Sony AC-148F dual mic pre mod?

[Bo Deadly]

I just picked up one of these:

  Sony AC-148F phantom power supply

The pics are not of mine but it's the same exact model near as I can tell. I'm reasoning that I could make a nice remote dual mic pre with it. It has a nice enclosure with all of the right connectors and space for more controls on the front, shielded bullet transformers, a shielded power transformer with 48V for phantom and hopefully enough juice for the amp circuits.

The question is, what circuit to put in? The single supply suggests something discrete but I could pretty easily make a virtual ground.

So what do you think? What should I do with this? Should I make a lot of fancy plans and then leave it in a closet and forget about it?

UPDATE:

Done. Created  a PCB for a dual THAT 1510 / THAT 1646 with small SMPS supply. Switches are 3 pos and use a smaller cap in the gain control to integrate low-cut directly into the 1510 CFP. LEDs trigger on +17dBu. Works sweet.

 

[Bo Deadly]

The question is, what circuit to put in? The single supply suggests something discrete but I could pretty easily make a virtual ground.

Actually I cannot make a virtual ground and also do 48V because 48V ground and virtual ground would be totally different but NOT floating relative to one another. I would be relying on the CMR of the input to filter out low frequencies (high frequencies are ok because the ground side of the 48V can be taken from downstream of the LC filter. But low frequency noise would be fully imparted between the chassis ground and virtual ground.

So it either needs to be single supply through and through or I need a new power supply with +48, +15 and -15. I need that Mean Well brick that nobody sells anymore.

 

[JohnRoberts]

Since most mic preamps are cap coupled inputs who cares what voltage the input thinks 0V is?

JR

 

[Bo Deadly]

Since most mic preamps are cap coupled inputs who cares what voltage the input thinks 0V is?

Right. But the incoming phantom powered signal is going to be relative to the chassis whereas the preamp signal is going to be relative to the virtual ground sitting at 24V. The problem is that noise between chassis and 24V cannot be filtered. The -15 relative to 24 can be filtered. But it will still be 100% between chassis and virtual ground.

Now if the preamp has good common mode rejection, then it will filter out that noise. But only so much. I was planning on using THAT 1510 and make it super quiet. I don't think I would be able to get to -128 dB+ or whatever would be considered good for that chip.

Agreed or no?

 

[JohnRoberts]

I never designed with a 1510 (after my time), but you should be able to make your V/2 supply clean and low impedance with active buffering if necessary, but be careful this doesn't introduce noise. .

That looks like a PS not a preamp box...

JR

 

[Bo Deadly]

Actually I think I may have figured it out.

I can AC couple the virtual and chassis grounds together:

So C16 in this schem is the coupling cap. It does need to be larger depending on load caps or low frequency filtering deteriorates. And positive noise is a little higher since couping the two grounds pushes all noise into the upper rail.

The 4 loads shown would be for two THAT 1510 and two THAT 1646. And then on the left are two loads for 48V phantom. So just the choke and capacitance multipliers could go on a little 90mm x 40mm PCB that would mount directly on top of a Mean Well APC-160-350. Height in total would be maybe 60mm. Not bad for 350mA of +-18 and +48 (with AC coupled grounds).