A Parallel feed single ended 2A3 stereo amplifier with separate power supply.


Martin Adams


Outboard power supply on the left. Stereo amplifier on the right.††††††† View of amplifier shows ĎHaltroní 76 input triodes in foreground, RCA red base 5692 driver in centre, Valve Art 2A3ís behind and Sowter plate chokes at the rear. The top plate is 4mm thick solid copper plate. Home made solid brass control knobs.

†††† †††††††††††††††††††††††††††

Amplifier and power supply connected by umbilical with industrial connectors. †††††† †††††††PSU with GZ37 rectifier, oil caps and B+ voltmeter.


Underside of amp, Sowter 8995 parafeed output transformers on the right.††††† Underside of the PSU, Custom made Sowter mains TX lower right.

††††††††††††††††††††††††††††††††††† Amplifier plinth is made from 18mm thick solid Beech wood joined together with dovetails in the corners

The power supply plinth is made from a pair of surplus cherry wood veneered drawer fronts with beech veneered particle board for the front and rear panels held together with galvanized steel corner braces and wooden side rails. The top plate is 3mm thick solid brass plate.

†††††††††††††† ĎCADí layout sheet glued to top plate ready for machining†† †††††††† ††††††Cutting the copper top plate using a hole saw on the drill press


††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††† †††††††††† Assembling the top plate

The amplifier is based on the JE-Labs Stereo SE2A3DX design with 76 input tubes, a 6SN7 (5692) driver tube and 2A3 output tubes but utilises a parallel feed output stage with Sowter 8982 plate chokes and Sowter 8995 parafeed output transformers with iron and mumetal cores. Signal coupling caps are Audio Note copper foil paper in oil types, parafeed output caps are MKP types from ĎAmpohmí in the UK. Resistors are 2 watt carbon from RS in the UK. The hum balance pots are 50 ohm 10 turn wire wound types.

The power supply is my own design, a custom mains transformer from Sowter feeds a GZ37 rectifier into a choke input filter using a surplus 10H Gardners oil filled choke and two parallel 50uf oil filled caps, from there the supply is split into left and right channels with a further 4.7H choke and 30uf of filtering per side.

The amplifier is all AC heated, but required a modification to the original JE-Labs design in order to get the hum down to an acceptable level.I used an extra centre-tapped filament transformer so I could use the centre tap to raise the filaments of the 76ís and the 6SN7 above cathode potential, this resulted in a super quiet amplifier, only 1 to 1.5 millivolts of hum/noise measured at the speaker terminal (with a Hewlett Packard 400E AC voltmeter) which is inaudible at the listening position with my 98dBLowther EX3 speakers.

The amplifier sounds fantastic with my Lowther Bicor 200ís, it will even drive my old Spendor BC1ís (around 83dB/1W/1m) to an acceptable level in my small listening room.

Since the pictures above were taken I have changed the output tubes to JJ Electronic 2A3-40ís from Eurotubes in the USA. The move from the Chinese Valve Art 2A3ís to the JJ 2A3-40ís has changed the character of the amplifier to something much more refined and detailed.

My thanks go to Bob Pletka at Eurotubes for the supply of the JJ Electronic 2A3-40 tubes at such a good price, these tubes are truly stunning.

Thanks for looking.

Martin Adams Ė Brackley, Northamptonshire, UK . 2004

E-mail: martinadamsATdsl.pipex.com (replace the ĎATí with Ď@í to email me)