CM7156

[Moby]

Hey guys. I decided to put back in order old DDR's Funkwerk Leipzig, VEB CM7156. Besides I know some historical details about this mic (smaller version of M7 capsule, ac761-EC92 tube,,,) I have no much more info about it. Google helped a little but not enaugh. Can somebody help with detailed votage-current values inside. How acurate is the PSU schemo I found somewhere? Also, what about 8 pin connector? I never saw similar one

 

[moamps]

Maybe these links will be helpful to you.


http://www.radiomuseum.org/r/funkwe_lei_netzgeraet_stv8556stv_855.html

http://www.neumann.com/forums/view.php?bn=neumann_genrec&key=1142526891&v=f

http://doctortube.de.tl/R.oe.hrenmikrofone.htm

The power supply looks simple to make. For the connector you should contact Microtech Geffel, IMO.

 

[Moby]

Thanks mate. Yes, I'm familiar with those links but thanks anyways. Good Idea to contact Geffel regarding connector. I tried all around withouth sucess. Yes, Psu is not a problem at all to build, I just wonder about voltage values (hand written) on the schemo. Are they correct? My first impreasion was that voltage is a too high for the  loaded state. Maybe im wrong , i still have  to calculate the tubes bias based on the mic schematics. 

 

[panman]

The correct voltages are given in the Radiomuseum-link:
"Netzgerät für Kondensatormikrofon CM7156, 210 V und 125 V Anodenspannung, 6,3 V~ und 4 V- Heizspannung."
210V and 6.3V for EC92, 125V and 4V for AC761.  Those voltages given in the OP are really too high. And probably would damage the capsule. Gefell always used 5.8V DC for the EC92 filament, but here it is 6.3V AC.  I suspect, that the noice level is higher because of that? Well, the trimmer probably helps some. The trafo-primary is also wired in an interesting way, AC-coupled to ground with C8. Wonder what the benefits of this are.

 

[Moby]

The correct voltages are given in the Radiomuseum-link:
"Netzgerät für Kondensatormikrofon CM7156, 210 V und 125 V Anodenspannung, 6,3 V~ und 4 V- Heizspannung."
210V and 6.3V for EC92, 125V and 4V for AC761.  Those voltages given in the OP are really too high. And probably would damage the capsule. Gefell always used 5.8V DC for the EC92 filament, but here it is 6.3V AC.  I suspect, that the noice level is higher because of that? Well, the trimmer probably helps some. The trafo-primary is also wired in an interesting way, AC-coupled to ground with C8. Wonder what the benefits of this are.

Yes, thanks, those numbers looks just right Yes it's possible to lower the EC92 filament voltage but it will probably lower the current and it will lead to out tx mismatch. Anyways it's woth of trying Sorry, where is that "C8" ??

 

[panman]

Sorry, where is that "C8" ??

Just follow the pin"5" to the PSU!

 

[Moby]

Sorry, where is that "C8" ??

Just follow the pin"5" to the PSU!

as I can see it's just R/C 10k/50u for EC92 ( 210V)

 

[panman]

as I can see it's just R/C 10k/50u for EC92 ( 210V)

Right! Now look again and you see the primary termination going straight to C8(50u).  And yes, there is supposed to come 210V along that wire, but that is DC and our signal happens to be AC. Got it?

 

[moamps]

... Those voltages given in the OP are really too high. And probably would damage the capsule...

These measurement are maybe done without microphone (load) connected.

The trafo-primary is also wired in an interesting way, AC-coupled to ground with C8. Wonder what the benefits of this are.

Output transformer's primary is connected between anode and Vcc. Theoretically, AC wise, it is perfectly the same where is connected: to Vcc or ground. Practically, doing it this way hum and noise numbers are maybe slightly  better.

 

[moamps]

... Yes, Psu is not a problem at all to build, I just wonder about voltage values (hand written) on the schemo. Are they correct? My first impreasion was that voltage is a too high for the  loaded state. Maybe im wrong , i still have  to calculate the tubes bias based on the mic schematics.

You should simply start with capsule bias which is 60-70V. So, the voltage at pin 4 is about 120V. If we assume anode current thru AC761 at 0,3mA (anode voltage is about 60V), current thru W10 is about 0,6mA. And voltage on him is  about 60V.  We get now the voltage of 180V on pin 5.
If we assume higher capsule bias, the voltages on pin 4 and 5 are proportionally higher. 

 

[Moby]

as I can see it's just R/C 10k/50u for EC92 ( 210V)

Right! Now look again and you see the primary termination going straight to C8(50u).  And yes, there is supposed to come 210V along that wire, but that is DC and our signal happens to be AC. Got it?

You confused me with therm "c8". As mentioned before C8 is just the part of the anode DC voltage RC filter, but yes, output tx is connected to the Vcc and it surely works like that. One benefit is to protect the primary winding from DC in case that C3 became leaky. DC current will be less than direct to gnd connection. Yes, it also uses the primary CMRR, but not ideally because there is anode resistor W5 involved into both DC and "AC noise". So it maybe helps a little but not important at all

BTW, i still hunt  the damned 8 pin connector I think that custom connector DIY is smiling :/

 

[Moby]

... Yes, Psu is not a problem at all to build, I just wonder about voltage values (hand written) on the schemo. Are they correct? My first impreasion was that voltage is a too high for the  loaded state. Maybe im wrong , i still have  to calculate the tubes bias based on the mic schematics.

You should simply start with capsule bias which is 60-70V. So, the voltage at pin 4 is about 120V. If we assume anode current thru AC761 at 0,3mA (anode voltage is about 60V), current thru W10 is about 0,6mA. And voltage on him is  about 60V.  We get now the voltage of 180V on pin 5.
If we assume higher capsule bias, the voltages on pin 4 and 5 are proportionally higher.

thx Moamp.. u save me some time and did my homework beer in ZG

 

[adamasnan]

BUMP!

Bringing this thread back, as I just got a CM7156 myself. Very good condition (unopened - I was the first it seems) but as of yet untested. I have the original power supply but no mating connector, so gathering suggestions for something suitable to replace it with (unless anyone knows of a source)

My mic is actually en route to Andreas Stenzel, but I am curious to hear of any testimonials? Can't fine very much out there.

Thanks!!