tubo wrote:
Before you just go off and start substituting things you should stop and consider what you want out of the buffer. You also need to define what you mean by "better". The buffer of the 4S, used here, was actually designed as a driver stage for a Baxandall tone stack with some very stringent design requirements (for load, Zo, and distortion). And against those requirements, it performs quite well. In this application, as an output buffer for the 4S, it also performs well given very low additive distortion and having very low intermodulation distortion given the potentially large input voltages coming from a firmly driven 4S preamp stage (which has very high gain as a preamplifier).
I know that this was indicated about what was posted:
ILoveHiFi wrote:
WTF AMPS one have much smaller cathode resistor and is pushing much more current through the 12au7. This allows for less distortion and heats up the valve more, lower distortion + more warm and postive valve sounds.
I find this take interesting, but rather speculative. It is a common misconception that higher current is simply a "good thing" regardless; but this is not a hard rule by any means. The WTF stage is biased to about 8.2mA at Vp=124v (Pp≈1.0W). Here, µ≈19.5 and rp ≈ 7.5kΩ giving an rb/rp ratio of about 2.0. This is enough to allow the feedback to fold-back the high distortion from the low rb (15.3kΩ) by about a factor of ≈12.5. Natural distortion of the WTF stage is about 4.3% so with feedback the CF stage distortion is about 4.3/12.5 or 0.35%. This is OK but not great for a CF buffer. However, the input voltage limits of the stage will still be rather low. And the amplification will be about 0.92 (≈ -0.7dB this is reasonable).
So the buffer you posted is ok, but not great as CF buffers go. But again, you need to define your desires for the buffer. Only once you've done that, can you determine which design will better meet your goals.
Just my thoughts on the matter. YMMV.
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