This is what I am playing with at the moment. I tested 1 ohm, 2 ohm and finally 3 ohms R-sense. Since I use a 2:1 stepdown autoformer (L1 and L2 in the schematic) the 3 ohm R-sense behaves like 1,5 ohms in a normal "OTL" source follower amp.
With 3 ohms sensing resistor the bass becomes more prominent and energetic. It is not a big change, but it is easy to hear when I remove the PCF by shorting out R-sense with a short cable and alligator clips. The amp sounds a little more sterile and "hard" with PCF.
I can't make up my mind if I prefer the amp with or without PCF in this case. It is a little bit like a tone control.
I would love to hear a F4 with some PCF. I guess a infinite damping factor or even a slight negative output impedance could make an interesting combination with some speakers which thrive with some vigorous control and muscle.
Positive current feedback does not only change the output impedance (damping factor) of the amp. There is a sonic difference between how the damping factor is created. Excessive negative voltage feedback does not sound the same as a massive brute-force source follower output-stage with lots and lots of parallel devices. PCF is another solution, with its own inherent qualities and character. I recommend everyone to explore them all. It is fun.
Cheers,
Johannes
I agree, although I tend to think it makes more sense using it on a circuit that actually could do with some real help, eg F3 or possibly F6.
F4 and source follower output stages with damping factors around 50 or higher don't need much help if any (at least not on my speakers). I always refer back to Papa's Zen philosophy which is something like a circuit should be as simple as possible but no simpler, for me that means if pcf is the simplest solution to get a damping factor better than lets say 50 then I'll certainly add pcf, if the circuit already has a damping factor better than 50 I'm probably not going to bother. I like dumb circuits.
On the other hand it is a lot of fun trying out new tweaks or ideas and everyone should have fun so I'm not going to preach against having fun, I'm always fooling around.
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"I would love to hear a F4 with some PCF. I guess a infinite damping factor or even a slight negative output impedance could make an interesting combination with some speakers which thrive with some vigorous control and muscle."
Hmm. . Well someday maybe there will be att least one...
/ Tord
Hmm. . Well someday maybe there will be att least one...
/ Tord
Now there's an interesting idea... Although, doesn't the F3 already have a little PCF with the Aleph CCS?
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