So I was sitting when all of a sudden a nasty tone started blasting through my speakers that made me jump. I did a little investigatigating and figured out it's my preamp. I took a picture of an oscilloscope hooked up to the output of the preamp but don't know how to interpret it. This is with nothing input to it just the power turned on.
The preamp has 2 NE5532 opamps and is takes a single power supply. It has a 12v regulator on the board. Let me know if there's more information needed. Thank you
That is very unusual to see these days. The frequency is very low...
Things to check are to look at the power rail with the scope (pin 8 of the 5532's) and make sure that is clean and constant. Also remove any input leads. See if the problem is affected by altering volume and tone controls.
A circuit of the preamp might help. The issue could even be physical damage like broken print causing the circuit to behave as an oscillator.
Things to check are to look at the power rail with the scope (pin 8 of the 5532's) and make sure that is clean and constant. Also remove any input leads. See if the problem is affected by altering volume and tone controls.
A circuit of the preamp might help. The issue could even be physical damage like broken print causing the circuit to behave as an oscillator.
Thanks I'll check the power supply. I removed the input wiring already and unplugged the output wires and that's what I hooked the oscilloscope to. I don't have a circuit diagram unfortunately. Adjusting the volume pot works as intended and amplifies the signal shown. I will try the tone controls next (Bass, mid, and treble) I don't use them and just keep them flat.
Photo?
Is it a circuit you have made, a pre-built board, a commercial pre-amp?
You sure know how to keep up suspense! 😵
Is it a circuit you have made, a pre-built board, a commercial pre-amp?
You sure know how to keep up suspense! 😵
Well done 👍 the blob probably caused a short between two points that actually turned the circuit into an oscillator.
That all looks fairly standard opamp circuitry. One possible design problem is the DC coupling from stage to stage and the wipers of the pots passing DC which can cause noise as you turn them. Using a different opamp would minimise that or eliminate the problem (the 5532 is about the worst opamp to configure like this with DC coupling). Use either an LM4562 or use a FET opamp like the OPA2134.
Why do the pots pass DC noise? I've experienced that before unfortunately. Does the DC voltage come from the power supply inside the 5532 itself?
You need to understand how the opamps 'input bias currents' effect and cause this issue. A small current flows out of each input pin (the values are in the data sheets) and that current develops a small volt drop across any resistance seen at the input. That voltage then appears as a DC voltage at the input and is therefore amplified by the the 5532 by a value determined by each opamps external components.
This unwanted voltage can be minimised by equalising the DC resistance seen by each input, something not possible with this circuit.
So here for example:
The opamp on the left will have a small DC voltage at the output. That is passed into the pot and the wiper is DC coupled into the opamp. So here you have to sources generating a DC voltage. There is the opamp on the left and the input pin of the next opamp. As you turn the pot the wiper will pass DC current and that often is the cause of noise in pots. Also as the pot is turned the resistance the + input sees will vary and that causes the DC voltage at the opamp output to vary as well.
These input bias currents are notably high for the 5532 and that is why its a bad choice here.
https://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf
So you either accept the issue or use devices to minimise the problem like a FET device which has essentially zero bias currents (zero to all practical purposes).
This unwanted voltage can be minimised by equalising the DC resistance seen by each input, something not possible with this circuit.
So here for example:
The opamp on the left will have a small DC voltage at the output. That is passed into the pot and the wiper is DC coupled into the opamp. So here you have to sources generating a DC voltage. There is the opamp on the left and the input pin of the next opamp. As you turn the pot the wiper will pass DC current and that often is the cause of noise in pots. Also as the pot is turned the resistance the + input sees will vary and that causes the DC voltage at the opamp output to vary as well.
These input bias currents are notably high for the 5532 and that is why its a bad choice here.
https://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf
So you either accept the issue or use devices to minimise the problem like a FET device which has essentially zero bias currents (zero to all practical purposes).
Thank you that was an excellent explanation. I've been learning about op amps for a few months now on my own and other sources never explained this in a way I understood. Thats possibly what made my preamp send out a jagged 14hz tone the other day. An input wire shorted to a pin of the volume pot and only when I shut off my TV it happened.
There's a much worse problem with passing DC bias current through a pot wiper. As the pot wears / dries / oxidizes out you can get to the state where the wiper and track have intermittent contact. If the current that biases an opamp input is interrupted the opamp's output will swing to the supply rail. This is very bad for your speakers/phones (and ears come to that). With a FET input opamp the swing is slower due to the stray capacitance of the circuit, and the problem is much much less.
For a bipolar opamp you should provide a separate DC path for bias current and add a capacitor on the wiper. This can be tricky to do without cost, space and compromising performance, so FET opamps are often used for volume and tone control circuits.
For a bipolar opamp you should provide a separate DC path for bias current and add a capacitor on the wiper. This can be tricky to do without cost, space and compromising performance, so FET opamps are often used for volume and tone control circuits.