Overheating with no input

[SSV]

Hey all,
My amp is getting really hot when there is no input supplied. It stays room temp when the input is connected and playing. It's a bridged lm12clk amp circuit. The schematic can be found here:

http://www.geocities.com/sapitopt/lm12.htm

Anyone got any ideas what would make this amp hate me so much or what could be the problem?

[pinkmouse]

See this thread

GC oscillation ?

[ashok]

Why is the power supply bridge rectifier connected to the output of the two chips ?

I guess it must be a goof in the drawing.

Your amp must be unstable . It heats up when it oscillates. With an input connecetd it probably stops and so stays cooler. You must check the output on a scope. You might be able to stop it by decoupling the power supply pins at the chip with low value film caps like 0.1uF to ground.

If the instability is gain related you might have to increase gain or put a low value cap ( in pF ) across the nfb capacitor.

[peranders]

If your inverting amp has it's input loose, you will get gain of 1 => Oscillations. This means that you input MUST have pulldown resistor and the value must not be more than your minimum gain for this IC (which I don't know what it is). Your input must sense ground somehow (through the signal source or through a resistor) all the time IF LM12 aren't stable with gain of 1. See datasheet for LM12 for this.

[SSV]

Thanks everyone for the help on this problem. The resistor across the input suggested by peranders cleared it right up! It's been a rough build. I've blown 4 lm12clk's then I found that my power supply was pushing the max voltage +7v. I just wouldn't suggest the +- voltage in that schematic. Think I'll go back to lm3886's)

[peranders]

I'll suspect that LM3886 is a much better or at least better IC because LM12 is old now.