Dynaco Stereo 400 Power Amplifier

View the
parts list,
pictorial diagram(very large),
MBI-400 pg. 1,
MBI-400 pg. 2,
MBI-400 pg. 3,
MBI-400 pg. 4,
MBI-400 pg. 5,
MBI-400 pg. 6,
MBI-400 pg. 7,
MBI-400 pg. 8
for the ST-400. The MBI-400 circuitry converts a ST-400, 410 or 416 to bridged monophonic operation at 600W. You may want to right-click (Mac users click/hold) to save these, as they will slow your browser down significantly while loading.

Kevin Boales has provided an excellent article on fixing an ailing ST400; give it a look!


The Dynaco Stereo 400 is a basic two channel power amplifier employing all silicon solid state devices. They include 46 transistors, 47 diodes, 2 silicon controlled rectifiers and 4 integrated circuits. The series-connected output circuitry is of full complementary symmetry design. The entire main amplifier is direct coupled. Its measured distortion levels approach the threshold of the most exacting laboratory test equipment, not only at the commonly specified full power ratings, but more importantly at very low power outputs as well. This is the result of circuit techniques which eliminate any discernible crossover notch at low levels, and contributes to the Stereo 400's freedom from listening fatigue. Its lucid, utterly uncolored neutrality reveals delicate nuances and musical shadings in the best pogram material - one result of achieving unconditional stability with varying loudspeaker loads.

Implicit in the design of a superb power amplifier is the responsibility to include superior techniques of protection for both the amplifier and the speaker load, if long-term trouble free operation is to be achieved. To this end, Dynaco has included more separate protective systems and features than have ever been employed in such a product.

These include an AC line circuit breaker (rather than a fuse); four separate power supply fuses; electronic volt-amp limiting to ensure safe operation of the output transistors; independent high temperature cutouts on each channel, which are mounted directly on the output transistors to assure maximum sensitivity; over 1000 square inches of radiating area for the mammoth heat sink; provision for accessory fan cooling with optimum efficiency; delayed turn-on to eliminate loudspeaker thumping; relay protection against DC signals at the output; load (speaker) protection fuses accessible on the front panel; input level controls to reduce sensitivity; input filters to control the bandwidth; and the exclusive DYNAGUARD™ adjustable dynamic power limiting circuit to protect the speaker load.

The Stereo 400 was engineered to accomodate a wide range of specialized needs in studio, laboratory and industrial applications, as well as in the home. It has been designed as two independent 200 watt (at 8 ohms) amplifying channels with a common power supply having a high degree of inherent regulation. For special requirements, a technician can easily convert it to a monophonic 600 watt (at 8 ohms) output.

Accessory provisions for mounting on a standard 19 inch rack, an alternative front panel which includes two output meters, a cooling fan, and an alternative power transformer for international AC line voltages are available at additional cost.


Power Output:

20 Hz to 20 KHz, both channels driven:

200 watts continuous average per channel @ 8 ohms;
300 watts continuous average per channel @ 4 ohms;
100 watts continuous average per channel @ 16 ohms.
600 watts rms @ 8 ohms.

Intermodulation Distortion: Less than 0.1% at any power level up to 200 watts rms per channel into 8 ohms with any combination of test frequencies. Distortion reduces at lower power levels. Typically less than 0.03%.

Harmonic Distortion: Less than 0.25% at any power level up to 200 watts rms per channel into 8 ohms at any frequency between 20 Hz and 20 KHz with both channels driven. Distortion reduces at lower power levels. Typically less than 0.05%.

Power at Clipping, single channel, 2500 Hz, less than 1% distortion:

235 watts @ 8 ohms;
350 watts @ 4 ohms;
135 watts @ 16 ohms.

Ppower bandwidth: 5 Hz to 35 KHz at less than 0.25% total harmonic distortion - half power output, 8 ohm load.

Frequency Response: +0, -1 dB, 8 Hz - 50 KHz @ 1 watt into 8 ohms; ±0.5 dB, 20 Hz - 20 KHz @ 200 watts.

Hum and Noise: Better than 100 dB below rated output, full spectrum.

Input: 50,000 ohm load; 1.6 volts for 200 watts @ 8 ohms.

Slewing Rate: 8 volts per microsecond.

Damping Factor:

Greater than 80 to 1 KHz into 8 ohms;
Greater than 30 to 10 KHz into 8 ohms.

Channel Separation: Greater than 60 dB by IHF standards.

Connectors: Inputs: phono jacks. Outputs: Color coded 3-way binding posts with standard 3/4" spacing.

Dimensions: 17" wide;, 14" deep;, 7" high.

Weight: Shipping weight 65 lbs; Net weight 54 lbs.

Power Consumption: 120 VA quiescent; 11 amps maximum; 50/60 Hz, 120/240 VAC.

Designed by:

Wade Burns (Dynaguard™), Jim Bongiorno, Erno Borbely, Harry Klaus, Hans Frank

Year Introduced:



$499.00 kit
$725.00 assembled


Dynaco's blockbuster 200 watt per channel amplifier, continuing the tradition of value for price and enthusiastic reviewer response. Featured Dynaguard™ speaker protection circuitry, sophisticated automatic output muting, and a colossal heat sink. The FAN-1 option further improved heat dissipation for PA and sound reinforcement applications. The MC-4 option added accurate power output meters and a range switch. The MBI-400 bridge kit allowed use as a single channel 600 watt amplifier.

This page created and maintained by Greg Dunn.
Copyright © 2000 Greg Dunn