FETs are horribly nonlinear things, and you're proposing to drive one into the low impedance load of a valve's cathode. By the way, the all-valve equivalent of this circuit is known as a cathode-coupled stage and was popular in oscilloscope circuitry. That doesn't mean it's linear, just that it can be made to have wide bandwidth. There's not a lot of point in making the 6922 linear by loading it with a CCS if the signal driving it isn't too good. If you were to use a valve cathode follower to drive the grid of the second valve, you would have good bandwidth, low input capacitance and distortion worthy of using a CCS load. Do you really need low input capacitance? Can't you just drive the grid directly?
EC, the line stage in my preamp (using a 2N5114 driving into a 6DJ8-type tube) has distortion below the residual of my analyzer (0.02%THD) at 5V out. This with no loop feedback. That's not what I'd call "horribly nonlinear." Nor is the line stage of a Berning TF-10, with a 2N5461 driving into a 12AT7 cathode- about 0.05% THD.
SY said:
I hear what you're saying, but I stand by my opinion of FETs being horribly nonlinear. I've achieved low distortion results using a FET as the bottom device in a cascode differential pair (0.05% THD+N at 20VRMS between anodes), which had me quite excited until I realised that a cascade of valve differential pairs would have given more gain and less distortion. I'd happily use your line stage, but 0.05% for a line stage is a bit much. BTW, is it a cathode-coupled stage?
The Berning schematic is here: http://www.davidberning.com/tf-10.htm
The stage I'm using has the same topology, but different (I maintain better) tubes and FETs.
Thank you, SY! FETs make superb current amplifiers. In many situations, a combination of tubes and FETs yields a better result that either device type alone would.
I'm generally leery of FET voltage amplifiers. I would consider a JFET (never a MOSFET) for voltage amplification only in special circumstances. Allen Wright has been quite successful with JFETs at the bottom of cascodes in phono stages. The impact of the FET is exactly what one hopes for, an improved S/N ratio that allows LOMC cartridges to be used without step up trafos without the introduction of unpleasant SS sonic artifacts.
Those of us with right-click buttons download the images, then flip them.
I've used this topology in my preamps for... oh, my, at least 25 years now, going back to the days before the TF-10 was commercial (we built prototypes into PAS-3 chasses). There are some circuit enhancements to improve performance, not the least of which is using different tubes and FETs, but the basic topology is quite sound. I noticed that Audio Research ripped it off a few years later in the SP11, though they biased it differently presumably to get around the claim language in Berning's patents.
Eli, I've also had good luck with FETs at the bottom of a cascode; I used this in the first hole of my phono stage, but was not thrilled with the performance in other stages.
I've used this topology in my preamps for... oh, my, at least 25 years now, going back to the days before the TF-10 was commercial (we built prototypes into PAS-3 chasses). There are some circuit enhancements to improve performance, not the least of which is using different tubes and FETs, but the basic topology is quite sound. I noticed that Audio Research ripped it off a few years later in the SP11, though they biased it differently presumably to get around the claim language in Berning's patents.
Eli, I've also had good luck with FETs at the bottom of a cascode; I used this in the first hole of my phono stage, but was not thrilled with the performance in other stages.
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