Opamp mains noise

Thread Starter

Alex Cristian R

Joined Feb 16, 2019
22
I am returning with another issue that I want to address in my journey of restoring the EKO Tiger P61.

I have noticed a constant buzz since i got it and i hope i can get rid if it. It's the double of the main frequency i am hearing 50Hz x 2 = 100Hz in my country.

I have shorted the input of the power amplifier to ground and i got total silence, so probably the power amplifier it's ok based on what people say on other forums regarding this test.

What would be the next step in finding the root cause? The organ has a bunch of opamps for filtering and effects but i don't know what to check and how.

I am attaching the original schematics of the organ and the current state of the circuits with the changes i have made so far.

Original circuit scan, best quality i have. Was too big to upload here. https://drive.google.com/file/d/1XCHp9CpVsF7pEQgyBwREcIZ_Nc8OggtP/view?usp=drive_link
 

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crutschow

Joined Mar 14, 2008
34,410
The 50Hz pickup is apparently caused by a ground loop between the signal source and the power amp, and the source of that loop would be difficult to determine in that huge circuit.

Do you hear the noise when the power amp input is left open (no connection)?

If not, then you could try isolation between the amp and its signal source.
A ground loop isolator, such as this at the input to the power amp, may solve the problem
 

Ian0

Joined Aug 7, 2020
9,803
If you have 100Hz interference it is probably caused by ripple on the supply, because that is at twice the line frequency,
Check the amount of ripple on the DC supply. If it is excessive, it may be caused by a faulty smoothing capacitor, especially if they are old.
 

Audioguru again

Joined Oct 21, 2019
6,688
Since the 100Hz stops when the input of the power amplifier is AC grounded (do not DC ground it if it has a DC bias voltage) then something ahead of the power amplifier is causing the 100Hz.
 

MisterBill2

Joined Jan 23, 2018
18,463
First question is " How is the amplifier working, ASIDE FROM THE BUZZ"?? Does it still amplify whatever program material you feed it?? OR NOT??. Second question: Do any of the controls have any affect on the noise, either amplitude or tone properties?
NEXT question: do you have access to either an oscilloscope, or to a signal tracer? Do you have access to a multi-meter, analog or digital?? The input to that power amplifier is connected to quite a few different points, including at least one that looks like an active DC level control. The "accomp volume" control.
SO, ONCE AGAIN,, does the adjustment of any other controls affect the volume level of the buzz??? That will help a great deal in the analysis of the amplifier output of this electric organ.
 
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MisterBill2

Joined Jan 23, 2018
18,463
Observe that the power supply feeds to the power amplifier are from a different segment of the power supply. So it might be that there is no longer quite enough filtering. So if you have an oscilloscope, use AC coupling and look at the DC power to the output transistors.
AND, based on the fact that it is there with the input open, it is likely to be on the power connections. OR on some other input besides the one you opened.
 

Thread Starter

Alex Cristian R

Joined Feb 16, 2019
22
@MisterBill2
Aside the buzzing the amplifier is working fine, it amplifies no matter what setting the organ has.

Probably not related but while i increase the master volume i start to hear some different noise which sounds like all notes played together. This noise disappear as i switch of the tones: FLUTE, VIOLIN, TRUMPET, CLARINET from the FILTERS & TABSWITCH CIRCUIT. There are some other transient brown noises i hear, they appear and disappear randomly, haven't figured out a pattern yet.

Also, as i increase the master volume i hear different harmonics of the mains noise. (all organ tones off)

Currently i have a basic digital multimeter, but i am waiting for the arrival of a two channel oscilloscope.

@crutschow
With the input open i started measuring at about -300mV DC. As i kept the multimeter probes connected it steadily dropped to -10mV in about 2 seconds. I shut it down and discharged the amplifier capacitors and repeated the experiment. This time the voltage started dropping from -900mV. Maybe the first time i didn't watch it carefully enough. While the probes of the multimeter where connected to the amplifier i could hear some radio station through the speakers.
 
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MisterBill2

Joined Jan 23, 2018
18,463
It seems like there may be a bit of electrical leakage, both across switches and elsewhere. So it may be useful to use compressed air to blow as much dust as possible off of everything electrical. That might possibly solve the problem, or at least change it a bit.
 

Thread Starter

Alex Cristian R

Joined Feb 16, 2019
22
Right now i don't have any access to any compressed air source.

Fortunately the oscilloscope arrived, is there anything i could do at this stage?

@MisterBill2, I have measured the power supply of the amplifier (label 67 in the schematics) i see a 100Hz sawtooth wave of about 1V. The DC voltage was around 24V. On the other one (label 68) i got a similar waveform but lower amplitude, around 300mV.
 

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Audioguru again

Joined Oct 21, 2019
6,688
The power supply on the huge schematic is too small to see any text or numbers. There appears two voltage regulators that should eliminate the large sawtooth outputs.
 

MisterBill2

Joined Jan 23, 2018
18,463
OK, that looks like about 35 millivolts of ripple, which is way more than it should be on a DC supply for audio use. It gives the impression that the load current is much more than it should be. Is that same ripple showing on the negative supply voltage line??
 

Audioguru again

Joined Oct 21, 2019
6,688
The larger schematic of the power supply and power amplifier still has blurry text that is scribbled instead of clearly printed.
The power amplifier circuit is a class-AB that should have a very low current draw when not playing near maximum output power. Then the ripple voltage on the 24V supplies should be very low. But maybe the trimpot (R7?) is worn or adjusted wrongly for high current (all the time) class-A.
 

Audioguru again

Joined Oct 21, 2019
6,688
Do not remove R7. Instead turn it with no sounds playing and measure the voltage across R1 or R3.
Use Ohms Law and the value of R1 or R3 to calculate the amplifier idle current which should be about 30mA with very small crossover distortion (when the amplifier plays low levels) but adjust R7 to be no more amplifier idle current than about 100mA.
 
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Thread Starter

Alex Cristian R

Joined Feb 16, 2019
22
While not playing the keyboard i got these DC voltages across R1 and R2 and calculated corresponding current.

VR1VR3IR1IR3
3.1mV8.7mV4.5mA12.7mA

I have also adjusted R7 up until i got 100mA. The buzzing got brighter in tone, same frequency.

When playing multiple notes simultaneously with R7 set to initial position the current through R3 increased proportionally with the master volume.
 

Audioguru again

Joined Oct 21, 2019
6,688
Since the buzzing loudness increased when the amplifier idle current was only 100mA then capacitors C1 and C2 (the main 4700uF filters) need replacement.
 
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