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Opamps stage design questions

Discussion in 'PC Audio' started by trodas, Jun 11, 2008.

  1. trodas

    trodas Member

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    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:

    [​IMG]

    Complete scheme:
    [​IMG]

    Input swichboard:
    [​IMG]

    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:
    http://www.diyaudio.com/forums/showthread.php?s=&postid=1535720

    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?
     
  2. dakiller

    dakiller (Oscillating & Impeding)

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    Remove C99, leave the resistor divider stage there but remove everything between it and the input to the first opamp and replace that with a coupling capacitor if you wish to keep one somewhere in the amp as well as a 100-470k to ground on the opamp input, this is a must have! To pick a capacitor, the corner frequency can be calculated as f=1/(2piRC), having it somewhere around 2-3hz is the best spot to aim for, C7 can go too. C9 and C13 is quite funny, really shows how bad a designer the person who did this was, remove both if you wish. Just check you opamp for any DC offset, as this will be amplified by the main amp stage, with a gain of ~30
     
  3. OP
    OP
    trodas

    trodas Member

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    dakiller - thanks for suggestions, these seems to be really helpfull.

    So, like me you suggest for killing the C99 100pF capacitor, hmmm. Some claim it is necessary to kill the ESD and RFI crap. Since I use good source and shielded wires, I dubt this can be issue and all in all is this capacitor definitively not good for fidelity.

    All? This is even more radical that I would do :o
    Because after the first divider (R129/R135), there is another one (R1/R3)... Altrough I definitively agree that having 2 dividers instead of one is crazy - but I think it would be nice to have one divider with result very close to the combined divider of these two - eg. 0.68 and with the resistance (11k in parallel with 43k) close to the 8759 ohms, witch in the end ask for divider made from 2870 ohms / 5900 ohms resistors.
    (resulting division is 0.68 and total resistance is 8770 witch is exctly as it was - unless I got something very very wrong)

    About coupling capacitors - what about to remove the C3 + C9 and keep only the C13?
    I measure the DC offset from my moded X-Fi with removed & shorted coupling caps, but it should be small-enough that it can stay into the opamps... what do you think?

    Interesting. First at all, recommended LM4562 circuit ( http://www.national.com/images/pf/LM4562/201572k5.pdf ) show a only 47k to the ground resistor. You recommend 2x to 10x value. Why?
    And why it all, when there is the divider (eg. also resistor to the ground) even w/o this?
    Also application notes on AD8599 opamps ( http://www.analog.com/UploadedFiles/Application_Notes/1023068910AN_937.pdf ) says that "This ac coupling is an easy way to block dc voltages associated with the input voltage (VIN).This is especially useful in high gain applications,where even a small dc voltage at amplifier input can limit the dynamic range or even result in output saturation. However, capacitive coupling into a high-impedance
    input without providing a dc path for current flowing in the positive input leads to problems."
    That basicaly says that the resistor is mandatory when the opamp input is separated by the capacitor and hence the DC ofset is killed on it... so where would the dc current originate then? It would be understandable that we need to provide a path for them, but from where it would originate? It can't pass capacitor! Hope this is not from the opamp... :o

    So, this is meant as capacitor to replace the C3 ...? 2x Pi (3.1415...) and what R...? The divider R as first part of the RC filter?

    I think too, I tend to want keep the C13 as only one separating cap in whole loop to kill the DC offset... what do you think?

    True... that would be killer :o
     
  4. dakiller

    dakiller (Oscillating & Impeding)

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    As the input switchboard already has a 1k and 10k, the rest is really redundant. You sources will see a 11k input impedance which is around the perfect amount to see. There is no further need to have any voltage division on the input unless you want to keep the overall volume/gain of the amp down, and if you do, just modify the 1k/10k to do so
    Fine, any DC offset present here will be small, and with the low gain of the amplifier, you don't stand to lose much output voltage swing from it, with +/-12v supply rails, 2v of offset isn't going to clip your line level signals here and C13 will then block it there after
    First, don't really pay much attention to the "Typical Application" diagram, it has little useful value, it is only an opamp and it has all the basic opamp properties. The reason for the resistor here is purely for safety reasons, as the input impedance of an opamp is very high, it takes very little current to bias the input and if the input becomes open circuit then it will typically saturate and peg the output to one of the voltage supply rails. While the 1k and 10k will normally keep it in check, there is mechanical switches between that and the opamp and if they were to fail or even mid switching when there is a moment when the opamp input is open circuit, it could be enough to saturate the opamp and you'll have +/-12v on the output. So putting a fixed resistor to ground will stop that happening and having around 10X larger than the main input resistors means that it doesn't have any significant loading effect.

    The application note you have there isn't really relevant for this

    I was initially suggesting to keep C3 and the 100-470k would be your R in this case, but to have C13, you need to take the input resistance to the TDA7269 as you R, which the datasheets say is typically 20k
     
  5. OP
    OP
    trodas

    trodas Member

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    Damn!
    I screw something up and I think I need help :o

    Todays I was get the precise DIP8 sockets, so I thought - what the hell - let's start doing first steps on the speakers. The first step was cut legs of all the 4 opamps, desolder the rest, clean holes, solder sockets. Push the LM4562 opamps inside. Looking great! :D

    I wanted to do things by small steps, so when something went wrong, I know what I had done, but... when I start soldering, I sometimes don't know when to end. So I removed and shorted C9, C10, C35, C36, C61, C62 and C64. All opamps output DC offset filtering caps... and all the unnecesary ones, because later are the C13, C14, C20, C21, etc.
    I remember czech guy recommending some mods on these speakers speaking about how the C64 0.1uF (!!!) capacity kill the bass of the thing, so a 22uF audio cap should be there.
    Nevermind.

    When I was in action (again, small steps... damn), then I took 7 pcs of 10uF 16V SMD Murata caps and solder them between pin 8 and 4 and ground for IC6, 7 and 9. For IC9, as it is powered from only positive voltage, only between pin 8 and ground (pin 4 in this case).
    I measured and double-checked no shortcut was made by this.

    And if meddling with computers teach me something, it is, that clean voltage is required and what better way to get the best voltage that add filtering caps - elyte ones on the voltage line, and ceramic as close to the pins of the IO in question, as possible.
    Maybe 10uF is overkill?

    Regardless, I was thinking about the C99, C100, C101, C102, C103 and C104, but I did not touched them ATM. Instead I soldered a six wires between the pins of the switcher, so the audio will be taken only from the 6 channels input now. I have no desire to use any other inputs, so...

    So I double check again for shortcuts and power on the speakers - first w/o anything. Sounded good, the start "boom" was very small, a improvement? Then connected thru the cable hell and... DAMN!
    A very strong noise from all the channels! WTF! :rolleyes:

    So I remember the guy from DIY Audio forums that was claiming that:
    ...and even I think it is a nonsense, I tried playing with the pot a lot, but no improve at all. Switching to the other inputs (just for show now) seems to lower the noise a little, maybe because only 2 inputs are taken into consideration? Dunno.

    Regardless, when I unplug the X-Fi, everything is fine. The X-Fi itself play well, tested on headphones I wearing now, but... as soon as I connect any signal to the speakers, the noise happens.

    So I hought that by hard-wiring the switcher I was connected something (how that can be, lol) to the signal path that is picking the noise, so I removed the shorts.
    And no change :(

    Then I get another idea. You said that the C7 can go too, and as I look at it, it can very well cause some oscilations like these (looks to me the noise existing in input is amplified a lot?), so... I removed all these, that mean C7, C8, C33, C34, C59, C60 and even C63.
    Also I finally move on the inputs and removed the C99, C100, C101, C102, C103 and C104.

    Any no change again :(

    Anything more to try, before I put all the C9, C10, C35, C36, C61, C62 and C64 back on? :confused:


    And on the side note - can it be, that most of these changes can't be made step-by step and component by componet and that some changes must be made in whole?

    Looks like it.

    As for the "leave the resistor divider stage there but remove everything between it and the input to the first opamp", that means (for the L channel) to remove these components:

    C99, C1, R1, R3, C3 and C117. And replace the R5 with 20k resistor - or the 33k can stay? Or move somewhere to the initially recommended 100 - 470k value?

    Damn, I was so happy yesterday that I finished the X-Fi mod ( http://forums.overclockers.com.au/showthread.php?t=540598&page=3 ) and it worked just right and played so well... so I have to screw something the next day, damn. Thanks for your efforts anyway :thumbup:
     
    Last edited: Jun 14, 2008
  6. dakiller

    dakiller (Oscillating & Impeding)

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    I don't really know what's gone wrong, could be many things. You really need to trace the signal path to see where it's going wrong. My first thought is that there is something wrong with the grounding and you've crossed something somewhere, try shorting the input to ground and see what that does. Also, could be the opamps are oscillating, they are very fast and can easily get temperamental to bad layouts (and I cant see this amp having a very good layout at all) see if you can pickup some generic ones and try them in there. I very much doubt that it could be any of the capacitors you've removed or added in.

    Next time, do it in smaller steps
     
  7. OP
    OP
    trodas

    trodas Member

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    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 :D
    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?
     
    Last edited: Jun 15, 2008
  8. OP
    OP
    trodas

    trodas Member

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    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 ... :o

    No wonder the amplifier did not play well...

    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, 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 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:
    [​IMG]

    See? No C117 from my amplifier...
     
  9. OP
    OP
    trodas

    trodas Member

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    Well, the circuit in question looks like this now:
    [​IMG]

    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:
    [​IMG]

    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? :(
     
  10. dakiller

    dakiller (Oscillating & Impeding)

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    I don't really have much else to add apart from trying some generic opamps and seeing if the problem is still there
     
  11. OP
    OP
    trodas

    trodas Member

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    Votage filtering for IC6, IC7 and IC8 - all channels input opamps in short, powered from +/-12V, now this is filtered:
    [​IMG]
    by 2x Panny FM 1000uF 16V ( P12366-ND ) caps.

    Voltage filtering on the opamps pins
    [​IMG]
    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?
    http://www.fairchildsemi.com/sitese...=sitemap+id&ia=1&text=NE5532&as=1&render=1&w=

    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:

    [​IMG]

    Comments? Suggestions? :confused:
     
  12. OP
    OP
    trodas

    trodas Member

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    Could it be, that the LM4562 / NE5532 opamps simply did not like so low values on the gain controlling resistors?

    I mean - in the Chu Moy's example they are 470k / 120k ... in my case they are 10k / 10k ...

    That is like 12x less at least.

    Could that be a reason to the oscilations, producing this nasty noise?!
     
    Last edited: Jun 21, 2008
  13. dakiller

    dakiller (Oscillating & Impeding)

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    No, it is the complete opposite, larger value resistors are more noisy
     
  14. OP
    OP
    trodas

    trodas Member

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    Well, why would Chu Moy used in Hi-Fi amplifier then more noisy solution?
    That does not make sense at all, unless...

    Anyway, I do hear very annoying loud noise all the time :thumbdn:

    How I can stop it then?!
     
  15. OP
    OP
    trodas

    trodas Member

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    Big success! :D

    I managed to quiet the R, L, RR and RL channels! :D 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) = 33k between input and ground to get the same resistance as 7269A
    TDA 7296 100k - (SUB) = 25k between input and ground to get the same resistance as 7269A

    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?
     
    Last edited: Jun 25, 2008
  16. OP
    OP
    trodas

    trodas Member

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    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... :thumbdn:

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

    trodas Member

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    WoW!

    NJM4558 operating current - 3.5mA typical ; 5.7mA maximum.

    What worry me is, that I can't find in the LM4562 spec what current it draw
    trodas: http://www.national.com/pf/LM/LM4562.html

    But there I find: http://www.head-fi.org/forums/f6/stax-srm-001-mod-thread-277049/#post3536731
    That the LM4562 draw 5-6mA/amp - but it is double amp (10 - 12mA) and that is true, when "amplifier current draw when using 3.7V lithium batteries"...!!!

    While using +/- 12V voltages, then we talking probably a hell of more current!

    I see the guy claiming that LM4562 draw notably more that others - (whole amp= 500m ) while with TL082 it is just whole amp current draw is 320mA.

    So instead of the stuuupid R11 and R12 resistors and nonexisting D4 and D5 Zener diodes (they are NOT present in reality!) I slap a 7812 and 7912 regulators there and we see what happnen then.

    With the TO-220 package, they should do about 1000mA, so heatsinks is probably not yet required :)
     
  18. dakiller

    dakiller (Oscillating & Impeding)

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    Location:
    3844
    Page 21 of the data sheets has the Supply Current graphs for the LM4562, 10mA
     
  19. OP
    OP
    trodas

    trodas Member

    Joined:
    Sep 5, 2004
    Messages:
    598
    Location:
    Czech republic
    True, how can I overlook it... but I use + and - 12V supply voltage, and not just 12 or 18V...

    And still, it is noticably more that NJM4558, so... I probably add a better grounding wire AND the regulators instead of resistors. Wish me luck :)
     
  20. davmax

    davmax Member

    Joined:
    Jul 1, 2008
    Messages:
    9
    Changing Op amps

    Trodas I read thread. Unfortunately you changed too much initially. You should first change only the op amps. I might say that there was very little to gain from changing them because each stage gain is only x 2 and they are voltage followers with high input impedance and flat frequency response ( modified by C117 and C7) any improvement would be input noise in proportion to amp spec.

    C7 and C117 and all equivalents on the op amps control the high frequency roll off at each amplifier not just the op amp. So should not be changed or removed. C9 should remain. C64 is essential to block 5 volts from the 78L05. C64 and C67 control the low frequency to the sub. Coupling caps C3 and C13 should not be removed until operation is like it was before changing op amps. Remove these later if desired. If C99 is 100pf leave it untouched it is not significant at audio frequencies.
     
    Last edited: Jul 1, 2008

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