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T.Bone SCT800/Apex 460 Mod Question
Old 7th December 2013
  #1
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Jazz Noise's Avatar
T.Bone SCT800/Apex 460 Mod Question

So I'm looking at modifying an SCT800 and I already know I'm going to alter the C9 and C10 RFI caps, but I wanted to ask about R5 as I'm not sure about what I'm planning on.

R5's job is obviously negative feedback, but I assume it also helps set the impedance seen by the capsule. So my question is what effect will raising that value have? Will it change the HF response as well as increase distortion? I've heard lower voltages on the capsule diminish HF response, so would I end up with a mellower mic with a higher THD?

Old 8th December 2013
  #2
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Matt Nolan's Avatar
 

I believe you are mistaken. R5 has nothing to do with negative feedback. To do that it would have to have some output signal at the other end of it, which it doesn't (unless the power supply is hopelessly poor).

You are right in thinking that R5 is to do with polarising the capsule but, as the capsule doesn't draw any static current, there is no static current through R5 (nor R10) and thus no DC drop. The capsule polarisation voltage is exactly the same as the voltage set by the divider R3 R4. It does seem mighty high at 96V, so re-jigging that divider could well be a good thing. Though you will lose sensitivity as well as brightness.

Capsule sound is more influenced by the air volume behind the diaphragm than the tension or voltage on the diaphragm, but there will be a small tonal effect from adjusting the polarisation voltage.

Many LDCs are polarised to around 60V, though the old CMV-563 was 80 or 85V IIRC and the Funkwerk Leipzig 7151 monster bottle mic had a whopping 105V. U87s are at around 40V - again, IIRC...
Old 8th December 2013
  #3
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Jazz Noise's Avatar
Quote:
Originally Posted by Matt Nolan View Post
I believe you are mistaken. R5 has nothing to do with negative feedback. To do that it would have to have some output signal at the other end of it, which it doesn't (unless the power supply is hopelessly poor).

You are right in thinking that R5 is to do with polarising the capsule but, as the capsule doesn't draw any static current, there is no static current through R5 (nor R10) and thus no DC drop. The capsule polarisation voltage is exactly the same as the voltage set by the divider R3 R4. It does seem mighty high at 96V, so re-jigging that divider could well be a good thing. Though you will lose sensitivity as well as brightness.

Capsule sound is more influenced by the air volume behind the diaphragm than the tension or voltage on the diaphragm, but there will be a small tonal effect from adjusting the polarisation voltage.

Many LDCs are polarised to around 60V, though the old CMV-563 was 80 or 85V IIRC and the Funkwerk Leipzig 7151 monster bottle mic had a whopping 105V. U87s are at around 40V - again, IIRC...
Doh! I'd overlooked the 1.5pF capacitor the last time, which clearly is the NFB. It seems to be attached to the tube socked though, so modifying it may be tricky. Reducing sensitivity would probably not be the wisest option for a little high end smoothing, so I'll probably avoid that. If they think this thing needs as high a voltage as they're giving it, it probably does.

Thanks for the reply.
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