Different BM800 Mods
A Couple Of Different BM800 Mods
Saturday, January 29, 2022
The two main problems with cheap Chinese mics are noise (hiss from the voltage regulator circuit) and limited dynamic range due to the type of JFET used at the input. For the simplest fix for these issues, see this article. The overload was addressed by using negative feedback to reduce gain.
Another approach is to replace the K596 FET, which uses non-conducting diodes at the gate to simulate a very large resistor. The problem arises with loud signals and sensitive capsules, such as the larger 25mm electrets used to improve sound quality. As the gate is driven near 1/2 volt or so, those diodes begin to conduct, shunting the signal to ground. If the FET is replaced with one without the diodes in a normal Schoeps-style phase splitter, the source will follow the gate voltage, and will remain reverse biased up to several volts output. That approach was used in this modded mic.
A third approach retains the K596 FET, but rewires it as a source follower with no voltage gain. It’s a little more complicated soldering job, but uses most of the OEM parts. Added parts are 2 resistors and an electrolytic filter capacitor. Since the FET is used as a common drain follower, the source and gate stay at the same voltage and the diodes do not conduct. These FETs designed for electret mic use cannot be “biased” and must be run at Idss, with zero gate-source voltage.
So the original circuit looks something like this:
Modifications:
Add 470 Ω resistor and 1000µF filter cap to quiet voltage regulator.
Move K596 FET to back of capsule and rewire as follower with 4.7KΩ load resistor.
This reduces capacitive load on capsule for more sensitivity.
Since FET does not invert signal, output is in phase with studio mics, and it isn’t necessary to swap wires to pins 2 & 3.
Pictures of the modded mic:
How does it compare with the charge amp version? (The beginner’s mic?) It’s more sensitive and detailed on first impression. It feels more lively, but that may be just because it’s louder. Loud always sounds better until it’s loud enough to hurt. It remains to be seen how this circuit stacks up against other variants of the Schoeps family of mics. The test will likely be drums and brass instruments to see how it behaves when driven near clipping.
All told, it’s a huge improvement over the stock BM-700 we started out with. The JLI TSB-2555B capsule is a big step up from the small OEM capsule. Minor circuit mods quiet the hiss and tame the distortion of the Universal Chinese Mic circuit. Oddly enough, the circuit board itself is laid out for this circuit topology, but I’ve never seen a mic assembled with the correct parts. This would be a very low-cost mic if you start with one of the ZRAMO mics in the previous article.
