MXL D.R.K. Condenser Mic as Donor, Alice, Royer??

[doorunrun]

I spotted this "not working/for parts" mic on eBay at a price I could not refuse. I also I thought it could make a nice donor body if need be.
There's not much written about the mic's performance or internals and I thought this would be a good time for it here.
Photos:




Anything familiar here? The IC on the daughter board is a OPA2134. The circular board with the switch holds the capsule's polarization voltage components, The mic can be powered by an internal 9V battery and the switch is to use it or not.

First time I've seen that type of capsule in a MXL mic. It seems like a strange orientation. At the diaphragm's center connection there's clear plastic with some holes, I think that's actually the backside of it.

I had it out earlier, I think I put it back together correctly. I can see how the fine mesh screen on the other side might be a good idea to protect the front.

Opinions? I'm measuring V+ on all the pins of the '2134 even pin 4 which is connected to ground. Whaaa??

Is it worth continuing to troubleshoot the amp? I'm not thrilled with the capsule and would like to replace it with a larger one.

Thanks!

 

[Khron]

First time I've seen that type of capsule in a MXL mic.

I don't know about that...

http://recordinghacks.com/microphones/MXL/990

 

[doorunrun]

I don't know about that...

http://recordinghacks.com/microphones/MXL/990

Thanks for straightening me out on it! I've got some reading to do...

BTW, I goofed up on pin 4 voltage. It's only about 1V lower than what's on pin 8, or V+.

 

[Khron]

That's not that shocking - OPA2134 sucks typically ~4mA per amplifier, so that dual would draw about 8mA (max 10mA). But the supply will have at the very least the two 6.8k resistors inside the preamp you plug the mic into, so those will drop about 27-34V on their own, leaving the mic with best case in theory, 21V or so. Plus whatever else gets dropped across the (i'm suspecting) pair of 2.2k resistors that tap the phantom power off XLR pins 2/3 inside the mic, and so forth. And that's assuming nothing's fried in there...

https://www.ti.com/lit/ds/symlink/opa2134.pdf

Some actual numbers WOULD help, though Like the DC voltage on XLR pins 2/3, and the actual voltages on the opamp pins.

Is there anything else on the "main" board, except those electrolytics, that pair of 1G(?) resistors and the opamp module?

 

[Voyager10]

The top board looks like a Schoeps-style Hartley oscillator to provide the polarisation voltage.

You could check the voltage on the big (220uF) capacitor on the board, typically 6-9V or so, and the voltage between the cathode (black striped end) of D6 and ground, should be 40-60V.

 

[doorunrun]

I will take some voltage measurements and report back; thanks for the suggestions.

I took the liberty of installing leftover MXL boards I had after updating a V57m with OPA-Alice and inverter boards from JLI.
It was a little tricky because the D.R.K. has tapped mounting holes only on one side of the rail: a pair up near the capsule and a pair near the read of the mic. Anyway I got them mounted and got the mic working sort of...

Audio would drop out fairly often. I turned out being the polarization voltage was not steady. I did not spend much time measuring it. I decided to change the capsule with one from the same said V-57M mic. I replaced the saddle and got the 25mm capsule mounted. After getting it wired up the polarization voltage was now steady. From what I could tell, the circuitry is about the same between the two oscillator boards.

I don't know what could happen to the D.R.K. capsule to cause it to pull the polarization voltage down like I saw.

In effect all the guts of the D.R.K. have been changed, for now. I'll take those measurements of the original board and post. Is there a recommended capacitor value that should be used as a substitute for the capsule?

Thanks!

 

[doorunrun]

The D.R.K. with new internals, a slightly modified MXL/Schoeps-style impedance converter and inverter board.

I have no interest in getting this mic back to battery operation. It would be nice to have the original circuitry operational again, maybe with a better opamp, OPA1642??

I was wondering if there might be other boards that can fit these rails? There's a little over 80mm of vertical space to play with. The cover is a straight brass tube ~43mm in diameter.

Cheers!

 

[Khron]

I was wondering if there might be other boards that can fit these rails?

Stay tuned, got a bunch of stuff in the oven...

 

[doorunrun]

Some voltages from the D.R.K.'s circuit board. My phantom voltage source is an '80s "Electronic Musician" magazine project/kit "Phantom Power Pre-Amp" which produces a no-load voltage of 38.6V in it's current state.
With it connected to the D.R.K. board the voltage between pin 1 and 2-3 is 22.3V.

OPA2134 voltages:​

Pin1: 18.97 Pin8: 19.38​

Pin2: 18.97 Pin7: 18.96​

Pin3: 18.94 Pin6: 18.96​

Pin4: 19.36 Pin5: 19.33​

​

I'm going to pull the IC, as we say in the South, "This jus' can't be good." It will also give a clearer picture of the circuit flow.

On the inverter board, as @Voyager10 suggested, the voltage across the 220uF cap is 6.75V and the cathode end of D6 reads 49.1V.

​

 

[Voyager10]

Before you do too much disassembly, this isn't showing that the OPA2134 itself is broken.

Any conceivable circuit will have pin 4 of the IC connected directly to XLR pin 1 (i.e. circuit ground). If you're reading +18V here, that connection is broken and it's stopping the IC receiving power.

It should be possible to trace the ground to find out where it's broken. Probably worth re-soldering all the joints between the main PCB and the IC module, in case the joints have cracked.

 

[doorunrun]

The D.R.K. daughter-board with OPA2134 chip removed:

A detail of the upper left of the image with emphasis on the state of the pad in the upper left.
The daughter-board's pin connects to the main-board's ground area. On the daughter-board, that's pin 4 of the opamp which generally would be connected to ground or a virtual ground, I guess, but there sure looks like a trace from the IC's pad to the daughter-board edge connector.

Another thing, that edge pad goes nowhere else.

Before all this surgery, there was no connection between the two points, my DMM read ~2M. I solder-wicked the area and as you can see now there's no way it's going to make contact without adding a jumper wire.

I'm hoping it was like that to begin with.

If the OPA2134 never had a V- (Gnd) connection could it be possible I'd get the kind of voltages shown in the earlier post?

Thanks again!

 

[Khron]

Can we see the back of that main board?

 

[Voyager10]

Yes, without a ground pin everything will be hovering near the +ve supply voltage, your measurements are entirely consistent with that.

Looks like that pin is supposed to be bent over to make contact with the PCB pad, and then soldered, but it isn't. Perhaps it was manufactured wrong and just sneaked through production test before breaking.

I'd say there's nothing to lose by re-making that connection, with a jumper wire if needs be, putting down a new op-amp, and seeing what happens. If you are able to use an OPA1642 or 1656 instead of the 2134, it may noticeably improve the noise performance.

 

[doorunrun]

Can we see the back of that main board?

Yes!

 

[doorunrun]

Close up of the Ground pin's connection to the daughter-board.

 

[doorunrun]

Yes, without a ground pin everything will be hovering near the +ve supply voltage, your measurements are entirely consistent with that.

Looks like that pin is supposed to be bent over to make contact with the PCB pad, and then soldered, but it isn't. Perhaps it was manufactured wrong and just sneaked through production test before breaking.

I'd say there's nothing to lose by re-making that connection, with a jumper wire if needs be, putting down a new op-amp, and seeing what happens. If you are able to use an OPA1642 or 1656 instead of the 2134, it may noticeably improve the noise performance.

Voyager 10,
That's my handiwork trying to find out why there was no connection. Originally the pin was in place, and there was some solder on it, but no connection between edge-pad and edge-pin.

I will order the 1656 as a replacement and properly connect the ground.

The other thing I'd like to mention is the somewhat screwy way the op amps' outputs are coupled. I haven't traced the circuit in much detail, but I assume one would want to use fairly decent coupling caps out to the XLR. I just don't see that on this board. Are those SMD MLCC caps doing it? If so, OMG!

Thanks!

 

[Voyager10]

I'm fairly sure this is basically an OPA Alice circuit, with the Schoeps-style oscillator for polarization voltage on the top board, but with the first op-amp (pins 1-3) as a non-inverting amplifier with a gain of 2, rather than just a voltage follower.

The output coupling caps are the electrolytics on the main board. There are 5 caps in all, which I think are: 2 output coupling, main supply decoupling, "mid supply rail" decoupling, and DC blocking for the noninverting amp.

(Looks like the signals go on and off the board to the capacitors, using the many connections to the daughterboard).

 

[doorunrun]

I'm fairly sure this is basically an OPA Alice circuit, with the Schoeps-style oscillator for polarization voltage on the top board, but with the first op-amp (pins 1-3) as a non-inverting amplifier with a gain of 2, rather than just a voltage follower.

The output coupling caps are the electrolytics on the main board. There are 5 caps in all, which I think are: 2 output coupling, main supply decoupling, "mid supply rail" decoupling, and DC blocking for the noninverting amp.

(Looks like the signals go on and off the board to the capacitors, using the many connections to the daughterboard).

Yes, that makes the most sense!

I think I'll remount the '2134 chip and see how it goes. I'd rather not invest in the upgraded chip especially if @Khron has something cookin'....

 

[Voyager10]

Apologies for the terrible artwork, but here's what I think is going on on the daughterboard:



Pins are numbered 1-12, anticlockwise from top right, like a regular IC.

I'd guess, therefore:
5 - input from capsule
4 - noninverting amp dc blocking capacitor
6 - circuit ground
3,8 - to output coupling caps
7 - mid supply decoupling capacitor
9 - main supply decoupling capacitor
10 - battery input?
1,12 - from output coupling caps
2,11 - to XLR

 

[Voyager10]

Assuming I've read it right and those resistors say '103' i.e. 10K, that's a very noisy circuit. Thermal noise in a 10K resistor is ~13 nV/rtHz, and they're all over the signal path.

The original OPA-Alice has 2K2s here. There's probably no point in a fancier op-amp unless you're inclined to change the 10K resistors as well.

I suspect @Khron's circuit will prove to be the right answer,