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Hotrodding the X-Fi: A Layman's Guide (No 56k) - Page 146

post #2176 of 2181
Quote:
Originally Posted by germanium View Post
 


Germanium...some added info for you.

 

I have not worked with opamps for a long time and in my day the LM725 was the hallmark of opamps. It was an instrumentation opamp meaning it operated very precisely. However, to get that precision, you had to add external circuits.

 

The X-Fi uses an NMJ2068 dual opamp, meaning it squeezes two opamps into one package. With a + and - input and output on each opamp, plus the required power supply inputs, all 8 pins are used up.

 

The LM725 was an 8 pin device as well but had only one opamp in the package. It had extra pins available with which external circuits could control the offset drift and supply frequency compensation.

 

If anything, the dual opamp used in the X-Fi suffers from a lack of that external control.

 

You might be able to remove the NMJ2068s and use some kind of daughter board to add 8 pin single opamps that allow external control. You would have to extend the NMJ2068 pads to the daughter board for input and output. You could take the required power supply voltage from elsewhere, or from the same pads.

 

If you get a data sheet for the NMJ2068 that shows an equivalent circuit, you will see the output being taken from between two resistors in between two output transistors. They are using the NPN/PNP complimentary symmetry arrangement which is a Class AB1 design. The AB1 has an inherent distortion at the crossover point between the +ve and -ve going output signal and it requires an output capacitor in some cases to smooth out the distortion.

 

The capacitor works much like a power supply capacitor, although it is actually coupling signal to the output. In a power supply, the output of the rectifier stage is a pulsating DC with the capacitor right across the stage output. The capacitor charges to the peak voltage of the positive DC pulse, but as the sinusoidal waveform of the DC pulse decreases, the cap tend to hold the voltage at the peak. Between pulses, the cap tends to hold the voltage flat causing a pure DC but it does drain off through the load, depending on the RC time constant.

 

When viewed through a scope, the DC has a ripple in it at either 60 hz or 120 hz. All RC-based circuits leave a slight ripple which can be removed with a regulator.

 

At the output of an audio amp, the cap is attached in series with the load to the neutral point between output transistors. The resistors you see on the equivalent drawing are the emitter resistors for each output transistor. As the input signal varies between +ve and -ve peaks, each output transistor will be on an equivalent amount of time. There is a point, however, between the +ve to -ve output signal transition (crossover point) where both transistors are off. In this region, the outputs from each output transistor do no coincide, causing a potential overlap which can be heard as a buzzing sound.

 

There are several solutions attempted but one of the simplest is to include an output capacitor. The cap can stabilize the crossover distortion region by maintaining the voltage till one transistor begins firing again. There are purely direct coupled output designs with the speaker connected right to the crossover point. Most amps I have seen, however, use a cap. If you omit it, you need to compensate using feedback and/or bias adjustments on the transistor input. Since there are none available in a dual opamp package, you can't get at them.

 

You would save yourself a lot of grief by leaving the caps in the circuit.

post #2177 of 2181
The fact is these coupling caps are not required & in most cases the quality of the coupling cap does in fact reduce output quality especially on these cards. The opamps in question are actually used in most pro audio equipment so the opamps should not be a source of bad audio & are more stable than some of the opamps people are replacing them with.

Every one of the opamps used in the Creative labs audio cards & the Asus cards are the ones recommended at least at some point by the DAC manufacturer With the exception of the main output of the low end X-fi cards which was chosen to be able to drive headphones as it is higher current capable. I found them to be plenty good once direct coupled.

The low end X-Fi card does have higher d.c. output when direct coupled than is safe for D.C. coupled amps which can amplify D.C. though but D.C. coupled amps are not likely to be used with these cards anyway as most would be priced way out of the typical price range of equipment used with such a card.  Any other type amp is ok to use with these cards when fully direct coupled.

The X-Fi Elite pro & the Asus Xonar D2/PM as well as the Xonar Essence ST & STX have very low D.C. ouput when fully direct coupled. The Xonar D2/PM is fully direct coupled from the factory. These card have such low D.C. output that that they can feed a D.C. coupled amp & maybe even reduce the intrinsic D.C. output of some of those amps by having a lower D.C. output than the D.C. present from their own input with nothing attached. This cannot be done with a capacitor coupled final output section by the way as the inputs D.C. output cannot reach the output of the final stage opamp of the soundcard if a capacitor is present in order the D.C. to be absorbed & be cancelled.

Having a schematic is definately helpful but not necessary if you know how to trace a signal through the circuit which is possible on the X-Fi cards with the DAC's that have a voltage type output which includes all the Cirrus Logic DAC's in question The X-Fi Titanium HD card has DAC's with current output & as such is not traceable with a volt ohm meter however they are very logically placed between the IV conversion opamps & buffer opamps & as such are easy to find but not so easy to get rid of but can be done with excellent results. Correction, by time the signal reaches the coupling caps it has been converted to a voltage type signal who have is tracable.

By the way I was not trying to start a flame war, Only correct the parts that I know are inaccurate.  

It appears however you are trying to start a flame war by posting the exact same post 3 times like that is going to make things any different. It doesn't so please refrain from doing so in the future.

The specs for the DAC's used in the low end X-Fi card does show a Coupling cap but not where Creative puts them. There are no coupling caps before the differential input opamps in Cirrus Logics data sheet for the DAC used on that card , only at the output. On the data sheets for the Titanium HD card DAC there are samples that don't include any coupling caps at all. In fact none of the samples shown include coupling caps at all.

Note that all of my amps are full D.C. coupled & can amplify D.C. & I use a modified X-Fi Titanium HD with the coupling caps eliminated & wire put in their place with excellent results
Edited by germanium - 1/6/14 at 10:55am
post #2178 of 2181

Dear Forum!

 

Just bought an X-fi Xtreme Fatality audio card (almost the same as Xtreme Music, but more Xram and a fancy led on the upper right corner)

 

It has some issues. The driver is installing ok, but the main (stereo) output has some distortion. I mean I cannot use it at all with distortion this high.

I am writing because I read before that it can be fixed by changing the caps or the OPA pn the PCB.

Can anyone help me? Has anyone meet the same problem?

 

Thank you for your helping in advance


Edited by Nemeske88 - 1/12/14 at 7:57am
post #2179 of 2181
Quote:
Originally Posted by Nemeske88 View Post
 

Dear Forum!

 

Just bought an X-fi Xtreme Fatality audio card (almost the same as Xtreme Music, but more Xram and a fancy led on the upper right corner)

 

It has some issues. The driver is installing ok, but the main (stereo) output has some distortion. I mean I cannot use it at all with distortion this high.

I am writing because I read before that it can be fixed by changing the caps or the OPA pn the PCB.

Can anyone help me? Has anyone meet the same problem?

 

Thank you for your helping in advance

This is not likely an opamp or coupling cap problem. You need to go through your settings & turn off all signal processing as that is likely causing an overdrive situation. Changing caps or opamps is not something that should be done by the uninitiated here. These are multi layer boards that are easily damaged. They also have very thin traces. The mods talking about here do not fix distortion issues caused be DAC's being overdriven by signal processing. They only improve detail & soundstage of an otherwise properly functioning card that is not being overdriven by signal processing.


Edited by germanium - 1/12/14 at 7:22pm
post #2180 of 2181

Well, not so happy this time.

Thank you for your answer. I will check it's software side, but I have no doubt that it is a hardware problem. I've read a recapping and opa changing procedure can be easily done by who is familiar with soldering even a bit.

 

Will inform you about this


Edited by Nemeske88 - 1/14/14 at 1:19am
post #2181 of 2181

Well, after 5 years of service my X-FI Fatality sound card (64mb XRam version) went dead. Hanged the whole PC and after restart could not detect it, even by populating on other slots on the motherboard. Also tried on a fresh install OS and could not detect it. Now am thinking if I should try to replace a few caps (power filtering cap ?) and see if I can revive it before shelling out for a new card...? The card is stock standard (i.e. with no  mods)

 

any ideas on what component could have went bust ....  or what component I could try replacing  .... before chucking it into the bin

 

thanks.

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