[quote author="kiira"][quote author="pstamler"]
Not well. The low-impedance output comes from the 100% feedback on the cathode follower, which reduces the output impedance by the same factor as the feedback. If the gain without feedback was 20dB, then you'd reduce the output Z by a similar factor, 10x. So far so good.
But think about what the gain without feedback would be if the tube was operating into 600 ohms. It would be quite low, and so the feedback factor would also be low, and so the impedance reduction (and distortion reduction) would be low. So you'd get an output that was higher impedance than you want, and higher distortion as well.[/quote]
I see. So the unsuitability for driving a load is a function of the design itself and cannot be gotten around. [/quote]
Well, yes and no. As PRR points out, you could use an 8k:600R transformer and drive a load nicely, but then your gain is less, about 11dB less if I calculate right. So your overall gain would be about 9dB if the original gain was 20dB. You could get around that by using something like a 12AX7 or, better, a 6SL7 as the first stage, which would bring your gain up to something reasonable.
However, you'd run into clipping problems. Assuming you want to put out +20dBu into 600 ohms; given the tranny, that means your cathode follower needs to be putting out about +31dBu. In my experience it'd be well into clipping by then.
Here's a possibility: see if you can find a 4k:600R tranny. That has 3dB less loss, so now you only have to put about +28dBu into it. Difficult but not impossible; to drive the lower impedance load, you'd put the two halves of the 5687 in parallel, using a second tube (again, I'd suggest a 6SL7) for the voltage amplifier.
Juggling like this is why push-pull outputs (as used in Ampex recorder outputs) and totem-pole circuits got popular.
Norman Crowhurst has an excellent explication of this in his "High Fidelity Circuit Design". (I basically stole his.)
Is that a book Paul? I have read all the stuff by NC that audioxpress has on their site - he explains things pretty clearly.
Yes; looking at the copy on my shelf I discover that in fact Crowhurst co-authored it with George F. Cooper; it was published by Gernsback in 1956, and was (at least in part) composed of reissues of articles they wrote for
Radio-Electronics, which later became
Electronics World, merged with
Popular Electronics and more-or-less disappeared. Look on
www.abebooks.com for a used copy -- and good luck. Or use interlibrary loan -- I'm sure there's a copy out there in a university library somewhere.
Peace,
Paul