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Old 10th June 2008, 09:39 PM   #1
trodas is offline trodas  Czech Republic
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Red face Opamps stage design questions

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:

Click the image to open in full size.

Complete scheme:
Click the image to open in full size.

Input swichboard:
Click the image to open in full size.

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/showt...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?
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Old 10th June 2008, 09:50 PM   #2
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My thought is you do actually need all those components.

C1 is ESD and RFI filtering - more essential every day!

C3 is input decoupling - at your discretion.

C9 and C13 are needed to stop loud pops from occuring if the pot wiper momentarily disconnects from the track such as when if rides over a bit of dust or fluff.
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Old 10th June 2008, 10:34 PM   #3
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Default Re: Opamps stage design questions

Quote:
Originally posted by trodas

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?
Modding analog circuits is a bit different from computer stuff, where voltages are frequently compatible across equipments and many 'mods' are software based anyway.

Analog circuits often behave in ways which are non-intuitive. Most people find that some years of study are required to acquire a basic understanding. In fact most electronics graduates leave university after 3 years with no clear understanding of how to build a transistor amplifier.

After you get some real understanding of what is going on is plenty soon enough to start modifying stuff, unless you are simply implementing modifications designed by someone else or substituting higher quality components. Even this can contain traps for the unwary. Changing components on the basis of half-understood ideas will almost certainly result in poorer performance if not complete economic destruction of the unit.

Doing things the recommended way is the recommended way of doing things.

Right?

w
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Old 16th June 2008, 09:46 PM   #4
trodas is offline trodas  Czech Republic
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Iain McNeill -
Quote:
My thought is you do actually need all those components.
I hope not, as they obviously killing the audio quality...


Quote:
C1 is ESD and RFI filtering - more essential every day!
But it also kill the quality of audio signal. I prefer my shielded wires (sme goes for the C99 that is not in the zoomed schematic, but if you look at the switchboard scheme... there you go)

Quote:
C3 is input decoupling - at your discretion.
I think unnecessary. I would decouple the signal and kill DC offset just before the amplifier - at C13 position. A Elna RFS 22uF 25V seems to be good for the job.
I think that even like 2V DC offset after the opamp is not going to clip my signal at all, when powering the opamp from +/- 12V lines, so...

Quote:
C9 and C13 are needed to stop loud pops from occuring if the pot wiper momentarily disconnects from the track such as when if rides over a bit of dust or fluff.
You sure? I think it is just a decoupling and the designer just took the recommended circuits for opamp and the amp and put them together, not realizing that he create triple decoupling...



wakibaki -
Quote:
Modding analog circuits is a bit different from computer stuff, where voltages are frequently compatible across equipments and many 'mods' are software based anyway.
Well, almost all my mods are HW based anyway. Just check there:
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D2
http://capsmod.net/forum/viewthread....extra=page%3D2
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://capsmod.net/forum/viewthread....extra=page%3D1
http://trodas.wz.cz/index.php?act=ST&f=16&t=337&s=

However the problem is, that capacitors in computer circuits are straight-forward used for voltage filtering. And there you almost can't screw things up, when replacing caps with better ones... with capacitance as well as specs bump.
Audio seems to be totally different horse.

At least I think I understand the voltage filtering part of the amplifier. Except for that there: http://tangentsoft.net/audio/hs-opamp.html
I read that the recommended voltage decoupling ceramic caps near the opamps pins is 0.1uF and one "need to go down to 0.01 F for faster op-amps"

I don't understand. I would like 10uF X7R Murata ceramics there! The specifications of ceramic SMD capacitors todays did not change much (if at all) with the increasing capacity. Hell, it is possible to buy a 100uF 6.3V 1210 size SMD ceramics now! (almost $4, but you can)
So, since I don't know about anything that could prevent this 10uF capacitor to filter well even in the Mhz range (specs looking damn good http://search.murata.co.jp/Ceramy/im...18X/C2ED01.pdf ) ...
So, why not use 10uF caps on the opamps legs?

Quote:
Analog circuits often behave in ways which are non-intuitive. Most people find that some years of study are required to acquire a basic understanding. In fact most electronics graduates leave university after 3 years with no clear understanding of how to build a transistor amplifier.
Good point. I have an idea, but dunno if that will work

Quote:
After you get some real understanding of what is going on is plenty soon enough to start modifying stuff, unless you are simply implementing modifications designed by someone else or substituting higher quality components.
Well, I just wanted basically to do that... but looks like the opamps started to oscilate

Quote:
Even this can contain traps for the unwary. Changing components on the basis of half-understood ideas will almost certainly result in poorer performance if not complete economic destruction of the unit. Doing things the recommended way is the recommended way of doing things. Right?
Well, that is why I asking there about opinions to get the recommended ideas - and modify them, of course. But I must say I thought it would be far more simple...


I 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!

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!

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. The C7 can go too, because it only limit the opamp gain on high frequency (that should prevent the oscilation in the first place!) and as I look at it, it can very well ALSO 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

Next thing to try - remove the C117 and all corresponding caps... or you guys got better suggestions?


Sadly I can't try back the original JRC4558 ones to see if they will oscilate or not... I cut their legs in order to prevent desoldering damage to the PCB...
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Old 17th June 2008, 03:14 AM   #5
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the reason for decoupling the volume control pot is quite simple... it's a very bad idea to have any DC across the volume pot, because it causes the carbon particles in the element to generate noise. the DC isolation is for the potentiometer, not the op amps. also your comment about resistors creating distortion, only applies to carbon composition resistors. metal film and carbon film resistors don't create distortion, though carbon film resistors are a little bit noisier than metal film. it would have been better if they had the pot in the opamp feedback loop. that would make the gain of the opamp variable, and they could have eliminated those "extra" capacitors.

the 100pf and 330pf caps across the inputs are to keep RF out of the input stages, because RF can cause some nasty sounding side-effects.

most engineers designing consumer electronics will not add unneccesary components to a circuit, because they must design first and foremost with cost in mind. that is why so much stuff on the market has a lot of "shortcuts" in the design, using the cheapest electrolytic caps, running 1 amp devices at 950mA, using 5 watt resistors to dissipate 4.9999 watts of heat.

the fact that there are so many caps in this design (more than i'm used to seeing) means that this was a squirrely design to begin with, and had to be "tamed" with bypass caps on the op amp inputs, integrating caps in the feedback loops of the op amps. and there are some caps missing from where they should be. zener diodes are somewhat noisy creatures, and 99% of the time should have bypass caps across them.
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Old 17th June 2008, 03:01 PM   #6
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Quote:
Originally posted by unclejed613
most engineers designing consumer electronics will not add unneccesary components to a circuit
Actually, once you've got it working, it's not that uncommon when cost reductions are required, or even before, to mess around seeing what you can take off before it stops working.

trodas, the Universe has seen fit to instruct you in some of the most fundamental engineering principles: -

1) If it ain't broke, don't fix it.

2) Only change one thing at a time.

I would have thought that even a digital modder would have known these...

w
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Old 17th June 2008, 11:49 PM   #7
trodas is offline trodas  Czech Republic
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unclejed613 -
Quote:
it's a very bad idea to have any DC across the volume pot, because it causes the carbon particles in the element to generate noise
Good point. But a AC signal is basically half-full of DC, so, no help there. The signal still has to pass thru the potentiometer...

Quote:
metal film and carbon film resistors don't create distortion
Vishay claim othervise:
http://www.diyaudio.com/forums/showt...postid=1535720
...and I would believe them.

But the main problem I have with this explaination lies in C13. When we have the DC offset gone thanks to the C9 - why add another decoupling cap just after the pot?!



wakibaki -
Quote:
1) If it ain't broke, don't fix it.
Well, it IS broken. Now... )) And I wanted to improve the speakers, so... I had to mess with them. At least I learn something...

Quote:
2) Only change one thing at a time.
Good point. Who would thought that this will be so hard. I just replaced opamps and... things go wrong



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?

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:
Click the image to open in full size.

See? No C117 from my amplifier...
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Old 18th June 2008, 07:57 AM   #8
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lol, I tried modifying a sound card once... it had 10uf lytics on that were the size of rice.... so I replaced it with nice elna audio caps...

Well it played beautifuly for 2 minutes then died.. PC locks up if I try to put that card in now....

I have one soundcard that is pre windows 95, A terrasound something... now that sounds sweet... very strong output, feels like twice the output of regular cards... I miss the good old days.
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Old 18th June 2008, 11:05 PM   #9
trodas is offline trodas  Czech Republic
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Well, the circuit in question looks like this now:
Click the image to open in full size.

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:
Click the image to open in full size.

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
Quote:
you may need to go down to 0.01 F for faster op-amps
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?
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Old 19th June 2008, 04:58 AM   #10
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Quote:
Originally posted by unclejed613
the reason for decoupling the volume control pot is quite simple... it's a very bad idea to have any DC across the volume pot,
Exactly, and you have to deal with the output offset voltage of the input op-amp 4558, =C9 (which will get worse if you eliminate all coupling caps) but also the input bias current of IC3, =C13. These combined effectively prevent crackly pots for the life of the product. I'd be interested in hearing your ideas on how to retain this feature while eliminating the series caps.

Have you got a spectrum on the noise? You said:

Quote:
And quess what. No change at all. Still noisy, oscilating.
Is the noise tonal? That might indicate a different problem. I was thinking random broadband noise due to thermal activity. Some scope shots or preferably an FFT with a full scale tone for reference would tell you a lot.

Are you sure you haven't changed the gain anywhere?
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