Scratchy pot on Tenghong low pass filter

In an attempt to slightly upgrade the quality of the lowpass filters I usually use, I got me two of these: https://nl.aliexpress.com/item/1005...t_main.47.21ef79d2zUlmi4&gatewayAdapt=glo2nld
The picture shows it next to the other filters I have in my spare drawer:
IMG_7393.JPG

The left one needs 12V DC, the other two between 9-0-9 and 12-0-12V AC power supply. Only the center one has 'proper' Voltage regulators on board.

I connected one board to a mono amp and used the combo to power my subwoofer in the living room, worked fine.
scratchy pot 01.JPG

But, after a few hours powered up, the volume potentiometer produced scratching noises when turned, loud enough to be heard over the music. Weird.
Turning the board off and on again the next day made no difference, the scratching was here to stay, no big deal as long as you don't turn it...

Out of curiousity, I swapped the first board for the second one and the same thing happened; after a few hours of working fine, with a silent operating pot, this one also started scratching.

I suspected DC offset, so I measured the Voltage over the outer legs of both pots: 4,45V DC on the first and 4,2V DC on the second board.
The first measurement was with the input and amp attached, so the DC could have come from my input signal (PC soundcard in my shed and the sub out in the living). The second board was measured with nothing else than the power supply connected.

I've contacted the vendor and asked for a schematic, to be able to check things out.

But this is odd, isn't it? Over 4V DC over the volume pot on both boards. No, I didn't measure prior to both boards developing the scratch..

Ideas, suggestions?

Regards, Jan.
 
Update: the vendor can't supply a schematic.
He did notice that the amplifier I use, has a single DC powersupply and the filterboard uses a double AC powersupply.
According to him, this might be the reason the pots get scratchy and I measure over 4V DC over said pots.

I never heard of different ways of powering different devices causing such issues.

Regards, Jan.
 
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Jan, what can be said about this...?
No schematic, vague help from the vendor.
You can't even be sure that the opamps, caps, even the resistors are genuine.
Download the datasheet of the NE5532 and try to reverse engineer the filter and post the schematic.
It won't be that complicated but resistor and cap values can't be seen from the pictures.
Hope this helps.

Hugo
 
Hi Hugo,

I made an attempt to draw all connections of the board on an A4 yesterday, this didn't improve my mood, I can say, too much glaring and fidling about with the multimeter...
Schema TENHONG low-pass (2).jpeg


The large capacitors are 35V 2200uF and the four smaller ones are 25V 1000uF. The four small caps on input and output are 3,3uF NP

I could measure the exact values of the resistors, but this would entail soldering them loose on one side, a similar 'nice job' as drawing the board.
I do have a close up of the resistors:
Weerstanden.JPG


I did download the NE5532P datasheet, but I have no training in electronics and don't have a clue how to draw a schematic (that someone else could read anyway).
On the Dutch forum I was suggested to swap the input IC for a TL072P, didn't make any difference regarding the scratching...

Regards, Jan.
 
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I understand, Jan.
You did a real nice job with the drawing but certain traces are impossible. For instance, the right handed IC has pin 6 & 7 common. That's an input tight to an output...
I'll be honest with you: find yourself a decent filter and your problems will be solved. :cool:
But I leave that advice to more knowledged people in that area.

Hugo
 
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Do a google search on "fake NE5532".
You can read all day...
I once had a small 'Ali' amp from a customer. Everything in there was fake. Caps measured all different en sometimes more then 100% of their nominal value.
Transistors, diodes, everything was fake. Resistors were all far off despite 1% tolerance markings. Just so you know :)

Hugo
 
Hi Hugo,

Getting a less cheap filter (from China) was intended to 'upgrade' from the $4-5 boards I also use (from China), from the cheap ones, I had only one failure in about 10 boards, so that wasn't too bad.

These Tenghong board at least looks better, but both developed scratchy pots after running for a few hours (and 4V DC offset).

I'm willing to invest in a better filter, but these are either out of my price range (Rentner), or way too complex for my limited building skills.

Regards, Jan.
 
Small update
The vendor noted that the amplifier I use, has a single DC powersupply and the filterboard uses a double AC powersupply.
According to him, this might be the reason the pots get scratchy and I measure over 4V DC over said pots.

I never heard of different ways of powering different devices causing such issues.
Small update: with the above in mind, I connected the filterboard to a dual AC powered amp, a TDA 7294, which also had a 12-0-12V AC transformer on board, so I powered the filter with it.
After powering up, the scratching was still present, most likely the pot has been damaged, I'll change it.
More disturbing was the fact I measured 8,53V over the outer legs of the pot (could be the higher Voltage, but still).

After changing the pot for a 50K one (which was a used one, I had lying around), after powering up, the pot immediately scratched when turned.
Measuring the DC after a few minutes, I started with 0,8V. After five minutes it was 1,8V, one Chris Rea song later I measured over 2V, a few songs later 3,52V.
Anybody have a clue about what's happening here?

@Hugo: I checked the pins 6 and 7, with and without the IC in it's socket: the are tied...

Regards, Jan.
 
Hi Charles, if you read the entire thread, both boards started with 'silent' pots, the scratching developed over a few hours powered on (exept after the last pot change).

Even with nothing connected to the boards but the power supply, the offset is there, ruling out DC in the input signal (or so I think).

How do you suggest I block the DC?

Regards, Jan.
 
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I checked the pins 6 and 7, with and without the IC in it's socket: the are tied...
I checked your drawing and to me the opamps are mounted the wrong way around.
Instead of pin 6 & 7 you'll have pin 1 & 2 tied wich makes sense.
You sould measure the supply voltage at pin 4 & 8, (or pin 8 if it's a single supply) not at pin 1 or 5. which could explain the DC on the pot.
The other opamp might be wrongly positioned as well.
Before turning the opamps, make sure to measure the correct supply voltages on the correct pins.

Edit: The above is wrong, so forget it.
Still, I have no idea why pin 6 & 7 are tied.

This is the pinout:

NE5532.jpg
 
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Hi Charles,

Not that I can see/measure; the left input goes via the 3,3uF NP, directly to pin 3 and the right input via the 3,3 uF NP, to pin 5.
If that would do the trick, it's relatively simple to add two resistors from the input caps to ground on the bottom of the board. Which value do you suggest?

Soldering is done with leadfree solder, which makes it a bit harder to remove parts like the pot...

Regards, Jan.
 
And nothing elese is connected to pin 3 and 5 ? If this is the case, I would suggest a 10k resistor from each pin to ground.

Regards

Charles
Done:
Pin 3&5 10K to ground.JPG


Power applied: starting at 1,3V DC it slowly creeps up, currently over 2V DC and rising.(11:38: 5,67V DC)
It doesn't seem to work in keeping DC from the pot.

Regards, Jan.
 
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