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Hi Jocko and group. Needless to say, *all* current feedback op amps need not be tarred with the same brush.

That said, one thing to look out for with CF amps is the dismal PSSR, due to the architecture. Thus, rock solid supplies are needed to get the most from them. If it sounds wunky, then investigate the PS first, I'd say (barring silly oversights such as high-K ceramics for decoupling, or worse yet, in the signal path). I know you wouldn't do that Jocko, but some might. And I surely have taken high-K ceramics from "perfect sound forever" CD players, where they were used as supply bypasses on the I/V and output stages.

Cheers,

Walt Jung

PS: On the 844, I once used it as an I/V, and the biggest limitation was/is the output stage. Sounds better by going around it, but then of course one is only using half the chip. Drive a clean buffer from the 844 pin 5, and it is a whole different matter.

Hello Walt,

nice to meet you again.
Regarding the AD844 output stage, that is what we were thinking about - to connect discrete diamond buffer on TZ pin (5). Also would like to try AD829 and AD744 with discrete buffer, but as voltage amplifier, not I/V.

I am quite interested in your "composite line driver" - have you continued in this work since the circuit was published?

Best regards,
Pavel Macura

Jocko I know the story.
Don't believe a word of it.....
[joke]

Great to see your posts here, Mr. Jung.

Based on the measurements I’ve done using AD844 open loop I can confirm that there is a problem with its output stage since I’ve found it adding about 0.5% (at 2V RMS and 1kHz) to the overall open loop THD figure which is quite poor for the diamond buffer, even taking into account relatively low bias current. I hope you can, being the insider, shed more light on the reasons behind.

Funnily enough, its subjective performance is still not bad at all and I clearly liked this chip better with than without this embedded buffer at signal path, and the following device I’ve used had relatively high input impedance (100kOhm). I’ve also tried external (discrete JFET) buffer against the embedded one and while I do think the better buffers can be done, it is still not the trivial task.

All the above assumes AD844 with no feedback applied. With feedback and when used like an I/V converter AD844 wasn’t that great and was certainly inferior to LM6181 (also CFA). IME OPA627 is soundwise better than both.

Pedja

Then there is the story of the Alexander-style amps I helpded a buddy "design".

One of the guys who heard it, and hated it, said "Hey, I know where I heard this sound before........it was that horrible CD player you made a long time ago." Funny thing is.......all the subsequent versions all had the similar characteristic, but I will be the first to admit I have no idea why.

At one time, I worried about that sort of thing. Too old to are now, right Elso?

BTW.......the production version of the amp sounded about the same way as well, but I was out of the loop at that point. Lots of my industry buddies said "Hey, you are friends with so-and-so. Someone needs to tell him his new amp sux."

I kept my mouth shut. Probably the only time in my life!

Jocko

I'm not an insider at ADI, really never was, even when I worked there. Barrie Gilbert designed the AD844.

CFAs with husky outputs can do well, the 811 and 815 come to mind.

Yes, you can run the 844 as a DAC terminator, with roughly a 50R Zin. The current IO of the DAC becomes reflected at pin 5, so an R from 5 to GND recreates a voltage. R > 50R makes for gain. You still need a buffer to drive any useful loads, however.

Hello PMA. The buffer at pin 5 of the 844 will be a challenge. I suggest low-C high current xstrs like PN2907A and PN2222A, running the ouput on the warm side (several mA). I'd *not* use the parts spec'd on my website, unless you are really careful. The 2N6715/27 have appreciably higher C than do the above types. See: http://waltjung.org/PDFs/WTnT_Op_Amp_Audio_2.pdf for the general idea.


The composite driver has an entire history documented on my website. See: April, 2006: 'A Collection of Composite Line Driver/Amplifier Articles for Audio Uses' on the Classic Articles page. The most recent is the latest, and is within Ch 6 of the ADI book.

Enjoy,

Walt Jung

So you did try it in the conventional (feedback) manner, and were not pleased with the results. When used in the open-loop manner, you have basically created the I/V stage I made, but in chip form.

Jocko

Mmmm... I have a sudden impulse to try the AD815 for I/V.

_________________
Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein

Hi Mr. Jung, it seems you are not a fan of ceramic capacitors for decoupling. Is this for only analog, or do you dislike them for digital use also? I can easily see why they are bad for signal path applications but they seem to be very suitable for decoupling applications. Would you recommend the use of something like SMD film (PPS) instead for high speed decoupling?

I tried. Got a red hot chip from oscillating and no sound....

Is there a capacitor across the feedback resistor? Current feedback op-amps are unstable with this capacitor.

Read the Walt Jung article. Walt has a trick with it can be done.
Link in this post:
http://www.diyhifi.org/forums/viewtopic ... 2594#12594

Aren't decoupling caps in the signal path, too? If not, why are they there?

Yes, I would recommend the chip style PPS caps, as I have used them succesfully. As long as you are careful soldering, of course.

I've heard that story before about digital bypasses not mattering. Right! (wanna buy a bridge?). Try taking out a few high-K ceramics and replacing them with PPSes of the same value, and see what you think. There is *no* advantage that I can see for a ceramic vs. an equal value PPS in chip form, other than price.

Walt Jung