The "Atwood Reference" Amplifier

Niyom Nakarin lives in the San Francisco Bay Area. He attended the third Randall amp class at the Randall Museum. His good mechanical skills were apparent in his class amp. However, he went all-out in this "Atwood Reference" amplifier, based on the Randall chassis and design.

Somewhere under this beautiful chassis is an original Randall chassis!

Here is the amp without the cage. The original tube line-up is used, although N.O.S. RCA Special Red and Amperex "Bugle-Boy" tubes are used. Note the special stand Niyom built to hold the amp both upright and upside-down.

This is a close-up of the top of the chassis. The transformers are in welded aluminum cases. Note the Cerafine filter capacitors. The BFT-1 transformer is in the can to the left. This is the only factory-wound transformer - all other chokes and transformers were designed and wound by Niyom.

This is the underside of the chassis. Things are very tight, since interstage transformers and a shunt-coupled (parallel-feed) output stage are used. The power supply filter chokes are in the middle, the output transformers are on either side and the interstage coupling transformers are the ones with the white wires on top. The gold cylinders are Niyom's hand-made coupling capacitors.

Here is a close-up of the chassis. The red-cased transformer on the right is the output transformer, which uses 80% nickel-alloy laminations. The shunt-load chokes are on the top of the chassis, where the output transformers would go in a stock Randall. The home-made coupling capacitors are at the left. According to Niyom, the dielectric is a combination of mylar and Kapton (polyamid), impregnated with mineral oil. These were chosen after extensive listening tests of various dielectrics.

This is a close-up of one of the interstage transformers. These also use nickel-alloy. It is capacitively-coupled from the driver plate resistor. The white wires on the top are connecting each layer together in a "Z-winding" scheme. Most transformers use "U-winding", where the direction of winding changes for each layer. In Z-winding, each layer is wound in the same direction and the connections between layers are made outside the transformer. This is more difficult, but significantly increases the high-frequency response.

Finally, here is the back, with a better view of the amp stand. The layout sticks very closely to the original Randall, except that only one impedance (8 ohms) is available.

Niyom's amp is one of the most amazing pieces of construction work I've seen. Not only does it look good, but it sounds excellent - probably the best of any amp using the single-ended topology. All of Niyom's hand-made transformers and capacitors (as well as the choice of other components) were made after extensive listening tests of many different variations. As with the original Randall amp, AC is used on all filaments and no semiconductors are used anywhere, yet the amp is completely hum-free (listening on 100dB+ efficient speakers). This is truly a work of art, both mechanically and sonically!

- John Atwood