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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 1st December 2011, 12:04 AM   #18551
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Quote:
Originally Posted by wayne View Post
Same here John.
Try this for an 'A' :
http://www.intersil.com/data/fn/fn475.pdf
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Old 1st December 2011, 12:13 AM   #18552
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by PMA View Post
I have had the same experience for ten years of experiments, even with fast opamps. Anyway, as a rule, FET input, low noise, low distortion and SR >= 50V/us were the best sounding, with several exceptions like AD844.
I have a hypothesis, FWIW - undegenerated bipolar inputs. I've recently played with AD847 as it turned up in a DAC I bought. It ended up sounding not bad at all - when I checked the datasheet its input LTP has emitter degeneration. AD844 doesn't have LTP input, despite being bipolar.
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Last edited by abraxalito; 1st December 2011 at 12:17 AM.
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Old 1st December 2011, 01:08 AM   #18553
scott wurcer is offline scott wurcer  United States
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John Curl's Blowtorch preamplifier part II
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Originally Posted by john curl View Post
It seems to be related to differential phase.
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Old 1st December 2011, 01:23 AM   #18554
Joachim Gerhard is offline Joachim Gerhard  Germany
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Level dependent phase shift ?
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Old 1st December 2011, 01:35 AM   #18555
john curl is offline john curl  United States
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Yes, Joachim, Video is VERY sensitive to it, so it is measured and on the spec. sheet. Guess what, the AD825 is measured and put on the spec. sheet, but it is fairly lousy compared to many other video amps. The AD797 does not have this measurement on the spec. sheet. Perhaps Scott can give us the measurement value. This is the heart of the matter. It is a standard test (video).
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Old 1st December 2011, 01:41 AM   #18556
CG is offline CG
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I have found these to work well for line level applications.

http://www.analog.com/static/importe...eets/AD845.pdf

For you fans of it, look at the differential phase and gain performance.

Also, look carefully at figure TPC 7. Some possible clues there.

If the impedances at the inverting and non-inverting inputs aren't pretty close to being the same, what happens with these devices when used as non-inverting amplifiers?
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Old 1st December 2011, 01:54 AM   #18557
john curl is offline john curl  United States
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CG, the AD845 does look good, in fact, very good, relative to differential phase. AND it only has an open loop bandwidth of 100 Hz or so. Interesting.
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Old 1st December 2011, 01:59 AM   #18558
Joachim Gerhard is offline Joachim Gerhard  Germany
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http://www.ti.com/lit/an/sloa040/sloa040.pdf
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Old 1st December 2011, 02:12 AM   #18559
ThorstenL is offline ThorstenL  Germany
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Hi,

Quote:
Originally Posted by abraxalito View Post
I have a hypothesis, FWIW - undegenerated bipolar inputs. I've recently played with AD847 as it turned up in a DAC I bought. It ended up sounding not bad at all - when I checked the datasheet its input LTP has emitter degeneration.
Yes. Undegenerated bipolars are really bad...

Quote:
Originally Posted by abraxalito View Post
AD844 doesn't have LTP input, despite being bipolar.
Plus, it is in normal operation quite substantially degenerated...

Ciao T
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Old 1st December 2011, 03:12 AM   #18560
john curl is offline john curl  United States
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I just had a long talk with Ron Quan, the guy who did the latest PIM article for AES.
He gave me the rundown on what is important in a video signal, and why many video amps are different from audio amps. He also measured the AD797. It came out pretty good in differential phase.
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