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?

{quote]So I think I better ask these who know about my ideas how to modify a existing design[/quote]

always a good idea




No.Capacitors are no more than filters so depending on how and where they are used will determine what the filter function is with the size of the cap determing at what frequency with the latter being a dead giveaway as to function and here is why.BIG value capacitors (in uF) filter LOW frequencies and small value capacitors filter high frequncies so the 0.1uF tells you the filtration is high up in the frequency band.

Next is filter type.A capacitor placed in series (that means everything goes THROUGH it) will be a "high pass filter" meaning at the point of cutoff everything BELOW will gradually die away and everything above will pass.

A capacitor placed in a "shunt" position,that is where it connects from the "hot" or signal lead to the "common" or ground lead,makes it a low-pass-filter so will in fact attenuate everything ABOVE the cutoff frequency.

so where does that leave you ?

Well the "C99" cap is there to filter out any RFI (low pass filter remember) from going past that point ans with a computer being one big mamma of an RFI generator not only a good thing but a neccessary thing so let it be unless or untill you want to improve on the quality of the capacitor.

looking at the section you want to upgrade I can see one screaming "fix me ! Fix ME !" in the schematic and that is the coupling capacitors at the input and output of the Op-amp itself.Again,a required part they being there to block DC (high pass so anything ABOVE......) from going into the op-amp input or being passed to the next stage due to any op-amp DC offset voltages but man,an electrolytic and likely a cheap one at that is audio quality death unless you step up to very expensive versions and then only when the value of the cap is so large you have no other options.

Fortunately that "10uF/25VDC" is readily available from a damn lot of venders in a quality part ranging from bargain prices to insane with the "insane" better left to ciruits that can actually make use of the increase in quality so if I may make a recommendation it would be to try for a WIMA MKP 10uF and that in the highest voltage version you can fit higher voltage capacitors sounding way better than their lower voltage counterparts for whatever reason.If you want to step up to a higher class look to something like an "Auricap" or "Hovland Musicap" or to shoot the middle gap a "Solen Fast Cap".Those last three may be a bit of a struggle to fit with the Solens being fat buggers and the other two long

As for the Op-amp itself the one you have is not really that shabby but if you must then the National chip mentioned is a possible as are the AD843 and AD845,smooth but yet with good "slam" is one way to describe them.....

hope this help a bit,good luck

rickmonster

[quote name='rickcr42' date='Jun 11 2008, 16:53' post='570586']
{quote]So I think I better ask these who know about my ideas how to modify a existing design[/quote]

always a good idea


[quote]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?[/quote]

No.Capacitors are no more than filters so depending on how and where they are used will determine what the filter function is with the size of the cap determing at what frequency with the latter being a dead giveaway as to function and here is why.BIG value capacitors (in uF) filter LOW frequencies and small value capacitors filter high frequncies so the 0.1uF tells you the filtration is high up in the frequency band.

Next is filter type.A capacitor placed in series (that means everything goes THROUGH it) will be a "high pass filter" meaning at the point of cutoff everything BELOW will gradually die away and everything above will pass.

A capacitor placed in a "shunt" position,that is where it connects from the "hot" or signal lead to the "common" or ground lead,makes it a low-pass-filter so will in fact attenuate everything ABOVE the cutoff frequency.

so where does that leave you ?

Well the "C99" cap is there to filter out any RFI (low pass filter remember) from going past that point ans with a computer being one big mamma of an RFI generator not only a good thing but a neccessary thing so let it be unless or untill you want to improve on the quality of the capacitor.

looking at the section you want to upgrade I can see one screaming "fix me ! Fix ME !" in the schematic and that is the coupling capacitors at the input and output of the Op-amp itself.Again,a required part they being there to block DC (high pass so anything ABOVE......) from going into the op-amp input or being passed to the next stage due to any op-amp DC offset voltages but man,an electrolytic and likely a cheap one at that is audio quality death unless you step up to very expensive versions and then only when the value of the cap is so large you have no other options.

Fortunately that "10uF/25VDC" is readily available from a damn lot of venders in a quality part ranging from bargain prices to insane with the "insane" better left to ciruits that can actually make use of the increase in quality so if I may make a recommendation it would be to try for a WIMA MKP 10uF and that in the highest voltage version you can fit higher voltage capacitors sounding way better than their lower voltage counterparts for whatever reason.If you want to step up to a higher class look to something like an "Auricap" or "Hovland Musicap" or to shoot the middle gap a "Solen Fast Cap".Those last three may be a bit of a struggle to fit with the Solens being fat buggers and the other two long

As for the Op-amp itself the one you have is not really that shabby but if you must then the National chip mentioned is a possible as are the AD843 and AD845,smooth but yet with good "slam" is one way to describe them.....

hope this help a bit,good luck

rickmonster
[/quote]

And why do you want coupling capacitors anyway? They do nothing positive to the sound and eventually require replacement. I just got done replacing 150 (about 1/5 of the total) caps in a Sony Betacam today. I'm a big believer in direct coupled amplifiers - like my Hafler DH-101 preamp. I'd be much more concerned with a junky old opamp like the 4558. It was pretty good in 1977 but there are WAY better amps. Check out the Analog Devices AD-712 or the Burr Brown (TI) OPA-2134 just to mention 2. Either should just 'drop in' to that circuit.

If you think you need the caps to get rid of DC offsets, you have bad amps. I've designed video systems DC coupled from photo sensor to the A-D converter with 30 MHz bandwidth for HDTV telecine with like 450 opamps and a total of 3 'coupling' caps that actually are in-line sample-and-hold capacitors - one each for red, green and blue.

C9 IS not required. C3 only block DC from the outside world which SHOULDN'T be there. If IC-3 has an input offset, C13 IS required otherwise it can go too.

G²

to keep Dc from getting INTO the opamp where it will be magnified by the gain factor of the circuit (for example X10 gain means the Dc will also be increased ten times) and at the other end to prevent any posibility of Dc reaching the next stage where it will cause real havoc not to mention an eventually 'scratchy" sounding volume control from the DC being applied to it.Using coupling capacitors is not somethng just to do but there because they are a safety valve.That measly Dc offset spec may look good on paper but take it to another stage with gain and it suddenly becomes large.




Beleive in direct coupling if you wish,your choice,but unless the circuit was DESIGNED to be direct coupled chances are there will be problems at some point.Capacitors,DC Servoes,fully balanced topologies with a closely matched bipolar power supply good.none of the above ?

Bad.

As for the 4558 being ";bad' just because it is old I beleive you have no clue of the how and why of "opamp rolling" because if you did you would realise you can NOT just drop in another part because the pins are the same and expect it to operate perfectly.For instance.Some of the high bandwidth types are notoriously unstable or very sensitive to RFI unless 'steps" are taken to compensate and those steps will vary from device to device.more than one person has thought they "bettered" a piece of gear until someone with a 'scope showed them the oscillations going on and why they heard a slight "brittle" quality or glaze over everything that they thought was just added 'detail".

no one ever died from this but more than a few opamps have after generating too much heat due to the full time high order oscilating and even that usually not noticed for what it is (slight noise increases over time up to and until opamp death)

do some research.Check just how many highly touted pieces of audio gear use the 4558.Listen.don't just read spec sheets and if you do then what you REALLY want in a good audio op-amp is not lightening fast speed but more like around 25-50V/uS which is damn near perfect for audio work where at BEST your opamp will be passing signals in the 1-3 volt range.other specs to look for would be current noise density,idling current (too low and there is no doubt the output will be pure class-B),how it responds to momentary signal overload (does it STICK to one supply rail then lock up ?) so overload recovery time is important as is 'settling" time,how low of an impedance can it safely drive without adding a buffer....


More than just "low noise and fast" because that COULD mean it is designed to work in the mHz range and will be crap at audio frequencies


but you knew that right ?

Rick out

Seconded. Many sound cards (and other DACs) have an appreciable amount of DC on the output. If you don't block this early in the chain, it will cause distortion in the gain stages (asymmetrical clipping), could cause oscillation and other problems. If it gets through the amp OK, the DC will effectively lower the Xmax of your woofer - causing distortion.

AC coupling in audio circuitry is generally reasonable, especially given the insensitivity of human hearing below 20 Hz.

The down side is that this usually involves fairly large capacitance values for which non-electrolytic capacitors are physically quite large and also quite expensive.

Some electrolytics have pretty good performance in audio applications, and it helps to use capacitors with higher voltage ratings.

The weaknesses of electrolytics are leakage, shortened lifetime and poor high frequency characteristics. High voltage rating helps the leakage and buying the best quality capacitors helps with the lifetime.

As for high frequency characteristics, I personally prefer to place a small non-electrolytic capacitor in parallel with the electrolytic, although you can find electrolytics that don't need this. The second capacitor adds very little expense and I just feel that this is good policy in general.

Or worse! I once built a preamp with an op-amp that was not stable at low gains. (It oscillated at radio frequency)... I hooked it up (with no input)... It was perfectly silent ... I thought that was good... Then, I heard a small noise from my power-amp... And, my power amp was blown! (I had to replace the power-output transistors).

I've got a similar story. The NatSemi datasheet for the part said that it was stable for gains over 10dB. After getting a cloned part, I implemented the circuit, and ended up with a bad preamp, and a pretty decent 15MHz oscillator. Looking at the cloned part's datasheet, much later, revealed that it was only stable for gains over 30. I should have listened to my electronics lecturer's warning: "only amplify the signal you want".

Probably the best thing you could use to replace the 4558 (more or less a 741 with internal compensation IIRC) would be a TI TL072 series opamp. - can't remember the name of the 8-pin version at the moment. That design is extremely low noise and works well to amplify audio. I've used them for years in homebrew audio gear and they work fine.