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Dynaco Stereo 400

Stereo Power Amplifier (1972-74)

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Dynaco Stereo 400


The Dynaco Stereo 400 is a basic 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.

It's 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 it contributes in large measure to the Stereo 400's freedom from listening fatigue.

It's lucid, utterly uncoloured neutrality reveals delicate nuances and musical shadings in the best program material - one result of achieving unconditional stability with varying loudspeaker loads.

Implicit in the design of a superb high 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 cut-outs 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 accommodate 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: 200 watts per channel into 8Ω (stereo), 600W into 8Ω (mono)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.25%

Damping factor: 30

Input sensitivity: 1.6V

Signal to noise ratio: 95dB

Speaker load impedance: 4Ω to 16Ω

Semiconductors: 46 x transistors, 47 x diodes, 2 x silicon controlled rectifiers, 4 x IC

Dimensions: 17 x 14 x 7 inches

Weight: 52lbs


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Reviewed Jan 12th, 2019 by novy07

Great power amplifier - Very robust and powerful



olschl8's picture

re: Stereo 400

Does anyone have a manual or review for the 400 Series II?

re: Stereo 400

I had this amplifier for many years. Also Dynaco preamplifier and Dynaco loudspeakers and Dual 1249 record player.
Very good sound. There were two fusions on the front. I think I had fusions of size 2 amp.
To protect the loudspeakers when I blasted Pavarotti.

Dynaco Stereo 400

Thanks for the schematics . This was very helpful .

download Dynaco Stereo 400

Thanks so very much. You have just saved another would be "boat anchor" from the landfill

re: download Dynaco Stereo 400

I want to thank who ever contributed this information. I just purchased this amp, and find this information
Very helpful. Thanks again, moduck.

sinclair2's picture

re: download Dynaco Stereo 400

Hi friends HiFi equipment,
topology of the output /power/ amplifiers and others half-circuit of Dynaco 400/410/ Stereo designed Mr.Erno Borbely. These amplifiers have a very nice sound and powerful performance.

re: download Dynaco Stereo 400

Jim Bongiorno had a hand in the design of the Stereo 400, as well. he was Dynaco's chief engineer at that time. I have had a Stereo 400 for 40 years and replaced the output transistors with, I believe the ON Semiconductor MJ21194/MJ21196 or MJ21193/MJ21194 family of linear power transistors. That seemed to be a significant sound improvement.

This website is not affiliated with or sponsored by Dynaco. To purchase Stereo 400 spares or accessories, please contact the company via their website or visit an authorised retailer.