Class A biasing for the AD797? - Page 3 - diyAudio
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Old 27th May 2008, 02:17 AM   #21
jcx is offline jcx  United States
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Quote:
Originally posted by nelsonvandal
About biasing monolithic opamps to class A. It's very easy to try with a resistor, CRD or JFET (maybe not as good as bipolar). I've done it a lot (not with AD797) and thought the world of it... until I tried it live - connecting/disconnecting it while playing. No bloody difference at all! It was all placebo.

BTW, what is the optimal implementation of AD797, resistor/capacitor values?
optimum Cn from my simulations looks to be very "tweaky", you are essentially canceling the internal Ccomp and any device parasitic C at that internal node some of which may be V dependant, hopefully at least the bulk Ccomp is a MOS cap

and the cancellation occurs through the current mirror which likely has some tolerance on current transfer ratio and a high frequency roll off

90% cancellation leads to a 10x greater loop gain around the output stage-Cn local feedback loop and corresponding distortion and output impedance reduction

to get to the ~200x max boost I found in my subsequent sims of the simplified datasheet circuit in the real world seems problematic - you could try to measure the output Z as you trim the Cn cap but you'd need sub mOhm resolution at ~100 KHz - a daunting measurement with nH lead inductance adding ~mOhm/inch of lead wire impedance - even varying position of the gnd ref probe point on a full 2 oz ground plane will make a difference at these levels
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Old 27th May 2008, 01:43 PM   #22
Bonsai is offline Bonsai  Taiwan
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Nice use of a Howland current pump JCX.

Scott, I seem to recall you mentioning that modern NPN and PNP IC process transistors really performed well and class A biasing was not neccessary to get good performance. I guess thnkgs in this area hav e changed a lot sinnce the late 70's and early 80's.
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Old 27th May 2008, 06:09 PM   #23
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Quote:
Originally posted by Bonsai
Nice use of a Howland current pump JCX.

Scott, I seem to recall you mentioning that modern NPN and PNP IC process transistors really performed well and class A biasing was not neccessary to get good performance. I guess thnkgs in this area hav e changed a lot sinnce the late 70's and early 80's.
Well they are now much more complimentary, but as I keep telling JC the world wants low power so most general purpose amps are fairly light on the bias. We have some performance at all costs differential A/D drivers that are different but they are all +-5 or less volts.
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Old 27th May 2008, 07:38 PM   #24
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I just want LOWEST 7th harmonic distortion. How can I modify the 797 to achieve this?
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Old 27th May 2008, 08:06 PM   #25
lineup is offline lineup  Sweden
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Quote:
Originally posted by john curl

I just want LOWEST 7th harmonic distortion.
How can I modify the 797 to achieve this?
Well, you can't
at least not without getting some other severe issues/problems around you neck

An IC, however good it is, is a 'rather' closed system of optimized semiconductors and passives
and AD797 is one of the best no doubt

I would say, it is enough difficult to accomplish a removal of the nasty 7th
in a completely open system = discrete devices
... but this you already know very well Mr. Curl, by experience

I take your enquiry as being a highly retorical one.


lineup discrete hifi regards from the audio basement
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Old 27th May 2008, 09:06 PM   #26
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Quote:
Originally posted by john curl
I just want LOWEST 7th harmonic distortion. How can I modify the 797 to achieve this?
Why don't you ask Bruce Hofer of AP?
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Old 27th May 2008, 11:16 PM   #27
KSTR is offline KSTR  Germany
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Quote:
Originally posted by Bonsai
Nice use of a Howland current pump JCX.
Hhm, I don't see a Howland, rather I see the "slave" opamp as a traditional INIC...

The original Sandman circuit looked like this (see attachment)

- Klaus
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Old 27th May 2008, 11:23 PM   #28
KSTR is offline KSTR  Germany
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And on the thread topic, a good amp like the AD797 might only benefit from class A bias under extreme and suboptimal conditions for it's supply and bypassing by making the load induced supply ripple more signal related that it is in class B (half wave rectified portions).

- Klaus
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Old 28th May 2008, 01:35 AM   #29
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It is best Scott, to ask the actual 'designer' of the AD797 before proceeding on my own. I WILL find out, one way or another, or change to a different part.
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Old 28th May 2008, 03:32 PM   #30
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Quote:
Originally posted by john curl
It is best Scott, to ask the actual 'designer' of the AD797 before proceeding on my own. I WILL find out, one way or another, or change to a different part.
What circuit, what frequency, what load, what level? Not a very well "engineered" question. If -130dB is only good for rock and roll so be it, I'm not of that school.
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