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Old 18th May 2008, 11:13 PM   #3361
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Default EC

Hi Forr,

Now, connect the non-inverting input of the output stage to node Y, instead of X. See what I mean?

Cheers,
Edmond.
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Old 19th May 2008, 05:05 AM   #3362
jcx is offline jcx  United States
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I was “inspired” by a recent comment by Scott Wurcer in the Blowtorch thread:

Quote:
Originally posted by scott wurcer

>Scott,
>
>In a sense, that 797 is too smart for its own good. How many >application designers realise/understand that capacitive error >correction?
>
>Jan Didden

In the beginning I never knew why anyone would want 1nV at gain of one. The part was designed for ATE and other instrument use, the rest was for fun. The rbb on the input devices was reduced below a critical threshold for the ft on this process, the "10 turns of wire on a resistor" input snubbers work best but marketing forbid this on the data sheet.


BTW I got the brush-off at the AES conference from DR. L, I guess I wasn't an insider. Also someone tracked me down to say that the distortion neutralization "had to be just feedback in disguse". Where have I heard that before?
I highly respect Scott's work but I am “one of those people” – My belief is that the AD797 distortion cancellation scheme is an interesting local feedback circuit that increases loop gain, as detectable by appropriate measurement of Bode's return difference T

I believe this sim captures the topology of the AD797 as presented in the data sheet description of the error correction scheme – obviously the transistor models and bias are crude approximations but the circuit appears to have plausible performance

in order to plot comparative graphs the circuit is duplicated and the Tain bilateral Loop Gain Probe is inserted in each

green is the measured loop gain with the 50 pF Cn distortion cap in place, red is with the value reduced to negligible level – both circuits with external feedback setting Av=10

over the ~1KHz to 1MHz region Scott's “distortion cancellation by simple subtraction” is readily visible as an added 20 dB of loop gain

the usual cost of higher loop gain is the reduced phase margin where the gain has to roll off faster than 1-pole, in this instance the cost is very neatly compensated by the lower slope of the gain curve just past the 0 dB gain intercept (higher intercept frequency with the distortion cancelation C helps too)

which isn't exactly the same as the AD797 data sheet claim: "Therefore, the distortion cancellation does not affect the stability or frequency response of the amplifier"

Click the image to open in full size.
the circuit was a pain to get started - bias point calculations seem to fail more often in LtSpice with very high gain circuits
I reduced Tools>Control Panel>Spice tolerance settings until some bias point was found, then I used the .savebias/.loadbais directives to get it to start up with default Spice accuracy settings
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Old 19th May 2008, 11:06 PM   #3363
forr is offline forr  France
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Default Re: EC

Quote:
Originally posted by Edmond Stuart


Now, connect the non-inverting input of the output stage to node Y, instead of X. See what I mean?
Hi Edmond,

You may think of the influence of the non-linearities of the input of the output stage on the source. Y having a lower impedance than X, you should mean lower distorsion.
Since Jan's publication, I started to read the whole thread with greater attention than previously, you may have already written something on this scheme but I did not find it yet.

Regards
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Old 20th May 2008, 06:57 AM   #3364
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Default Re: Re: EC

Quote:
Originally posted by forr
Hi Edmond,

You may think of the influence of the non-linearities of the input of the output stage on the source. Y having a lower impedance than X, you should mean lower distorsion.
Since Jan's publication, I started to read the whole thread with greater attention than previously, you may have already written something on this scheme but I did not find it yet.

Regards
Hi Forr,

Perhaps I was a bit cryptic, but I'm only trying to say that if you move the input of the output stage from X to Y, you get essentially the same topology as I have dropped here last year. For convenience, I'll put it here again. The only difference is that two functions, emitter follower and current conveyor, are combined in only one transistor pair: Q6/Q7. This works fine as long as the base currents of the next stage are small WRT the feedback currents through R10/R11.

BTW, don't build this circuit as there are better or simpler ways to lower distortion. This thing was only meant as an example how to circumvent the need for a the large bias voltage (22V) and boosted power supply for the front-end as in HEC.

Cheers,
Edmond.
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File Type: png ops-ec91-pub.png (44.0 KB, 733 views)
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Old 21st May 2008, 07:26 AM   #3365
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Default Re: EC

Quote:
Originally posted by Edmond Stuart
Hi Forr,

Now, connect the non-inverting input of the output stage to node Y, instead of X. See what I mean?

Cheers,
Edmond.

Yes that will certainly work, but it takes that input buffer/ current conveyor outside the ec loop. The nice thing about Forr's idea was that it also corected errors of that input stage.

@ Forr:
You show the amp and summer blocks as unity-gain connected opamps. That is no requirement; one of the nice things of ec (for me ) is that you can explore low-gain, open loop circuits for the Vas as well as the output stage.

Jan Didden
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Old 21st May 2008, 07:37 AM   #3366
GK is offline GK  Australia
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Hmmm........ Did any of you guys catch this link in another thread?

http://www.onahighernote.com/luxman/?c=4&id=28

Seems Luxman got their own version of "EC".

Cheers,
Glen
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Old 21st May 2008, 09:59 AM   #3367
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Default ODNF

Quote:
Originally posted by G.Kleinschmidt
Hmmm........ Did any of you guys catch this link in another thread?

http://www.onahighernote.com/luxman/?c=4&id=28

Seems Luxman got their own version of "EC".

Cheers,
Glen
Hi Glen,

With 70ppm distortion, I wonder if their ODNF circuitry is really worth a patent. Or is it just another marketing trick?

Besides, a claim like this: "This new technology also ensures that need for a DC servo circuit is alleviated, again improving sound quality." makes me even more suspicious.

Cheers,
Edmond.
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Old 21st May 2008, 10:03 AM   #3368
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Default Re: Re: EC

Quote:
Originally posted by janneman

Yes that will certainly work, but it takes that input buffer/ current conveyor outside the ec loop. The nice thing about Forr's idea was that it also corected errors of that input stage.

...............

Jan Didden


Cheers,
Edmond.
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Old 21st May 2008, 10:17 AM   #3369
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Default Re: ODNF

Quote:
Originally posted by Edmond Stuart
[B]With 70ppm distortion, I wonder if their ODNF circuitry is really worth a patent. Or is it just another marketing trick?[B]
It's sure worth a patent if it costs little In Europe it would probably out of the question. In the US very, very small and unsignficant things can be patented.
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Old 21st May 2008, 11:07 AM   #3370
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Default Re: ODNF

Quote:
Originally posted by Edmond Stuart


Hi Glen,

With 70ppm distortion, I wonder if their ODNF circuitry is really worth a patent. Or is it just another marketing trick?

Besides, a claim like this: "This new technology also ensures that need for a DC servo circuit is alleviated, again improving sound quality." makes me even more suspicious.

Cheers,
Edmond.

Hello Edmond

The amp looks like a balanced configuration, this type of amp does not need to use a DC servo like umbalanced amps. But it seems as though they have tossed the no dc servo benefit with the ODNF circuitry which is a bit confusing.

The THD spec is only at 1KHz 250Watts 8ohms interesting they dont quote 20Khz performance .



Regards
Arthur
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