INA 163 low noise inputs
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Samuel Groner
Well-known member
Thanks for correcting me. Not sure I truly understand this - if I look at the AD797 datasheet (o/l gain curve), it seems like a global feedback problem to me, no?
Samuel
bcarso
Well-known member
[quote author="Samuel Groner"]
Samuel[/quote]
Well of course I could be full of Sh*t :razz:
Actually, looking at it just now, iirc the new datasheet is quite a bit more detailed than the one I saw, which merely had the discussion (retained in the new one) about the lack of damping with the low r sub bb' input transistors. The inclusion of the open loop gain/phase plots I don't recall from the time I used the parts (about eight years ago).
So---it looks like the effect of the parasitics is to boost the open loop gain, effectively, while the rest of the amp is going over the cliff in terms of stability margin. So my analogy to grid stoppers is somewhat bogus, at least inasmuch as the tube situation is a local oscillation and not a global one.
I used the parts with a gain of +2.00 (+6.02dB), and as I say added to the fun by having additional phase shift, however small, in AD811 buffers so I could drive some heavy loads. I devised a complex compensation network empirically, and the beast was still on the verge of getting tetchy.
Thanks for correcting me. Not sure I truly understand this - if I look at the AD797 datasheet (o/l gain curve), it seems like a global feedback problem to me, no?
Samuel[/quote]
Well of course I could be full of Sh*t :razz:
Actually, looking at it just now, iirc the new datasheet is quite a bit more detailed than the one I saw, which merely had the discussion (retained in the new one) about the lack of damping with the low r sub bb' input transistors. The inclusion of the open loop gain/phase plots I don't recall from the time I used the parts (about eight years ago).
So---it looks like the effect of the parasitics is to boost the open loop gain, effectively, while the rest of the amp is going over the cliff in terms of stability margin. So my analogy to grid stoppers is somewhat bogus, at least inasmuch as the tube situation is a local oscillation and not a global one.
I used the parts with a gain of +2.00 (+6.02dB), and as I say added to the fun by having additional phase shift, however small, in AD811 buffers so I could drive some heavy loads. I devised a complex compensation network empirically, and the beast was still on the verge of getting tetchy.
Samuel Groner
Well-known member
Thanks, clearer now!
Samuel
Samuel
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Guest
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[quote author="TedF"]
We used to bury a dynamic mic in the middle of blankets, seal the box and put it in the cupboard under the stairs!
.[/quote]
Please tell me you got the idea for that from a certain studio up the Holloway road
We used to bury a dynamic mic in the middle of blankets, seal the box and put it in the cupboard under the stairs!
.[/quote]
Please tell me you got the idea for that from a certain studio up the Holloway road
No, :shock: but it was just a little after Holloway Road.... Joe's blankets were hung on the walls; spaced away with egg-boxes.
I was developing mic amps for a console for a sound stage in Malta; they wanted some specially quiet mic amps for distant micing with dynamics. The FET front end amp sounded great, but it did not like transients! I think I recall that we ended up with a very simple 'ring-of-three' design with switched gain. Most stuff was class-A in those days, and with 24V single rail. (this was long before low-noise op-amps.)
I was developing mic amps for a console for a sound stage in Malta; they wanted some specially quiet mic amps for distant micing with dynamics. The FET front end amp sounded great, but it did not like transients! I think I recall that we ended up with a very simple 'ring-of-three' design with switched gain. Most stuff was class-A in those days, and with 24V single rail. (this was long before low-noise op-amps.)
