Neumann SM2 - v2 schematic? + some PCB labelling photos for posterity

[VanguardAudioLabs]

Working on two Neumann SM2s that are within 3 S/N of each other (1448 and 1451). Had several problems to fix, mostly involving age. Eventually got them to spec, EXCEPT for the front diaphragm voltages and backplate voltages.

According to SM2/SM23 schematic, the front diaphragms/AC701 grids should be at +2.4V, backplate at +41Vdc rear diaphragms at 0-79Vdc.

The SM2 i was working on kept reading 0V front diaphragms, 38V backplates , 0-79V rear diaphragms (These mics had Grosser and Schaal power supplies included, respectively).

After doing some circuit sleuthing I realized that they had grounded the front diaphragm/grid feed. So I opened the other mic and found the exact same thing. neither of these microphones appear to have been serviced previously, which would point towards this a factory install. The wiring also appeared to be the same type and gauge as the rest of the internal wiring.

Eventually I got smart enough to measure the cathode of each tube, which was holding at 1.6Vdc instead of the spec'd 4V.

My guess - apparently at some point Neumann started running these more like SM69s - AC701 grids were at 0V and cathodes were at 1.6V. Ultimately provides the same result with the grid/cathode seeing a 1.6V difference (4V minus 2.4V).

Does anyone have a v2 of the SM2 schematic, so we can add it to the technical docs archive? I've done some cursory searches of the forum and Google and came up empty.

Also attaching the quick labelling I did of the upper PCB I had to reference while working on these mics, since i've yet to find a service manual for the SM2 as well.

 

[RuudNL]

I don't know if it helps, but this is the official schematic.

 

[VanguardAudioLabs]

I don't know if it helps, but this is the official schematic.

Yep, that’s the one I’ve got…with +2.4Vdc on pins 1 and 4 (front diaphragms/tube grids) and the +4V on the cathodes (supplied from heater).

When I get to my computer I’ll post theSM69 tube schematic; it runs the cathodes at 1.6V and the front diaphragms/grids grounded. That’s how these SM2s are set up, so my assumption is at some point Neumann changed the way they did the SM2 to be more like the SM69’s style.

Either way, the cathode ends up +1.6Vdc with respect to the grid. But until I wised up and measured the cathode on these SM2s, I was a bit confused as to why the grids were grounded!

 

[RuudNL]

Neumann SM69 schematic.

 

[MagnetoSound]

Fixed bias vs self bias … there is an audible difference, and of course SM69 is a little less fussy about heater supply.

Either way, the cathode ends up +1.6Vdc with respect to the grid. But until I wised up and measured the cathode on these SM2s, I was a bit confused as to why the grids were grounded!

It’s a mistake to view the grids as grounded. Simply referenced at 0v.

 

[VanguardAudioLabs]

It’s a mistake to view the grids as grounded. Simply referenced at 0v.

Yes, that's true with the 150M resistors there. Initially I was thinking someone had modded the mic and the supply voltage was grounded instead of a full schematic change. I haven't fully gone through the mic to see if the bias configuration is changed as well (was more concerned about getting the capsule voltages fixed up, and all that was concerning issues in the upper boards and the Grosser power supply). Just getting the first mic finished up and testing, but i'll definitely do a bit more digging in the second mic.

 

[VanguardAudioLabs]

In other news, the second SM2 has what appears to be gold diaphragms.

First photo is of the first SM2 with nickel (pre cleaning), second photo is the second SM2. The second one definitely had been serviced at some point (by someone with mediocre-at-best soldering skills) so not surprising to find a re-skin here.

 

[VanguardAudioLabs]

The plot thickens. This is essentially an SM69 pcb internally. But the tuchel pins are wired like an SM2 (shared heater and B+ feeds). The bodies are SM2 bodies. There’s also some internal wiring for the shared capsule voltages (which were fed separately on an SM69) and there’s a missing 1M resistor that’s replaced by a jumper, I assume because of the voltage sharing arrangement of the backplates. Attached are two shots of SM69 internals from coutant.org



this leaves me two possibilities:

These are modified SM69 internals that someone adapted to SM2, finding (somehow) two spare SM2 bodies and capsules.

OR the simpler answer, which is that Neumann built some SM2 with modified SM69 internals.

I found another photo of an SM2 on Saturn sound that would corroborate this.

 

[VanguardAudioLabs]

And here's a photo of an early model SM2, i assume following the official SM2 schematic, as reference:

 

[VanguardAudioLabs]

Quick modification of the SM69 schem to show how it’s wired for an SM2. Might be useful for posterity for anyone with a later SM2 that uses the SM69-style PCB.

I did this in about 10 minutes so keep in mind there may be errors, but hopefully a handy reference.


Note that on the SM2 PSUs I have here (Schaal and Grosser PSUs) pins 3 and 5 are simply grounded.

Voltages are approximated between the two SM2s I serviced (37.9 and 38.8V were the measurements on the two mic backplates), but maximum polarization voltage should be calibrated to double the backplate voltage.

 

[VanguardAudioLabs]

Also for posterity, the internals of SM2 serial 1448 (identical for serial 1451), using the SM69-style (self-biasing) circuit and essentially a modified SM69 PCB.

 

[Dark1]

Yes, Neumann changed the biasing scheme on their AC701-based mics over time, incl. KM5x, SM2 etc., the final version being SM2c. Neumann should be able to not only date your mics, but also send you the correct schematics, if you email them.