03-02-2021, 02:10 PM
(02-02-2021, 05:56 PM)Kalee20 Wrote: Apologies - I missed the words 'and isolated' in your earlier post. For me, isolated filament supplies are definitely non-preferred.
I'm very much aiming to have just ONE filament supply. Eventual aim is to have a switchmode converter supplying HT, LT, and GB. And while adding extra outputs is simple in principle, filtering thereof is very much not - and I'm looking at this AF amplifier being part of a rod-pentode radio.
My apologies if I have missed something earlier but, although it was with an RF circuit rather than audio, in the past I did experiment with a RF differential preamplifier using 1j29b's with cathode bias and heaters supplied via switch mode isolation. I did find the noise level limited small signal performance even with fairly good power supply filtering. Obviously this would be less of a problem with lower frequencies and a power amp but yet another nagging issue to be addressed.
I do presume that directly heated valves would be less resistant to heater borne noise than indirectly heated with their greater thermal inertia. Possibly another good reason not to float the heaters, but to ensure a low impedance signal path to ground?
Tracy
