Hi Charles,
Now I have to show my ignorance and ask you where in my drawing I can measure those values...
Regarding the supply Voltages of the IC's; I suppose you mean the values at pin 8 and 4 relative to ground?
Regards, Jan.
Now I have to show my ignorance and ask you where in my drawing I can measure those values...
Regarding the supply Voltages of the IC's; I suppose you mean the values at pin 8 and 4 relative to ground?
Regards, Jan.
With only the power supply connected (as the boards are currently not hooked up to a signal source and power amp + speaker), using a toroid transformer which delivers 13-0-13 V AC, I measure the following Voltages:
Left IC ground to pin 4: -16,38V DC
Left IC ground to pin 8: +16,4V DC
Right IC ground to pin 4: -16,4V DC
Right IC ground to pin 4: +16,38V DC
Black probe on power supply 0, red probe on the pins.
Regards, Jan.
Left IC ground to pin 4: -16,38V DC
Left IC ground to pin 8: +16,4V DC
Right IC ground to pin 4: -16,4V DC
Right IC ground to pin 4: +16,38V DC
Black probe on power supply 0, red probe on the pins.
Regards, Jan.
Hi Jan,
I tried to trace the circuit using Microsoft Paint, but I don't have a full image of the back side of the PCB, can you post that?
Here's what I've been able to do so far:
The front traces are red, the rear traces are light blue and green circles to ground.
I tried to trace the circuit using Microsoft Paint, but I don't have a full image of the back side of the PCB, can you post that?
Here's what I've been able to do so far:
The front traces are red, the rear traces are light blue and green circles to ground.
Hi Chrisng,
Thank you for giving this a go.
There's a picture of the back side in Post #19: https://www.diyaudio.com/community/...-tenghong-low-pass-filter.401145/post-7401923
I have added the 10K as suggested by Charles.
I'll added two pictures without the resistors today, the board being black doesn't help photographing it so the traces are visible properly... Hence two pictures at a slightly different angle...
This board is where I replaced the scratchy pot (which changed nothing), removed it and have not put it back, as this is the board where I'll de-solder all resistors on one side, to be able to measure them properly.
Depending on what I find in deviating values, I may remove all componentes and re-populate the board with locally sourced components to see/hear/measure if that makes any difference.
Regards, Jan
Thank you for giving this a go.
There's a picture of the back side in Post #19: https://www.diyaudio.com/community/...-tenghong-low-pass-filter.401145/post-7401923
I have added the 10K as suggested by Charles.
I'll added two pictures without the resistors today, the board being black doesn't help photographing it so the traces are visible properly... Hence two pictures at a slightly different angle...
This board is where I replaced the scratchy pot (which changed nothing), removed it and have not put it back, as this is the board where I'll de-solder all resistors on one side, to be able to measure them properly.
Depending on what I find in deviating values, I may remove all componentes and re-populate the board with locally sourced components to see/hear/measure if that makes any difference.
Regards, Jan
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Hi, Jan
This is the schematic based on the PCB photos, please check if there any error. I hope someone with insight can make suggestions to improve this circuit design and eliminate scratch noise.
This is the schematic based on the PCB photos, please check if there any error. I hope someone with insight can make suggestions to improve this circuit design and eliminate scratch noise.
How about add a capacitor (10µF BP) to the volume pot and see if that removes the scratch noise?
The third op-amp's positive input has no DC path to anywhere according to your schematic. If that is true, then it needs fixing. If the capacitors at the output of the first two op-amps are DC blocking rather than filter capacitors, shorting them should do the trick.
Hi Chris,
Thank you for drawing the schematic! I do have a 10uF BP at hand, so I could add this to the board that has the volume pot removed, using a bit of flying wiring and see if this does the trick.
As I'm not skilled in reading schematics, can you confirm that the 10 uF would go onto the left connection of the 100K pot in my drawing and your Paint schematic?
Regards, Jan.
I connected the 10uF BP as described, using 10 cm long wires for the two other connections and the scratching is gone!
The free running wires do cause hum when I go near them, but I expect taht to be gone when the pot is in it's usual place without the extra wiring.
After five minutes of running, I measure just over 2V DC over the outside legs of the pot, slowly rising. I'll have to measure again after it has been powered on a bit longer.
Regards, Jan.
The free running wires do cause hum when I go near them, but I expect taht to be gone when the pot is in it's usual place without the extra wiring.
After five minutes of running, I measure just over 2V DC over the outside legs of the pot, slowly rising. I'll have to measure again after it has been powered on a bit longer.
Regards, Jan.
Update: After four hours of playing continuously, the DC over the volume pot has risen to 9,54V DC.
I'll let the board run for another couple of hours and measure again.
Still no scratching when I turn the volume pot.
Regards, Jan.
I'll let the board run for another couple of hours and measure again.
Still no scratching when I turn the volume pot.
Regards, Jan.
Hi Marcel,
Lacking the ability to properly read the schematic, where would I find said capacitors and where do I short them to?
Ground, or bypass? Please let me know.
Regards, Jan.
I think they are the two 33 uF electrolytic capacitors. I meant connecting one pin of the capacitor to its other pin using a short wire. I guess that's what you mean by bypass.
The usual fix is adding a capacitor on the wiper, so no bias current flows between track and wiper. You'd need to provide an alternate path for the bias current to keep the 2nd opamp happy.
With the existing circuit any bouncing of the wiper over rough points in the track could break the path for bias current, causing large swings in the opamp output. Don't route bias current through a wiper contact if the opamp is bipolar. FET opamps have so little bias current that stray capacitance reduces the problem of bias-current interruption.
Further a cap on the wiper can have a much smaller value as the load impedance is an opamp input.
Hi Mark,
I suck at reading schematics, so I need indications/directions as to which component you mean and where to find it on the board, also noting which IC leg number is involved.
1) I understand I should put the capacitor on the wiper, but 4) which value?
2) How do I do that?
3) I have a couple of TL072, which IC should I replace by it? The one in the input path?
@marcel: there are four 3,3uF NP capacitors on the board, whic ones do you suggest I bypass? The ones on the input going to pins 3 and 5?
Regards, Jan.
Offset has crept up to: 10,96V DC. Shutting down the amp for today.
Will bypass the capacitors tomorrow.
Regards, Jan.
Will bypass the capacitors tomorrow.
Regards, Jan.
Assuming this is true, have I damaged both boards by connecting them to a single power supply amp?
Reason I ask, is the fact the scratching is remedied by adding a 10uF BP capacitor to the pot, but the DC offset is stil present and slowly rising as long as the amp is under power.
I can do more measuring when the new board arrives, which the vendor offered at a 50% discount.
The new board will not be connected to a single powersupply amplifier...
@Mark Tillotson: I'm going to short the 3,3uF BP caps, following Marcel's suggestion. Taking one off the board and replace it with a wire bridge is easily done. Is 3,3uF small enough to use it on the wiper?
Regards, Jan.
I've replaced the two 3,3uF BP with wire bridges and used one, soldered onto the wiper of the pot by extending the outer legs to create room for the capacitor to fit underneath.
Not the neatest job, but it'll have to do for testing, before I start scratching out copper traces.
Next: re-assembling and power up, fingers crossed...
Regards, Jan.
Not the neatest job, but it'll have to do for testing, before I start scratching out copper traces.
Next: re-assembling and power up, fingers crossed...
Regards, Jan.