Chinese BM800 review / mod / upgrade (mod circuit & boards usable in sE 2200A)

[Khron]

<Details and photos can also be found in the "part 1" and "part 2" posts over on my blog >

Because of Mr. Spencerleehorton, i decided to spring for a few of these uber-cheap BM800 / BM700 electret mics - primarily as body donors.

I went for the blue/silver version, at least because it seemed less garish than the black/gold one And i specifically looked for one with  METAL shockmounts because plastic shockmounts (would) just give me the heebee-geebees... So i ended up grabbing four of these:

http://www.ebay.co.uk/itm/291654240021?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

They updated the price since i had bought'em, at the end of january. So four of these, including shipping to Finland, ran me a grand total of 71,13$ / 47,52gbp, so roughly 18$ / 12gbp each. They were sent as two separate packages - one arrived this week apparently without getting checked by customs, while the second one arrived into the country on friday (12.02) and DID stop at customs. Go figure... On the bright side though, it only took about 2 weeks to get here from China, so still - could be worse



Either way, moving on: first impressions.

1) Cast chassis - quite impressive, in fact. Most of the body is one cast piece - from the XLR connector "neck", all the way to the top plate to which the capsule mount is fixed. This means it's pretty much (mechanically) dead, as opposed to mics like the Apex 435 / Alctron HSMC001, which are highly resonant (ie. if you knock them with a finger, you'll feel the vibrations for a good quarter/half a second).

2) Gold ring is quite loose. Purely a decorative / an aesthetic component, it's a bit too narrow for the space between the tube of the body and the headbasket, so it jangles a bit when you move the mic. Couldn't care less, it will get removed anyway, to allow room for the cutouts for the pattern / pad / high-pass switches.

3) Two-layer headbasket mesh - this is a slightly "mixed bag", as it were. The inner layer isn't soldered in or anything, only fixed with one(!) dab of glue, near the top of the headbasket. Bad side = long term, it might(?) be prone to wiggling loose and/or rattling. Good side = removing it is easy as pie

4) Circuitry's no big deal - an analysis & tests can be found over here: http://audioimprov.com/AudioImprov/Mics/Entries/2014/12/18_Another_Chinese_mic_Circuit.html

5) This one's a bit of a downer - the capsule mount's just a crap little moulded piece of plastic, so... I'm gonna have to get a bit creative when it comes to mounting the new capsules

I tried to make a half-decent "montage" of various views of the mic. The mic itself pictured is already with the inner layer of mesh removed.

Now, the plan is to stick a pair of group-buy CK12's into two of these bodies, along with a modified version of the stock circuitry (which is itself a modified Schoeps circuit). I decided to keep the input design, only with a J305 FET. The "separate" high-pass / phase-splitter PNP there makes for an easy implementation of a switchable high-pass with a single-pole switch. Later i think i might also post the circuit, for the scrutiny of minds brighter than mine ;D

I'm planning to get a few PCB's manufactured (signal board, dc-dc converter & switch-board), and designing those has been what i've spent the bulk of the last couple of days on LTspice for tweaking component values and topology details, and then cramming everything onto suitably-sized boards, while keeping an eye on mechanical constraints (mounting holes, board width, component height etc).

Not a whole lot to show just yet, but i'll post updates as soon as i have any

 

[ln76d]

I hope you don't mind another little review

On aliexpress you have a lot  different color options of BM 800.

You schould check this one also
http://www.aliexpress.com/item/High-quality-condenser-microphone-two-model-head-for-choosing/32303588010.html

It looks better than on the pictures
Size is similar to the mxl 2006.
Headbasket is a little bit shorter and fit mxl body.
Mesh is dual layer - i removed one and it looks better
Pipe, bottom cap  and xlr housing are made from aluminium (unfortunatelly) -  most alctrons for example also
It could be great donor for cheap ela m251 because of headbasket shape.
Diameter is 50mm.
I'll be making new pipe for it - 1.5mm steel, longer (10cm) - so the xlr thread will protrude only a little beyond the bottom cap.
Mounting rails inside are pretty big, so there's option to change it for little bit longer without any problem.
I had in plans to make fet847, but right now i changed plans to make C414E circuit in it. Fet847  need to wait
Definitely worth the price

 

[Khron]

Interesting, that might be worth keeping in mind

 

[homero.leal]

ln76d, do you know if this mic is has an electret capsule, or a LDC?

I know that you can't trust 100% on the product description, but is reads:
"3. Using pure gold coating vibrating head, special cartridge structure design."

Thank you in advance, and best regards!

HL

 

[ln76d]

It's electret, pretty cheap.
Small capsule inside aluminium housing which is painted (anodized?) "gold".
It's good as cheap donor body

 

[Khron]

Ok, some updates

I got around to fiddling with the circuit & PCB designs a bit more these days, and managed to  make the boards usable both in these BM800's, as well as the sE 2200A mics (i've collected 5 of them, so... ;D ). MAYBE the boards might fit other similarly sized mics, but i'll leave that up to others to find out, if they're curious (Apex 480 perhaps?).

I've chosen to use an assembly / interconnection method that's a hybrid between the stock 2200A, and the AKG Perception 200. The switch PCB (with the HPF / pad / pattern selector switches) gets soldered perpendicular to the main audio PCB. The DC-DC converter board plugs into a 3-pin 0.1" header that's on the back of the audio PCB.

For the audio PCB i've drawn up two designs - one modeled after the stock 2200A (JFET + PNP follower driving an output transformer), and one based on the stock BM800 circuit. Both designs feature switchable pads (~10dB) and high-pass filters. The DC-DC board provides both positive and negative voltages, so the end-result, if chosen, can be a multipattern mic (cardioid / omni / fig-8).

I'll have a batch of 10 of each of the 4 pcb's manufactured in China (hey, it's CHEAP ;D ), so it might take a good few weeks for actual physical results. In the mean time, i've kinda-sorta made a couple mock-ups with 1:1 printed pcb's, just to make sure things fit together, that the mounting holes are lined up properly, and that the switches poke out roughly where they should.

The switches will be mounted on the TOP of the switch pcb's, not on the bottom like in the 2200A's, for ease of (dis)assembly. My boards will be shorter than the stock 2200A's, so removal shouldn't be a problem, even without removing the transformer "can".

 

[Khron]

Here's the schematic for the audio board, modded Schoeps version.

I've hidden the layer with the component names & values purely for clarity Otherwise the PCB view would've been a mess
Board size comes out at roughly 48x37mm. The four holes in the corners are 2mm (the 2200A mounting screws are M1.6, i believe), and the ones in the middle are 2.8mm (to fit the M2.5 BM800 mounting screws).

 

[Khron]

And a view  of the switch PCB - about 29x34mm.

I had to reduce the width (from a symmetrical 34mm down to 29mm) to make it fit between the BM800's mounting rails.

 

[Khron]

Audio board, modded 2200A version.

Multiple output cap footprints (2.5mm / 5mm / 10mm pitch)

 

[Khron]

And the DC-DC converter board.

Partially SMD, but with several footprint options, for adjustment options (for example the input series dropper resistor).

 

[Khron]

As per Homero's tips, i added a resistor and a cap, from the "main" supply voltage (mid-point of the resistors from the XLR pins 2 & 3), to provide capsule bias voltage even without a DC-DC converter.

Either way, surprisingly quickly, the PCB's got manufactured and delivered and they arrived today

 

[homero.leal]

Great Khron,

Quick question... from what PCB manufacture provider  you ordered the PCB's? The ones i have used are not very quick...

And also, have you already built a mic? How does it works?

Cheers and regards!

HL

 

[Khron]

The two most affordable ones i've found are Itead and Smart-Prototyping.

From these, i've had the switch pcb, the DC-DC board and the Schoeps board done at Itead, and the last board and some class-D amplifier boards done at S-P.

I didn't go for the cheapest-cheapest shipping option with either of those, but Itead charged me about 35$ in total, and S-P 36$ (the 10 amp boards were 12$ out of that).

Haven't yet put those to use Aside from work, i've been messing around with the real life version of a 2-transistor buck converter, that's a part of the circuitry on that class-D amp board. Not sure if i'll get to assemble a mic this weekend, but perhaps next week.

Speaking of which, i should put together an order of extra mounting screws and stuff...

Great Khron,

Quick question... from what PCB manufacture provider  you ordered the PCB's? The ones i have used are not very quick...

And also, have you already built a mic? How does it works?

Cheers and regards!

HL

 

[Khron]

Does anyone know what's the "official" / "industrial" name for those teflon(?) insulated-pin standoff-thingies?

I'm not even sure what to look for, at the big electronic parts distributors

http://cdn.recordinghacks.com/images/mic_extras/cad/GXL2200-pcb.jpg

Those doohickeys where the JFET's gate and one end of the 1G resistors are soldered to...

 

[elskardio]

Does anyone know what's the "official" / "industrial" name for those teflon(?) insulated-pin standoff-thingies?

I'm not even sure what to look for, at the big electronic parts distributors

http://cdn.recordinghacks.com/images/mic_extras/cad/GXL2200-pcb.jpg

Those doohickeys where the JFET's gate and one end of the 1G resistors are soldered to...

Turret lugs?

http://ca.mouser.com/Search/ProductDetail.aspx?R=11218virtualkey53400000virtualkey534-11218

 

[Khron]

That looks like just the thing - nice catch, thanks!

Not necessarily what i'd call "cheap" though ('round about a euro something a pop)

 

[elskardio]

Cheaper alternative...

http://ca.mouser.com/ProductDetail/Keystone-Electronics/11251/?qs=sGAEpiMZZMtzcnMBgC2bs2tXIcWHTfKartiZeljXkao%3d

 

[Khron]

With due apologies to all parties concerned, for zombie-fying this thread...

Not sure if i'll get to assemble a mic this weekend, but perhaps next week.

Ehrm... Well, it "only" took me half a year (longer)... ;D But hey, better late than never, right?

Well, i finally got around to putting a franken-BM800 together. I went with one of my modified-Schoeps boards - strongly inspired by the stock BM800 circuitry, as well as the standard JFET+PNP circuitry often found in cheap transformer-coupled condenser mics. All these were fiddled with and tweaked in LTspice, to determine some of the component values for obtaining the intended response from the circuit (mainly the high-pass and the pad options).

Speaking of the electronics - not meaning to step on midwayfair's toes, but in case anyone's interested in the updated versions of my take on these circuits and boards, i'd gladly share them

The capsule i used was the 34mm AKG version of a few that were subjected to, or rather, were the result of, some very naughty things, shall we say... ;D

In case the inherent presence peak of the capsule turns out to be bothersome, the circuit and the board have the resistors in series with the output PNP bases, precisely to facilitate implementing Henry Spragen's solution for that.

Either way, i've put up a whole bunch of photos and further details over on my blog.


Sorry, but the one photo per post is just a hindrance, in cases like this...