MXL 990 modification

[MartyMart]

First off thanks to Gus for the 603? info.
Picked up a 990 for £54 and though not wonderful stock, it wasn't awful.
Changed out 4 x 22n ceramics for AVX box poly's but had to go up to 47n's
as I was out of 22n's
Also swopped a pair of 220n's with AVX
The cap between capsule & fet was an 820pf ceramic, so I squeezed a mica
cap in there of the same value.
I didn't mess with the fet or the bias setup at all.
As I only had one 990, I couldn't do an A/B after the changes, but from memory,
it seemed a little sweeter and had a touch more low end.
It craps out if you get too close - no pad ! but from 6 inches through a pop
shield it works well.
For a total cost of perhaps £2 , well worth doing.
It wont replace my "Blue Baby Bottle" mic or my Original NT-2 but it will be fine
for some duties and perhaps on some voices.
Thanks to all who've contributed to the "cheap mic" threads and in particular
Gus
Marty.

 

[Rossi]

If you're talking about the 22 nF caps at the output, 47 nF is not the way to go. In fact, Wavebourn said, and if I remember correctly others agreed on that, those the quite hefty bandwidth limiting caps introduce some distortion in the treble range, as they effectively lower the impedance at high frequencies. So the output stage has to drive a heavy load -> distortion.

On my ribbon preamp, which in its early stages was similar to the Schoeps circuit, some knowledgeable people here suggested to replace my 22 nF caps by something like 1 nF.

If you look at the last posts of Wavebourn's thread, it may be a good idea so see if MXL dropped the 47u lytic between the output transistor junction and ground. I also agree with Gus that matched output transistors (as in the original schematic) do make a difference. I did a simple Hfe matching using transistors from the same batch (same date code), and it did make the sound more transparent.

 

[MartyMart]

Thanks for the tip Rossi, I'll have a read of that post, so a reduction in value
of the 22n's at output is recommended ....
I have everything from 1n to 15n available in either AVX or Panasonic ECQ.
MM

 

[zapnspark]

A clarification.
I have never run across a commercial microphone, with the Schoeps-like circuit, that omitted the 47 uf 50 V electrolyic capacitor at the collector junctions. It's only DIY microphones that are based on the SD schematic that have omitted this part.

ZAP