Dynaco Stereo 120 Power Amplifier

By popular request, the Dynaco TIP mod for the ST-120; this update is supposed to improve the reliability and bandwidth of a stock ST-120 several fold.

View the
power supply schematic,
amplifier schematic,
service notes1,
service notes2,
PC board layout,
pictorial diagram
for the ST-120. You may want to right-click (Mac users click/hold) to save these, as they will slow your browser down significantly.


The Dynaco Stereo 120 is an all silicon transistor basic power amplifier for use with separate preamplifiers such as the Dynaco PAT-4 or PAT-5, or for use with tape recorders or tuners such as the FM-5 or AF-6, which have their own volume controls. The Stereo 120 contains two 60 watt amplifiers on one chassis with a common power supply.

Although the Stereo 120 is a solid state device containing transistors and similar semi-conductors, it has been designed to be used under normal conditions without special safety precautions, just as if it were a high grade tube amplifier. Thus it can be connected to source and speaker components and used with confidence in all conventional arrangements. There are no circuit breakers, speaker fuses, or other resettable devices to impede the use of the Stereo 120 under any reasonable conditions of use or abuse. This is achieved by using novel circuits (on which patents are pending) which automatically and instantly protect the amplifier.

The components of the Stereo 120 are of the highest quality to protect against failure, both now and for many years in the future. All parts are used conservatively with close tolerances to assure continued proper operation, and etched circuit modules have been pretested under actual use conditions to ensure that every unit, after assembly, will meet the specifications normally associated only with laboratory prototypes.

The specifications of the Stereo 120 speak for themselves. The distortion and noise, up to levels well beyond those required to drive any conventional amplifier, are comparable to the finest tube designs while the high power distortion remains inaudible. Specifications do not reveal all the facets of superior audio performance, however. In use with varying program material, the Stereo 120 achieves its design goals of sonic ease and naturalness always sought but rarely achieved in solid state designs. There is remarkable clarity and an impression of direct contact with the original without the extra brightness or stridency which, unfortunately, is sometimes attributed to high fidelity sound, but rather there is an impression of limitless range and effortless handling of the highest power peaks.


Frequency Response: 5 Hz to 100 KHz ±0.5 dB.

Power Output Rating: 60 watts average continuous power per channel into 8 ohms at any frequency between 25 Hz and 15 KHz at less than 0.5% total harmonic distortion. Distortion reduces at lower power levels. This is in accordance with F.T.C rating requirements, including preconditioning.

Intermodulation Distortion: Less than 0.5% at any power level up to 60 watts per channel into 8 ohms with any combination of test frequencies. Distortion reduces at lower power levels.

Noise: 95 dB below rated output, unweighted with shorted input; 100 dB down by IHF standards.

Damping Factor: Greater than 40 from 20 Hz to 20 KHz.

Separation: more than 70 dB from 20 Hz to 20 KHz.

Input: 100,000 ohms; 1.5 volts for 60 watts output.

Semiconductor Complement: 15 transistors; 15 diodes.

Size: 13" by 10-1/2" by 4".

Weight: 20 lbs.

Maximum Power Consumption: 400 watts.

Designed by:

Ed Laurent (power supply by Sid Lidz)

Year Introduced:



$209.00 kit
$289.00 assembled


One of the first modern high power transistor power amplifiers (60 watts per channel), featuring capacitive output coupling and a regulated power supply.

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