« Reply #46 on: August 07, 2006, 06:38:20 PM »
I looked at the circuit (homerecording.com forums). I should comment that the "4.7k mod" we (Chris Church and I) do is a little different than this one. In particular, we *cut* the traces on the WM61 capsule and put the 4.7k resistor between the source of the FET and ground. This acts as "negative feedback" to reduce the gain of the mic, which is the reason why it can handle high sound levels.. Aside from this the circuit looks good. Very nice work with the FET...
Yes I am aware of the Linkwitz mod, I never meant to imply my circuit uses it. I did test it, but I could not derive a substantial benefit vs. my floating design, and the difficulty of soldering to the case (well, difficult for me, anyway) caused me to toast the occasional capsule. However I have never been able increase SPL beyond 126dB, at that point the capsule seems to hit a brick wall and clip irrespective of supply voltage, FET configuration, etc. Linkwitz reports 135dBSPL, but I can't duplicate that myself. Of course my test gear also cannot much exceed 126dBSPL without increasing harmonic distortion significantly itself, so . . . however I have had good success tweaking the circuit further from the homerecording.com circuit to reduce THD up to the 126dB point.
The design of the 1A was also to enable use with low input impedance pres, and to some extent as I push the envelope on SPL, that ability gets degraded.
Try a search on homerecording for a thread by Marik, he did a lot of experimentation with this capsule in respect to the Linkwitz mod and some interesting variations of his own. Those ideas were influential in my design.
The trick, that Marik posted I think, was to (carefully) modify the capsule as follows:
- use copper "solder wick" to remove the excess solder from both tabs on the WM61
- find the tab connected to the ground with copper
- carefully cut (with an exacto knife) across that tab. You should see the pins of the FET protruding on the back. Cut the tab so the FET pin is separated from the part of the tab which goes to the case
Now you have three parts (one on the one tab, and two on the cut tab) to connect to. And you don't have to solder to the aluminum case.
It only took me ten or twenty tries to discover this, or more accurately, discover what Marik was doing and that it really worked!
Happy hacking...
Richard
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Mics: Sennheiser MKE2002 (dummy head), Studio Projects C4, AT825 (unmodded), AT822 franken mic (x2), AT853(hc,c,sc,o), Senn. MKE2, Senn MKE40, Shure MX183/5, CA Cards, homebrew Panasonic and Transsound capsules.
Pre/ADC: Presonus Firepod & Firebox, DMIC20(x2), UA5(poorly-modded, AD8620+AD8512opamps), VX440
Recorders: Edirol R4, R09, IBM X24 laptop, NJB3(x2), HiMD(x2), MD(1).
** This individual has moved to user "illconditioned" **
