Converting balanced input to unbalanced?

Question

A friend of mine bought a used Biamp CPA130 to replace his broken stereo power amp.

The problem is that it requires a balanced input, and the output from his preamp is unbalanced.

I found on the internet one way to connect an unbalanced output to an unbalanced input by connecting the negative input to ground then connect the center conductor of the coax coming from the unbalanced output to the positive input, and the shield to ground.

The salesperson where he bought the amp is telling him that if we wire it up that way, we will destroy the amp, and is trying to sell him converters that cost $150 for both channels.

I think the salesman is just trying to get him to spend more money, I looked at the schematic for the CPA130, and it doesn't look like it will hurt anything, however I quit working on amps many years ago, and I'm a bit rusty with that stuff now.

Is there any reason why I shouldn't connect it that way?

Daniel K · Accepted Answer

You can connect unbalanced sources to that amp.  If the guy at the store actually read the manual for your amp, he would know that:

http://www.hnny.nl/archief_biamp/manuals/manual-cpa130.pdf

Page three "Input Barrier Strip".

Very easy.  Won't hurt a thing.

Anonymous · Answer

that method from internet is really ok, if this is about a true proper unbalanced input. (but a bit sub-optimal, you get half the sig (or twice the noise) and don't overdrive your input)

however don't mistake 'ground' for the household ground (of your house electrical power)... In this case 'ground' means signal ground, that you find on the outer border conductors of your rca-input connectors on that amp... but maybe this is obvious to you.

figure 2-1 of http://www.jensen-transformers.com/an/an003.pdf discusses this.

(and you see more 'true' solutions there. but transformers in the audio range are more difficult/$$$ then an active solution i guess)

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Converting balanced input to unbalanced?

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