Dual-diaphragm LDC Circuit Repair/replacement (Is this the right section to ask?)

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-known member
Mar 15, 2023
The mic in question is a Boya BY-M1000/Movo VSM-7 Multipattern condenser, obviously can't compare to some of the top microphones around, but for the value proposition (got a pair of this for less than USD90 from my country Malaysia) & as my learning tool to various mic recording techniques, this thing's hard to beat, though I wished it will have just a tad more sensitivity/headroom, A little backstory of what happened: I opened up one of the microphone a couple weeks back to see the circuit, and decided to try changing the bigger capacitor to see how it will alter the sound. First time I did, accidentally broke the solder joint between back diaphragm & pattern switching circuit. OK, I figured, its no big deal, easy to solder back, and I decided to extend the wires while I'm at it. Assembled it back, and the cardioid mode now in bi-directional mode (I'm guessing the back diaphragm was not grounded/opened properly during switch), and I proceed to open back up to try and resolder the capsule's wirings again.

THEN now this is the first time circuit board fell onto the floor along with the kind of heavy XLR output shell after I disconnected the diaphragms wiring. I probed the component connections (except for the SMDs), doesn't look like there's any significant damage, and I figured I might as well try swapping out the 50V DC capacitor while I'm at it. This thing's hard to remove, and I think I left my solder on the circuit board for more than half a min at times just to melt the tin and suction it out (the remainder tin inside I attempted to use a thin drillbit to punch a hole, I messed this joint's direct connection to GRD on the circuit, so I wired an extra connection for the capacitor to go to Ground). Also accidentally removed the R11 SMD resistor (think its a 6.8k resistor.. I measured 6.5k+) in the meanwhile and proceed to lose it. : ( Circuit fell another time just as I was done soldering a new cap. Replaced the lost R11 with 2 series resistor, and wired everything back.

I must have damaged the circuit board quite badly because now when I plug it back in, I hear nothing but random sputters & farts, some kind of short oscillating tone sometimes, & if I crank the gain high up I will get white noise here. Switches does absolutely nothing, and the low cut switch will occasionally pop if i switch it back and forth.

TLDR: Got greedy & attempt to mess with the Multipattern mic's circuit, and proceeded to screw up badly & killed the circuit. But the shell & diaphragm (3 wire LDC) is totally fine, not sure about the switches.

Anyway, since I'm already at this stage (I had other DIY projects planned besides my current mic booster mod), I figured I might as well attempt some options below while I try to understand the circuit, just need opinions:

Option 1: Attempt to repair & study the circuit. I'm starting to dislike SMD components due to my lack of skills at handling tiny components like these, and I can't quite tell at first glance what's the circuit based on. From what I have read so far, LDC mics are supposed to be polarized by the circuit, and the pattern switching is done by either by polarizing the backplate with +/- DC voltage or leaving it open (was looking at one of Neumann's multipattern circuit). However, in my probe the switching seems to do something a little bit more complicated, and I suspect the LDC's not being polarized with enough voltage to begin with (maybe that explains the sensitivity issue)

Option 2: Replace the circuit completely. I have access to Aliexpress where I live, so ordering budget replacement circuits is fairly easy (quality concerns aside). But I'll need to modify the circuit a little so that I can retain the pad switching & the low cut switching. But I might change the pattern switching to just retain Cardioid & figure 8 pattern as I don't have much use for the omni pattern. I'm also thinking maybe a DIY circuit kit is a good idea, since I can learn how to assemble a circuit from the ground up, but I'm limited by the spacing inside this shell (I can either mount a thin PCB of about 30mm*52mm, or roughly just above 40mm by 45mm). Maybe I'll need to rehouse the capsule & switches to another body/shell, and repurpose the original body for other uses.

So yea, shoot me, point me to a place where I may be able to source for good quality kits/parts, or laugh at me all you want. Learned my lesson of don't fix things if they ain't broken, I'm open to any feedbacks at this time. Pics of the mic in attachment.


  • 1680153066146.jpg
    6 MB · Views: 1
  • 1680153066206.jpg
    8.1 MB · Views: 0
  • 1680153066378.jpg
    9.3 MB · Views: 0
  • 1680153066350.jpg
    6.5 MB · Views: 0
  • 1680153066316.jpg
    9 MB · Views: 0
  • 1680153066290.jpg
    9.7 MB · Views: 0
  • 1680153066262.jpg
    8.4 MB · Views: 0
  • 1680153066232.jpg
    9.9 MB · Views: 0
  • 1680153066183.jpg
    5.9 MB · Views: 0
  • 1680153066165.jpg
    5.8 MB · Views: 0
  • 1680153066131.jpg
    6.2 MB · Views: 0
Oh yes good catch. I didn't notice that. I'll have to probe it again to see if it is.

That kind of damage will quite likely break the connection right? What do you suppose the R1 is doing for the circuit? (Asking because this is my first time dealing with this circuit, its connected between the Omni & cardioid pattern poles of the pattern switch)
Last edited:
What're the odds the capsule biasing / pattern switching is similar to the CAD GXL3000?

That's also got fig-8 in the middle ("off") position of the pattern switch, and a bunch of 51meg (51 and 6 zeroes after) resistors.
Hmm could likely be. I'll probably only be able to find time over the weekend to sort it out. I'm also in the midst of waiting for some parts to arrive for my other project...
@Khron it seems you're likely right about the circuit being some kind of CAD GXL 3000 variant. I took a measurement of R1, it is indeed giving a reading of 50.2MOhm while the switch is engaged at figure 8 mode.

Also the number of inductor & transformer count is exactly the same as the GXL, I think the voltage multiplier stage is done by the IC in the circuit. I attached more pictures I took of my 2nd mic to measure. (If you're interested)

Welp, it looks like I need to buy some 22M Ohm & megaohm resistors locally so that I can replace the previous damaged R1 and retest to see if it fixes the issue. I think the wire joint to the mic capsule may also need to be resoldered too.
View attachment 1680285978421.jpg


  • 1680285978406.jpg
    4.5 MB · Views: 0
  • 1680285978387.jpg
    4.9 MB · Views: 0
  • 1680285978371.jpg
    3.9 MB · Views: 0
  • 1680285978338.jpg
    3.8 MB · Views: 0
No, the oscillator for the capsule bias is done with that transistor and two double diodes in the second photo.

That IC is an opamp, and is quite likely the impedance adaptor itself - input to pin 5, output on pins 6-7. The other half seems to also be connected as a unity-gain buffer, but with the input grounded; i don't see any trace coming off the output (but i could be wrong).
I see. Anyhow, I'll update again once I replace that damaged resistor first and see if I could bring it back to life.
Ok guys an update: I think I'm inclined to declare this circuit as a lost cause.

I've attempted to replace the damaged resistor with an exact 51M ohm value using a series of 22M & 1.5Mohm resistors, and I got the value to around 51.4Mohm. However, during the process of soldering on the resistors, I've not only discovered another broken trace on the 1st power filter cap, I think I also twisted and broke some wire soldering joints a few times which I resoldered back.

Anyway, the end result this time is i got this constant clicking noise and the sputter noise that I experienced the last time.
So now, I think i'll keep the salvagable parts (capsule, body, some capacitors & resistors), but I'm ready to just replace in a new circuit (either 48mm*45mm or 26mm*56mm max in size, I might be able to squeeze it i can find a circuit board that happens to be 50mm*31mm)... This marked for me the disappointing part of my otherwise good day : (
Last edited:
So looking up on Aliexpress I think I found some polarization circuits that may just barely fit into the body of the microphone. I found 1 polarization circuit that gives the option of +60 & -60V

So from what I have been reading, the multipattern of 3 conductor capsules work by either disconnecting the back diaphram from the circuit, polarize it with +60V to make it omni (possible to connect in parallel to the +ve pin out of voltage plate with front diaphragm?), or -60V for bidirectional. My question is regarding the switching pattern: Is it as simple as using a 3 position SPDT (on-off-on) switch like the bOya mic to change the polarization?)

Also, I haven't trace the circuit from the pic, but description on the models say that they are based on schoeps style variant for both the polarization plate & main circuit:
Circuit:FETTransistor field effect circuit,SchoepsExpansion circuit

Additional questions are basically: at which stage is it best to implement the attenuation pad & the HPF circuit (I plan to add a 2nd order HPF circuit to clean up low end, I might change the attenuation pad to a LPF since I think its more useful to tame the highs on some of these chinese LDC mics), is it better to do it between the capsule & the circuit, or between circuit & XLR out, or in the middle?
Last edited:
So looking up on Aliexpress I think I found some polarization circuits that may just barely fit into the body of the microphone. I found 1 polarization circuit that gives the option of +60 & -60V

link? I can't find anything similar.
I've been looking up the schematic of Russell Technologies RT-GB02 Polarization schematic.
Just wondering if I'm able to use a 3pin 3 position switch to do cardioid/bipolar/omni figure by polarizing the LDC differently.
I know having a switch at T5 (connected to the back LDC) will give me cardioid & bipolar, however I'm trying to solve a little bit of a logical puzzle here since I know to polarize t5 properly with a +60V I need to flip D3 & D4 around, where should the switch go to achieve that?

I have though of connecting the back diaphragm straight to T4 where the front diaphragm is, but i'm not sure if this is ideal to polarize the LDC with positive voltages since current will be divided.

(Crossed out K1 & C6a according to the schemo instructions to get +60/-60 V at T4 & T5 respectively. Just want to ask around here first before I decide to contact Russell Tech directly)
Russel Tech Polarization board.JPG
Why couldn't / shouldn't they? And/or how would you expect to get omnidirectional response, another way? Although of course, that does depend entirely on how the capsule is connected to the circuit and bias voltages / pattern switch...
You can, but once again, it depends how the capsule is connected to the rest of the circuit.

Compare the U67, U87A, CAD GXL3000, C414...