Some time ago, I bought a pair of McCrypt BM-700 microphones
. (Also sold as ISK BM-700
) They were cheap, but sounded amazingly good!
I saw that Kirstein
had the same type of microphone on sale for an even lower price than I bought them for, so I bought two more of them.
Now the type number was Pronomic CM-22. At first sight they looked exactly the same as the BM-700's I already had.
But to my surprise these CM-22 microphones were very
noisy. The BM-700's were extremely quiet, so there must be something wrong, I thought.
Here is a handdrawn schematic PRR found for me on the internet:
At first, I suspected the FET after the capsule. The microphones use a no-brand
2SK30A, so I replaced it by a 2SK170BL.
While I was busy, I also replaced the 0.1 mfd capacitors by 1 mfd, to get a better LF response.
I readjusted the bias for the same voltage on the drain as I had originally measured (= +5.6V, voltage over the FET stage = 7.5V, Uds=3.7V), but I was surprised that he noise level was still high, you could still "hear the sea"...
When I changed the bias on the FET, I reached a point were the noise seemed to 'fade out'.
Microphone sensitivity remained the same, and no distortion was heard.
To check this, I injected a signal from a generator though a small capacitor (6.8 pF) to the gate and measured the distortion after the microphone amplifier. Maximum output suffered slightly, but distortion figures were still very low.
I ended up with +4.6V on the drain.
What could be the explanation that the noise depends so much on the FET bias? I think the difference in s/n is in the order of 15 dB, compared to the state they were in when first delivered! Now they sound very good, but I am just curious...