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Old 19th January 2013, 06:59 AM   #121
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Quote:
Originally Posted by georgehifi View Post
I wished they had old school multibit dacs though, then I would look/search for them s/h.
I find it particularly ironic that they don't, given Charles' staunch anti-feedback stance. He lets it under the radar when its hidden inside digital chips
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Old 19th January 2013, 07:14 AM   #122
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yes I've had an exchange with him where he denies local degeneration is negative feedback, and in the next line, same post, implicitly uses Blackman's Theorem to answer another question

hard to understand the position - is it bald faced lying to keep his marketing message intact?
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Old 19th January 2013, 07:22 AM   #123
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No, I'd not say lying, just pretty common cognitive dissonance management. Never underestimate the power of denial.
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Old 19th January 2013, 12:22 PM   #124
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844 is already quite linear, yet isnt the best one on measurements. You could either wrap it with feedback (the linear circuit plus mild feedback approach) or make it beefier to accomodate larger current swings as they did in ayre.
Third option is to make it discrete and thus have a control on its current biasing and fanout.
4th option is to implement some distortion management circuitry...
Or just leave it alone and use proper dac with low current swing. Biasing the input current round zero is essential.

Dont forget that with no-nfb thing you have lots of passive filtering options.
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Last edited by s3tup; 19th January 2013 at 12:25 PM.
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Old 19th January 2013, 12:27 PM   #125
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Quote:
Originally Posted by georgehifi View Post
Here is a little from the now obsolete AD846 datsheet also can be used as current feedback opamp, seems it was the forerunner to the AD844, also had a TZ type pin 5. but was mainly used for bandwidth limiting compensation or as a low mA output point but they don't say what the output impedance is at that point either, looks to be similar architecture though.

Cheers George
I've seen used AD846 on ebay if they are that desirable.

So, what's left to do with the AD844 circuit? Is someone going to make a PCB for a group buy or something? I'm not sure if I'm interested myself, just curious.
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Old 19th January 2013, 12:31 PM   #126
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Why would you want a pcb for 8-legged dip device?
Especially with so many options of implementations available?
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Old 19th January 2013, 01:41 PM   #127
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Quote:
Originally Posted by s3tup View Post
Why would you want a pcb for 8-legged dip device?
Especially with so many options of implementations available?
Then don't do it if you are not interested.
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Old 19th January 2013, 09:23 PM   #128
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Thinking of trying this after, either from the AD844 TZ pin or from it's output buffer.
If I come from the TZ pin so as not to load it down because it has such a weak output will I have to remove R4 C2 & R6 on the attached buffer?
If I come off the AD844 output buffer I can leave R4, C2 & R6 in place?
Oops think we've gone over this a few weeks back, a little hung over this morning, big night.
Cheers George
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Last edited by georgehifi; 19th January 2013 at 09:30 PM.
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Old 19th January 2013, 09:44 PM   #129
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Hey George,

You're aware of this thread?

DIYHiFi.org • View topic - I/V convertor output buffer

I have these boards as well and I'm getting high dc offset: +50 and +80mV.

How about you?

Using the trimming circuit will give you less offset but the FETs aren't well matched so the performance will suffer.

My upgrade path will be better matched 246/107 FETs then a more full Cascode.

Cheers,
Jeff
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Old 19th January 2013, 09:59 PM   #130
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Thanks Jeff if it all works fine then yes there are matched pair/quads I can get off ebay, just need to know first if it all works out and if the total gain is enough and if the dc trim pot works as well, as I have something against coupling caps.

Cheers George
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