The spec from that ETM review is probably trying to say that at 2V RMS output the overall THD measures about 0.015%. I'm assuming this is at 1khz but it could be a 20Hz to 20khz measurement as well, hard to say.
That being said, you have a 12AU7 in a common cathode gain stage (1st stage) direct coupled to a cathode follower. This is basically the Bottlehead Foreplay (and loads of other preamps designed) from years ago, except that it doesn't have current sources substituting for the plate resistor of the 1st stage and doesn't have a current sink for the cathode resistor on the cathode follower (2nd stage). Replacing those resistors with well designed current sources will improve the performance. C10 on the 1st stage ensures that the stage gain will be high, i.e. 10X or 20dB in this example.
It's hard to say what the max voltage (peak to peak) will be without measuring it directly but I would guess it will easily do 10VRMS (28Vpp) into a high impedance load, for example 50K ohms. Note that the output impedance of this preamp is about 1.5K ohms. An F4 has an input impedance of about 47K ohms so possible a good match.
Hope this helps,
Anand.
That was perfect and amazing, thank you 
I think I'm going to build a really big chassis with lots of room for tube pre projects. I love the Pass projects but always feel as though not having spent enough time with tubes has left a really big knowledge gap I would benefit from filling.
That, and I have numerous 5814/12AU7/E80CC tubes leftover from preamps I no longer own.
Thank you Anand!
I think I'm going to build a really big chassis with lots of room for tube pre projects. I love the Pass projects but always feel as though not having spent enough time with tubes has left a really big knowledge gap I would benefit from filling.
That, and I have numerous 5814/12AU7/E80CC tubes leftover from preamps I no longer own.
Thank you Anand!
Yes, I have had the Minimax for quite some time. It works with the F4, but curious regular headphone amps seem to sound better. Like the DNA Stratus sounds better than the Minimax -> F4 with my modified LCD-4's. Which doesn't really make sense because the Stratus was made for electrodynamics and the F4 should drive the heck out of the LCD-4 if it has what it needs.
I got a Bottlehead Moreplay to try instead but haven't had the chance to yet, I need to extend a shelf on my rack to set it up. Not enough space.
From what I've been told, the Moreplay should have more voltage output and should be a good match for the F4. I'm curious how it would compare to an Aikido with the F4.
The F4 affords that nice flexibility doesn’t it?
My recommendation is to build a test bed using a well designed B+ supply and filament supply. Try Aikido 8 pin and 9 pin versions. Also try Broskie’s other designs (CCDA, etc…).
I’m even curious about 300B based designs like Bottlehead’s 300B Pre, Transcendent’s Masterpiece and Tom Christiansen’s DG300B Pre (with different output transformer turns ratio). And the Aegis of course. The DNA Stratus might sound great to you because it is transformer coupled. Have you popped it open and snapped some pics? Enquiring minds since this is a diyaudio forum. I wonder what vendor he uses for his transformers .
Best,
Anand.
My recommendation is to build a test bed using a well designed B+ supply and filament supply. Try Aikido 8 pin and 9 pin versions. Also try Broskie’s other designs (CCDA, etc…).
I’m even curious about 300B based designs like Bottlehead’s 300B Pre, Transcendent’s Masterpiece and Tom Christiansen’s DG300B Pre (with different output transformer turns ratio). And the Aegis of course. The DNA Stratus might sound great to you because it is transformer coupled. Have you popped it open and snapped some pics? Enquiring minds since this is a diyaudio forum. I wonder what vendor he uses for his transformers
.Best,
Anand.
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