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Old 14th April 2012, 09:25 AM   #11
marce is offline marce  United Kingdom
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Try the decoupling as stated above, it may help, but a full schematic would help, with the IC supplies and decoupling. Look at the signal return in respect of the computer jack, it is a wire with an inductor on the end, I would try connecting it to the supply ground and see if the noise goes away.
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Old 14th April 2012, 09:32 AM   #12
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If the upper and lower circuits are isolated, I would wonder what that ground between the voltage regulators does (besides making an ugly ground loop). It doesn't appear to be seen by the op amp, only the + and - 5V for the op amp power. Does this ground connect somewhere inside the volume control block?
The diode bridge is mirrored and has a resistor load, which I assume is just hasty error.
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Last edited by sofaspud; 14th April 2012 at 09:35 AM.
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Old 14th April 2012, 06:07 PM   #13
DF96 is offline DF96  England
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Hum which goes away when you touch something is almost certainly due to RF oscillation getting modulated by a rectifier diode.
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Old 14th April 2012, 06:44 PM   #14
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More complete schematic
Sorry about the earlier schematic. There are a few errors. Like I said, the circuit works as intended, it's just really noisy.
The zeners in this schematic are used instead of the voltage regulators (my circuit sim doesn't have voltage regulators)

Under what circumstances should the power be decoupled? I wouldn't consider decoupling the power to, say a 7400 chip.

What would be picking up the RF noise? Would it be the transformer?


Thank you so much to everyone who has been contributing to this thread. I greatly appreciate it.
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Old 14th April 2012, 10:11 PM   #15
DF96 is offline DF96  England
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That circuit contains an op-amp follower. All followers are prone to instability, especially if they see a whiff of capacitance at the output. Add a resistor at the output, as close as possible to the opamp output pin. The value will depend on your load impedance, but somewhere around 100 ohms might work.
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Old 14th April 2012, 10:47 PM   #16
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Quote:
Under what circumstances should the power be decoupled? I wouldn't consider decoupling the power to, say a 7400 chip.
Every circumstance,IMO.

7400 series stuff should have a decoupling cap next to each IC,preferably right on the power pins. Remember,digital stuff is all square waves,with very quickly changing voltages,and lots of harmonic content.
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Old 14th April 2012, 11:03 PM   #17
DF96 is offline DF96  England
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I seem to remember Texas TTL books recommend one ceramic cap per chip, plus one electrolytic per row of chips. All this with careful design of ground and supply rails to maintain low resistance and inductance. Modern opamps can be faster than 7400 logic, so may need better decoupling. Don't use a faster opamp than you really need.
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Old 15th April 2012, 04:20 AM   #18
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Ok. I tried the decoupling capacitors and they had no effect on the noise. (I tried one from +5 to -5 and when that did nothing, I tried one from +5 to ground and from -5 to ground.) Then I tried the 100 ohm resistor in series with the output, and the noise is gone....WHY???? I can't think of a reason why this would work, but it does--very well.
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Old 15th April 2012, 03:37 PM   #19
DF96 is offline DF96  England
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As I said
Quote:
Originally Posted by DF96
All followers are prone to instability, especially if they see a whiff of capacitance at the output.
A bit of cable is sufficient to give enough capacitive load for instability. The resistor stops the follower from seeing the capacitor.
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Old 15th April 2012, 03:51 PM   #20
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Wow. I am only using about a 6 inch cable to connect from the op amp to my computer. Is there capacitance at the input jack of the laptop?
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