Current limiting resistor question for a project

[chrisdski]

The pre amp is an Acoustic Image Focus supplied with +6DC and +12DC 400mA. The power amp being supplied is an IcePower ASX50 BTL +/- 3.3 V input voltage. I don’t know the output voltage from the preamp but I can measure it.

[Mick Bailey]

As the preamp is using +6v and +12v and not a split supply it's worth taking a look at the preamp before deciding on what circuit to use to invert the signal to match the headroom.

Does it use opamps - if so, what types are used?

My initial thought is to power your signal inverter from the 12v and use the 6v as the reference voltage. The circuit you show is for a dual supply and needs the grounded + input connecting to 6v via a 1M ohm resistor instead. You also need a blocking capacitor on the output and maybe also on the input if the preamp doesn't already have one. For opamps, a TL071 would work fine, or TL072 with the unused side not left floating.

[chrisdski]

There might be some off the shelf solutions - thoughts?

[Mick Bailey]

The sparkfun board has a fixed gain of - 4.7 set by on board resistors so isn't as straightforward as it appears.

The dedicated BTL board looks like it plugs straight in and uses a dedicated IC specifically intended to convert an unbalanced to balanced output. As it uses a split rail supply the headroom available should be excellent. Check that the auxiliary supply is the same with your particular amp as the 125ASX2.

[x-pro]

What is the power supply for this amp in your schematic?

I was interested in the supply voltage of the amplifier according to the inverter circuit you suggested in your post. 14.
But, it is not important now.
Mick Bailey made it perfectly clear to you

[chrisdski]

The sparkfun board has a fixed gain of - 4.7 set by on board resistors so isn't as straightforward as it appears.

The dedicated BTL board looks like it plugs straight in and uses a dedicated IC specifically intended to convert an unbalanced to balanced output. As it uses a split rail supply the headroom available should be excellent. Check that the auxiliary supply is the same with your particular amp as the 125ASX2.

The asx50btl, which I have , is +/- 25v DC 400mA. The 125asx2 is +/- 24v DC 250 mA. Should work fine. I ordered one.

[chrisdski]

I received the BTL board. Wired it all up and it works great. Had to find a grounding issue, which turned out that I daisy chained the pre amp ground thru the voltage converter grounds. I know, big no no. I rerouted it to the central grounding post. Quiet as a mouse now. So as a summary: I replaced a blown power/amp board out of an Acoustic Image chorus amp head. I used an IcePower asx50btl that I had on hand (the asx125 would be a better choice), used the +25v off the auxiliary power on the ice power and using small voltage converters generated 6 and 12 volt for the preamp, and 32 volt for phantom power. I used the eBay BTL board that is made for IcePower boards to take the preamp output into the IcePower signal input. Thanks everyone for your guidance.