You can share the winding between the rectifier and the 6SN7. This schematic is actually similar to a Bottlehead Foreplay II. I would mention, however, that if you don't bias the filaments for the 6SN7 up around 60v, you will get significant noise. Also, R7 can be tied to the other plate without the R3/C3 combination. With a cathode follower, this just isn't necessary.
A significant difference between that circuit and a FP is the presence of a global NFB loop.
Frankly, I'm not at all comfortable with loop NFB being applied to the I/P grid, without a DC blocking cap. being present. Also, the NFB ratio varies with the setting of the volume control. The value of the O/P coupling cap. is too small. To work into the IHF "standard" 10 KOhm load, a minimum of 3.3 μF. is needed. TANSTAAFL always applies and I suspect that kit is not a good value.
Jim McShane is finally getting to work on a back burnered project that could fill this need nicely. Look here.
Frankly, I'm not at all comfortable with loop NFB being applied to the I/P grid, without a DC blocking cap. being present. Also, the NFB ratio varies with the setting of the volume control. The value of the O/P coupling cap. is too small. To work into the IHF "standard" 10 KOhm load, a minimum of 3.3 μF. is needed. TANSTAAFL always applies and I suspect that kit is not a good value.
Jim McShane is finally getting to work on a back burnered project that could fill this need nicely. Look here.
Yeah, it seems odd to put feedback in a preamp that doesn't have a whole ton of gain to begin with. I try to stay out of the feedback arguments, however, but in this case it makes very little sense. Removing the feedback and that extra RC stage in the power supply ought to make this a listenable preamp. The coupling caps could always be changed later, I see plenty of solid state and tube amps alike with 50+k input impedance now.
-Paul
-Paul
I don't think this circuit is likely to work very well. As pointed out by others, the 10k input resistance will load that 100k pot very heavily. Further, there's not very much global feedback - you either want none or shed-loads of it. I suspect this circuit was developed rather than engineered.
I used global feedback like that at one point on Bruce Rosenblit's original Mu Follower 12AU7 circuit. It worked okay, but the problem is that the volume pot is (part of) the bottom resistor of the feedback voltage divider, so that the amount of feedback you have depends on the volume setting. That's the reason for the 10K additional resistance in the input circuit; it's not a 'stopper', since it's outside the feedback loop, it's there to stabilize the feedback circuit somewhat.
In any case, you don't get really a lot of feedback, about 3dB most (volume control in the middle position). You can leave it, and the 10K resistor out of there, it won't change the sound or drive much. That circuit is much more common than a Foreplay, by the way. It's about nine tenths of all the linestages ever made. It actually has pretty good technical performance, though it can be difficult to dehum. I just don't like the sound of cathode followers.
Aloha,
Poinz
In any case, you don't get really a lot of feedback, about 3dB most (volume control in the middle position). You can leave it, and the 10K resistor out of there, it won't change the sound or drive much. That circuit is much more common than a Foreplay, by the way. It's about nine tenths of all the linestages ever made. It actually has pretty good technical performance, though it can be difficult to dehum. I just don't like the sound of cathode followers.
Aloha,
Poinz
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