closest to the panel where each component is most likely to be placed. For example, the audio input jacks are usually on the back panel, yet the input pads are near the front of the board. The reason is that when laying out the components on the board, we favored short traces and a minimum of layer changes to layouts that would have made for shorter hookup wires.
We made this tradeoff because hookup wires are flexible. Bad sound due to a poor PCB means you have to throw out the whole PCB, but bad sound due to suboptimal hookup wiring is fixable by using a better hookup scheme.
It is possible that these longer hookup wires could act as antennas, bringing interference into the amplifier. I haven’t found any such problems in the amps I’ve built on this board. If you do encounter a problem, the single easiest and most effective fix is to twist related wires together: the wires going to each set of input or output jacks, for example. You could also look into things like rerouting each wire’s path from board to panel component.
If you want to adjust the default multiloop resistor values, you should first read Walt Jung’s article Op-Amp Audio - Minimizing Input Errors.
The outer loop’s resistor values should be lower than the inner loop’s values. As a rough rule of thumb, make R3 five to ten times lower than R5. Also, remember that the lower these resistor values are, the more current your amp will use. I personally try to make R3 no lower than about 220 Ω, which means that R4 would be 2 kΩ for a gain of 10; then, R5 would be 1.2 kΩ or higher, making R6 120 kΩ for a ~100× inner loop gain. At the same time, you don’t want to go too high with these values, either, because that increases the chances that noise will get amplified along with the music signal. Avoid resistances over 1 MΩ if you can. Not only do larger values risk noise pickup in the amp, large-value 1% resistors tend to be expensive.
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