One nice user pointed out, that the brum is not like 5 - 6kHz, but rather 100Hz!
Now when I yesterday at night read this, I was like... wow! Why I never thing about blown-out rectifier from the initial failure when I place the ceramics wrong on the wrong opamps legs and the whole thing oscilate unbearably loudly!
WoW!
Even the slovak moder of these amps suggested replacing these rectifiering diodes with higher rated ones - so I was like - yea, that has to be it, you got it!
So, luckily, I have 10 pcs of these 6A diodes 50V ( http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=6A05-TPMSCT-ND ) so I get right to it today. Took 8 pcs of them, yes, they are HUGE, and started with the B3 rectifier block, powering the TDA 7269A for the L, R and RL, RR channels. And... no change. Then I exchanged the rectifier block B1, powering the noisy center speaker and... no change at all.
Damn. And it looked so so promising...
Few scope ripple measuring on the amp.
C37, C39 - 69.9 - 70mV AC ripple (+19, -19V on them)
C41 - 41.5mV AC ripple (+29V)
C42 - 38.9mV AC ripple (-29V)
C49 - 31.2mV AC ripple (+16.2V)
C45 - 0.6 - 0.7mV AC ripple (+12V)
C46 - 0.6mV AC ripple (-12V)
C26 - 0.8mV AC ripple (+5.04V)
opamps IC6, 7 and 8 (+/-12V powered) on each 4 nd 8 pins has 0.9mV AC ripple
opamp IC9 (+15.8V powered) has ripple 30.2mV on pin 8...! 100Hz ripple too, BTW.
(still the sub seems to be quiet - at least very much, compared to the cursed center...)
The center is weird anyway. At some point only the center speaker was noisy (not the L/R, RL/RR ones) and then I measured what is ON the speaker anyway and found this:
CENTER speaker - 8.23V DC (!) and 6.4mV AC ripple, noisy. Weird.
RIGHT speaker - 0.2mV DC, 0.9mV AC ripple, quiet.
Errata
I don't know how I made it happen, but my previous statement that the L/R and RL/RR speakers only made strong noise (much stronger that with opamps board presend) when the opamps board is disconnected was FALSE.
When I unplugged today just the signal wires, all of them, the amp was quiet. In all channels. At least on balcony and when I have only one testing speaker that I just connect to different outputs...
What I like is that this somewhat confirm my determination to remove them - eg. I was right about them being unnecessary and possibly even bad for the sound...
What I did not like is, that this mercilessly kill my idea about how the terrible noise is spreading to other channels... damn
Reducing the noise.
Since the 12k resistors in parallel to the main 50k pot surpressed the noise nicely to notably lower levels, I tought that I "quiet" at least the center. So from the output of C61 to the ground, I added the 12k resistor as before. And quess what. No notable change. Since before I had these resistors where w/o the cap, then I put it, next time, on the C61 input, not on the oputput - so it will be directly on the opamp, not only after the cap.
This, together with the very high DC offset (8V?!) is sure a good hint to where to get the source of the problem, however... no luck. On the opamps outputs are very low AC ripple levels. VERY low...
Making better ground for the opamps PCB.
As part of attemt to cure the noise, it was suggested to make the grounding of the opamps PCB better. So, I took a nice, strong wire (originally a PC PSU black wire) and SOLDERED it to the ground on the opamps PCB as well, as on the back PSU/amps section.
So upon diassembling, I need to unsolder it from the PCB first.
No change at all. Sadly.
Attempt to cure the 30mV AC ripple in IC9 opamp voltage supply.
First I thought that the ceramic 10uF 16V Murata cap must be dead, so I desolder it from under the opamp and check and... it is fine. About 11.5uF capacity. So I solder it back and added another 10uF 16V Murata SMD ceramic cap on the legs of the added Samxon GC 470uF 16V cap shown there: http://img357.imageshack.us/my.php?image=geniussw51htcapaddedbk9.jpg
(on the very same pic you can clearly see, that the resistors R109, R111, R156 and R157 are not present)
Still, adding another ceramic does not fix the voltage at all, still about 30.2mV ripple there, in clear contrast to the 0.9mV ripple on other there opamps. Why is that, well, I don't know.
From the ripple measuring on the power caps it is clearly visible that the C37/C39 give almost twice the ripple the new caps show. Su'scon caps are crappy bad caps, that is why. However the 30.2mV ripple on the 10 000uF Samxon KM 25V cap is present all the way to the CENTER amp - TDA 7360.
I wonder why the Samxon GC did and the ceramic caps not help to make the voltage cleaner. This is beyond my understanding. Perhaps the better choice there will be some "softer" caps, that are also optimized for lower frequency, like Samxon KM or RS, or GF, or GK, or GT... Just not the superfast super-low-ESR ones.
Perhaps time to try the Panny FM there, if they works so well on the other there opamps?
I don't know.
Now when I yesterday at night read this, I was like... wow! Why I never thing about blown-out rectifier from the initial failure when I place the ceramics wrong on the wrong opamps legs and the whole thing oscilate unbearably loudly!
WoW!
Even the slovak moder of these amps suggested replacing these rectifiering diodes with higher rated ones - so I was like - yea, that has to be it, you got it!
So, luckily, I have 10 pcs of these 6A diodes 50V ( http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=6A05-TPMSCT-ND ) so I get right to it today. Took 8 pcs of them, yes, they are HUGE, and started with the B3 rectifier block, powering the TDA 7269A for the L, R and RL, RR channels. And... no change. Then I exchanged the rectifier block B1, powering the noisy center speaker and... no change at all.
Damn. And it looked so so promising...
Few scope ripple measuring on the amp.
C37, C39 - 69.9 - 70mV AC ripple (+19, -19V on them)
C41 - 41.5mV AC ripple (+29V)
C42 - 38.9mV AC ripple (-29V)
C49 - 31.2mV AC ripple (+16.2V)
C45 - 0.6 - 0.7mV AC ripple (+12V)
C46 - 0.6mV AC ripple (-12V)
C26 - 0.8mV AC ripple (+5.04V)
opamps IC6, 7 and 8 (+/-12V powered) on each 4 nd 8 pins has 0.9mV AC ripple
opamp IC9 (+15.8V powered) has ripple 30.2mV on pin 8...! 100Hz ripple too, BTW.
(still the sub seems to be quiet - at least very much, compared to the cursed center...)
The center is weird anyway. At some point only the center speaker was noisy (not the L/R, RL/RR ones) and then I measured what is ON the speaker anyway and found this:
CENTER speaker - 8.23V DC (!) and 6.4mV AC ripple, noisy. Weird.
RIGHT speaker - 0.2mV DC, 0.9mV AC ripple, quiet.
Errata
I don't know how I made it happen, but my previous statement that the L/R and RL/RR speakers only made strong noise (much stronger that with opamps board presend) when the opamps board is disconnected was FALSE.
When I unplugged today just the signal wires, all of them, the amp was quiet. In all channels. At least on balcony and when I have only one testing speaker that I just connect to different outputs...
R109, R111, R156 and R157 are NOT even present in reality, much less connected. Same as, for example, the Zener diodes D7 and D8 drawn in dashed line box, are not present in reality in the amp, the resistors R109, R111, R156 and R157 are not present at all.
What I like is that this somewhat confirm my determination to remove them - eg. I was right about them being unnecessary and possibly even bad for the sound...
What I did not like is, that this mercilessly kill my idea about how the terrible noise is spreading to other channels... damn
Reducing the noise.
Since the 12k resistors in parallel to the main 50k pot surpressed the noise nicely to notably lower levels, I tought that I "quiet" at least the center. So from the output of C61 to the ground, I added the 12k resistor as before. And quess what. No notable change. Since before I had these resistors where w/o the cap, then I put it, next time, on the C61 input, not on the oputput - so it will be directly on the opamp, not only after the cap.
This, together with the very high DC offset (8V?!) is sure a good hint to where to get the source of the problem, however... no luck. On the opamps outputs are very low AC ripple levels. VERY low...
Making better ground for the opamps PCB.
As part of attemt to cure the noise, it was suggested to make the grounding of the opamps PCB better. So, I took a nice, strong wire (originally a PC PSU black wire) and SOLDERED it to the ground on the opamps PCB as well, as on the back PSU/amps section.
So upon diassembling, I need to unsolder it from the PCB first.
No change at all. Sadly.
Attempt to cure the 30mV AC ripple in IC9 opamp voltage supply.
First I thought that the ceramic 10uF 16V Murata cap must be dead, so I desolder it from under the opamp and check and... it is fine. About 11.5uF capacity. So I solder it back and added another 10uF 16V Murata SMD ceramic cap on the legs of the added Samxon GC 470uF 16V cap shown there: http://img357.imageshack.us/my.php?image=geniussw51htcapaddedbk9.jpg
(on the very same pic you can clearly see, that the resistors R109, R111, R156 and R157 are not present)
Still, adding another ceramic does not fix the voltage at all, still about 30.2mV ripple there, in clear contrast to the 0.9mV ripple on other there opamps. Why is that, well, I don't know.
From the ripple measuring on the power caps it is clearly visible that the C37/C39 give almost twice the ripple the new caps show. Su'scon caps are crappy bad caps, that is why. However the 30.2mV ripple on the 10 000uF Samxon KM 25V cap is present all the way to the CENTER amp - TDA 7360.
I wonder why the Samxon GC did and the ceramic caps not help to make the voltage cleaner. This is beyond my understanding. Perhaps the better choice there will be some "softer" caps, that are also optimized for lower frequency, like Samxon KM or RS, or GF, or GK, or GT... Just not the superfast super-low-ESR ones.
Perhaps time to try the Panny FM there, if they works so well on the other there opamps?
I don't know.
Well, I think this ripple is pretty normal stabilization ripple and up to 30mV is pretty good result, after just four diodes and one cap... yep, bridged with 100nF ceramic cap too...
Work-in-progress report.
Todays I tried again to kill the cursed noise, and, well... What I did was that I put back the C117 & siblings and changed the C7 & siblings to the original 100pF value.
Then I moved the 12k resistor that was quieting a bit the noise from center speaker from the output of the C6 to the input of the C6.
Then I tried to fix the high ripple on IC9 opamp. While others (IC6 - 8) enjoy only 0.9mV ripple, thanks to the regulators and caps, this one is suffering at 32mV of ripple. I know, I know, power supply ripple rejection >80dB fix that, but I still did not quite like it. So... so the filtering cap Samxon GC 470uF 16V I changed the 1000uF 16V Panny FM and also added an another 100nF ceramic cap before the Panny FM one. On the bottom of the Panny FM cap is another 10uF Murata 16V X7R SMD cap and the last 10uF SMD Murata is on the opamp leg 8 against ground.
Ripple get lowered to 27mV
Next move - add a toroid L filter coil in the IC9 input voltage. I will kill the ripple on IC9, somehow!
Then I tried the another opamps I get on my hands in meantime. A big thanks to French user pilli from diyaudio forums for sending my 4pcs of the NJM4558 opamps.
So I put these original opamps into the almost identical circuit, and quess what. NOISE! Still the same, or... maybe even louder one. Damn!
This confirming my belief that something must be damaged, somehow, when I placed the ceramic caps wrongly... because there was now not a significant difference between the circuit before I touch it, and now.
Yes, the diodes on most of the power supply lines are now 6A, yes many caps are replaced for better ones, yes the 10uF CapXon bad caps are replaced with 22uF Elna RFS audio caps... but that it is. No other change. Well, okay, except these regulators providing clean voltage!
I also tried the OPA 2132 opamps todays. Same terrible noise. Definitively no opamps change could cure that, I'm affraid.
Opamps outputs voltage/ripple/frequency on no audio output from X-Fi
-------------------------------------------------------------------------
CENTER: -5,8mV DC, 18,6mV AC 33kHz
RL: -3.0mV DC, 34mV AC 83kHz
L: -1.8mV DC, 7.9mV AC 1.6kHz
R: -1.8mV DC, 8.1mV AC 1.8kHz
RR: -3.0mV DC, 33.8mV AC 81kHz
SUB, first stage (IC8 pin 7): -7.2mV DC, 73.7mV AC 111kHz
SUB, second stage (IC9 pin 7): 7.45V DC, 9.6mV AC 0Hz
Interesting, huh?
My X-Fi is a bit into the negative voltages, so that could be the source of them:
X-Fi output DC offset measuring with LM4562 opamps
---------------------------------------------------------------
L: -190mV DC, 10mV AC
R: -174mV DC, 10.2mV AC
RL: -210.2mV DC, 10.1mV AC
RR: -204.7mV DC, 10mV AC
CENTER: -209.5mV DC, 10mV AC
SW: -231.9mV DC, 10.2mV AC
X-Fi output DC offset measuring with AD8599 opamps
---------------------------------------------------------------
L: -219mV DC, 9.5mV AC
R: -205mV DC, 9.8mV AC
RL: -209mV DC, 9.7mV AC
RR: -205mV DC, 9.3mV AC
CENTER: -210mV DC, 9.6mV AC
SW: -232mV DC, 9.7mV AC
Conclusion - X-Fi opamps stage design does produce the -200mV DC offset. That probably can be cured by just increasing the positive opamps supply voltage from 5V to 5.2 or 5.4V ...
But the second stage of the SUB with the 7.45V is kinda interesting one. I did not pretend that I understand the circuit. If someone can, please... explain.
I also again measured the voltages/ripple on the speakers.
CENTER: 8V DC, 36mV AC 70kHz (next measuring show 13mV AC, 12kHz)
RL: -122mV DC, 100mV AC 110kHz
RR: -125mV DC, 105mV AC 112kHz
L: -183mV DC, 86mV AC 119kHz
R: -158mV DC, 42mV AC 70kHz
I was a hell lot worried about the 8V DC on center speakers, but davmax is saying that this is normal, since the TDA 7360 is in bridge configuration and hence on pins 8 and 4 should be about half of the powering voltage, witch is almost 16V (very close to it) and half from that is 8V, so, it is okay then.
Nothing still does not explain the noise.
Moving the 12k quieting resistor on the C61 input seems to INCREASE the cursed noise. AND change it - before the move, most of the noise come from center. Now the noise from L/R speakers overshadow the CENTER noise, witch is sort of bad... definitively worser that before. The resistor had to go back on the C61 output.
What if the output amplifiers are somewhat damaged and sending the oscilations back to their inputs...? Could that be a result from the misplaced ceramics 10uF caps?
How to explain that the amp is quiet when only single speaker and output by output is connected to it to check the silence, when the opamps are disconnected?
How to explain the terrible noise w/o opamps from L, R, RL and RR speakers when connected? And quiet center and sub at the same time?
Damn, I'm lost.
Work-in-progress report.
Todays I tried again to kill the cursed noise, and, well... What I did was that I put back the C117 & siblings and changed the C7 & siblings to the original 100pF value.
Then I moved the 12k resistor that was quieting a bit the noise from center speaker from the output of the C6 to the input of the C6.
Then I tried to fix the high ripple on IC9 opamp. While others (IC6 - 8) enjoy only 0.9mV ripple, thanks to the regulators and caps, this one is suffering at 32mV of ripple. I know, I know, power supply ripple rejection >80dB fix that, but I still did not quite like it. So... so the filtering cap Samxon GC 470uF 16V I changed the 1000uF 16V Panny FM and also added an another 100nF ceramic cap before the Panny FM one. On the bottom of the Panny FM cap is another 10uF Murata 16V X7R SMD cap and the last 10uF SMD Murata is on the opamp leg 8 against ground.
Ripple get lowered to 27mV
Next move - add a toroid L filter coil in the IC9 input voltage. I will kill the ripple on IC9, somehow!
Then I tried the another opamps I get on my hands in meantime. A big thanks to French user pilli from diyaudio forums for sending my 4pcs of the NJM4558 opamps.
So I put these original opamps into the almost identical circuit, and quess what. NOISE! Still the same, or... maybe even louder one. Damn!
This confirming my belief that something must be damaged, somehow, when I placed the ceramic caps wrongly... because there was now not a significant difference between the circuit before I touch it, and now.
Yes, the diodes on most of the power supply lines are now 6A, yes many caps are replaced for better ones, yes the 10uF CapXon bad caps are replaced with 22uF Elna RFS audio caps... but that it is. No other change. Well, okay, except these regulators providing clean voltage!
I also tried the OPA 2132 opamps todays. Same terrible noise. Definitively no opamps change could cure that, I'm affraid.
Opamps outputs voltage/ripple/frequency on no audio output from X-Fi
-------------------------------------------------------------------------
CENTER: -5,8mV DC, 18,6mV AC 33kHz
RL: -3.0mV DC, 34mV AC 83kHz
L: -1.8mV DC, 7.9mV AC 1.6kHz
R: -1.8mV DC, 8.1mV AC 1.8kHz
RR: -3.0mV DC, 33.8mV AC 81kHz
SUB, first stage (IC8 pin 7): -7.2mV DC, 73.7mV AC 111kHz
SUB, second stage (IC9 pin 7): 7.45V DC, 9.6mV AC 0Hz
Interesting, huh?
My X-Fi is a bit into the negative voltages, so that could be the source of them:
X-Fi output DC offset measuring with LM4562 opamps
---------------------------------------------------------------
L: -190mV DC, 10mV AC
R: -174mV DC, 10.2mV AC
RL: -210.2mV DC, 10.1mV AC
RR: -204.7mV DC, 10mV AC
CENTER: -209.5mV DC, 10mV AC
SW: -231.9mV DC, 10.2mV AC
X-Fi output DC offset measuring with AD8599 opamps
---------------------------------------------------------------
L: -219mV DC, 9.5mV AC
R: -205mV DC, 9.8mV AC
RL: -209mV DC, 9.7mV AC
RR: -205mV DC, 9.3mV AC
CENTER: -210mV DC, 9.6mV AC
SW: -232mV DC, 9.7mV AC
Conclusion - X-Fi opamps stage design does produce the -200mV DC offset. That probably can be cured by just increasing the positive opamps supply voltage from 5V to 5.2 or 5.4V ...
But the second stage of the SUB with the 7.45V is kinda interesting one. I did not pretend that I understand the circuit. If someone can, please... explain.
I also again measured the voltages/ripple on the speakers.
CENTER: 8V DC, 36mV AC 70kHz (next measuring show 13mV AC, 12kHz)
RL: -122mV DC, 100mV AC 110kHz
RR: -125mV DC, 105mV AC 112kHz
L: -183mV DC, 86mV AC 119kHz
R: -158mV DC, 42mV AC 70kHz
I was a hell lot worried about the 8V DC on center speakers, but davmax is saying that this is normal, since the TDA 7360 is in bridge configuration and hence on pins 8 and 4 should be about half of the powering voltage, witch is almost 16V (very close to it) and half from that is 8V, so, it is okay then.
Nothing still does not explain the noise.
Moving the 12k quieting resistor on the C61 input seems to INCREASE the cursed noise. AND change it - before the move, most of the noise come from center. Now the noise from L/R speakers overshadow the CENTER noise, witch is sort of bad... definitively worser that before. The resistor had to go back on the C61 output.
What if the output amplifiers are somewhat damaged and sending the oscilations back to their inputs...? Could that be a result from the misplaced ceramics 10uF caps?
How to explain that the amp is quiet when only single speaker and output by output is connected to it to check the silence, when the opamps are disconnected?
How to explain the terrible noise w/o opamps from L, R, RL and RR speakers when connected? And quiet center and sub at the same time?
Damn, I'm lost.
Attempt to clean the IC9 opamp voltage. Since the change of filtering capacitor from Samxon GC 470uF 16V to Panny FM 1000uF 16V and added the 100nF ceramic bring down the ripple on the +16V IC9 voltage from 32mV to 27mV, I tried go futher todays.
So I thought - what about LC filter? If I add a L coil, the voltage might be clean! So, I did. I made my first toroid filter, sadly the wire is 0.8mm thick (a bit too thick, IMHO):
...and result? No change in behaviour. And, what is worser, the voltage ripple on IC9 increased from 27mV to 28.3mV...!
Damn!
I thought things has to be better, but... Probably the coil impedance is too low for the 100Hz - or it might even help the ripple...
So I thought - what about LC filter? If I add a L coil, the voltage might be clean! So, I did. I made my first toroid filter, sadly the wire is 0.8mm thick (a bit too thick, IMHO):
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
...and result? No change in behaviour. And, what is worser, the voltage ripple on IC9 increased from 27mV to 28.3mV...!
Damn!
I thought things has to be better, but... Probably the coil impedance is too low for the 100Hz - or it might even help the ripple...
Muting is not working. Pin 10 of TDA7295. Big buzz that fades a bit. First guess - you have a problem in your power supply.
How are you making your measurements? Have you got a scope? If not, buy one of these: - PC oscilloscope or something like it so you can see where the noise is, and where it is not.
Look at the PCB with a magnifying glass. Look for shorts or broken tracks or bad solder joints. Start at pin 10 of the TDA7295 and work towards the PSU. Check every connection pin-to-pin with a continuity meter.
Look for wrong components or wrong way round components. You can look at these a dozen times and still miss a mistake.
Years ago I built an amplifier with a Sinclair IC-12 chip when they first appeared . ~ 1971. It suffered from a momentary loss of output that occurred when there were crescendos in the bass. I cannot remember how many times I tried without success to find the fault, since it was a non-trivial expense for me in those days.
About 10 years later I came across the PCB and circuit and idly started to examine it. Almost immediately I spotted an incorrect electrolytic cap value. I was stunned that I could have gone through the motions of checking the circuit so many times, and still have missed it. I replaced the cap and the fault was gone.
When I worked on production test as a student, we did repeated tests on complex military radios. Each type required the assembly of a different test rig. Sometimes you would get stuck and couldn't get the results you wanted. If someone did get stuck and ask for help, often one of the other testers could come over, look at the setup, change one thing (doh!), and everything would work. It happened to everyone.
Get someone else to check the circuit, even someone with no electrical knowledge. I don't imagine you've got a girlfriend?
w
Work in progress update - sumarizing what I did
- replaced all electrolyte caps, all audio one are 22uF Elna RFS 25V ones now
- replaced all rectifiers diodes to 6A 50V ones
- replaced all the regulators with zener diodes, using 7805, 7812 and 7912 chips
- added the last missing 1N4001 diodes tor the "latch-up" troubles, as datasheet suggest
- completely reworked the opamps section, new OPA 2132 opamps utilized, added many voltage filtering caps
- added 10uF 16V SMD ceramic on the voltage legs / ground of the TDA 7360 (CENTER channel, datasheet suggest 100nF)
- added two 10uF 50V SMD ceramic on the voltage legs / ground of the TDA 7296 (SUB channel, datasheet suggest 100nF)
- replaced the center amp, TDA 7360 to a new one
Results.
The damn noise is just a bit stronger, not fixed at all. I fear that the "sharper" more intimidating noise is result from the added ceramics to the TDA CENTER/SUB channels amps, resulting in way better / sharper amps characteristics - witch is why I felt the noise as increasing.
Can be also that the center TDA 7360 need burn-in time.
So far, this is very depressive result.
Next move?
Well, it is then either the two TDA 7269A for L/R and RL/RR channels, or the TDA 7296 in SUB channel. Hence I think that I will start with the TDA 7269A, because they was emiting the very strong noise, when the whole opamp section is diconnected, so...
They can be the culprit.
Also I pick them, because they has few legs and hence are easier to desolder and replace...
Any suggestions?
- replaced all electrolyte caps, all audio one are 22uF Elna RFS 25V ones now
- replaced all rectifiers diodes to 6A 50V ones
- replaced all the regulators with zener diodes, using 7805, 7812 and 7912 chips
- added the last missing 1N4001 diodes tor the "latch-up" troubles, as datasheet suggest
- completely reworked the opamps section, new OPA 2132 opamps utilized, added many voltage filtering caps
- added 10uF 16V SMD ceramic on the voltage legs / ground of the TDA 7360 (CENTER channel, datasheet suggest 100nF)
- added two 10uF 50V SMD ceramic on the voltage legs / ground of the TDA 7296 (SUB channel, datasheet suggest 100nF)
- replaced the center amp, TDA 7360 to a new one
Results.
The damn noise is just a bit stronger, not fixed at all. I fear that the "sharper" more intimidating noise is result from the added ceramics to the TDA CENTER/SUB channels amps, resulting in way better / sharper amps characteristics - witch is why I felt the noise as increasing.
Can be also that the center TDA 7360 need burn-in time.
So far, this is very depressive result.
Next move?
Well, it is then either the two TDA 7269A for L/R and RL/RR channels, or the TDA 7296 in SUB channel. Hence I think that I will start with the TDA 7269A, because they was emiting the very strong noise, when the whole opamp section is diconnected, so...
They can be the culprit.
Also I pick them, because they has few legs and hence are easier to desolder and replace...
Any suggestions?
Can I have the circuit you use, currently.
If it is not power supply hum (like ground-loop)
it may be that you have some problems with CHIP input pins.
Any Op-Amp, JFET input or BIPOLAR input
has got either Gate or Base connected to input PIN.
If not is any gate/base stopper resistor,
you may have to put like 1 kohm resistor into Input PINs.
This is very common practice in op-amp schematics.
If not put such base-stopper for all input pins
at least you may benefit from having such
at where the INPUT Signal comes into amplifier first input transistor.
If it is not power supply hum (like ground-loop)
it may be that you have some problems with CHIP input pins.
Any Op-Amp, JFET input or BIPOLAR input
has got either Gate or Base connected to input PIN.
If not is any gate/base stopper resistor,
you may have to put like 1 kohm resistor into Input PINs.
This is very common practice in op-amp schematics.
If not put such base-stopper for all input pins
at least you may benefit from having such
at where the INPUT Signal comes into amplifier first input transistor.
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