[doobooloo]

I just got hold of a pair of AD797s, but they're not doing any good in my META42.

Basically, there is sound, but on top of 2V of DC.

It's quite perplexing, as I've read threads about simply swapping opamps between AD797 and OPA627 and others in the Emmeline HR-2...

What do I need to do to make it work on my META? Thanks in advance!

 

[digi01]

I have used 797, meet the problem like yours

Ad797 is Cranky Op-Amps like LM6171 of what I had once used,you need to add a small film capacitor inline with the feedback resistor (R4).
theses two articles,inside there is solution.

http://www.tangentsoft.net/audio/hs-opamp.html

http://headwize2.powerpill.org/proje...=meier_prj.htm

BTW:The sound is very fine that 797 utilizes it well, the sound is very fashionable, magnificent , It is sheer to work up the sense!

CHEERS

 

[jeffreyj]

Quote:

Originally posted by doobooloo I just got hold of a pair of AD797s, but they're not doing any good in my META42. Basically, there is sound, but on top of 2V of DC....

It basically comes down to the inner loop producing substantial DC offsets due to its high gain. The impedances seen by each input of the inner loop op-amp need to be matched, especially if bipolar input op-amps are used. Especially, especially if fast bipolar op-amps are used!

It's a pathological problem with having a low loop gain in the inner loop to correct errors (low loop gain means more of the op-amp's Avol is being used, leaving less to correct errors).

 

[tangent]

Right...the DC offset is due to input current imbalances. Unfortunately, I don't think anyones' bothered to try and figure out how to balance input impedances (so the currents get balanced) in a Jung multiloop topology. No doubt it can be done, but someone's got to sit down and do the figurin'.

 

[ppl]

Due to the High Input Bias currents this device is going to have a changing DC offset with Volume control posistion. A DC servo is the only viable solution and this is not acomidated in the META42. Also the AD-797 has only an 18 Volt per uSec Slew rate with High open loop gain. Combine this with the over 80 MHz bandwidth and you invite problems. The AD-797 is great for Mic and Mag Phono But line level stages like a headphone Amp are better served with a FET Input device like the AD-8610