Pad circuits in mic head amps?

[mhelin]

I've got a pair of RØDE NT1 mics which I'd like to use as overheads, but they came without the pad circuits. So which is the best way to implement one? The most usual one must be using a shunt capacitor from capsule to ground which makes a voltage divider. In this case as NT1 uses the Schoeps circuit it may be that it's the PNP output stage which is overdriven first (assuming the FET is biased properly), so it would be also possible to put a resistive voltage divider between the FET and the bipolar(s). Third option would be to lower the bias voltage to the capsule. I didn't found any discussions on this subjects in meta, so I'm asking you what are the benefits and pitfalls (if any) of the different methods. I guess the best option would be no pad circuit at all, but in that case a "Royer mod" of G7 type tube circuit would be needed, right, or is it possible to get large output drive/voltage swing from basic 48V phantom using some other than tube circuit. How about transformer output vs. transformerless circuits, which one's got better headroom?

 

[dale116dot7]

Use the capacitor across the capsule. Dividing later doesn't prevent amp overload. Lower polarizing voltage works too but will change the sound a bit. Of course, multipatterns do this already.

 

[mhelin]

I used a 220 pf cap across the capsule, it did work great with NT1. I only had a ceramic cap available, does it make a difference if these are changed to polyprop?

Btw. The AKG C451B seems to use both the capacitor and a resistive divider (active only in -20 dB pad position):
http://www.akg.com/mediendatenbank2/psfile/datei/97/C451B4055d1ffc4013.pdf

 

[Samuel Groner]

I only had a ceramic cap available, does it make a difference if these are changed to polypropylene?

No, ceramic NP0/C0G is of excellent quality.

Samuel

 

[Gus]

How close are the NT1s to the real schematic?

http://www.omnipressor.com/MicSchLib/Schoeps.pdf

Look close at the schematic around the DC to DC output and the input fet.

 

[mhelin]

It's different, the DC/DC output goes first via 1M to 0.47uF cap to ground, and from that cap via 1G resistor to capsule. Capsule is connected thorugh 1 nF polypropylene (yes, a WIMA FKP3 is the standard part there) cap to FET. So there's no cap from DC/DC output to the drain of the FET, and NT1 has two 1G resistors. Otherwise they are identical, R5 and R6 on NT1 are 100k. I have replaced the 0.1 uF caps with 1.0 uF ones and removed the 470p caps between the bases and collectors of the PNP's, also removed the HF-disk from the capsule, sounds fine to me now.

 

[Gus]

look at jumper a and b

 

[mhelin]

[quote author="Gus"]look at jumper a and b[/quote]
In the jumper position B (standard version) the gain is -3 dB. The 0.47 uF cap kind of connects the capsule in feedback loop of the T1 FET. But this doesn't work with NT1 as it is connected like this MX603:

http://www.omnipressor.com/MicSchLib/MXL603S.jpg