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I am really struggling to do the DIY mic project and it has gotten to the point where I am making a post about it. I started with different JFETs but with troubleshooting i have decided to completely start over from the beginning with all the same parts (other than the 22uf caps which i have ceramic caps rather than bipolar electrolytics incase that matters). It has been a nightmare.
My current state is that the THAT1512 for some reason is drawing tens of milliamps causing massive overheating of the chip and burning of the 100R resistors in series from the +- 15V lines. I have noticed i can get signal from the mic to the PC by touching the ground plane or the gate on the JFET but I have not been able to get anything from the capsule itself.
I have an oscilloscope and a cheap multimeter if that would help with troubleshooting. i have hooked a capsule directly to the oscilloscope and i can see clear signals from it when i blow or talk into the capsule so luckily those are still working. Any help would be appreciated as i am at my end in terms of knowledge.
Thank you
Posted : 26/03/2026 12:33 am
Noise - probably high-frequency stuff. The THAT is a wideband amp (it has to be to deal with audio signals that have complex harmonics and for internal feedback) but that means if ANY leaks into the inputs, you can kiss bye-bye to any usable signal.
I've been there, got so annoyed with the t-shirts that I redesigned the whole thing from the ground up.
That's not to say this one doesn't work (it does) but it's a lot of trouble potentially which Matt (and I) missed the first time around. I came to it late but even with my experience I got caught on several rather nasty snags.
Anway,
There are some DC tests you can do with the THAT in and out of circuit that are listed (albeit pages back) on the official topic. Start at the latest page and work back from there. We have had a few people track this one down.
AC and DC offsets at the output AT the THAT1512 are useful too.
Take everything I say with a pinch of salt, I might be wrong and it's a very *expensive* way to learn!
Posted : 26/03/2026 5:55 pm
Topic starter
@marcdraco Thank you very much for your reply.
You said Noise is probably what is causing it (and probably high frequency) and that raised some red flags in my mind.
1. Is it possible that it isn't working due to me not having it enclosed in a faraday cage yet? (seems far fetched to me but i really dont know this stuff and putting it in a fully coper mesh box didn't get it to work and heat still accumulated)
2. what frequency is "high frequency" Mhz? khz?
3. can really just some noise cause a chip like the THAT 1512 to consume tens of milliamps?
4. are there any known issues with the schematic in the video other than the flipped caps on the 5v rail?
I am going and more carefully reading through the official thread again so I hopefully don't waste any of your time and thank you for helping me with this.
This post was modified 5 days ago by StormFire451
Posted : 27/03/2026 7:30 pm
marcdraco reacted
All good, and same apology to you too for my dilatory replies.
1. Extremely likely, yes.
2. Probably just background radio, stuff from space... (seriously) it's broadband and the JFET head is an (albeit) simple aerial. Mains hum is also a serious culprit but you'd hear that at your digitiser. As I recall [so I checked] the 1512 is good up to around 7* MHz at 40dB of gain - bonkers on its face, but quite normal with a high-end design.
3. Absolutely it can. If you integrate" energy over time (in your head) you can see that regardless of frequency, the amount of power that's used is similar over time and that at higher frequencies the parts have less time to cool down! So they are working harder.
4. I don't know that there are in issues in the schematic that I used (from the web V1.0) but a short circuit or busted chip could also cause this issue.
* I had it as 8 MHz in my head but hey-ho
Take everything I say with a pinch of salt, I might be wrong and it's a very *expensive* way to learn!
Posted : 29/03/2026 9:26 am