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After playing with DHT preamps (26, 71a, 4P1L, 46), I ended up staying with a CX301a version of it. I liked its warm sound and tone so I decided to stay with it. It’s been over 2 months so far that I haven’t returned to my precious 26 preamp (I will probably revisit my 26 preamp based on latest updates from Andy Evans).As I’m working on a version of the 4-65a SE amp by Michael Koster, I started investigating options and ways of improving the 01a as will be the first valve of my amplifier.
Using curves plot with my curve tracer and Dmitry’s composite model, I got a very well matched spice model for the 01a. I modelled in LTSpice two versions of the gyrator loaded preamp with filament bias and Rod Coleman’s superb filament regulator: 1) Version 1: Using Anatoly’s PNP-FET Gyrator
2) Version 2: using classic cascoded-FET gyrator with mu-follower output.
Interesting results came out of this first simulation test. I think I haven’t managed to optimise (again) Anatoly’s gyrator despite have managed to keep good VDS (38V) to keep the CoSS low. I get slightly worse THD (0.03% versus 0.016%) compared to the MOSFET version. Looking at output impedance, version 1 has 10K @1kHz whereas the version 2 can get as low as 734 ohms but impedance raises significantly at low frequencies (70k version 1 versus 80k version 2). This can be reflected in the frequency response where version 1 performs better than version 2. Output impedance (Version 1)
Output impedance (Version 2)
At high frequency both preamps perform very well up to 1MHz despite version 2 performing slightly better. Frequency response (version 1)
 Frequency response (version 2)
 At this point, it looks like if output capacitor of version 2 is increased in value to improve the LF rolloff, it seams that overall performs better than version 1 as THD and output impedance are better?
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