Negative psu for germanium preamps

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hereforever

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Joined
May 12, 2014
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85
Location
Brooklyn, NY
I have a pair of germanium mic preamps from an old Broadcast console that run off -48V

While testing them I've been using an International Power +48V off the shelf supply, floating
+48V of the psu to 0V on the amp
and
0V from the psu to -48V on the amp.

I'd like to rack these up with phantom power but am a little bit confused about how to properly wire the power supply grounds & shielding correctly.  Maybe I'm overthinking it

Is this the correct way to wire it up?
0V from the psu to audio module B- (power rails)
+48V from the psu to audio module 0v & chassis

And if I use a SMPS option like the Meanwell for phantom (i.e. separate power supply) just wire it up as usual with 48v to phantom switch/resistors and 0v to audio module 0v/chassis?
 
No,
I dont think mis-using phantom power in this way is likely to end well  even if it initally appears to work ;D

I think you got it right to begin with , separate supply rail correctly ground referenced .
 
If the power supply is isolated (meaning the negative side of the output has NO DC path the neutral line of the input) then you could, in theory, connect the -48V to the chassis and pin 1 phantom return the chassis and then NOT connect 0V of the preamps to the chassis. And anything referenced to ground with a DC path to pins 2 and 3 of the inputs or outputs (like coupling cap drain resistors) would have to be changed to reference -48V.

Post a schematic of the preamp and at least a block diagram of how the PS, preamp, inputs and outputs are all connected as described above and maybe we can direct you further.
 
hereforever said:
Maybe I'm overthinking it
I don't think you are.

Is this the correct way to wire it up?
0V from the psu to audio module B- (power rails)
+48V from the psu to audio module 0v & chassis
That is correct, but I doubt you can use the same PSU for phantom, unless you're prepared to do a lot of mods.

And if I use a SMPS option like the Meanwell just wire it up as usual with 48v to phantom switch/resistors and 0v to audio module 0v/chassis?
Do you mean a second PSU for phantom only? That is what you need to do, however the phantom current drain, even with the addition of a pilot light, will be problematic. Most SPMPS require a minimum load of about 20% of their nominal; if not they enter in hiccup mode, which produces a very noisy output.
I believe one member here found a supplier for a 48V 5W open-frame. This would need a shunt resistor of about 2.2kohm 2W.
Another solution would be an isolated DC/DC converter 48V to 48V, but I don't see any in the usual catalogues.
Finally, don't forget that a simple linear PSU is probably the most DIY friendly solution. There are many kit suppliers.
 
Sorry my first post was a little unclear.  I edited the original post to make clear that I intended to run phantom power from a second PSU.  I made a block diagram drawing to show the connections as I have them now

The preamp has input & output transformers

I'm still looking at power supply options but looking through some old posts it seems some members have used the Mean Well RS-15-48 https://www.meanwell.com/productPdf.aspx?i=1
 

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hereforever said:
I have a pair of germanium mic preamps from an old Broadcast console that run off -48V

While testing them I've been using an International Power +48V off the shelf supply, floating
+48V of the psu to 0V on the amp
and
0V from the psu to -48V on the amp.

I'd like to rack these up with phantom power but am a little bit confused about how to properly wire the power supply grounds & shielding correctly.  Maybe I'm overthinking it

Is this the correct way to wire it up?
0V from the psu to audio module B- (power rails)
+48V from the psu to audio module 0v & chassis

And if I use a SMPS option like the Meanwell just wire it up as usual with 48v to phantom switch/resistors and 0v to audio module 0v/chassis?
I have thought about this over the years and indeed it can get a little tricky.

The microphone doesn't know the difference as long as it sees 48V between 2/3, and 1.

If the preamps is using electrolytic phantom blocking capacitors they will need to have their polarity reversed as the nominal DC input voltage will be several volts negative.

further and the tricky part is that the mic 0V, often tied to the mic body/handle will now be sitting at -48V, this can cause nasty noises if inadvertently shorted to ground. 

For clean signal that -48V now effectively mic signal ground needs to be clean wrt audio ground.

Patch bays, snakes, and lots of installed wiring does not expect -48V on pin one so issues can arise. bonding pin 1 to chassis which is good practice will cause issues with grounded chassis.

etc, etc, etc....  There is reason this is not already done.

JR   
 
hereforever said:
Sorry my first post was a little unclear.  Yes, I meant that I intended to run phantom power from a second PSU.  I made a block diagram drawing to show the connections as I have them now
That's correct.

I'm still looking at power supply options but looking through some old posts it seems some members have used the Mean Well RS-15-48 (https://www.meanwell.com/productPdf.aspx?i=1)?
The issue with a 15W PSU is going to be even more of a problem. You'll need to draw a few watts in a resistor (or a lamp) to make sure the PSU doesn't go into hiccup mode. You're going to use effectively the PSU at less than 3% of its capacity?
 
abbey road d enfer said:
The issue with a 15W PSU is going to be even more of a problem. You'll need to draw a few watts in a resistor (or a lamp) to make sure the PSU doesn't go into hiccup mode. You're going to use effectively the PSU at less than 3% of its capacity?

I have two channels but yes 10 ma per channel is only 3% of the .3A rating of the Mean Well.

The preamps have a lot of gain.  I could probably bear to ditch phantom altogether and just use an external phantom supply if I ever need it
 
hereforever said:
I have two channels but yes 10 ma per channel is only 3% of the .3A rating of the Mean Well.

The preamps have a lot of gain.  I could probably bear to ditch phantom altogether and just use an external phantom supply if I ever need it
KISS...

JR
 
Since the pre is transformer isolated in and out, there is no reason not to call the -ve rail analogue 0V (the two rail will be connected for ac by the decoupling caps). Then you can run the phantom off the same single power supply.

Cheers

Ian
 
ruffrecords said:
Since the pre is transformer isolated in and out, there is no reason not to call the -ve rail analogue 0V (the two rail will be connected for ac by the decoupling caps). Then you can run the phantom off the same single power supply.
That would mean disconnecting pin 1 from the chassis  :( or lifting the audio ground from the chassis (best IMO); I don't know how feasible it is. Lack of data.
 
abbey road d enfer said:
That would mean disconnecting pin 1 from the chassis  :( or lifting the audio ground from the chassis (best IMO); I don't know how feasible it is. Lack of data.

Not necessary. The negative rail is analogue 0V and also -ve of phantom. This gets connected to chassis as does pin1 of the XLR in and out.

Cheers

Ian
 
abbey road d enfer said:
Is it? In the absence of a schemo, I assume the ground is positive with a germanium piece.
I maybe wrong... as I said, lack of data. If the chassis is floating, that's easy.
Yes, the 'normal' ground is positive but it is/can be connected at AC to the negative rail by capacitors. So analogue ground can be the negative instead. It does no matter which rail you use as your reference if, as shown in OPs schematic, the ins and outs are all transformer isolated.

Cheers

Ian
 
ruffrecords said:
Yes, the 'normal' ground is positive but it is/can be connected at AC to the negative rail by capacitors. So analogue ground can be the negative instead. It does no matter which rail you use as your reference if, as shown in OPs schematic, the ins and outs are all transformer isolated.

Cheers

Ian
Unless you have more info than what has been given, you're speculating on the possibility to disconnect the audio ground from the chassis. I'm not sure it's the case (pun not intended).
 
abbey road d enfer said:
Unless you have more info than what has been given, you're speculating on the possibility to disconnect the audio ground from the chassis. I'm not sure it's the case (pun not intended).
It is based on the diagram the OP gave in post #4.

Cheers

Ian
 
Thank you very much for the help & patience.

Chassis of the preamp modules is not connected to any of its i/o pins.

Ian, I redrew my block diagram and hopefully I correctly understood what you were suggesting?

There is a pin for T1 shield on the i/o of the modules which I added to the diagram.
Ian, you also mentioned connecting pin 1 of the XLR out to chassis - is this necessary?  That wouldn't cause a potential ground loop with whatever the output shield gets connected to further on?
 

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