Tone Control - Volatage range for TL072/LF252-N - Page 3 - diyAudio
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Old 17th May 2013, 12:29 PM   #21
Xoc1 is offline Xoc1  United Kingdom
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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!
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Old 17th May 2013, 12:34 PM   #22
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I will try to add caps. Do I need to add caps to both rails? Will 100uF be enough?
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Old 17th May 2013, 04:27 PM   #23
Mooly is online now Mooly  United Kingdom
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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.
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Old 18th May 2013, 03:49 AM   #24
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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.
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Old 18th May 2013, 06:26 AM   #25
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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.
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Old 18th May 2013, 08:48 AM   #26
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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.
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Old 18th May 2013, 09:15 AM   #27
Mooly is online now Mooly  United Kingdom
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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.
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Old 18th May 2013, 09:33 AM   #28
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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?
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Old 18th May 2013, 10:16 AM   #29
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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.
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Old 18th May 2013, 10:26 AM   #30
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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.
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