asus stxII with sparkos discrete opamps

Both the Sparkos and the Burson op amps are very demanding on the power supply.  The newest Burson are less demanding, but still require a good amount of capacitance around them and good strong and smooth voltage from main power supply.  This is something that the computer PS just cannot provide.  I'm not sure why so many people think that Bursons sound good on this card -- maybe a placebo affect??  Without a good power supply and closely mounted larger size capacitors, these discrete Class A op amps can sound bright/thin.

I once tried one of the MUSE op amps and the sound was very full, but a bit dull on the high frequencies. (can't remember if it was MUSE01 or MUSE02)  I attributed this to the very slow slew rate.  It may work out in some situations, but not others.

The best op amp I would recommend for a card like that are the single channel OPA827 SMD opamps mounted on a "single-to-dual" adapter.  Cimarron Technologies will sell these with the op amps already mounted/soldered onto the adapter for $33.50 each.  Just make sure you choose the "Pins are installed" option.

https://www.cimarrontechnology.com/browndog-020302-single-to-dual-op-amp-adapter

You can have him install other op amps onto the adapter, but I have found the OPA827 is the best compromise of resolution and FET sonic signature.  These monolithic op amps are also not as demanding on the power supply and work well on many platforms.

As far as burn in, yes, any electronic component will require a level of burn in.

I have tried the AD787BRZ op amps (including soldering a 50pf compensation cap across pins 6 & 8 of the op amp - which was challenging).  At first I thought this was one of the most cleanest and transparent op amps without adding any coloration.  However, after extensive listening, I found they sounded just a bit dull and actually a tiny bit dirty.

I have not tried any other AD op amps (such as AD8610).  Those other op amps have a max power supply voltage of +/-13V or lower.  The equipment I test in has a +/-15V power supply, so I can't test these other op amps.

By the way, I have tried and worked with a great many op amps over the years trying to find the best configuration.  I have ended up running a combination of Burson V6 Vivid fet-based op amps and Sonic Imagery 994Enh bipolar op amps (not recommended for your STX!!!!).

The Burson's are excellent resolution and have a slightly warm character, even though high frequency resolution is amazing.  The Sonic Imagery are the most demanding on the power supply and are just HUGE op amps (requires space).  They also require big capacitors soldered directly onto the op amp for power supply and a very strong main power supply, but they are absolutely amazing sounding!   It's like putting a mini-Krell analog circuit into your DAC/preamp for $94 per dual-channel op amp circuit!  Both Burson and Sonic Imagery are fully discrete Class A analog circuits.

The STX II uses two MUSES 8920 for the DAC I/V stage. These have a slew rate of 25V/us (on the fast side).

However, the output opamp is a MUSES 8820 (outputs to the RCA connectors) has a rather slow slew rate of 5V/us. This will roll off the high frequencies rather drastically.

The OPA827 op amps (that I recommended) have a slew rate of 28V/us, but they are actually very excellent sounding with good bass and great resolution for a monolithic op amp.  They are also more forgiving and easier to implement than other "touchy" op amps, such as the more expensive OPA627 (slew rate 55V/us, but requires 0.1uf MKP capacitor soldered directly across power pins for stability) and stuff like OPA211 (which is a very sharp op amp, but actually came across somewhat bright and harsh).

You could also try the new Burson V6 Vivid op amps.  They are better, sharper and much higher resolution than OPA827, but they -could- have a possibility of coming across slightly thin/harsh in some systems without good power supply.  I would say to try the OPA827 first and if they are still too slow/dull, go for the Burson V6 Vivid.

In my experience, op amps even in the I/V position will affect the sonic signature.  However, the I/V op amps are placed right at the DAC chip output.  The DAC chip is outputting straight DC pulses.  These are essentially square waveforms.  The purpose of the I/V stage is to amplify these square waveforms and also provide some sort of smoothing to turn these into the curved analog waveforms of true sound.  The slow slew rate of the 8820 may be helping to smooth these straight line square waveform pulses.  On the buffer output stage, the waveform is already somewhat smoothed, so the slow slew rate is having even more of an effect to reduce the high frequency response.

I was just indicating that a slow slew rate may not reduce as much treble clarity as much when in I/V because it's responding to straight line square waveforms.

That being said, yes, a slow slew rate will ultimately result in poor treble. It's slowing down the waveforms so the midrange will get thicker and bass will get thicker/warmer. However, if you slow it down too much, it gets too warm and messy. There's no clarity or "separation of instruments".



I think you still have to have a minimum slew rate in audio.  However, you don't want a fast slew rate for I/V position.  The buffer/output stage can have a high slew rate, though.  I think the OPA827 is an excellent I/V op amp.  You could also try OPA2132 if you wanted to for I/V op amp.  They are relatively cheap at about $12 each from digikey (get the OPA2132P model).  The OPA2132 is the most popular op amp to be used for I/V position.  Even my Krell S1200U home theater processor uses opa2132 for I/V, but it has it's own fully discrete Class A output stages.

For output, if you wanted more clarity you could try Burson V6 Vivid.  Slew rate is around 40V/us.  (somewhere between 36 and 49).

However, your new OPA827 probably need to be burned in some.

Hi Johnny,

Based on our conversation, I have gone back to try different opamps for I/V in my reference modified DAC. The OPA827 appears to be the best in this position due to the slower slew rate when compared to my other reference op amps. However, when I use OPA827 in the output buffer, the sound actually slows down some. It’s not as clean/clear and doesn’t have as much attach or high frequency resolution. I found putting Burson V6 Vivid in the output buffer really helped in this case. The V6 Vivid is definitely better than OPA827 for output buffer. It is much cleaner and clearer. Midrange and high frequencies are faster and better represented. I don’t know how you are liking your current sound, but you might try getting a couple V6 Vivid for your output stage on the STX.

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Just a thought.

The Xonar Essence STX is a 2-channel audio card, unless you have the model with the 6 channel extension card that has 6 RCA analog outputs.  Do you have this extension card?

That being said, the extension card really doesn't have good power supply capacitors.  Technically speaking, the "gain" of op amps is defined by the negative feedback circuit.  However, it is said that the Sparkos op amps have the most "perceived" gain.  They don't work well in your situation, however.   You could try using one of the OPA827 op amps for the sub channel I/V.  You could also try buying a OPA2132P from digikey.  They are $11.51 each.  Digikey part number is OPA2132P-ND.  They have a slower slew rate of 20V/us and may give a fuller waveform for bass.  I would suggest buying a few for the bass I/V and output buffer.