The Heater Supply
Using a 24VAC wallwart to attain a higher 85V B+ (as runeight discussed) coupled with support of 6.3V and 12.6V
heated tubes presented an early hurdle for the project. With no center-tap, this meant deriving these low voltages from an approximately 35VDC
supply. A transportable amp implies being carried about, and this posed both size and heat constraints. Conventional linear regulation, such as a power resistor followed by a linear regulator, could not be employed.
Our solution is below…. A switch-mode heater supply, which as far as I can tell, could be a first for DIY headamp projects (although not without precedent for DIY audio). Reasons for this may be: 1) common, low-cost switcher implementations (without ripple filters) are less desirable for audiophile applications; and 2) early, lower-frequency devices require larger passive components and could present issues too close to audible range.Edit: We found that C3H needed to be a larger value
to eliminate very faint hum when using 6.3V/high draw tubes w/efficient cans.
My desire to avoid SMD solutions, especially for prototyping, drove us to National’s (150kHz) LM2595
step-down (buck) switching regulator. Listening to this implementation in the POC with ripple filter (L1H / C1H), I was unable to detect any audible differences between SM heater and use of a battery for heater supply. The significant ripple reduction we see through use of a post-ripple filter is similar to that seen in figure 17 on page 20 of the LM2595 spec sheet
National’s Simple Switcher tools were used to drive associated component values and selection. The design uses 0.8A parts and supports up to 600ma 6.3V or 12.6V heaters. In feedback, R1H (9k) alone yields 12.6V and switching R3H (7k) in parallel with R1H yields 6.3V. The LM2595 seems a good fit and requires no heatsink
in our implementation. Builders will want to follow the BoM components here, e.g. use of low impedance caps, and proper choke and diode values and ratings.
Higher frequency (260kHz+) SMD switchers could be considered in a future revision to save even more space.
This heater circuit coupled with runeight’s use of capacitance multiplier filters in HV and LV sections makes for a small, high-quality power supply.