Altrough I'm reasonably skilled moder of mainbords and GFX cards and all electronics stuff, I did not play into the audio field much.

So I think I better ask these who know about my ideas how to modify a existing design.

This one:



Complete scheme:


Input swichboard:


As soon as the signal hit the switching part of the amplifier, it is shorted to ground with C99 cap, a 0.1nF one. I think this is completely unnecessary blurring of the sound, as capacitor in general act to prevent voltage changes, so it has to "blur" a little the amplitude to prevent fast and rapid changes of it.
Do I get it right?

Later the signal go thru a 0.9 resistor divider, witch is probably used to put some small load (11k to ground) on the audio source. Is this value optimal for the X-Fi equiped with LM4562 opamps...?

What about keeping just ONE of these dividers and adjust it to the 0.68 as the result of two dividers (0.9 and 0.76) after themselves are in the end.
The aim is less distortion in resistors - or even using a audio grade resistors such as Vishay Audio Resistors:
diyAudio Forums - Vishay Audio Resistors

I also fear that the combination of R and C components can create a slight RC filter that in the end make the "blur" effect of the C99 stronger a little. Right?

After it pass thru the switch, here come another ground-shorting cap, a C1. Now with 0.33nF capacity.
Why?

Then come another resistor divider, this time 0.76 and directly after him a first decoupling cap, a C3 - 10uF 25V.
As far as I understand audio, the blocking caps is necessary for the filtering of the DC offset. What if my X-Fi has very low DC offset? Is not no cap better for audio that ANY cap, even quality audio one?

I think the C3 is entirely unnecessary one. I think only one decoupling cap in the whole spekers (or none) is best solution - and placed directly before the output amplifier.
Right?

And it get worse. Just after the opamp, there is another decoupling cap! A C9 - again 10uF 25V for all except CENTER and SW channels. First thing I did not like is that the capacity on the output is same as on input - should not be bigger? Maybe is the level of signal not that high still, but... it just did not feel right.
Second thing I did not like at all is the fact that we already removed the DC offset before the opamp, so, why now? Sure, a badly balanced of sucking opamp could produce some DC voltage at the output, but... why not balance it better or remove it and use quality one instead that does not need second decoupling?

I think with the LM4562 or perhaps better AD8599 I can remove these.
Right?

And right after the potentiometer we have another decoupling cap - a C13! In fact, he is in serial circuit with the C9, witch bring the ending capacity down to half... not to mention that with the huge resistance between then the impact on the signal can be high.
I hope I'm wrong on this one, but... IIRC the most clean voltage filering is a RC way. Only with the problem that it's output voltage differ with different current - so current has to be always the same and stable...

I think the designer of this speakers just put together the recommended way of the used circuits and then these double-triple decoupling caps are the result.

Suggestions?

Trodas

C3, C9 and C13 are absolutely necessary. These are DC blocking and removing these will completely upset the DC voltages throughout the circuit and I guarantee it will not work.

Take the first stage with C3. What's the first thing that happens when you remove this? You effectively put R5 in parallel with R3 giving you 16.5K and effectively halfing the DC level that was here previously, altering the DC level at pin 3 of the 4885D. The consequences go on. These capacitors are there for a reason, they block DC and allow AC at audio frequencies through. C1 is just a decoupling capacitor, value choosen, to remove high frequency noise and have little effect on audio. Removing these decoupling capacitors will just allow more noise into the amplifier and reduce audio quality.

All the components in this amplifier have been specifically included for a purpose or they would not be there. You can increase or decrease values slightly to for instance increase the bass or reduce the treble to suit your listening preferences but if you start removing them you are just asking for trouble.

Hope this helps.

putting capacitors in series in the signal path will only have a negative effect on the sound depending on:

* the type of capacitor
* having too little capacitance (like <3uF) will start to have a measurable bass rolloff and phase distortion...

in addition to what Paul has already said, for C3/C4, C9/C10, C13/C14 you could try some Polypropelene, Polycarbonate, Polystyrene film type capacitors instead, or even Polyester/Mylar film (not quite as good), all should sound better than electrolytics for DC blocking... finding these types of capacitors in the small size of the electrolytic while still maintaining the capacitance will prove to be a challenge if you don't have the space

oh, and the LM4562 does have DC offset on the output, it's enough to cause problems on other equipment or parts of a circuit connected to these op-amps... my modded X-Fi has significant DC offset on the output, enough to make it impossible to connect an amp without DC blocking caps on the input without severe distortion that will easily destroy headphones and speakers... I may replace the crappy (shorted on PCB, silver trace) electrolytic caps on the X-Fi outputs with Poly film caps and remove the shorts

hey the 4558D, same as on Audigy/X-Fi right?

I think switching op-amps (to AD, TI/BB, NS) will have a much bigger change in sound than switching caps... although I'd almost certainly do both

The problem

Plaing with the amp, like tracing the signal, is very hard to do, almost extremly. It is even hard to diassemble, and can't really run in such state... that just added to the trouble...

Nevermind, how to stop the opamps from oscilating is the question. I did not have any other opamps to do replacement ATM and for like weeks, till I can recieve the new/different opamps

That is not good. Not to mention the aim was to improve the quality, so, use new opamps is the goal. Possibly AD8599 later, however it looks like the samples will took like a month (!) to get there, and I need 2 shipments - only 2 pcs per one... (likely 4 shipments, if I use AD8599 on X-Fi too...)

So the question is, how to change the circuit to stop the oscilation from happening?



Shorting to ground? Not need. Unplugging the X-Fi is enought and there is silence then
Suddently. Obviously the opamps are oscilating, why then hell I did not figure that out myself...

X-Fi output DC offset measuring:
--------------------------------------
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

Good? Bad? Terrible? Usable?

What does worry me is the constant output signal noise... I cant hear it, but around 5.2 to 5.6kHz signal is there... Even the X-Fi is not playing anything ATM. I used the X-Fi control panned to check on the signals, when mesuring AC voltage, then letting the card say "left, right, center..." is clearly visible on the scope

PS. come to think, you are right. No capacitor removal (and certainly not AFTER the opamps) could trigger such noisy oscilations as I experiencing. Well, what about the C117 then? It is this a part of the "to be removed" stuff from the amp too?

Hoooray! I figured what I did wrong!

It is very embarrassing, but these 10uF SMD ceramics I added to the opamps legs, well... I added them to the legs 1 and 5, not 8 and 4 ...

No wonder the amplifier did not play well at all...

So, when I figured this silly mistake made, I fixed it and - hoooray - it now play MUCH better. There is only one catch - it is still very noisy. There is, as soon as the amplifier is powered on (w/o connecting to my X-Fi), still noise in all channels :thumbdn:

Much less that before, but there it is. So I put all my changes back. I give back all the ceramic blocking caps I removed before, even these for the opamps feedback (witch should not be there IMHO), and no change.
So as last desperate attempt I even removed my voltage filtering caps, but quess what - no change at all. It is not these caps...

Just instead of the original C9, C10, C35, C36, C61, C62, C52, C64, C3, C4, C29, C30, C55 and C56 10uF 25V CapXon crap caps I used Elna RFS 22uF 25V audio caps. (true, C52 is 1uF and C64 is 0.1uF by original design, but all these caps are used only for the DC offset filtering, so they can be bigger... at least C64 does not limit the bass line now... and the noise is in other channels anyway...)

And of course, instead of JRC4558 opamps are there now a LM4562 ones...

So, can anyone tell me, how to get rid of this little (3x hoooray!) oscilation ... ? What about the C117? I never seen that capacitor in any design... and the C7 might also trigger the oscilation, right? :confused:

I read some there:
http://tangentsoft.net/audio/hs-opamp.html
...but I did not yet come to any practical solution. Help?

A good example of opamp circuit:


See? No C117 from my amplifier...

Sorry, you are wrong. Putting capacitor will ALWAYS have a negtive inpact on audio signal, regardless on what. It is only the degree of severity we are talking there.

Check there: http://tangentsoft.net/audio/hs-opamp.html
I quote from the Non-Solutions


Of course this is audiophile point of view and most people did not notice the changes, or does not have a good speakers/ears/amplifiers or even audio source to begin with to notice the difference. In most cases they miss everything mentioned.



Of course we use AUDIO grade capacitor with reasonable capacity. No question there. Surely I will not tolerate a known bad caps CapXons in my audio path. Not to mention they are not audio caps either...



Truth, but you can't fit them there:

So at least I want to remove all the unnecessary ones I can.



The question is more like with opamp does not have a DC offset
I quote again:


X-Fi output DC offset measuring:
--------------------------------------
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

My headphones did not noticed a thing, also possibly because their resistance is tiny (hence the DC offset voltage will be far, far lower) compared the Megaohms of the inputs of my scope.
Of course I'm aware that there IS a DC offset.



How much you measured? Comparable to mine? Worser? Better? Mine is like 5.2 - 5.6kHz. Yours?



Only if the amp does not have inside any deblocking caps already. With they always have for just this reason... They cannot predict to what audio source these amps get connected...



I dubt that help. Please note that these decoupling caps are between the DAC and the opamps. They are NOT after opamps and as you say and as we all know - opamps produce DC offset. Especially when they are mediocre, witch is the cause of LM4562 ones.
These are recommended by guys like that are placing useless tantalous caps into X-Fi... So... go figure. I will put there the AD8599 anyway. The Hi-Fists seems to love them... 2pc (samples) already recieved.



True. Same ones. Just DIP8, but that does not make a difference.



Not sure. When I switched the opamps on my X-Fi, there was almost no change at all. Till I removed and shorted the cursed bad caps Jamicons. Then there was a significant change, so...
Once there are too many caps, then changing the opamps is useless.

Changing will be quite easy also, because look what I did:

Opamps nicely in sockets, all modifications reversed back to original. Still noisy. Not like 1:1 to the signal, as it was thanks to my silly mistake, but like 0.1:1 to the signal. Useless.

So todays work - add a voltage filtering caps missing in the design near the opamps:


This 470uF 16V Samxon GC suxxka is supporting now the +15.8V for the IC9 - a subwoofer-dedicated opamp.

No change. Noise from subwoofer still comming.

Later I added two pieces of Panny FM 1000uF 16V caps to filter these +12V and -12V for the IC6, 7 and 8 - eg. for the rest of the opamps.
Even I think I did good job, no change on the noisy behaviour.

So I started again (sigh) removing weird looking caps, started with C117, continuing on the C118, C119, C120, C121 and C122.

And quess what. No change at all. Still noisy, oscilating.
I beginning to think that these LM4562 was either damaged (but all of them? and also - audio is playing right now as I type and well, except the noise, of course) or too cranky to be used in this design.

What to try now?

Put back the 10uF 16V Murata X7R 1206 caps near each the opamps legs? Or use the much smaller values (0.1uF or even 0.01uF) as suggested there?
http://tangentsoft.net/audio/hs-opamp.html

I still not understand why use so small values, because as I look into the Murata specs, these ceramics go way up to GHz, regardless of capacity and when come to voltage filtering, then the more capacity the better...?!

If not help, then remove the C7 and siblings? (that mean C8, C33, C34, C59, C60 and C63)

And what if that does not change a thing?

trodas

I am totally baffled as to where this thread is leading!

You intially say you are fairly well into electronics and in the next sentence you want to remove critical components from the amplifier and expect it to work. You say you are not into op-amps, the op-amp is the one of the major components in the feild and found everywhere.

I take it this amplifier is noisy and you are trying to find out why. Why not follow the circuit through with an oscilloscope, starting with the power supply (usually the main culprit), and see if you can find a what stage the noise is being introduced. Changing all the capacitors and adding others is admirable but it could just as easily be a semi-conductor that is at fault.

True, but todays I tried different opamps and the same resuts... So, not good. Perhaps the replacement opamps did not like much gain zero...?


Votage filtering for IC6, IC7 and IC8 - all channels input opamps in short, powered from +/-12V, now this is filtered:

by 2x Panny FM 1000uF 16V ( P12366-ND ) caps.

Voltage filtering on the opamps pins

Done by 10uF 16V Murata X7R 1206 caps, ( 490-3911-1-ND ) 7x of them.

No change in behaviour. Still the damn noise is there. Side note - upon powering the amplifier on, the noise seems to none, then sharply the amp become very noisy and then the noise get lowered to what it is for the rest of the time. That happen in like 0.3 sec.

Is this significant?

Another question - the opamp gain is determined by the R9 / R7 size ratio, right? In the Chu Moy's example this is 3.91 ...
In the Genius case it is 1.
Does that mean the opamps gain is ZERO?
Should not that pose a problem for the majority of opamps?
What is the minimal gain to keep the amp stable?

As for trying the other opamps, well, does Fairchild NE5533 count as generic opamp?
Fairchild Semiconductor - Site Search - Operational Amplifiers

I just recieved 4pcs of the DIP8 ones today and - no change at all.
In a desperate attempt to cure the problem, I even desolder the shorts I made on the input switcher, because the RR channel was sometimes nonpresent - eg. the contact is not good anymore there...
No change again.

To make things real simple, I included the switch part in the schematic and keep just the L channel in it:



Comments? Suggestions? :confused:

to be brutally honest, I think you are trying to sugar-coat a turd...

it's an expression that means you are starting off with something crap, and no matter what you do to it, it will still be built on the basis of crap...

it might be good practise in amp/speaker modification, but I think the crap-aspect means the project is going to bear rather unappealing fruit

I thought it was 'polish'?

And yeah, I don't really see the point completely..

Loosely translated I think that means buy a new, better quality one?

Well, what I'm really saying is that it will be hard to get an improvement in sound on a speaker set that is so bad to begin with... it's like trying to volt-mod and overclock a Geforce2 MX, or trying to super-charge a Fiat Panda...

while you can do both as a practise project, the actual best result is still so bad that it won't be rewarding or as much of a learning experience as if you tried to modify something that's half-decent to begin with... so in my examples this could be volt-modding and overclocking an 8800GT, and super-charging a Merc/BMW along the lines of Touring Cars...

Well, you can see it a different way too.
What about practice before screwing up something valuable?

Eg. better practice on voltmoding the GF2 cardie that on 8800GT, right?

On the sidenote, these speakers have a reasonably good output and hence modification should only improve the results.


...and....

Big success!

I managed to quiet the R, L, RR and RL channels! At least for like 30% of the volume, witch is what I use daily. So, this is very good for me - if it was not that the center speaker brum is not gone ... (and subwoofer also product some noise, but at very low level, since the noise is like 5 - 6kHz and the subwoofer is optimized for much lower frequency anyway)

What I did over the original schematic?

Removed C117, C118, C119, C120, C121 and C122.
Replaced the C7, C8, C33, C34, C59, C60 and C63 with 12pF caps (original 100pF).

That almost instantly kill most of the noise from the R, L, RR and RL channels. Adding R100 resistors instead of C9, C10, C35, C36, C61, C62 and C64 seems to helped a bit too.

No hearable pot noise as others suggested that this is why these caps are there in the first place.


Current situation.
-----------------
The noisy center is VERY ANNOYING. However hear this - the the volume pot is at like 30% of volume, the R, L, RR and RL channels seems dead silent. But as soon, as I increase the volume, then the center channel noise quiet and all other channels become noisy...!!! :wired:

This is nuts.

It, however, I think clearly demonstrate one thing. That the oscilation is happening because of the output resistance.

I'm I right?

Also caps size on the opamp output seems to play a role, too. Notice that I complain about the noise in the center channel most. (because at 30% volume are the other channels dead silent, so... even later I wand them to be silent at ANY volume, right now I want get rid of the noise at all costs - except for unpludging the center speaker, that it is ) Then notice the C5 cap capacity. 0.22uF for center output from opamp?! Are you kidding me! I want there 22uF 25V Elna RFS cap and I think it will stop the noise - at least in the 30% volume settings.
Also notice the C116 cap - a 470pF one. It is NOT present in the recommended TDA 7360 schematic. I vote for removal...

Input resistances
TDA 7269A - 20k (R, L, RR and RL)
TDA 7360 - 50k (CENTER)
TDA 7296 100k - (SUB)

So, do I get it right that these input resistances are maybe too high, and that cause the oscilations, because high input resistance mean high voltage and that cause high feedback and that, possibly, cause these oscilations?

I replaced almost all the audio decoupling caps with the Elna RFS 22uF 25V ones. All the input ones, and all four for the R, L and RR, RL channels. Can that be significant too?

Yesterday FedEx delivered the 4pc of RC4580 opamps and 4pc TLE206 opamps, listed my TI as compatible with NJM4558. Both produced very similar oscilation noise, so, no help. Probably not THAT compatible...

So, I got the idea that I can make the opamps to run with like 10k load. That mean put a resistor 12k parallel to the 50k pot = 10 or 9.6k load resistance for opamp.

I tried it for center channel first and the noise it did really limit a lot the oscilations for center channel, but only at given volume, so I tried it for all channells and it make matters a LOT worse after power-on, but in just a short while 10 - 15 sec the noise deacrease to level witch is very nice.
Still noisy, but much more enjoayble. now.

I also tried bump the capacity of C7 & siblings from the 12pF to 330pF, but no change. Maybe it is even slighly worser now...

Maybe the grounding is not perfect and the voltage supply suxx too? Dunno.

If you get excessive noise in any amplifier system the power supply is the first thing you should look at and confirm correct operation.

Regards

Paul

You are right. I will add a strong nice wire between the opamps ground AND the back PSU, as it looks like these are 4x inpuit volotage vires to the opamps stage, but only 1 (or two) ground wires. That suxx.


Work in progress update
I removed all the 12k resistors added on all the opamps outputs to ground.
I put back C9 & spol. deblocking caps.
Caps C41 & C42 are now nice Nichicons VR 4700uF 35V suxxkas.
Caps C47 & C48 are now Rubycon XYF 1000uF 16V, taken from one old PSU for DVD player, looks genuine and sure better that Su'scons
Replaced R89 and D8 (near C26, top right part of schematics, close to the front L/R repros) with a L7805 regulator, 1uF ceramic on input (16V cap and the voltage there as well) and 100nF on the output. PCB under R89 was very much darker, so I consider that as safe replacement. Also the output was like 5.3V and not 5V as it should be.

Resulting changes.
Removing the 12k resistors increased the noise level to the before-known situation. Not good, not cool. Caps have no change on the subject, only the C64 seems to put the bass line under control (it was weird, overblown and just blurred before...).
I regret removing the resistors and I planing on puting them back to battle the noise.

Still weird.
It still act kinda weird. When I power the amp up, it lack the before-mod strong SUB kick, but very shortly a strong (oscilating?) noise come from all speakers, and it slowly fade (as things heat-up?) to more acceptable levels. Weird at least. Still lack caps to do complete recap and starting to fear the caps was not the issue, as from the main power caps only C37 & C39 remain as original bad caps...

Futher testing.
I'm somewhat confused that when I power the amp on my balcony, where I solder and work on it, there is no noise from the subwoofer. It might not be even when I power it on with all the speakers, because they simply produce too much noise, that I can't be sure about that. I'm sure about all the 5 other channels, tough.
So first I power the suxxka w/o my PC/watercooling pump nearby and so on. No change.
Then I got idea. I unplug the PCB with the opamps from the rest of the amp. That way, only the output stages are "in game" and they should be very quiet, no noise, as it was before? Right?
Well, wrong.
SUB and CENTER seems dead-quiet, but these L, R and RL, RR channels are full of - wait a minute - MUCH stronger noise that WITH the opamps (and resistors on their inputs to ground!) ...!

What I think of that?
I strongly beginning to suspect that when I at first connected the ceramic 10uF caps to the wrong opamps pins (1 and 7 to ground for all the there opamps), it has consequences. The oscilating noise was unbearable, true... So that lead me to question the TDA7269A amps.
CENTER and SUB use different amps, but the L, R and RL, RR channels use the two TDA7269A ones. What if they are somewhat damaged, so they produce the noise all-by-itself? It is normal that amp produce so strong noise when not connected to any source? I doubt that, and at least CENTER and SUB are fine then.
Their voltage filtering caps (C41 & C42) are quality new Nichicons from Digikey now, so... can't be a issue there.

This would ALSO explain the noise in SUB and CENTER channel too. These channels are interconnected by R109, R111, R156 and R157, so the noise CAN get there by this way.

It would ALSO explain why the resistors helped to battle the noise. Of course opamps does NOT need so low load, but if the noise come the other way, then these quiet it down for obvious reasons. 12k is reasonably low, so it helped...

I think I should remove the R109, R111, R156 and R157.
If that kill the noise from CENTER and the very little noise from the SUB, then these TDA7269A are damaged and replacing them fix the problem. Higher quality parts drop-in replacement (okay, I willing to add few components as well, but the basic pinout has to be the same) suggestion welcome.



oshifis -

Heh. I probably already did. It is "loudest noise comming from amplifier" or "worsest dB ratio"... Actually this is no anymore about the difference between signal levels of noise and signal - but more likely now about between the signal and noise...


AndrewT -

You could be right. But check on the maximal ouput current as well, 26mA. That is on the top of the 10mA, probably.
Each way around, there was SIGNIFICANT, at least 2x higher (if not more) volume after I added these 7812 and 7912 regulators AND kicked the R11/R12 off the amp.
So that was a good idea and good change in the end. However the resistors should be keept there... the noise is not significantly higher

This is a little attempt to show how the amp behave on power on and power off/on by remove control. Turn the volume up to get an idea what I hear now all the time...
http://www.youtube.com/watch?v=iiPEJ2KgnQk

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...=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 :mad:

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=g...capaddedbk9.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.

Have a close look at C49, C41, C42, C37, C39, C47 and C48. These are all power supply smoothing capacitors and could be causing your problem. If you have a 10000uF or 4700uF capacitor of adequate voltage rating just connect it across the existing capacitors and see if it makes any difference.

Well, exactly my idea too. I had already replaced the C41 (10 000uF Samxon KM 25V), C41 & C42 (2x 4 700uF Nichicon VR 35V) and I can't wait to get 2x Samxon KM 6800uF 25V to replace the C37 7& C39.

Bad caps Su'scons already replaced by Rubycon XYF 1000uF 16V, taken from one old PSU for DVD player, looks genuine and sure better that Su'scons Bottom line - since instead of the R11/R12 and D4/D5 are now the L7812/L7912 regulators and the output has like 0.9mV ripple, I say the problem is not there. Besides, there is maybe too much bulk capacitance, so there should be the 1N4001 diodes added to protect the regulators, as shown in the datasheet, see figures 25 and 37.
http://www.alldatasheet.com/datasheet-pdf/...NICS/L7812.html

But no change at all, when the recap. Damn.

True is, that on the replacement caps is way lower the ripple - eg. they are filter the voltage better.

It must be something burned out by the oscilation when I wrongly placed these ceramic caps, or... I don't know.


Testing made todays.
I wanted to verify that altrough the sub and all channels are quiet, when I unplug the opamps on balcony, it oscilated like mad when all speakers are connected.
When no speakers are connected to ALL the outputs of the L, R, RL, RR and CENTER - just I connecting the one speaker to each one channel separately and find the amp dead-quiet. That was on balcony, when I worked on it.
This time I do it when everything is connected.
And quess what! It do oscilate. Only the L, R and RL, RR channels, but much louder that when things are connected! The behaviour does not change when my PC of powered off and unplugget from power completely, eg. no pump is running nearby. Just the amp.

Coud be a blown cap like the C16/C17 that is inside of the speakers, that is causing these problems?
But that would explain only the L/R channels oscilations. What about the LR/RR channels? Blown out these TDA7269A? But how come they play music well, if they are damaged? I don't know and that is what is making me mad

I mean - it is a simple circuit...!