Our symmetric, single-tier solid-state power amp kit, PA50H1 utilises a standard solid-state power amplifier topology that has been around since 1970. It’s the foundation for the famous Leech amp, Ampzilla, Bryston, Curl and many other fully complementary circuits that followed and continue to be produced to this day. The simple circuit description is thus: complementary differential stages feed a symmetric voltage amplifying stage (VAS) followed by two complementary cascaded emitter-followers (EF2). An adjustable bias regulator maintains the idle current set by the user. Twelve transistors total.
The use of symmetric supply rails eliminates the need for a bulky electrolytic coupling capacitor to the speaker. Such caps are a common source of distortion. Their elimination provided one of the first leaps in performance for solid-state amps back in the day. PA50H1 represents performance of the early 1970s, but surprisingly most commercial products today have about the same performance, with THD in the hundreds of parts-per-million. The amp clips smoothly and can be used equally well for hifi or musical instruments.
PA50H1 operates from symmetric plus/minus supply rails, in a configuration known today as an “H1” supply because there is only one absolute voltage level with no rail switching. The nominal design power is 50W, but lower or higher outputs can be obtained using lower or higher supply voltages. For a given power output, linearity is slightly better with higher voltages – as is the case with most circuits – and idle heat will increase with supply voltage for any given idle current per Ohm’s Law. One could build the amp for 15W at 8-ohms with +/-20V supplies, or for 140W with +/-63Vdc.
There is a single pair of output transistors – 200W devices as supplied. Alternate devices of lesser capability and cost may be used by hobbyists wishing to build a low-watt amplifier. PA50H1 may be idled cool to warm and requires a heat sink for safe operation. For average music use, thick chassis panels may suffice for heat dissipation, whereas harder use will require additional heat sinking and proper air flow. Thermopads are supplied in the kits and NO thermal grease is required.
All components are ordinary off-the-shelf widely available types, with preference for metal-film resistors, metal-oxide and wire-wound power resistors, NP0-C0G ceramic caps, film caps and long-life electrolytics. The printed circuit board has 2-ounce copper traces – the industry standard in 1970 but twice the present standard – and is a compact 80 x 65mm, 3.15″ x 2.55″. The power transistors add 20mm to the long dimension in the assembled form. Height with typical components and spacers is approximately 35mm.
London Power’s matching power supply is the PSU-H1. The basic supply can be used to support one PA50H1 as a mono amplifier, or two PAs for stereo, or multiple PAs as required, depending on the power transformer (provided by the user).
PA50H1-PCB is just the printed circuit board and notes.
PA50H1-COMP is the PCB, notes and all components that go on the board and mounting hardware; suitable for supply voltages from +/-20V to +/-63V and power output up to 140Wrms 280Wpeak into 8-ohm; requires assembly, mounting and wiring.
PA50H1-BLT is fully assembled and tested; suitable for supplies up to +/-63Vdc; power up to 140Wrms at 8-ohms; includes notes; requires mounting and wiring.