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Re: Ooops.

Posted by Henry Pasternack on March 4, 2006 at 13:06:06

In Reply to: Re: Ooops. posted by BBeck on March 4, 2006 at 10:08:25:

Hmmm. Doing some rough estimates with the 6DJ8, I figure a voltage gain of around 300 for a single-ended cascode with 10mA bias current. It would be half that in the differential amplifier driven from one input. This is compared to an absolute maximum voltage gain of mu = 33 without the cascode. I agree you're throwing away some potential gain running a 50K total load, but you certainly get a lot more gain than you would with a single tube.

With respect to linearity, I agree the steep load line is a less linear one if you need to develop the full output voltage across that load impedance. But assuming the same plate load resistance and AC output voltage, the current swing (and hence the distortion due variation of gm with current) should remain the same. On the other hand the signal voltage on the plate is only about one-third of what it would be otherwise, which should actually lower distortion. It isn't clear to me that you're better off trying to get the same gain with a pair of voltage amplification stages in cascade. Maybe someone can post some quantitative distortion numbers.

The PSRR of the differential amplifier is good if you are taking the output diferentially. But the output stage grids are referenced to ground, so if you have (for instance) LF noise on the B+ supply I believe you may see some modulation of the output tube bias points.

I don't see why you need to provide DC feedback from plates to the tail CCS. The 100K plate resistors to ground "spoil" the current sources and should allow the stage to settle into a stable operating point without any "fighting" between the upper and lower current sources.

I built an amp with a cascade differential driver once and thought it was pretty darned good. I never tried a cascode diff, though, but I'm curious about it, not the least reason being Allen Wright's fondness for the circuit.

-Henry

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