I recently bought a ASUS Essence STX, although it's far better than the integrated soundchip on my mainboard, I'm thinking about changing better op amps on the card.
The must-be-changed ones are those 2 JRCs in the I/V stage, it seems to be a lowcost solution, I can't even find the datasheet on the net.
I find some good op amps but choosing one from them seem to be a difficult trade-off on the electrical characteristics, the followings are what I found:
OPA627 (or similar ADA4627): Maybe the most well known audio op amp ever,
I found the PCM1704 from TI uses OPA627 in the I/V stage in the datasheet example diagram.
AD8620: Has similar characteristics from the OPA627/ADA4627, but has a little bit higher input bias current and a little bit lower dynamic performance.
AD827: A very fast op amp, originally designed for video purposes, but many people also use it in audio systems.
It really seems good for DAC output I/V conversion.PCM1794 datasheet says that
"Dynamic performance such as the gain bandwidth, settling time, and slew rate of the operational amplifier affects the audio dynamic performance of the I/V section."
But its noise is high in some extent for modern op amps, and it's bipolar-input,
I dont know why when it comes to DAC output buffer, some people will say that FET-input is better, could someone explain the reason?
LT1364: Similar to but faster than the AD827, and has the same questions.
LME49990/LME49720/AD797: These low noise and distortion series may be ideal for the preamp stage, many people also put them in the I/V stage and got satisfactory results.
But the dynamic performance is not so good as those ones above.
(I don't know how fast is really just enough for this kind of application.)
Or some people could recommend other good ones for me?
The must-be-changed ones are those 2 JRCs in the I/V stage, it seems to be a lowcost solution, I can't even find the datasheet on the net.
I find some good op amps but choosing one from them seem to be a difficult trade-off on the electrical characteristics, the followings are what I found:
OPA627 (or similar ADA4627): Maybe the most well known audio op amp ever,
I found the PCM1704 from TI uses OPA627 in the I/V stage in the datasheet example diagram.
AD8620: Has similar characteristics from the OPA627/ADA4627, but has a little bit higher input bias current and a little bit lower dynamic performance.
AD827: A very fast op amp, originally designed for video purposes, but many people also use it in audio systems.
It really seems good for DAC output I/V conversion.PCM1794 datasheet says that
"Dynamic performance such as the gain bandwidth, settling time, and slew rate of the operational amplifier affects the audio dynamic performance of the I/V section."
But its noise is high in some extent for modern op amps, and it's bipolar-input,
I dont know why when it comes to DAC output buffer, some people will say that FET-input is better, could someone explain the reason?
LT1364: Similar to but faster than the AD827, and has the same questions.
LME49990/LME49720/AD797: These low noise and distortion series may be ideal for the preamp stage, many people also put them in the I/V stage and got satisfactory results.
But the dynamic performance is not so good as those ones above.
(I don't know how fast is really just enough for this kind of application.)
Or some people could recommend other good ones for me?
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I have the Essence ST, same DAC circuitry as STX I believe. I changed to LM6172 and also reduced the size of the caps in the feedback loop. Much smaller caps work fine with LM6172 because of its phenomenal slew rate.
<edit> As for why people say FET input sounds better - its most probably about RF immunity of the LTP input. Best to do away with LTP all together in my view, the LM6172 has no LTP so FET vs bipolar becomes irrelevant.
<edit> As for why people say FET input sounds better - its most probably about RF immunity of the LTP input. Best to do away with LTP all together in my view, the LM6172 has no LTP so FET vs bipolar becomes irrelevant.
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Well... Is it proper to reduce the capacitor value? Isn't that used to filter the high-frequency noise come out from the sigma-delta DAC?
There is an idea comes out from my brain: Transimpedence amplifiers do the job of "I/V conversion" so is it good to use those op amps with these kind of characteristic (such as low input resistance?)
Maybe the ADA4898 will work well?
High dynamic performance combined with low input resistance:
55V/µs, 65MHz (-3dB freq.), Ri=5kΩ.
Has someone tried this?
Maybe the ADA4898 will work well?
High dynamic performance combined with low input resistance:
55V/µs, 65MHz (-3dB freq.), Ri=5kΩ.
Has someone tried this?
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Very sound advice there from Chris.
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
The OPA2134 gives very good results in many applications and is pretty docile in use.
If its proper to seek for better sound, then yes.
Isn't that used to filter the high-frequency noise come out from the sigma-delta DAC?
Its possible to add more stages to the downstream filter if you're worried about the HF noise. In my way of thinking its mainly there to ensure the opamp doesn't go into slew limiting (highly nonlinear).
It's been a while now since I had my Essence STX (experiment with external power & clock / cap upgrade went wrong and busted my unit), but the best ever opamp swap that I made was to change the opamps in the buffer stage to LME49710HA (metal can version).
You'll need an adapter; LME49710HA is a single opamp, you'll need two to replace the one in the buffer position. The ones I used were from Brown Dog : Single-to-dual Op-AMP Adapter - DIP version (p/n 021001). You'll need to shoehorn the legs of the metal can into the appropriate pins.
If it sounds like a bit of effort, it is kind of. But I think you'll find that it's well worth it.
From what I recall, there's quite an extensive thread on upgrades / tweaks to the Essence STX over at head-fi. That's where I got the metal can idea.
You'll need an adapter; LME49710HA is a single opamp, you'll need two to replace the one in the buffer position. The ones I used were from Brown Dog : Single-to-dual Op-AMP Adapter - DIP version (p/n 021001). You'll need to shoehorn the legs of the metal can into the appropriate pins.
If it sounds like a bit of effort, it is kind of. But I think you'll find that it's well worth it.
From what I recall, there's quite an extensive thread on upgrades / tweaks to the Essence STX over at head-fi. That's where I got the metal can idea.
Yeah, the stability of op amps when they driving capacitive loads is always a crucial problem, especially for audio DAC I/V stage because it's followed by a filter stage. And that's why I think OPA627 or ADA4627 is not proper for this position, AD8620 can be more stable, AD827 can be very very stable, and the others... should be tested instrumentally.
Maybe put a capacitor in the feedback loop can suppress but not to eliminate this kind of problem.
Maybe put a capacitor in the feedback loop can suppress but not to eliminate this kind of problem.
Driving an RC filter - where the R is into the high hundreds of ohms - does not constitute a capacitive load. That's because capacitors only cause stability issues by introducing phase shift at higher frequencies - with the series R the phase shift doesn't appear at the opamp output but on the C.
So, when driving the LPF, capacitors are not big problems for I/V stage?
Then what causes the op amps ringing and the instability?
So, when driving the LPF, capacitors are not big problems for I/V stage?
Then what causes the op amps ringing and the instability?
One reason is when the amplifiers phase margins are compromised but there's not always a simple one size fits all answer. Wide bandwidth high slew rate opamps are also very sensitive to physical PCB and track layout where the inductance of a few mm of copper print can cause problems. It's not the opamps fault, it's just not a suitable device for these (what are in reality) low frequency applications and the PCB layouts used.
You say at the beginning that the opamps are JRC cooking grade devices. I still feel something like the OPA2134 I mentioned earlier would be a worthwhile improvement and should at least be stable.
Thx Mooly, I'll try that.
If the unstable problems are caused by physical circuit layouts, then good layout techniques and well-decoupled power supply rail would eliminate these kind of problems?
If so, then I think the problem will be not so serious for STX. ASUS is a PC board-devices manufacturer, they would be able to handle such kind of issues. At least we can see that ASUS Xonar products have amazing instrumentally tested results compare to other competitors on the market.
If you really want to try something fast anyway, I'd recommend LME49720 or OPA1612 which have excellent specs and the datasheets do not indicate they require any special decoupling or layout to remain stable.
This doesn't mean they will be stable, but at least it's not asking for anything past a 0.1uF ceramic and removal of the ground plane under the inputs or outputs.
Actually, are these socketed DIP op-amps? Unless you plan to use an adapter I'd just stick with the LME49720.
This doesn't mean they will be stable, but at least it's not asking for anything past a 0.1uF ceramic and removal of the ground plane under the inputs or outputs.
Actually, are these socketed DIP op-amps? Unless you plan to use an adapter I'd just stick with the LME49720.
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I've tried various combinations using the following parts:
OPA2134PA
OPA2604AP
LM4562NA
NJM2114D
AD826AN
I ran RMAA with each of the above and there was almost no difference between the results. (percentage wise there was zero difference, however the graphs were subtly different, probably normal variation that you'd get between tests with the same parts fitted) In the end i stuck with 2x AD826 and an LM4562 as my favourite combo.
OPA2134PA
OPA2604AP
LM4562NA
NJM2114D
AD826AN
I ran RMAA with each of the above and there was almost no difference between the results. (percentage wise there was zero difference, however the graphs were subtly different, probably normal variation that you'd get between tests with the same parts fitted) In the end i stuck with 2x AD826 and an LM4562 as my favourite combo.
Right - because the opamp only 'sees' those caps through a series resistor.
Then what causes the op amps ringing and the instability?
If you're seeing ringing on the I/V it could be because the slew rate limit is being exceeded. Or, more likely because the DAC has output capacitance which is degrading the phase margin. Or perhaps just the opamp being used has marginal phase margin when used at noise gain close to 1, as found in I/V. Could even be poor power supply decoupling.
Just guessing here
For the I/V stage, I can recommend the ad8620 as well. The AD8620 in the I/V and the twin LME49710HA (metal can) for the buffer position resulted in the best combination for me AFAIR.
The LM4962NA and the LME49720NA are the same, more of a renaming to keep it in line with the rest of the series (from LM4962 to LME49720). There IS a big difference tho, between the dual metal cans and the single chip, so that is what I always recommend.
The LM4962NA and the LME49720NA are the same, more of a renaming to keep it in line with the rest of the series (from LM4962 to LME49720). There IS a big difference tho, between the dual metal cans and the single chip, so that is what I always recommend.
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Hey guys, I recently tried some op amps. Though there are many threads about this topic, I'd like to share my test results, the headphone used is the Senn HD595
First I place 3*LME49720HA, the soundstage is very wide, the transient of bass is fast and not too rush, but not very solid, it's like that the sound is "absorbed" very quickly. For classical instrumental pieces, it sounds very beautiful and airy, a good choice for baroque and classic period flute & string concerti. The treble is a little harsh for bruckner's symphonies, especially for the brass section.
Latter I put the LME49900 in the LPF, and tried the OPA2107 & OPA2132PA in the I/V section.
The OPA2107 has dark and soft sound (the burr-brown sound?), comfortable and not too harsh, and still has good soundstage and details. But it's a little dragging, the transient is not so good as others. I think OPA2107 is to churches what LME49720 is to modern philharmoniker halls. He's gentle and mysterious but not a rock guy.
The OPA2132PA has solid, concentrated bass and moderate treble, good transient and details. It has relatively flat THD characteristic (0.00008% under 1kHz and rise to about 0.0002% at 20kHz). Good for romantic and late-romantic period symphonies.
First I place 3*LME49720HA, the soundstage is very wide, the transient of bass is fast and not too rush, but not very solid, it's like that the sound is "absorbed" very quickly. For classical instrumental pieces, it sounds very beautiful and airy, a good choice for baroque and classic period flute & string concerti. The treble is a little harsh for bruckner's symphonies, especially for the brass section.
Latter I put the LME49900 in the LPF, and tried the OPA2107 & OPA2132PA in the I/V section.
The OPA2107 has dark and soft sound (the burr-brown sound?), comfortable and not too harsh, and still has good soundstage and details. But it's a little dragging, the transient is not so good as others. I think OPA2107 is to churches what LME49720 is to modern philharmoniker halls. He's gentle and mysterious but not a rock guy.
The OPA2132PA has solid, concentrated bass and moderate treble, good transient and details. It has relatively flat THD characteristic (0.00008% under 1kHz and rise to about 0.0002% at 20kHz). Good for romantic and late-romantic period symphonies.
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