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Author Topic: Eddie Ciletti on slew rate, op amps and output transformers  (Read 30527 times)

bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #60 on: December 05, 2006, 03:38:51 PM »

maxdimario wrote on Tue, 05 December 2006 19:49


once you reduce distortion artifacts to -60 dB by using feedback it's obvious that the amp will sound 'neutral' in the sense that you will have very low thd and ruler flat response amongst other things.

unfortunately, there is a fine element which dissapears. Basically the 'liveness' of which I was writing about in the posts above.

An amplifier that takes something away, anything, by any description, is not transparent either. If an amplifier is undetectable it neither adds nor takes away.
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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #61 on: December 07, 2006, 12:13:48 AM »

define 'undetectable'.

the source must be straight from a mic or close to it, and not from a cd, in order to tell..

my favorite test circuits have been mic pres and turntable pres with 50-60's records because of the simple recording path etc.

I never heard a circuit that was undetectable, can you quote me an example?
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dcollins

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #62 on: December 07, 2006, 12:44:19 AM »

maxdimario wrote on Wed, 06 December 2006 21:13

define 'undetectable'.



Undetectable by who?  Or is that what you mean?

Quote:


I never heard a circuit that was undetectable, can you quote me an example?


Max, what about the AD-797?  Looks pretty good on paper.

http://www.analog.com/en/prod/0%2C2877%2CAD797%2C00.html

It's "Ultralow Distortion!"

DC

bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #63 on: December 07, 2006, 02:49:39 AM »

I've not yet heard any IC op amps that fit the description of undetectable (I still need to put the LM4562 to that test), but I've got a discrete circuit that is neither measurable nor audible. Frightening amounts of feedback, btw. Something like 120dB around the output transistors, 70dB overall (at 20kHz). A simplified version was published in AES preprint 6294.
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Jim Williams

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #64 on: December 07, 2006, 11:08:31 AM »

dcollins wrote on Wed, 06 December 2006 21:44

maxdimario wrote on Wed, 06 December 2006 21:13

define 'undetectable'.



Undetectable by who?  Or is that what you mean?

Quote:


I never heard a circuit that was undetectable, can you quote me an example?


Max, what about the AD-797?  Looks pretty good on paper.

http://www.analog.com/en/prod/0%2C2877%2CAD797%2C00.html

It's "Ultralow Distortion!"

DC




That part has been out for years. It's pretty bland sounding. They have a new one, the AD8599 dual which has similar noise specs but greater gain bandwidth. The new BurrBrown OPA211 is also similar in noise, 1 nv/hz/sq and is easy to work with and it has a 55 mhz gain bandwidth, greater than the 797 without the stability issues. A dual version is also in the works.

So now all these manufacturers have a new arsenal of quality opamps to replace the tired old TL07X and 553X opamps. Don't expect to see them. With low bit audio and compromised sonics being the norm, giving up on a 15 cent opamp for a $2 opamp isn't going to happen. I expect these new devices to be used in medical and industrial applications, not audio. The LM4562 is going to be a dissapointment for National which made the extra effort to release the parts in a 8 pin dip and TO-99 can. The rest of these are surface mount only.
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #65 on: December 07, 2006, 11:30:44 AM »

Well I certainly hope they will keep making the LM4562 for some time. Even though I've not yet done an in/out comparison it's been sounding better here than anything it replaced.

The description "bland" fits my experience of the 797 quite well.
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zmix

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #66 on: December 07, 2006, 01:58:41 PM »

Dave,
Interesting that you have brought up both the MC340XX and the AD797 series. I've had extensive time listening to each. Recently I had one of my power amps break down, it's a boutique solid state amp, over designed, current sources on the current sources sort of thing. It turns out that the one opamp in the circuit latched up, sending 60 volts DC to my woofer.
I peeled off the heatsink on the opamp to discover it was an AD797. I replaced the device in each channel with an op amp I prefer the 'sound' of and viola, every annoying harsh glaring quality of the poweramp was gone...  not replaced by euphonic coloration, mind you, just lacking the harsh edgy quality I've endured all those years.

CZ

Terry Demol

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #67 on: December 07, 2006, 05:30:22 PM »

Bruno Putzeys wrote on Thu, 07 December 2006 16:30

Well I certainly hope they will keep making the LM4562 for some time. Even though I've not yet done an in/out comparison it's been sounding better here than anything it replaced.

The description "bland" fits my experience of the 797 quite well.


WRT 'bland' ditto here.

So Bruno, how can -120dB across the audio band sound 'bland'?
Or are we just looking for 'not bland'?

Another experience with the venerable 797 was with mic pre's.
Fully balanced, fixed gain of around 100, head amp for ribbon mic
used for stereo blumelin recording of small classical ensembles.
I didn't use compound stages as the 797 has good  OLG
for closed loop gain of 100.

A balanced discrete zero feedback circuit was much better.
We were -not- looking for colour, and we had the live source
for reference. I was frankly shocked how transparent it was.

cheers

Terry





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Dan Kennedy

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #68 on: December 07, 2006, 07:22:31 PM »

Hi Terry,

Long time, no see.

Zero feedback, as in loop, or local (degenerative) or none of either?
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bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #69 on: December 08, 2006, 04:05:45 AM »

Terry Demol wrote on Thu, 07 December 2006 23:30

So Bruno, how can -120dB across the audio band sound 'bland'?
Or are we just looking for 'not bland'?

That's always the question. Like the most neutral speaker is bound to lose in a quick shoot-out, the most neutral amplifier is also generally found to be rater middle-of-the-road. Only when one listens longer does one hear that the "correct" amplifier does deliver all the goods, it only doesn't emphasise any of it. So to me, "bland" is not always an indication of a problem.

The 797 seems a different shade of bland though. I don't pretend to know how an amp with these figures could sound anything but "totally not there" - I hope to find out some day. I do have some indications though. In a noninverting circuit, several input stage error sources (small though they may be) are outside the feedback loop. One unbalanced line amp I've been toying with recently consisted of a pot followed by a discrete 10x gain stage. High frequency (6kHz) THD was markedly dependant of the wiper position, even if the input amplitude was adjusted to get the same output each time. As could be expected, THD was worst when the pot was half-way (we're talking about -106dB at 4Vrms). The impedance mismatch at the two amplifier inputs made the effect of the nonlinear input impedance of same stand out.

I then rewired the circuit to inverting, placing the pot in the feedback circuit and grounding the noninverting input. Even though this results in variable loop gain, the distortion became nearly constant (around -120dB) up to +20dB gain. Since both amplifier inputs are held at ground potential, the input capacitance is not modulated.

This line stage is now as undetectable as you can get in an unbalanced setting. In its previous state it was, well "bland", and there was a clear loss of detail when it was inserted.

I plan to do some further experiments with IC op amps to see if this finding can be extended to explaining some of the extreme colourations in IC op amps that nevertheless have good specs. In another context, I have been using AD797's and OPA627's as the first stage in an integrator chain (discrete 1-bit converter) and the difference was extremely small. Again, the inputs were used as a virtual short. Anyone who has toyed with these 2 op amps will attest that in normal gain stages there are no two more different sounding op amps around.

Whether that counts as an explanation is still an open question, but these two bits of anecdotal evidence suggest that inverting (virtual-short) circuits are less sensitive to these colourations, giving some indications as to where they come from.
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Dan Kennedy

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #70 on: December 08, 2006, 08:19:11 AM »

I've used inverting stages preferentially for years, with the reason coming from the vague feeling of not so hot common mode performance of many opamps, couple this with a distrust of "distortion cancelling" inputs(after all, how do it know?) and there you go...
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dcollins

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #71 on: December 09, 2006, 01:49:01 AM »

zmix wrote on Thu, 07 December 2006 10:58


I peeled off the heatsink on the opamp to discover it was an AD797. I replaced the device in each channel with an op amp I prefer the 'sound' of and viola, every annoying harsh glaring quality of the poweramp was gone...  not replaced by euphonic coloration, mind you, just lacking the harsh edgy quality I've endured all those years.



That's what I'm getting at.  

Barring abject failures or dumb implementations, how can one opamp be both "bland" and "edgy?"

What did you replace the AD797 with?

An MC34081?

I swear there is some way to quantify these differences nowadays, but there is obviously nothing wrong with running the one that sounds better!

The 797 is also prone to "break into song" in a circuit that was working fine a minute ago....

But I think it can work.  

Am slowly, slowly, turning to discrete versions....must resist....

DC




bruno putzeys

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #72 on: December 09, 2006, 03:26:06 AM »

dcollins wrote on Sat, 09 December 2006 07:49

Barring abject failures or dumb implementations, how can one opamp be both "bland" and "edgy?"

I would indeed strongly suspect that something else (as in "look at the system level") is going on here.
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maxdimario

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #73 on: December 09, 2006, 08:54:16 PM »

bland and edgy...

makes sense to me. clean with no balls and a plastic-sounding top end.. plastic as in plexiglas.

that is the sound of a lot of clean opamps.

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Terry Demol

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Re: Eddie Ciletti on slew rate, op amps and output transformers
« Reply #74 on: December 10, 2006, 05:20:10 AM »

Dan Kennedy wrote on Fri, 08 December 2006 00:22

Hi Terry,

Long time, no see.

Zero feedback, as in loop, or local (degenerative) or none of either?


Hi Dan,

Likewise.

ZFB as in no glob, no interstage, no local (as in cfp etc),
just degen.

You know I know that degen is, strictly speaking, not ZFB Smile

Cheers,

Terry  


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