+1When cost isn't that critical, i don't mind "guilding the lily" as much as the next person, but on the other hand, i also prefer to keep things within reason. And/or to have good (or at least sane?) reasoning for whatever design choices i make
Throwing parts into a circuit willy-nilly, "just because", does not count as "good", nor "sane". Not in my book, anyway...
As far as fear of over-voltage on C5, you can put a 130 Volt Zener on the output of your B+ (in the supply) and that will act as an emergency brake, assuring that nothing over 130VDC EVER gets to your mic. This will be true even if the supply in powered on when no mic is hooked up, then you see the issue and plug in the mic...! The zener will not conduct during normal operation of the mic (so long as your supply is actually designed to deliver 120V into the mic). Therefore, there will be no zener noise to worry about. It's just an insurance policy and costs about 25 cents. C5 can then be sized for, say, 160V, or even less.Thank you all for your suggestions...
Love the internet
Thank you for your suggestion with the emergency brake zener for the PSU, but I just ordered some fancy 400V Zn MKPs hand selected by the seller to 0,1-0,4% tolerances with way lower ESR than the popular ERO caps. Let's see how they play.
Sorry to say, that schematic is proprietary (now owned by someone else). I can give you ideas, but not exact parts, values, or diagram.
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Thank you very much, Terry. This makes sense to me, a highly fascinating and promising concept and I think I would even be able to get it done, but maybe your last sentence is the most important one: It really sounds like building another mike.Sorry to say, that schematic is proprietary (now owned by someone else). I can give you ideas, but not exact parts, values, or diagram.
So long as the voltage tolerances on your parts - such as C3 - are adequate, you could raise the B+ supply to 200-210V. If you do, recalculate your voltage divider that provides your capsule polarization so that you retain 60V on the backplate (R5 and R5). You'll also have to limit your variable polarizing voltage for the rear diaphragm so that it can only receive 120V max. For a test of the sound, just disconnect the rear diaphragm and autidion the mic in cardioid mode. If you like the sound, then futz with changing the voltage supply to the rear diaphragm.
And one more IMPORTANT thing. Try lowering the heater voltage to about 5.2V. Listen, and see if you prefer that sound. The change will effect the harmonic content of your output signal, and it will also slightly lower the amplification factor of the tube. You might find a heater voltage you prefer to 5.2, but that's a very good starting point for fine-tuning the sound of your mic.
I realize this sounds like building another mic. Guess how most of us got into building the many, many mics we have built
. It has become kind of a running gag for her to ask: "Is your microphone done now?" and I go: "Yes!" and she goes "Like... for real? Really, really truly done?" and I go: "Yeah." Break. "Well, maybe there's just this one more tiny thing I need to take care of..." and she goes like: "I knew it, it is NEVER EVER going to end".
What did you not like about the neutrik transformer? I wish I could wind my own transformer, too.
Yes, its a (huge!) CL resonance between the cap and the output transformer.
Alright; got it and understood. Thank you very much. I was partly aware of that issue, but expected to valuewise be able to find some sweet spot lower than 1uF, where the resonances would not be severe, still giving me at least a minimum of a high pass filter behaviour. Which obviously isn't the case.
You can lower R3 (1G) if you havenāt already. A value of 60meg should make a noticeable difference. But donāt go too low. I think 30meg is the lowest I would go.
