audio signal generator
Here's my basic design for a 555-based audio signal generator. I will need this for troubleshooting the McGrath Mini amplifier prototype. In particular, the issue of lower-than-expected gain in the effects loop.
I carefully traced my wiring in this section of the circuitry, hoping to find some obvious error, but there was none that I could find. The circuit appears to match the schematic. This is only partly reassuring, because the design embodied in the schematic is my own, never before tested, based purely on theory and my speculative understanding of tube circuits with feedback. However, I researched triode feedback stages in several different trusted sources online, and this seemed to confirm that I'm on the right track and the gain should be approximately as I have calculated.
So, my plan is to send a test tone into the input of the amp, using the circuit shown above, and disconnect the top of the master volume pot from the circuit, then use a test lead connected to this point as my probe to trace the test tone through the circuit. Specifically, I'll be looking to hear a volume level drop of roughly the right magnitude on the output side of the effects send resistor divider, and then volume increases of the right magnitude on each of the successive effects return gain stages. The exact signal levels probably won't be important to measure, it'll be clear if the levels are close to what they should be, or way off the mark. Somewhere, one or more levels should be noticeably lower than expected.
I have shown the signal generator with two output jacks, one for "line level" and one for "guitar level", but actually I think I will use a three-way DPDT switch and a single output jack, to switch the levels and also control the power. "line level", "off", "instr level".
The usual frequency for testing generic audio equipment tends to be 1 kHz, but I am designing this generator to use lower frequencies, mainly because 1 kHz is extremely annoying to listen to for long periods of time, and also it is near the top of the guitar pitch range. I'm providing three frequencies, separated by octaves: 686 Hz, 343 Hz, and 171 Hz. These are all comfortably in the midrange of the guitar's range.
Power will probably come from a cannibalized USB cable: i.e., 5V.
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