Signal Tracer - Requesting input on requirements

[clearchris]

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:



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

[Duranitron]

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!!!

[redwire]

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.

[clearchris]

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.

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?

[clearchris]

Re: Signal Tracer - Requesting input on requirements

Progress is being made. Here's what I have so far, I need to add in a place for a 9v battery and a headphone jack. It took me a while to get around to making the enclosure parametric, I have used this box design 3 times already (didn't anticipate that) and wanted to make it easily changable for implementing new designs.

I need to get a stereo audio switch, right now I only have a mono audio switch, though I'm not certain this little amp can drive two speakers, even if headphone sized.

Looking at this, I think I'm also going to inset the speaker a bit and have screws come through the front to protect the speaker a bit and allow for a grill to be installed.

Comments anyone?

[clearchris]

Re: Signal Tracer - Requesting input on requirements

I like this better after adding the audio jack switch, it pushes the banana jacks up a bit, away from the amp board. I try to keep to a 120mm height limit, it's getting crowded. I need to devise a way to capture a power led with the plastic, I'm going to put it in the mirror image (along X axis) spot from the audio jack switch. I have another device and the led shines through the plastic, it's a cool effect.

Going to start printing some small pieces to test fit, I haven't used these safety banana jacks, the speaker or the power switch before. Power switch is going to be a pain, getting the right fit on the wings is always hard.

[redwire]

Re: Signal Tracer - Requesting input on requirements

It looks good so far. I use a 1MEG potentiometer with built-in on/off switch, one input BNC (so I can use a scope probe) and power-on LED. Input is not on the front face, but on the side of the box.
I did one with the knob on the top, that works too.

I worry about stabbing the speaker when plugging in a test lead?

[clearchris]

Re: Signal Tracer - Requesting input on requirements

What is the 1Meg pot for? Is that inline with the probe? Do you need the granularity of a multiturn pot, or just a single turn is ok? When the pot switches "off", is that open circuit?

My current device has a cap inline with the probe to act as protection, is the 1meg pot instead of the cap?

Also, do you have a link to a bnc jack? I don't have a probe, so I don't know the exact type, I'll probably do two versions, one bnc and one banana for no-scope people like me...

There are also plenty of grille designs up on thingiverse, but I'll probably design one that doesn't require supports. It will be an attachment, not built into the case.

[clearchris]

Re: Signal Tracer - Requesting input on requirements

BTW, here's the general form factor I'm going for.

[redwire]

Re: Signal Tracer - Requesting input on requirements

Here is my schematic for a simple LM386 signal tracer. Sorry I thought I'd posted something.
The high value (1MEG) pot is so you don't load down circuits when you are probing them. The signal tracer is sensitive, having a voltage gain of up to 200. You can listen to signals, noise, hum, RF, bad connections etc.
The 47nF input cap blocks any DC from the circuit you are probing, even for tube guitar amps at high voltage, or ESD etc.
Pair of diodes is a clamp to prevent surges from damaging the LM386. 1nF cap to stop AM radio from being picked up.
Most important is a 2.2uF X7R across the IC's power pins, the IC likes to oscillate without it. You also need star grounding for the battery(-), speaker(-) and LM386 GND pin, those three to one spot is best.

For the input I have a mono 3.5mm (but didn't use it) and BNC connected. The chassis mount BNC is just a low dollar part from fleabay or ali.
I really like using a scope probe instead of wires.

For a case something like a handheld Hammond RH enclosure with battery door.

[clearchris]

Re: Signal Tracer - Requesting input on requirements

Thanks! Looks somewhat similar to what I have, and I think I can add what's not on there. I'll have to add some more space on the bottom to accommodate some extra components.

Edit: I can pick up a BNC connector locally, maybe I'll do two banana jacks and the BNC connector.

[redwire]

Re: Signal Tracer - Requesting input on requirements

It's pretty close to what I posted but unrefined.
In that schematic 100uF output capacitor at 8 ohm speaker is 200Hz rolloff, and 0.1uF input+10k pot 159Hz, so that circuit has terrible bass response but is cheap. 1,000uF cap on power is a bit huge. Needs the RF cap 100pF-1nF at pin 3 and the diode clamp.
Usually those little LM386 boards have quite big capacitors good for 40-50Hz so you can hear bass. Many have poor pcb layout, never ground the pot at the speaker lol.

[clearchris]

Re: Signal Tracer - Requesting input on requirements

Thanks! I'll replace the output cap with a 300uf as yours has. I have the rest of the ceramic caps in my cart at mouser to add them to the board. Also found a 1meg pot (assume audio taper).

BTW, the 47nf 400v input cap is a ceramic, right? I noticed you used the one notation for the other ceramics except for that one.

[redwire]

Re: Signal Tracer - Requesting input on requirements

I used a 47nF 400V film cap there, less distortion than ceramic.
If you're not doing HV tube gear a 250V or less part is good, stereos are maybe 90V max.
I think 330uF-470uF 16V or 26V is fine for the speaker cap.

[clearchris]

Re: Signal Tracer - Requesting input on requirements

I used a 47nF 400V film cap there, less distortion than ceramic.
If you're not doing HV tube gear a 250V or less part is good, stereos are maybe 90V max.
I think 330uF-470uF 16V or 26V is fine for the speaker cap.

Film cap works. Not much cost savings to go with lower voltage, so I'll go big, though truly, I never want to see a tube on my bench. Most of my audio is from the beginning of the solid state age, when solid state meant superior and not super cheap.

The speaker output cap seems to have a similar role, is there a reason you went with an elecrolytic on that one instead of film?

I really need pcb capability, this board is going to be all hacked up.

[redwire]

Re: Signal Tracer - Requesting input on requirements

When that cute guitar player brings over a guitar amp for you to take a look at, you'll be happy your test gear could handle it
It happened to me, a very old VOX AC30 straight out of the Beetles, I used the signal tracer to find a cracked open coupling cap.

The signal tracer output cap must be electrolytic it's around 330uF, no film caps are that high in value.

I think you don't have to trash the pc board, most parts are tacked on top of connections (i.e. diode clamp) and the only cut trace is the extra surge resistor. The input cap can solder to the input connector.

[clearchris]

Re: Signal Tracer - Requesting input on requirements

When that cute guitar player brings over a guitar amp for you to take a look at, you'll be happy your test gear could handle it
It happened to me, a very old VOX AC30 straight out of the Beetles, I used the signal tracer to find a cracked open coupling cap.

The signal tracer output cap must be electrolytic it's around 330uF, no film caps are that high in value.

I think you don't have to trash the pc board, most parts are tacked on top of connections (i.e. diode clamp) and the only cut trace is the extra surge resistor. The input cap can solder to the input connector.

Heh, that's pretty much the only scenario I could see looking at a piece of tube equipment.

[clearchris]

Re: Signal Tracer - Requesting input on requirements

The bnc connectors arrived, I have a working cutout for them. I'm going to make two versions, one for banana plugs and one for BNC. There just wasn't enough room to fit everything and keep it in a small envelope.

The grill was a serious pain to make, I learned a different part of freecad to make it. I think I'll slap an rca jack in the back in case I want it to do double duty as a speaker.

Otherwise, going to start to do some test prints tonight. Overall, I'm pretty happy with how this is turning out.

Redwire: Can you get me a outer diameter measurement on one of those BNC jacks on the probe? I realized I don't have any, so I don't know how to space it.

[clearchris]

Re: Signal Tracer - Requesting input on requirements

Was watching some tutorials yesterday to figure out how to do the grille quickly (and took longer) and saw something about rendering. Everything looks better rendered. Too bad I couldn't figure out how to include the 3rd party STEP files.

[redwire]

Re: Signal Tracer - Requesting input on requirements

Had to laugh, as usual BNC missing a critical dimension.
I measure the mounting threads at 9.2mm (3/8"-24UNS) but with 8mm flats to stop it from spinning around. Like the banana jacks. I'd use a tight hole and file it out.
A probe is fat, I measure 19mm barrel so it would hit your raised grille?