Reviving two arcane Dual MC312 microphones

[antipattern]

I somehow ended up with two old Dual MC312 microphones that I would like to convert to 48V phantom power so the can be used with modern equipment. Capsules and bodies were made by the German manufacturer MBHO, but likely when the company was still under old ownership. They no longer have any documents regarding this microphone.

The capsule seems similar to the one used in MB/Peerless MBC 540 and also the Telefunken TC 600 microphone. The latter looks very similar but uses an external power supply. Given their pedigree, they could be really nice pencil condensers.


The microphones were originally made for an arcane Dual tape recorder via a stereo jack. The microphone has a feeding current of 0.25 mA and expects 18-20V voltage. When I picked up the microphones originally, I thought that the stereo jack that came with them would be used to feed T or phantom power, but I was wrong - the input appears to be unbalanced. Tip 1 + 2 appear to be positive, and 3 provides the ground.

I originally made a thread on Gearspace, but figured GroupDIY might be more appropriate, since the conversion would be more complex than the phantom power conversion I originally planned, and it’s been ages that I have been doing anything else apart from like-for-like replacement. I made a sketch of the circuit that shows the microphone’s internals in their current form.


- From my current understanding, the best way to go about it would be to put in a DC-transformer? Are there any readily available and recommendable for such a project?
- What would be the best way to go about this conversion?
- Is there anything else that would be advisable to change when working on the circuit?

 

[Khron]

Could that pair of diodes actually be an NPN transistor? Or actually, it better be a JFET, with that 1G resistor going to ground.

That's one very odd way of drawing a schematic, by the way

Volts are a measure of voltage, not current

Why would you need a "DC-transformer" though? Transformers only work in AC, anyway.

 

[antipattern]

Maybe I should have picked a smaller project to get started

I must admit I have had no formal training so my understanding is very limited and my drawings a little bit odd
But the current != voltage was a rewrite error on my part, oops

The pair of diodes could indeed be something else, but showing up as diodes in my tester.

From my understanding, a transformer would be recommended to isolate the microphone's internal circuit and provide a balanced XLR input.

 

[Khron]

Transformers aren't 100000% mandatory. I wonder what your tester might say, if you tested that component while removed from the circuit.

 

[antipattern]

Actually, that reading is with the component removed from the circuit while tested...

 

[Khron]

That could (and should) still be a JFET. With no voltage applied, there IS a conduction channel from drain to source (which the meter shows as a 25 ohm resistance between pins 1 and 3). They need a voltage at the gate lower than the one at their source, in order to stop conducting.

https://en.wikipedia.org/wiki/JFET
PS: Schoeps and Oktava circuits are prime examples of transformerless circuits

 

[analogguru]

My bet is that the part in doubt is black has 3 terminals and has written on it: K30(A)-R

That would be a FET of the type 2SK30(A)-R.

The Grundig GCM319 and GCMS332 were built similar. Here some more infos and discussions:
https://fr.audiofanzine.com/les-mains-dans-le-cambouis/forums/t.574160,grundig-gcms-332.html
https://www.mikrocontroller.net/topic/435995
https://worldradiohistory.com/Archive-Company-Publications/Grundig/Grundig-1974-1.pdf

 

[rock soderstrom]

Given their pedigree, they could be really nice pencil condensers.

Member @MicUlli has designed a circuit exactly for this type of microphones.

Thread 'A very versatile input stage for SDC mics' A very versatile input stage for SDC mics

 

[MidnightArrakis]

I somehow ended up with two old Dual MC312 microphones that I would like to convert to 48V phantom power so the can be used with modern equipment. Capsules and bodies were made by the German manufacturer MBHO, but likely when the company was still under old ownership. They no longer have any documents regarding this microphone.

The capsule seems similar to the one used in MB/Peerless MBC 540 and also the Telefunken TC 600 microphone. The latter looks very similar but uses an external power supply. Given their pedigree, they could be really nice pencil condensers.
View attachment 104262

The microphones were originally made for an arcane Dual tape recorder via a stereo jack. The microphone has a feeding current of 0.25 mA and expects 18-20V voltage. When I picked up the microphones originally, I thought that the stereo jack that came with them would be used to feed T or phantom power, but I was wrong - the input appears to be unbalanced. Tip 1 + 2 appear to be positive, and 3 provides the ground.

I originally made a thread on Gearspace, but figured GroupDIY might be more appropriate, since the conversion would be more complex than the phantom power conversion I originally planned, and it’s been ages that I have been doing anything else apart from like-for-like replacement. I made a sketch of the circuit that shows the microphone’s internals in their current form.
View attachment 104261View attachment 104263View attachment 104264

- From my current understanding, the best way to go about it would be to put in a DC-transformer? Are there any readily available and recommendable for such a project?
- What would be the best way to go about this conversion?
- Is there anything else that would be advisable to change when working on the circuit?

Based upon -- Rock Soderstrom's -- comment.....there's this from member "MicUlli":


HAVE FUN!!!

/

 

[antipattern]

Thanks everyone for the incredibly helpful suggestions. It looks like MicUlli's circuit is exactly what I need. And apart from the 2G resistor, all parts should be relatively easy to get hold of in this part of the world. My only question would be, at the risk of sounding very stupid - where is the connection to the microphone capsule?

 

[rock soderstrom]

Ck* on the left side is the capsule.

 

[antipattern]

My bet is that the part in doubt is black has 3 terminals and has written on it: K30(A)-R

That would be a FET of the type 2SK30(A)-R.

Actually, it reads IIP638, i.e. maybe 2P638? Either way, I could not find any information based on either part number.

The link to the Grundig mics is interesting, I wonder whether these were also built by MB(HO), since Grundig is also listed as one of their clients, but I haven't seen the MB-typical pencil condenser variety.

 

[antipattern]

Mea culpa on Ck*, should have had coffee before posting...

From what I can tell, all capacitors are non-polarised. Are there any recommendations on what types to use?

 

[rock soderstrom]

From what I can tell, all capacitors are non-polarised. Are there any recommendations on what types to use?

It only looks like that because the simulation software from which the schematic comes makes no difference.

In fact, C2/3/6/7 are electrolytic (or tantal?) capacitors. Besides the technical specs like capacity and max. voltage, the physical size will play a role since you have little space in the mic.

For the smaller caps MKP/MKT or NP0 Ceramics are fine.

 

[MicUlli]

The part of the circuit right from the xlr pins is only for clarification of the mic input device. C2 and C3 are electrolytics with 50V nominal voltage, C4 and C5 are ceramics NP0 (best) or X7R (for this purpose sufficient) 100V nominal voltage, C1 is MKT 50V.
Unfortunately the original MC312 circuit has only 200MOhm resistors. If it would be a TC600 you would have 2x1GOhm for usage as 2GOhm in series.
I would recommend to use a breadboard ("Lochraster") cut to 14x42mm, all the parts fit perfectly.
Yust have finished my 16th MBHO mod and thinking of designing a pcb..
BR MicUlli

 

[rock soderstrom]

Yust have finished my 16th MBHO mod and thinking of designing a pcb..

Good idea!

 

[MidnightArrakis]

Here is a redrawn schematic that I created in order to -- try -- and show a more linear signal flow, i.e., "Input" on the left and XLR "Output" over on the right. In addition, you will more easily notice that all of the electrolytic capacitors have "polarity" markings and the transistor have pin names. Also, please notice that most all of the component REFERENCE DESIGNATORS have changed due to their new locations within the circuit schematic. This is done to maintain a logical and sequential component flow instead of ending up with something like having R1 next to R12 on one side of the schematic and then finding R2 and R3 over on the opposite side of the schematic. That makes no sense (at least to me).



Now.....even though I am no -- Circuit Designer" by any stretch of the imagination and I have read several of the comments in this thread about how "unordinary" this circuit is.....my "gut" feeling about how R1, R2, R3, R4, C1 and C2 are all connected just does not sit well with me. The 2nd schematic shown below is an attempt to show what "MicUlli" was probably attempting to achieve, but got some of the connections messed up. If I am wrong here, please let me know, OK???



While it may not be obviously apparent by just quickly looking at the original schematic, should you take a more closer look and take the time to do some "mental juggling" of the - original - components of R8, R9, R10, R11, C6 and C7.....you will notice that all of these parts make up two separate sets of "parallel" connected parts. In the first schematic, I have "untangled" everything and have shown them as they really are, which also basically shows that XLR Pins 2 & 3 are connected together. In the second schematic, I have rewired things to be where each set of three 3 components are each connected to a separate XLR pin. Again, if I am wrong about any of this.....let me know, OK???

ARE WE HAVING FUN YET???

/

 

[kingkorg]

The 2nd schematic shown below is an attempt to show what "MicUlli" was probably attempting to achieve, but got some of the connections messed up. If I am wrong here, please let me know, OK???

I believe i answered this already in another thread, the part of the schematic you are referring is emulating the mic pre, it is not part of the microphone schematic.

 

[MidnightArrakis]

I believe i answered this already in another thread, the part of the schematic you are referring is emulating the mic pre, it is not part of the microphone schematic.

So.....this is all that is needed?.....


/

 

[MicUlli]

Unfortunately the JFET pinning is wrong. You have to exchange D and S. And YES, more is not needed..