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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.

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Inhaltsverzeichnis

Seite 1 - Dynaco 400 Assimilation

1Dynaco 400 AssimilationThe Dynaco 400 was the Tyranasaurus Rexof power amps in 1973. I wanted one badlybut I was bringing up children and settledfor

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2Replace the old Dynaco PC30 power distribution board in front of the transformer with the newcombined power-on and voltage regulator board, PCB-1. Dr

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3The two power amplifier modules from Marchandbolt directly onto the base of the oversizedDynaco heat sink. They sit an inch above the floorof the cha

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4For now, I am very happy with therebuilding effort. In many respects,the rebuild performs better thanthe original amplifier and runsmuch cooler. For

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5Parts List - Dynaco 400 Assimilation1 TH1 Thermisistor 2.5 Ω - 8 A Inrush limiter – (Thermometrics CL 30)1 TZ1 MOV - 250 V - 2,500 A Transient volta

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6Making Printed Circuit Boards for the Dynaco 400 AssimilationIt’s easy to make good looking printed circuitboards using the photofabrication techniqu

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