Cheap power supply for AC701 tube microphones

[danfrank]

Hi,
I just joined this forum and am currently working on power supplies for tube mics... I know this is an old thread but it has been resurrected so I'll just add my 2 cents...
I was given an AKG power supply that needed a complete rebuild for a C60 mic. The voltages required are 4 volts for the AC701 filament and 120 volts for the B+ supply.
What I ended up doing is using a couple of 3 terminal regulators to regulate both voltages. For the B+ voltage, I used a LT783 regulator which works well for the 120 volt B+.
The filament supply needs something that either ramps up slowly or a constant current regulator, which basically does the same thing. As luck would have it, the LM317 can be configured as a current source. See attached pic.

Has anyone used these regulator configurations for old tube mics? I don't see any drawbacks but please chime in if I'm overlooking something.
The advantage of using regulated supplies is a super quiet noise floor.

 

[danfrank]

I finished the mods to the C60 power supply and it is dead quiet. The LM317 current regulator for the filament works like a charm... It takes 30-45 seconds to come up to voltage and the voltage at the microphone is 3.74 volts, perfect!
The LT783 makes for a super quiet B+ regulator also.

 

[bremusound]

I finished the mods to the C60 power supply and it is dead quiet. The LM317 current regulator for the filament works like a charm... It takes 30-45 seconds to come up to voltage and the voltage at the microphone is 3.74 volts, perfect!
The LT783 makes for a super quiet B+ regulator also.

Sounds very promising. Would you share the schematic of your PSU circuit?

 

[RuudNL]

Don't use a LM317 for a KM5x microphone!
Without extra filtering it is too noisy, because the grid bias is derived from the filament voltage.

 

[ursonate]

@Ruud Hello Ruud,

I have a Neumann NKM, a Neumann NKMa and a Neumann NN48 power supply. The Stabilyt cells in all power supplies are defective. Would it be possible to install such a board with a voltage regulator in these power supplies? And since I don't trust myself to do such a clean job: Could you make such a circuit board (for a corresponding fee, of course) for my power supplies?

 

[RuudNL]

The main problem is that there is usually not very much space inside the Neumann power supplies.
I have added a stalbilisation/protectioion circuit to a couple of existing power supplies, but in those cases I had the power supply here to find the best place for the additional circuit and also to fine adjust the voltage and the voltage level where the protection should kick in.
(If you want more information, please send me a private message.)