I know that Studer used something like this with an external PP output stage in their first solid state A-80's. Worked OK, compared to the next generation A80 that used another approach. I showed Willi Studer the measurement comparisons, myself. (however, little was done about the situation)
Scott, I really don't know what you are referring to, when you denote fet input op amps, with regard to my opinion.
As you know, I prefer fet input over bipolar input in many applications. However, I use the AD797's in the input stage of my ST1700B analyzer, because they are so quiet, with great success. In fact, I helped Dick Sequerra do the same thing a few months ago.
I find that there is a difference when using IC's for measuring equipment (where they work well) and for audio, (where they sometimes fail to sound right). I learned this by relying on you, Scott, and your jfet input op amp, the AD712, and unfortunately, having to remove it from the input of my first Parasound power amp effort, when reviewers heard a problem. I got the criticism of my boss, for putting it in, in the first place, and trusting that it would sound OK. And I had to make an alternative version that we still use today for Parasound power amps. Once bit, twice shy, they say! '-)
As you know, I prefer fet input over bipolar input in many applications. However, I use the AD797's in the input stage of my ST1700B analyzer, because they are so quiet, with great success. In fact, I helped Dick Sequerra do the same thing a few months ago.
I find that there is a difference when using IC's for measuring equipment (where they work well) and for audio, (where they sometimes fail to sound right). I learned this by relying on you, Scott, and your jfet input op amp, the AD712, and unfortunately, having to remove it from the input of my first Parasound power amp effort, when reviewers heard a problem. I got the criticism of my boss, for putting it in, in the first place, and trusting that it would sound OK. And I had to make an alternative version that we still use today for Parasound power amps. Once bit, twice shy, they say! '-)
When it comes to IC's, I can't get more than a 'B' rating using IC op amps exclusively. I have certainly tried hard enough. Good: connectors, board layout, passive parts like caps and resistors, thick aluminum shielding, super power supply regulation, etc., etc. Ticked my boss off. He wants 'A's' from me, not 'B's' from my efforts, but it was not to be, no matter how hard we tried. After all, the JC-1, and the JC-2 were 'A's', why not the JC-3? Well, guess what, every input IC that I tried sounded different. I settled for a device sounding as good as any other and cost effective as well. I use no ADI parts in the JC-3. However I did design an AD825 into a power amp fairly recently, with great success. Can anyone figure out why?
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.
Why it was successful or why you designed it into a power amp?
Maybe this -
"High output drive capability
Drives unlimited capacitance load
50 mA min output current"
Or this -
"High Speed
41 MHz, -3 dB bandwidth
125 V/µs slew rate
80 ns settling time"
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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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).
I have found these to work well for line level applications.
http://www.analog.com/static/imported-files/data_sheets/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?
http://www.analog.com/static/imported-files/data_sheets/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?
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.
Hi,
Yes. Undegenerated bipolars are really bad...
Plus, it is in normal operation quite substantially degenerated...
Ciao T
Yes. Undegenerated bipolars are really bad...
Plus, it is in normal operation quite substantially degenerated...
Ciao T
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.
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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