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In Reply to: OPamplifiers, Replacements posted by groman on January 05, 2002 at 02:37:11:
If you look at schematic over AD797
in the DATASHEET you will notice:Only two stages, before output buffer.
The second stage is a differential common base configuration,
with current mirror that follows the signal.
This gives a very good working conditions
for the transistors.
Also is a commonbase configuration very fast.
This is my favorite OPamp,
when it comes to design.
AD797 DATASHEET in pdf-file:
Follow Ups:
Hi,These designs are based around the "Diamond Transistor" structure and dispense finally with the ill concieved (for AC applications anyway) differential circuitry.... Oh yes, distortion levels in the audio rival anything out there, drive ability is hugely improved and the bandwidth/slewrate so far beyond good and bad that adding a slewrate limiter on the input finally no longer introduces measurable distortion in the audio range....
Later T
Hi Thorsten,the term "diamond transistor" was coined by Burr Brown, if I remember correctly. I guess you are referring to the four transistor emitter follower that is found both as an output stage and as a buffer to the non-inverting input of most current feedback op-amps?
This design has excellent slewing performance, but usually has a THD in the -90 to -100 dB range, provided that device matching is excellent and loading low.
I use it as a buffer to a discrete bipolar VFB amp. Inherent low distortion plus some overall voltage feedback makes a good preamp.
Greetings,
Eric
Hi Thorsten,
What's wrong with the AD797 architecture?.
Double diamond I thought this was the LH002 style buffer also described in the Walt Jung Paper you are referring to.
ELSO
maybeUntil then,
we can wait for him to join
the ones
how seek for knowledgeand are happy for company
along
the way
of lifelife is to short
to bother with foolsgroman
Hi,BTW, I have ultimatly given up mostly on solid state - it's too much like hard work to get good sound from it.
But if you can, compare differential and diamond transistor (current feedback) based amplification circuits both measured with complex signals (noise loading) and subjective. You may be surprised by the results. I certainly was. Also look at the various thermal distortion components in differential vs. diamond transistor.
Later T
My post here above, could better
have been left unwritten,
as it doesn't add anything
to discussion.
But my words might have caused you
to lose your temper.
I am impressed by your maturety
and your patient manner.Thank you for your contribution
to this, I have learnt a great deal more.a little sad
groman
Hi Thorsten,
With the current feedback opamps I tried AD811, OPA603 and LT1026 and AD815Y, I did not hear anyting special compared to normal differential input opamps like AD 826, AD817, AD827 or AD847.
I did hear better voices and mids on FET inputamps like AD711, AD823 and OPA 627.
Logical conclusion for me to build my own discrete opamp with FET differential inputs, cascoded by the way.
ELSO
Hi,Well, I can perfectly live with that. The results of subjective evaluation are rarely unambigous. I have compared differential and rail symmetrical SE (diamond transistor and related) circuit structures in both power amps (discrete) and monolithic Amp's, with a great preference for the rail symmetrical SE ones.
But like any other circuit, so many variables influence the design that many differnet conclusions CAN be arrived at. BTW, in the end I find the OPA627/637 (sonically) slightly "foggy" and overly "warm" compared to a wire bypass, unlike the AD811 and LM6181 which I cannot tell from a wire bypass.
Later T
ThorstenIf you cannot tell the difference between an AD811/LM6181 and a wire bypass, how do you conclude that solid state audio is fatally flawed? Surely this is the test of any sonic degradation introduced by an amplifier?
Regards
Terry
Hi,> If you cannot tell the difference between an AD811/LM6181 and
> a wire bypass, how do you conclude that solid state audio is
> fatally flawed?Good question. Maybe I should not be as sweeping with my statements. Maybe more accurate would have been that "most" of solid state audio is fatally flawed.
> Surely this is the test of any sonic degradation introduced by
> an amplifier?Yes and no. I use the bypass test more to get a handle on a given items influence on the sound. I was very shocked to for the first time have something that I could not reliably tell apart from a level matched bypass. In the end - whatever the background and reasons, I still feel Valves sound better to me, for whatever reasons.
And as the job of my stereo is primarily to make me enjoy music, Valves win.... ;-)
(remember - foolish consistency....)
Ciao T
Hi Thorsten,
That's very interesting as I found my discrete opamp outperform the AD811 and the OPA627.
Yes the ambiguity of subjective evaluation is a problem, as f.a. I am not in contol of my own biorhytm etc. But in the end it is the gorgeous sound that counts , not the specs or the measurements.
ELSO
Hi Elso,One more question on this. Do you ever try to evaluate active or passive components in the classic wire bypass test? If not then maybe some of our differences in experience/view stem from that. To me it is one (but not the only) test I like to do.
Later T
Hi Thorsten,
One can not compare a amplfier with a straight wire bypass as the wire has no gain!
How would you compare the AD811 as a IV converter with a straight wire?? The wire does not convert current to voltage.
I did compare coupling caps this way and this led to the omission of a lot of coupling caps in the DAC and preamp. Best cap is no cap. From the current output of my DACchip(AD1865) to the poweramp I have not one coupling cap!
I usually try a new part in my circuit and live with it for a while. Then change back to the original situation.
Later....?
ELSO
Hi,I think it is a valid comparison to see how much a given part/circuit will colour the sound. If different gain is present you need to match that somehow, which of course introduces problems of it's own.
I still feel it worthwhile to try such procedures, as they give you a hande on the "sound" of a device/topology as much on it's own as is possible. I draw most of my design/modification elements from that type of test combined with in circuit comparisons. I find it interresting that it is possible to "subtract" system sound from the in circuit comparisons and to hear the same sonic signature superimposed that was heard first in the wire bypass test.
Of course, I could be completely deceiving myself and in reallity everything sounds exactly the same and I imagine everything.... ;-)
Ciao T
Elso,
You would need to do the A/B where you do not need gain, say after the IV converter. If you add a circuit after the IV converter and it sounds the same as not having it then the circuit is pretty good(as a buffer). How a circuit behaves as a IV converter might be way different from using it as a unity gain buffer or even as a straight times 3-10 line stage. Lots of high frequency junk coming from a current out DAC. Many circuits cannot handle it. Wadia has a new zero feedback current mirror circuit that they like much better than op amps for their IV converter.Yes, no coupling caps is best, but sometimes a simple circuit with a coupling cap will sound better than a complex circuit without.
By the way, I sent you two CS8420s about one week ago. Did you receive them yet?
Ric Schultz
Hi,> What's wrong with the AD797 architecture?.
It's differential.
Later T
I had LM6171 datasheet
but had to download AD811No schematic in AD811 pdf.
LM6171 is great
has something of the symetric structure
found in current-feedback amplifiers.But that is hard for me to realize,
if I have not got matching complementary transistors.I guess that is why diffrential input stages
has been popular (too popular) for so long.
Easier to find matching NPN+NPN PNP+PNP
than NPN+PNPWellbuild CFB-amplifiers are great!
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