duffyweber
Member
Posts: 220
I Am A(n): Professional Voice Actor, Semi-Professional Voice Actor, Audio Engineer
Pronouns: Any OK
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Post by duffyweber on Jan 8, 2017 5:55:59 GMT -8
***NOTE: Kira/Mods, if this is off-topic enough to be moved to the off-topic board, please do so.***
Okay, so this is a bit of an esoteric question, but any of you out there actually build mics, or know someone who does?
I know a lot of performers like to get their mics tweaked/custom capsules, etc. but I also have a selfish reason for asking.
I've a couple old schematics from out of production Soviet mics that I'd LOVE to have clones of, and I'll be blunt: my soldering skills just aren't up to the job. I get hum off the super-high-ohm (500KOhm/750KOhm) resistors, and its PC board looks like a scene in miniature where someone had a snowball fight with lead globs. Any refs would be TOTALLY appreciated.
Cheers, all!
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Post by StringStorm on Jan 12, 2017 3:38:06 GMT -8
I made three ribbon mics that sound as good as any other microphone. Sold them all. The only problem is finding a transformer for it. You can build your own transformer but screw that. Too much work.
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Post by JonoVO on Jan 16, 2017 12:35:24 GMT -8
Would love to craft my own mic in the future like it was a lightsaber......or make a mic lightsaber...I think I might have a good idea on my hands.
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Post by benedict on Mar 29, 2017 16:45:19 GMT -8
I've a couple old schematics from out of production Soviet mics that I'd LOVE to have clones of, and I'll be blunt: my soldering skills just aren't up to the job. I get hum off the super-high-ohm (500KOhm/750KOhm) resistors, and its PC board looks like a scene in miniature where someone had a snowball fight with lead globs. Any refs would be TOTALLY appreciated. I build microphones, but with very few exceptions they've been for field recording rather than for voice. That being said, don't give up on your Soviet clone just yet! If these are condenser mics, the noise is likely coming from the high-impedance end of the circuit (what you identified as the super-high-ohm resistors), just as you said. The most common culprit is contamination rather than the solder job itself. When soldering mics, be sure to clean the bejeebers out of the PCB when you're done soldering. Solder flux, finger oils, even dust can change the impedance and introduce hum. I start with CRC electronics cleaner and finish with methanol. Once clean, I don't touch the high-Z end of the circuit. A good conformal coat will help prevent (or at least reduce) future contamination. The second most common culprit is RF noise on an unshielded circuit. Be sure every part of the mic body is in good contact with pin 1 on the XLR, from the body tube to the headbasket. Paint, loose screws, corrosion, etc. can compromise that contact. Unfortunately that last bit makes mics tough to diagnose on the bench. Once the body tube and headbasket are removed, the mic will almost always hum.
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duffyweber
Member
Posts: 220
I Am A(n): Professional Voice Actor, Semi-Professional Voice Actor, Audio Engineer
Pronouns: Any OK
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Post by duffyweber on Mar 30, 2017 4:00:28 GMT -8
Thanks for all the feedback, guys! (no pun intended!) ; ) If anyone else is curious, this is the schematic, including a mod to drop the necessary voltage from 70V to the 48V you get from today's phantom power. And all the original Soviet parts in the list have been replaced with modern equivalents that you can get your hands on. And read. (I'm terrible yet with Cyrillic characters, but I'm learning slowly.) hulahulamoocow.com/Oktava_MK18/GandSchematic2-Oktava_mk-18.pngAnd thanks, Benedict! I think I'll give it another go one day soon when I get a little free time on my hands. Cheers! -Duffy
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Post by benedict on Mar 31, 2017 18:10:36 GMT -8
That's actually a really clever design! Thanks for sharing the schematic.
A couple of other thoughts, all of which may affect the vintage sound aspects of the mic...
Capacitors in the signal path are a common source of noise. For non-polarized caps, C0G film capacitors are a safe choice. Wima makes good ones. Ceramic capacitors (the big brown discs) are microphonic, and can cause issues. Tantalum capacitors have a tendency to give a "whooshy" character to a mic (not in a good way). I try to avoid both.
I normally see polarized capacitors used for supply filtering, but it looks like there are a couple in the signal path (C7, C8). These are another source for noise. Using polystyrene electrolytics here may help. (For filtering I've had good luck with Nichicon capacitors. Not sure what they'd do in the signal path.)
Do you know what VT1 was in the original microphone? Also, do you have a translation of the note on R10 and R11? The bias voltage on a FET typically needs to be dialed in for each individual FET, even when sticking within the same model number. Changing to another FET will almost certainly require different values for a couple of the resistors, possibly including R5, R7, and R8. Bias resistors of 1Gohm aren't at all uncommon in FET designs.
Aaaand at this point I'm getting wildly out of my depth. I build mics, but I'm not a designer by any stretch. I understood most of that schematic, but I couldn't help looking at some parts of it and thinking, "Here there be dragons!"
Cheers,
Tom
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