Stereo SRPP Hybrid Headphone Amplifiers:
This board holds two hybrid SRPP headphone amplifiers. The 12Vac SRPP HPA output stage is actually a small power amplifier in its own right (a Zen single-ended amplifier). It consists of one IRF630 power MOSFET that is loaded by a constant-current source, which is made up of an LM317P voltage regulator configured as a constant-current source. A two-resistor negative feedback loop sets a gain of 1 (unity) for the inverting output power amplifier. Each channel sees a 200mA idle current flow, which is enough to deliver 3Vpk of output voltage into 16-ohm headphones and twice that into 32-ohm drivers.
This FR-4 PCB is extra thick, 0.094 inches (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated-through 2oz copper. The PCB is 3.6 inches by 7.5 inches, with four mounting holes, which prevents excessive PCB bending while inserting and pulling tubes from their sockets.
The key advantage of the SRPP 12Vac is that it can be operated from a lowly 12Vac wallwart power supply, yet develop a fairly high-voltage B+ for the triodes. Where the typical tube-based line-stage amplifier requires a B+ of 200V to 400V, the SRPP 12Vac can get by with only a B+ voltage of 100Vdc. The two tube heaters are all placed in series and this heater string is powered by a portion of the B+ power-supply voltage. Thus, a single low-voltage 12V AC wallart or internal power transformer is all that is required to power the SRPP 12Vac.
Unlike the Aikido circuit, the SRPP offer a relatively poor PSRR. Thus, the SRPP 12Vac PCB holds four cascaded RC (resistor-capacitor) filters to smooth away B+ voltage power-supply ripple. DC voltage heats the two heaters. In addition, the constant-current source in series with the output MOSFET and heaters prevents huge current inrushes through the heaters at start-up, when the heaters are cold and offer little resistance; this will extend the tube’s life. In addition, the heaters are shunted with a zener diode to ensure equal and accurate voltage.
The part kit includes every part needed to populate the PCB except for the tubes. Very high-quality parts are used, such as Panasonic and polypropylene capacitors, and super-fast rectifiers. Includes two heatsinks and TO-200 mounting hardware, along with all solid-state devices. Kit comes with Panasonic 470µF non-polarized electrolytic output coupling capacitors in parallel with a 10µF Vishay polypropylene capacitor, which implies a -3dB low-frequency cutoff of about 10Hz with the 32-ohm load. With 16-ohm headphones, we would expect a 20Hz low-end bandwidth.
A 12Vac wallwart cost less than $20 from Jameco.com and delivers up to 2A. Jameco Part no.: 2227604 Or, a 12Vac transformer can be used to power the 12Vac SRPP HPA. A DC 12Vdc power supply cannot be used.