Hi,
So I've had a KM84 circuit I've been tinkering with, and built it inside a standard Chinese 22mm body. I managed to fit it all in, even if it meant removing the 10dB pad switch and C9. Schematic below, courtesy of the great audioimprov blog:
My problem is that the only 22mm capsules i have measure around 52pF. They're fine capsules sound-wise, but my problem is that with this charge amplifier circuit I'll be gaining up too much, since the voltage gain will be about 13 (52pF capsule and a 4pF negative feedback capacitor C2).
The transformer I'm using is a 7:1 ratio, and I'm worried about saturation, especially since the transformer is smaller than the Cinemag sizes. I'm using a 2N3819 for T1 (biased with a trimmer).
So here's my questions, and forgive me if they're naive:
1) Can I simply increase C2 to a 5.8pF capacitor, returning the voltage gain to the original spec of around 9?
2) If I do the above, will this create a high-end rolloff similar to the negative feedback people implement on U87-type builds?
2) Would the other solution be to use a higher-ratio transformer like a Cinemag CM-13114 (12:1 ratio)? Or would I still be in danger of saturating the transformer?
Thanks! 🙂
So I've had a KM84 circuit I've been tinkering with, and built it inside a standard Chinese 22mm body. I managed to fit it all in, even if it meant removing the 10dB pad switch and C9. Schematic below, courtesy of the great audioimprov blog:
My problem is that the only 22mm capsules i have measure around 52pF. They're fine capsules sound-wise, but my problem is that with this charge amplifier circuit I'll be gaining up too much, since the voltage gain will be about 13 (52pF capsule and a 4pF negative feedback capacitor C2).
The transformer I'm using is a 7:1 ratio, and I'm worried about saturation, especially since the transformer is smaller than the Cinemag sizes. I'm using a 2N3819 for T1 (biased with a trimmer).
So here's my questions, and forgive me if they're naive:
1) Can I simply increase C2 to a 5.8pF capacitor, returning the voltage gain to the original spec of around 9?
2) If I do the above, will this create a high-end rolloff similar to the negative feedback people implement on U87-type builds?
2) Would the other solution be to use a higher-ratio transformer like a Cinemag CM-13114 (12:1 ratio)? Or would I still be in danger of saturating the transformer?
Thanks! 🙂
I don't see a problem with that. I mean, the pad function ups that to a nominal 19 pF without (hopefully) anything bad happening, and a bit less gain could only be beneficial to distortion.
BTW, finally a mic circuit where even I can make out how it works. 😉 Oh, and I love these old 1970s multicolor schematics.
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