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Old 21st January 2013, 03:13 PM   #31
SY is offline SY  United States
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No, the input impedance is 100k if that's the grid input resistor. The grid becomes a virtual ground.

Agreed, a pentode is a better choice, but revintage's challenge was for an ECC88.
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Old 21st January 2013, 03:40 PM   #32
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As I explained, I meant a simple inverting triode stage with plate to grid feedback resistor. It's an anode follower, strictly speaking, only if it's unity gain.

100k input resistor, 400k plate to grid resistor. I'd CCS load the plate and use an LED in the cathode, but there's a lot of other options to get the open loop gain up to near the mu of the tube.

So... we have the same output impedance, same distortion, high input impedance, but no opamp and supplies.
If you refer to the Boegli articles he showed that the anode follower was useful because you could have other than unite gain with the circuit. The anode follower with unite gain is a fish out of sea water so to speak. The cathode follower thus gain favor in use in tube circuits. The anode follower is helpful if it is used as other than unite gain . As Charles' article showed .
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Old 21st January 2013, 03:50 PM   #33
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I haven't seen the Boegli article, but in my own designs, I use cathode followers; I named that as one of the options here as well.

This exercise was just for the benefit of revintage- and, of course, the children. Single stage, gain of 4, inverting, using an ECC88.
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Old 21st January 2013, 06:17 PM   #34
Cassiel is offline Cassiel  Libya
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My preamp is an anode follower: -25 dB NFB, gain is 2, output Z is 260 Ohms. Input Z is 100K, high enough for today's standards. I don't know what revintage means by limited bandwidth.
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Old 21st January 2013, 08:04 PM   #35
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I don't know what revintage means by limited bandwidth.
Gain of the stage is 4, so the effective Cin is 4*Cag + Cgk. Cag is about 1.4pF, Cgk is about 3.5pF, so the 100k input resistor feeds an effective capacitance of about 9pF. Using f3 = 1/(2piRC), the stage has a -3dB frequency of about 175kHz. Fine for audio, maybe less so for video.
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Old 21st January 2013, 08:11 PM   #36
Cassiel is offline Cassiel  Libya
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the stage has a -3dB frequency of about 175kHz. Fine for audio, maybe less so for video.
Mine is 154kHz, I think I'm fine.
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Old 21st January 2013, 11:24 PM   #37
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Hmh, let me briefly go back to the beginning of the thread since it seems no one mentioned one possibly obvious view of revintage's circuits...

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You do realise that the whole point of negative feedback is that a resistor has much less distortion than an active device? Putting active devices in the feedback chain seems perverse.
Actually, how about using the same type active device to provide an equal-but-oposite nonlinear function by putting it into the feedback loop, thereby implementing an error cancellation scheme? There is prior art (supertriode)...
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Old 21st January 2013, 11:30 PM   #38
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No, the input impedance is 100k if that's the grid input resistor. The grid becomes a virtual ground.
Only if the gain of the triode was 'infinite' as it is, it's closer to 20 or so without NFB - so the input impedance is higher than 100k. Also, with a 400k feedback resistor the gain will be lower than 4 for the same reason. Still, this does not change appreciably the principle that is being shown.
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Old 21st January 2013, 11:52 PM   #39
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Actually, how about using the same type active device to provide an equal-but-oposite nonlinear function by putting it into the feedback loop, thereby implementing an error cancellation scheme? There is prior art (supertriode)...
That was actually commercialized some years back (maybe the 1980s?). Can't remember the name of the company- they did things like put multiple solder joints, opamp buffers, and phono cartridges in feedback loops.

Open loop gain with a CCS load and a 400k feedback resistor (as load, not returned to grid) will be quite close to mu (35), so the input impedance won't be much higher than 100k- but as you point out, it WILL be a bit higher and gain will be a bit lower than 4.
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Old 22nd January 2013, 12:04 AM   #40
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Hmh, let me briefly go back to the beginning of the thread since it seems no one mentioned one possibly obvious view of revintage's circuits...
Really? My joke on the first page covered this aspect. :-)

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Actually, how about using the same type active device to provide an equal-but-oposite nonlinear function by putting it into the feedback loop, thereby implementing an error cancellation scheme? There is prior art (supertriode)...
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