Need Help with Audio Amp Balanced Input Circuit

[prwiley]

I can generally get simple circuits to work, but to call me a noobie would be generous.

I am trying to do something I thought would be simple: connect an audio amplifier (this one: 20W Bridged Audio Amplifier ) with a balanced input circuit (this one: Balanced Input/Output Assembly Guide – DIY Recording Equipment see schematic below).

I have the two circuits connected to the same 12volt power supply (this one: RS-35-12 - MEAN WELL - TRC Electronics ) pins 4 and 7 of U1 are connected to the 12V+ and 12V- of the power supply. Pin 1 of U1 is connected to the ground of the power supply. U1 is this chip: http://www.thatcorp.com/datashts/THAT_1240-Series_Datasheet.pdf

I hope someone experienced will be able to suggest what's going on from my description of what's happening.

I find that when I connect the OUTGND and the OUT+ of the balanced input circuit to the +/GND inputs on the amp, I get a buzz out of the speaker and the audio signal is very distorted and is only just audible. When I disconnect the OUTGROUND from the GND input of the amp so that only the OUT+ is connected from the balanced input, I get fairly clear audio out (there is some AC hum, but I'll leave that issue aside for the moment).

My question is: why does the audio sound better when only the OUT+ is connected?

My presumption is that is has something to do with the how the power supply is connected to both circuits, but I really don't understand how/why and would really like to know.

Can someone please explain what they think might be going on. I hope is that it's obvious to someone with more experience. The amp circuit itself does not seem to be an issue because I get the same behavior if I swap in a completely different amp.

Many thanks is advance for solving/explaing this puzzle.

 

[epicyclic]

Your power supply is single rail only and suitable for the amplifier but not the balanced line receiver which requires both a +ve and -ve supply and would have an associated common. You have been misled by the power supply output terminal marking which states +12V and -12V .The -12V terminal is just the negative of the +12V output . The ground of the power supply is just a protection ground .
The amp input ground is actually an extension of the -12V terminal .

 

[prwiley]

Thanks for the reply.

So, would be correct to say that what is happening when the GNDOUT from the balanced input stage is connected to the input GND of the amp is that the audio signal is shorted to ground, which would explain the distortion?

 

[epicyclic]

You are not shorting the output .With the grounds unconnected the balanced receiver ground is probably floating around +6V and is able to give the full waveform output . When you connect the grounds you inhibit the bal. rec. from giving the negative part of the output waveform .