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Dynaco 400 Assimilation
The Dynaco 400 was the Tyranasaurus Rex
of power amps in 1973. I wanted one badly
but I was bringing up children and settled
for a more affordable Dynaco Stereo 80,
which pumped out music for the next
twenty-odd years. It also survived three
teenaged boys. In 1998, I spotted a Dynaco
400 in a used equipment shop, paid way too
much and dragged it home--all sixty
pounds! Unfortunately, my `Dynasaur 400'
was seriously flawed. It had a nasty habit of going DC and destroying speakers. I spent more money
unsuccessfully attempting to repair a unit with too many previous ham-handed repairs and
modifications. Eventually I decided it was too old and complicated to fix and too big to use as a
doorstop. Resistance was futile; it needed to be assimilated with newer technology.
I decided on a rebuild that would retain the outward appearance of this classic amplifier while
replacing the innards with new components. After some looking around, I chose to upgrade using a
couple of compact 200 W all-MOSFET amplifier modules from Marchand Electronics in Rochester
N.Y. You can buy the PM224 modules as fully assembled and tested units, as kits with the PC boards
and all parts or as bare PC boards. See the Marchand site for details, specifications and schematics.
The rebuild is straightforward. Strip everything but
the massive Hammond power transformer from the
old chassis. Remove the faceplate and hinge the front
part of the chassis forward. Clean the wiring from all
front panel controls. Only the power switch and pilot
light will be functional in the new amplifier. The old
Dynaco yields about two hundred dollars in parts--a
1,000 VA transformer, heat sink, rectifier bridge,
circuit breaker, switch, speaker fuses, connectors and
a sturdy chassis with lovely big rubber feet. On a
good day, you can pick up an old, one-channel-good
Dyna 400 for fifty dollars on E-Bay; you'll have talk to
the Teamsters about getting it to your place.
Despite the utility of the Dynaco's original fifteen amp
power switch, I chose to use a triac to turn the amp
on, believing that MOSFETS are more susceptible to
switching spikes and noise than the transistors in the
old amp. Audiophiles are, as Nelson Pass notes, a
superstitious group. The triac power-on circuit is the
same as that used in the Pass/Thagard A75. The
voltage regulator circuit is taken from Pass'
`Complementary Zen' design. Thanks for both.
Right, top - the bare chassis with Hammond transformer; bottom -
PCB 1 mounted and voltage regulator MOSFETS bolted to the
transformer shield.
1
Regulated 65 V power supply for Marchand PM224 power amplifier modules
Replace the old Dynaco PC30 power distribution board in front of the transformer with the new
combined power-on and voltage regulator board, PCB-1. Drill a couple of holes in the transformer
shield to bolt down the large voltage regulator MOSFETS, as shown in the picture. I used two old
heat sinks as washers on the top of these devices, but they really don't generate a lot of heat. Place
insulating pads or mica washers underneath them. The large capacitors, C6 and C7, need to be bent
over parallel to the board so that they clear the front panel--leave their legs long and cover with heat
shrink wrap. Connect the power switch and pilot light. The original Dynaco pilot is a neon light with a
dropping resistor. Connect it across the transformer primary.
The front panel tilted forward for access to voltage regulator PCB 1
2
The unregulated side of the supply in my amp sits
at about