Neat boards 
As Xoc1 says, there seems to be a lack of decoupling although whether that will have any effect on the hum depends on many things. Try a cap across the two rails (+15 to -15) and not connected to ground. Its more for stability than hum. If it does alter the hum then you have a major problem with the supplies feeding it.
The rails need to be reasonably clean (preferably using either series regulators or, just as good, a simple resistor and zener shunt regulator). I suspect it will be in the wiring up of all this in where its all going wrong.
As Xoc1 says, there seems to be a lack of decoupling although whether that will have any effect on the hum depends on many things. Try a cap across the two rails (+15 to -15) and not connected to ground. Its more for stability than hum. If it does alter the hum then you have a major problem with the supplies feeding it.
The rails need to be reasonably clean (preferably using either series regulators or, just as good, a simple resistor and zener shunt regulator). I suspect it will be in the wiring up of all this in where its all going wrong.
I'll try to use a 7909 regulator for the -12V rail. Haven't yet tried a decoupling cap. I'll just see how it all goes down later.
I guess the main problem is the PSU itself. This thing is very noisy. Without a signal input on the main amp, it outputs a hideous hum. It's worse than the 60hz mains hum.
I guess the main problem is the PSU itself. This thing is very noisy. Without a signal input on the main amp, it outputs a hideous hum. It's worse than the 60hz mains hum.
Crap. More issues. Caps did not help at all but I managed to reduce the hum to lower level.
My main amp is going nuts now. Without the preamp, one of the two channels is putting out a significantly louder signal even when source volume is at minimum(not zero). With the preamp connected, the output is low even with the source and volume knob maxed out.
Dammit. This is frustrating.
My main amp is going nuts now. Without the preamp, one of the two channels is putting out a significantly louder signal even when source volume is at minimum(not zero). With the preamp connected, the output is low even with the source and volume knob maxed out.
Dammit. This is frustrating.
Try the main amp with it all connected as it used to be. If thats not right then something has happened. When experimenting with direct coupled circuits such as the tone control you always have to be aware that any glitches or connecting and disconnecting supplies can put a large DC voltage momentarily on the output. That could be enough to cause damage if the the main amp is DC coupled too and it tries to put large DC voltage across the speaker.
Possible I suppose, but it doesn't immediately suggest a zapped chip. Without knowing the amp first hand its impossible to give definite answers really.
Is there a volume control ? Could that have been damaged by DC being applied to it. If there is a volume control then try swapping the feeds from it and see if the fault changes channels.
Is there a volume control ? Could that have been damaged by DC being applied to it. If there is a volume control then try swapping the feeds from it and see if the fault changes channels.
Bump!
One of the channels is definately fried. I changed all the feedback components to no avail.
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Regarding this schematic:
I see that the first op amp is non-inverting and after the tone control stage the op amp is inverting. Do I need to add another inverting op amp to recover the signal?
One of the channels is definately fried. I changed all the feedback components to no avail.
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Regarding this schematic:
I see that the first op amp is non-inverting and after the tone control stage the op amp is inverting. Do I need to add another inverting op amp to recover the signal?
I notice a little bass boost but other than that I can't tell any difference.
You're right Mooly, I fried one channel of the chipamp together with one of the op amps when I was experimenting with it. I replaced them and everything sounds nice now except for a little distortion. I'm planning use the old chip in BTL mode for a sub.
You're right Mooly, I fried one channel of the chipamp together with one of the op amps when I was experimenting with it. I replaced them and everything sounds nice now except for a little distortion. I'm planning use the old chip in BTL mode for a sub.
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