I am trying to create a compressor around Jack Orman's Q&D Compressor 2. I have the compressor working fine, but I need to add balanced ins and outs and phantom power pass-thru from the mixing console to the mic.

This is the schematic I have done in Express SCH - http://www.champagneband.co.uk/compressor.gif

I am not experienced in circuits and the like, and really need all the help I can get and you can give

Here are some comments:-

• C2, C3, C7 & C8 must be non-polarised, since phantom power may be turned off.
  
  
• I am not sure about C1. I was going to have it polarised, but then if phantom power is off and the switch is in the off position, then polarity might reverse - I just don't know.
  
  
• D1 and D2 are to stop C7 & C8 damaging IC3 as they charge when phantom power is on. I do not know if I need something similar for C2 & C3. I think I might since IC1 can only take +-15v at its inputs in common mode (+-28v in differential).
  
  
• IC1 is configured for gain of 1/2. IC3 provies gain of 2. IC 2 has variable gain.
  
  
• With the DC blockers on IC1 and the gain of 1/2 I do not think I need a DC blocker between IC1 and IC2. IC2 can have gain of 60dB so I would like to have a 1uF film cap on the output, but the datasheet for IC3 says that it should be driven directly by an opamp or buffer. If no-one can tell me it is unnecessary, I think I will start with a 1uF or 10uF cap between IC2 and IC3 just to be on the safe side.
  
  
• The power supply is something I have cobbled together from various sources. I thought about using the phantom power to run the ICs, but the voltage may vary down to 9v and it may be off altogether depending on the settings on the desk. The 4 9v batteries at least allows the unit to be portable to some extent.
  [/list:u]
  That's about it. Well, does it stand a chance of working?
  
  Cheers,
  
  Rexel[/url]

Hello!
micpreamps!:) my second passion.. ok, i'l try to help You

C2, C3, C7, C8 - non polaraised 100u caps are very expensive. Even "big ones" in the world of mic preamps (f.e. Grace Design Model 101)  use lower uF caps, and often two 22u in parallel to achieve 44uf. But in many preamps I saw only electrolytic caps, sometimes with 100n in parallel for better performance.
D3-R11 - that won't work ;)
Protection against pikes from switching phantom PS :better way is to add at every in/out line, near IC's a pair of 1n4148 to +U and -U. Everything that is above +18V, -18V will be clipped. here's an example with some hi-band filtering and RFnoise protection.
I think it's better to set the IC1 gain to 2. Attenuating low level mic voltage at the first stage isn't good for noise performance.
Use DC blocker after 2166, it has near 2.5V internal bias voltage if i remeber correctly..
For 48V Phantom power - maybe use some DC step up converter, voltage multiplier etc.. I always power up my preamps with small 24VAC xformer. That gives me +/-15V for IC's and with simple diode-capacitance voltage doubler followed by discrete regulator (tranny+R+zeners) +48V DC for phantom power.
Regards
Peter

D3-R11 - Doh!  :? Pretend you never saw that, ok?

I can get general purpose non-polar electrolytics up to 100uF and beyond from RS Components, but I suspect they are not the best. The Data Sheet is fairly quiet as regards eg. ESR. If I just ensure that the phantom power is always on, I can use electrolytics. I will parallel them with a smaller cap as you suggest.

I will need to have a good look at your schematic for the spike protection, so I am sure I understand it. I am sure I could use the noise and RF protection as is. I had wondered if it was worth using that kind of filtering, since unless the filters are very well matched they will affect the similarity of the noise over the balanced cables and reduce CMRR performance in  IC1.

If gain of 1/2 is not a good idea, then I might as well use the SSM2141, which gives unity gain. With the SSM2166 in there, there is no need for extra gain stages.

Thanks for the note on the DC blocker. Good to know.

I was trying to avoid having to actually generate phantom power at all, since the mixer already does it, and just bypass it through the compressor. I will have another think.

Thanks a lot for the comments - very helpful and much appreciated.

Cheers,

David