312 Preamp on single supply

[RSRecords]

Has anybody run a 312 style preamp off a single power rail?
I want to do a single, stand alone mic pre and I was thinking of trying to run it off a single +32v rail.

I'm a little unsure of how to power the 2520 off a single supply rail, though. I assume it would be like any opamp with resistors between the V+ and the IN+ and the In+ and the V-

 

[RSRecords]

could I just place 100k resistors between the V+ and Com and V- and Com since everything is referenced to the common anyway? Seems too easy...what am I missing (besides proper education)

 

[Bo Deadly]

Sure. Just make a voltage divider with two 10k resistors to make a "virtual ground". Add a 100uF cap from the virtual ground to real ground. Then you reference anything that needs to be at 0V to the virtual ground. Although note that there can be no DC into / out of the virtual ground. It's only good for AC. But the 312 has no DC source / sink to 0V so it will work. So from the 312 schematic you would connect COM (pin 3) and gain trim (pins 9 and 10) to the virtual ground. Not that the cap on pin 10 cannot be connected to real ground because that would reverse polarity on it and it would be damaged.

 

[abbey road d enfer]

A 100uF cap for the half-voltage would shift the LF response because it would be in series with C3 AC-wise. It would also be in series with the xfmr primary. I would use a much larger cap there.

 

[jarvis]

Hey,

for hints on how to run a 312-style preamp on a single supply maybe take a look at the JLM Audio Baby Animal schematic. I think it's pretty much exactly what you're looking for and more:

Single opamp, impedance balanced out:
http://www.jlmaudio.com/Baby%20Animal%20Mic%20Pre%20with%20JLM14%20with%20JLM99v%20or%20Hybrid%20on%2048v.pdf

Dual opamp balanced:
http://www.jlmaudio.com/Baby%20Animal%20Mic%20Pre%20with%20JLM14%20&%20OPA2604.pdf

This is the schematic, there is also a thread in their forum with different single supply designs i.e. a Cohen double balanced style preamp or a beefed up version of the BA with 2 opamp stages for a litte more gain:

http://www.jlmaudio.com/forum/viewtopic.php?f=2&t=123&sid=3cc212eed2c161f9c488a069e4432d81

Best
Jannis

 

[Bo Deadly]

A 100uF cap for the half-voltage would shift the LF response because it would be in series with C3 AC-wise. It would also be in series with the xfmr primary. I would use a much larger cap there.

True. 470uF would do it probably. But looking at this again, gain trim pins 9 and 10 would not be connected to VGND. R2 is the path to VGND so a pot just gets inserted there. Presumably something like a C5K.

 

[RSRecords]

Here's a copy of the schematic of the pre I'm using. It's a bit more simple. I think I just need to add the10k resistors. I'm a little unsure of where to connect the xlr Pin 1 especially if I add phantom power.

 

[Bo Deadly]

Here's a copy of the schematic of the pre I'm using.

What do you have already built and what are you building? Do you have an actual PCB card with these components on it?

 

[RSRecords]

What do you have already built and what are you building? Do you have an actual PCB card with these components on it?

Yes, the card was given to me already built. I'm waiting on an edge connector to wire it up. I did power it up with out the opamp but with the 10K resistors. I was measuring +/- 12v (bench supply only does 24vdc) at the correct locations. I'm using a little stand alone chassis so I'm going to use an external smps at 32 vdc.