The Power Supply

The CTH power supply is mostly straightforward. The LV supply uses a fixed 24V regulator and the HV supply uses a voltage multiplier identical to the one used in the SOHA II amplifier. The heater supply, however, is different. It uses a switching regulator to drop the voltage from more than 30VDC to either 12.6V or 6.3V. The regulator does this with very high efficiency making it possible for the regulator to run without a heatsink even when supplying 600mA.

Heater Supply - by Chris Forster

Using a 24VAC wallwart to attain a higher 80V B+ 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.

CTH Heater Supply

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 National's LM2595 datasheet.

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.

This heater circuit coupled with the use of capacitance multiplier filters in HV and LV sections makes for a small, high-quality power supply.

LV Supply Pre-Regulator Filtering

The only issue with the LV regulator is that a 24VAC walwart, when rectified, makes only about 33VDC. There is not much voltage for a filtering section. In the CTH we've used a 1000μf first filter cap and then a simple darlington capacitance multiplier. This multiplier only drops a few volts but it takes out most of the ripple before the regulator.

CTH Pre-Regulator Filter Section