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Old 10-30-2020, 11:21 AM   #4
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Join Date: Dec 2013
City & State: San Antonio
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Default Re: Signal Tracer - Requesting input on requirements

Originally Posted by redwire View Post
Over the years I've made a few audio signal tracers with LM386 and they are a really great tool.

I have high input impedance 1MEG (not a crappy 10k pot) and headphone jack and protection diodes/input capacitor to protect the LM386 if you are tracing signal at higher voltages.
My latest design I added a JFET input and RF detector to find AM signals and LED indicator if ultrasonics are present. I use BNC input jack so a scope probe fits, and 3.5mm speaker output jack, 9V battery.

Mr. Carlson's Super Probe is insanely high gain and super sensitive. I think a bit too much distortion though, if you were trying to troubleshoot that in something, hard to hear the extra fuzz. Schematics he sells on Patreon but can be found on the web. He uses LM4871 (good to 5.5V max, not 6V no four batteries please) for the audio power amp.
When would you be tracing audio at higher voltages? Amps or tubes?

Headphone jack is a good idea, haven't thought of that.

Why would you want to trace ultrasonics, if it's by definition something you can't hear? Does it interfere with speakers or other things that cause audible problems?
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