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Old 22nd January 2013, 01:13 AM   #41
ilimzn is offline ilimzn  Croatia
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Join Date: Feb 2005
Location: Zagreb
Quote:
Originally Posted by SY View Post
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.
Happily I was not referring to that. The 'supertriode connection' was done by a Japanese guy, don't recall the name, he has a rather well documented website on this, WITH measured curves. The most basic one is connecting a vacuum diode (or diode connected triode) in a plate-to-grid feedback scheme. Of course, you have to choose your diode for appropriate Rp, and this basically means the input impedance of the circuit is very low, but it measures very much like an idealized triode. Of course, one can use the actual triode as an active element implementing a non-linear resistor by providing a few extra components, and... well, you see what I mean, surely.

Quote:
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.
Ah, sorry, missed the CCS bit. BTW that configuration (usually with LED in cathode) is my favorite 'simple line amp' and it does indeed work very well. It does have a 'hidden snag' - the output C forms a high pass filter dependent on the load. The gain of the actual circuit increases as the C and load provide a roll-off as the feedback is taken after the C, compensating the voltage drop on C. As a result distortion increases quite a lot at LF (also, capacitor distortion, which increases as voltage drop on C becomes non-negligible, of course subject to cap type, makes it worse). It is VERY helpful to keep the value of the output C high, to get the cut-off frequency lower, and also keep cap distortion well away from the audio band. Also, although this may not seem obvious at first, an input cap is advisable. It should be chosen for a cut-off equal or higher to the output part calculated for the most pessimistic (low impedance) load. One would wonder why add an extra component, but things become clear if one expects any kind of subsonic content in the input signal. This part of the signal will be amplified much more than useful audio, shifting the operating point of the tube far more than the useful audio signal, increasing intermodulation - sometimes quite drastically and even unexpectedly catastrophically if the amp gets close to clipping (which would only be easily seen if one looked at the signal before the output C!). In such cases it's better to filter out the subsonic component before it enters the amp.
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