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Old 05-03-2020, 10:52 PM   #1
clearchris
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Default Signal Tracer - Requesting input on requirements

Budm and redwire recently introduced me to using a signal tracer. I used a pair of amplified speakers and a special built probe, but well, my speakers need to go back to my computer, and I ordered one of these mono amps from ebay.

https://www.ebay.com/itm/1Set-LM386-...O/383526526009

It helped me find a problem in record time, so seems like it deserves a place on my bench, so I'm going to design a 3d printed box for it. But I haven't used one except for once. What features should it have?

I was originally going to make it USB powered, but it seems that it can go louder at 9v, so perhaps some banana plugs instead for input power. Or just expose the barrel jack on the side. Or both!

For output, I think an internal speaker would be nice, I found this super tiny computer speaker attached to an old motherboard, that's an option, I could also buy and build in probably up to a 3" or so speaker. It would be nice to switch the speaker on and off so an external speaker could be used if the internal one is inadequate to the task. So maybe the super small one to keep it tight, add a jack for audio out and switch to go between the two...

So, radio jacks or phone plugs? Should the output be protected in some manner? I saw a youtube where someone recommended a 2.7ohm resistor on the positive line out.

Should input need different levels of protection? Budm suggested a "10K resistor in series with 0.47uf cap" to work on a preamp, would other values be required for different devices? These could be built in with different jacks, that wouldn't take up much room and could be useful. I really hate digging in my box for resistors...

I wouldn't imagine you would need an onboard power supply, but I could maybe be convinced.

I'm also thinking it should be made safe to hook up to a computer to run an audio analyzer program.

What would your ideal signal tracer have? Or at least your ideal signal tracer that isn't larger than say 100mm^3.

Full design will be posted when finished. Here's the board I have:

monoamp_cropped.jpg

Here are some examples of my other design work.
https://www.thingiverse.com/thing:4249306
https://www.thingiverse.com/thing:4126979

Last edited by clearchris; 05-04-2020 at 12:10 AM..
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Old 10-24-2020, 10:59 PM   #2
Duranitron
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Default Re: Signal Tracer - Requesting input on requirements

Im looking forward on this.. keep it up...signal tracer and injectors are indeed valuable in troubleshooting audio equipment and the like.. Good Job!!!
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Old 10-29-2020, 01:31 PM   #3
redwire
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Default Re: Signal Tracer - Requesting input on requirements

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.
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Old 10-30-2020, 11:21 AM   #4
clearchris
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Default Re: Signal Tracer - Requesting input on requirements

Quote:
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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