Hi Zeebit
Looking at your schematic and PCB I see no decoupling capacitors on the power supply rails for the op amps. I wonder if this is contributing to the hum problem!
Looking at your schematic and PCB I see no decoupling capacitors on the power supply rails for the op amps. I wonder if this is contributing to the hum problem!
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.
A 7909 is a negative 9 volt reg. Always evaluate hum and noise with the input shorted (or at least connected to some source component) and not just floating.
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.
I think one side of the main amp is kinda toast. I tried running the main amp only one channel at a time. I used a car battery but the unusual loudness on one channel is still present. Is the chip itself that is damaged?
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.
The volume control is a pot added after the tone control stage. I ran the amp alone by itself and the problem persisted. I cant think of anything to do at the moment.
Dammit. I need a break.
Dammit. I need a break.
You have to try a new chip then although its worth checking any feedback components as these set the gain.
Yep, but its so simple a circuit that its just to much of a coincidence that all this has happened from experimenting with the preamp. Something has happened somewhere down the line.
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.
_______________________
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?
If the phase reversal bothers you then all you need do is reconfigure the first opamp as an inverting buffer. Just ground pin 3 and apply the input to pin 2 via the input cap and a series 10K resistor. Then add a new 10K resistor between pin 2 and pin 1.
Try it 🙂
All you have to do is swap polarity of both your speaker leads. That does exactly the same.
All you have to do is swap polarity of both your speaker leads. That does exactly the same.
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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