#12 Cheap chinese BM700 / BM800 microphone modding / upgrade (to say the least).

<Part 2 can be found right over here>

Last year, after hearing (or rather, after reading) about these cheapo BM700 / BM800 microphones, over on the GroupDIY forum, i decided to get a few myself. They were cheap enough (under 20 bucks a piece), and i figured they would, at the very least, make for reasonable enough donor bodies.

Some credit is due in no small part to Mr. Henry Spragens, whose microphone section on his blog served as a considerable portion of inspiration for this endeavour. Said blog is a fountain of knowledge, testing and ideas for anyone interested in condenser microphone modding or even building from scratch.



I did a little write-up containing my preliminary observations in a forum thread of my own, which i don't intend to repeat here, in its entirety. Either way, this last week or so, sprinkled throughout several evenings, i finally managed to put together the first one.

The capsule i went for was the 34mm AKG version that resulted from some of my... previous experiments, shall we say. Slight niggle is, the saddle i was somewhat forced to use isn't holding the capsule quite symmetrically inside the headbasket, front-to-back, but that could (or at least might) be taken care of at a later date. This would, of course, only be of concern when using the mic in the figure-8 pattern (and perhaps less so in omnidirectional).

The way the circuit boards go together was inspired by the stock sE2200A (main PCB and switch PCB soldered together in a T-joint), and the AKG Perception 200 (DC-DC PCB connecting to the main PCB via 0.1" pin headers / sockets). The mounting holes on the edges were placed such as to fit both into these BM800 mics, as well as the above-mentioned sE2200A's (of which i have several).

Before you freak out - no, i didn't install the 1M bias trimmer for the JFET gate (resistor). On one hand, because it was too tall for the body tube to fit over it, and on the other hand, because i couldn't be arsed to arrange the entire biasing setup at the time.

You might be relieved to know, though, that i DID bother matching the resistors pertaining to the Schoeps-like output stage, as well as the betas of the two output PNPs - if memory serves, one had 214 and the other 215. Close'nuff...

Due to limited imagination capabilities (most likely because of the late hour i was doing this at), i just "had to" sketch a cross-section on paper, just to make it clear to myself just how deep the cutouts into the body tube needed to be, in order to clear the switches when fully assembled. Hey, if it's stupid and it works, it ain't stupid!..

As described in the forum thread, the circuit in this particular version is a hybrid of several ideas and developments. It all started from the classic Schoeps circuit, an analysis / description of which can be found HERE. That was then altered by the Chinese who designed the stock circuitry in these BM800 (and other cheap side-address electret) microphones, details about which can be found HERE, along with some improvements.

I was determined to have the maximum amount of flexibility with these circuits (ie. this Schoeps-based one, and another version for low-ratio transformer-coupled mics), so with the aid of LTspice and a bit of elbow-grease & patience, i figured out where and how to shoehorn in a switchable pad, as well as a high-pass filter.