Current limiting resistor question for a project

[chrisdski]

I have an Acoustic Image Focus guitar amp that has a blown power amp board. I am replacing it with an IcePower 50ASX2BTL. The IcePower has 2 auxiliary power outs of +/- 25V DC unregulated with a 200mA max current draw. I am going to use these outputs to a step down transformer to power the pre amp at 12 and 6 volt. What I need to know is the value of any current limiting resistor that will be placed between the IcePower aux power out, and the step down transformers to keep the voltage at 25v and limit the draw to 200mA max. The IcePower has 400mA fuses on the power out, and they require the current draw to stay below 200mA. Excuse my ignorance if I am not approaching this correctly. I’m coming up with a 120 ohm 5 watt resistor, and I would wire this in line between the IcePower and the step down transformer.

[The Dude]

You can't step down DC voltage with a transformer. I'd probably build circuits with dropping resistors and regulators.

Edit: I'd hate to see a brand new amp module go up in smoke!

[Mick Bailey]

I wrote up a post then saw The Dude's reply. Are you certain the outputs are DC?

[Helmholtz]

Yes, it's +/-25VDC unregulated. What is the current demand for the +12V and +6V supplies?
https://shop.icepoweraudio.com/wp-co...asheet_1_9.pdf

[g1]

You could use step-down DC converters instead of transformers.

[chrisdski]

Excuse my lack of knowledge on this subject. These are what I propose to use to step down the voltage, and they are converters . Regarding the current draw of the preamp- I don’t know. I’ve been in contact with the son of the Acoustic Image founder (who recently passed away) and he doesn’t have that info. Schematics currently are not available. If the current available is insufficient, the preamp just won’t power up, correct? Am I able to limit the current to 200 mA between the IcePower and these converters using a current limiting resistor?

[Mick Bailey]

If there's insufficient current then the preamp is likely to run out of headroom and distort. A resistor in the supply drops voltage according to load. Without knowing what the demands of the preamp are it's possible that a resistor that would fully protect the supply would affect the preamp sound adversely. You really need to know what the current demand is under quiescent and maximum signal conditions.

[chrisdski]

Thanks Mick, that’s very helpful. I’ll get in contact with Acoustic Image again and see if I can get pre amp specs.

[Mick Bailey]

If the preamp current consumption in use is well within the limit of your supply it may be better to fuse the supply with 200mA fuses. That way you don't get any variable voltage drop as you would with a resistor and you protect the IcePower board.

[chrisdski]

I have been able to power up the preamp within the fuse specifications. I have signal from the effects loop send. I am working on the signal to the power amp. What is the component signified by the triangle with the -1 in it?

[The Dude]

I believe it's a gain stage with gain of 1 and the minus means inverting. So, basically it's an inverter in this case.

Edit: You are basically making a single push/pull amp out of 2 amps. Each amp is doing a half cycle- one the negative and the other positive. So, one of the two amps needs to be fed an inverted signal, much like the phase inverter in a tube amp.

[chrisdski]

Thanks for the reply. I have no idea how to invert the signal. Can you suggest a component or simple circuit to accomplish this?

[Mick Bailey]

You could use a passive circuit using a centre-tapped transformer, or an opamp phase inverter. You could perhaps as an alternative use an unbalanced to balanced DI box, but you'd need to know the impedance of the preamp output and ICE inputs to get the correct unit.

[chrisdski]

So a circuit like this is what I will need? What would be a good transistor to use? Suggested R values?

[x-pro]

Try with a 10k rating.
What is the power supply for this amp in your schematic?