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AD8397 vs. OPA551, which one sounds better?

post #1 of 12
Thread Starter 
I have only used OPA551, and was not impressed by the sound. But the 551 has great output capabilities (both current and voltage).......

Have anyone compared the AD8397 and OPA551 before? how much better (if any) is the 8397's SQ?
post #2 of 12
AD opamps tend to be fast and agressive. if you have heard the 8620, the 8397 is similar sounding with more refined highs. but it is a cranky opamp so becareful.
post #3 of 12
OK I will bite.
The sound quality of the AD8397 is much better.

Now does that really tell you anything?
post #4 of 12
depends on whether you need the V swing, the AD8397's 300mA current should be fine for any headphone load within its V limits 1.2+ Wrms into 32 Ohms

if the OPA551 extra V swing doesn't buy you anything then the AD8397 has ~ 20x more gain for feedback error correction at most audio frequencies, and the output stage is designed for low distortion with MHz signals, pretty much eliminating crossover distortion concerns, the AD8397 should be far more accurate than the OPA551 - whatever that means for "sound"
post #5 of 12
Anybody try opa552? Should be better than the 551, though admittedly only stable at gains over 5.
post #6 of 12
I used the OPA552 in the output stage of the original Corda, after adjusting the gain for 5. With OPA627's in the first stage, overall the sound was quite good. I just did this last week so I need to listen to it some more, but so far I'm pretty happy with it, and it's a huge improvement over the original Corda. I was comparing it to the Sudgen Headmaster which blew away the old Corda. I couldn't use the AD8397 because the Corda uses +/- 15v supplies and it is set up for single channel amps. But I have heard the AD8397 in various portable amps and the 552 in the Corda didn't have the slight edginess that sometimes comes with the AD8397.
post #7 of 12
I prefer the AD8397, and I'm not particularly surprised that I found this. The 8397 is specifically designed to drive low impedance loads with high linearity. If you need a buffer to tie to an op-amp, you can try the LMH6321. If you just want to use an op-amp, the AD8397 is one of the best.
post #8 of 12
Thread Starter 
Thanks. The only thing special about these two is their output capabilities. If I have to use a buffer I wouldn't use either one of them....

It is nice to know the 8397 sounds better than the 551/2.

Now can anyone tell me how much worse the 8397 is comparing to the AD843? I have the 843 in my buffered amp, so I know what it sounds like....
post #9 of 12
I don't think AD8397 takes a backseat to any opamp. It is a superb chip with few peers, but needs to be operated in a circuit designed with it in mind.

Being a bipolar opamp with significant input bias current, all resistor values must be kept low (but not too low) and reasonably balanced between the inverting and non-inverting input sides, or you'd have severe DC offset issues.

It's also a cranky chip and could become unstable easily when not in its element. It is more picky about PCB component layout and power supply decoupling than many other opamps, and my experience is that it isn't really unity-gain stable (even though the datasheet implies otherwise). Since it has no output short circuit protection, its high output current capability is both a blessing and a curse. It could be easily damaged, for example, if the headphone plug is partially unplugged while the amp is playing. An output resistor is prudent to provide protection, but the resistor should be wrapped within the feedback loop in order for it not to adversely affect the low output impedance. A ferrite bead should also be used at the output to isolate cable capacitance, because the opamp could also become upset with a capacitive load. High supply voltages (i.e., Two 9V batteries or more) also cause the opamp to run hot, and is apparently a cause of additional instabilities.

Many of these are lessons learned by Morsel and myself during the original MiniĀ³ v1 prototyping phase. Tangent had also encountered a high problem rate with the PINT amp and decided to pull the project as a result.

This is why I don't advocate the use of AD8397 as an opamp-rolling candidate in any amp not designed for it. If you're designing your own amp you should have proper wideband instrumentation to determine whether the chip is performing as it should.
post #10 of 12
Ditto Amb's comments on the AD8397 - no one could state it better.

Back to the OPA551, it works very well as a current buffer in some instances - as Amb as often championed. The original revMH Millett is one of those instances and the OPA551 is superior to a single BUF634 in those positions. My own listening comparisons with stacked BUF634's are very close, with perhaps the stacked BUF634's having only the slightest of edge. That's very good performance. (This is documented in many places, but you have to jumper a couple of the pins on the OPA551 to make this work.)

The OPA551 also works well in the SOHA (Brown Dog dual adapter required), where only a unity gain opamp for current amplification is desired, another perfect application for the OPA551.

All that said, its performance as a pure opamp is not outstanding, IMHO. It is pretty slow and lacking in snap and detail.
post #11 of 12
Thread Starter 
Thanks guys
The tube-hybrid amps are strange cases thoug: the BUF are buffers and work best with a controller, while in the tube-hybrid there is not opamp to control the BUF634, so the BUF's will have compromised performence. Now with the OPA551, even when configed to a follower, is still an opamp so it's the control itself, that is probably why the 551 followers are better than the uncontrolled BUF.....

I wonder why the 8397 is only in SOIC package, doesn't make sense. With 300mA output potentials they ought to be available in a bigger package, at least a DIP-8, otherwise how are you gonna handle the heat?
post #12 of 12
AD8397 is not so cranky, I don't populate any output resistors because it lowers damping factor significantly. I have never had any oscillations on AD8397 but I use it at gain of ~5. Also, you could eventually use ~2.2uH choke iductors for output buffering outside of the feedback loop to be sure no oscillations will occur in the headphones but it is actually not a problem because headphones are not capacative but slighlty inductive loads which op-amps like. All you need is to build proper feedback loop to fully compensate its quite big input bias currents. Low noise PINT shows well how to choose the resistors. Regarding SQ it's a very good chip, a bit on the bright side but smooth, powerful and giving impression of big sound sources. If you want to satisfy it completely you need to experiment with virtual ground channel circuits providing at least ~650mA current efficiency to fully match A8397 capabilities. Good results give AD744, OPA637, OPA228, AD825, AD8065, OPA134, TLE2141 as ground channel drive stage, depending on your liking. The resulting sound signature might be more interesting than AD8397 alone or you can try it as a buffer in multi-loop designs.
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