Dynaco Stereo 70 Power Amplifier

Description:

The Stereo 70 uses a circuit arrangement based on that of the Dynakits Mark
II and Mark III which has become world famous as one of superior quality while
essentially simple and trouble-free. The new 7199 tube is used as a pentode
high gain voltage amplifier directly coupled to a cathodyne phase inverter.
All parameters are adjusted for minimum distortion. This type of phase
inverter has the unique advantage that its operation is independent of tube
aging so that no adjustments are required in maintaining optimum performance.

An internal capacitive feedback loop balances the phase inverter at the
highest frequencies, and the arrangement provides accurately balanced driving
signals to the output tubes. These tubes are type EL-34s (one pair for each
channel) operating well below their maximum ratings. The output tubes use a
fixed bias arrangement which is set through use of Dyna Biaset (pat. pending)
which provides optimum linearity of the tubes and minimizes the effects of
unbalanced components. The connection of the output tubes includes a small
percentage of screen loading which improves the regulation of the output stage
and makes it comparatively uncritical of load impedance.

The impedance match and bias conditions utilized in the output stage provide
minimum distortion over a very wide dynamic range. In addition, 20 dB of
negative voltage feedback lowers the distortion to an unmeasurable proportion
at normal listening levels and to less than 1% intermodulation distortion at 35
watts output on each channel.

The use of a minimum number of phase-shifting stages along with careful
transformer design makes it practical to have unconditional stability at both
high and low frequencies. This feature of the design means that there is no
tendency toward motorboating or oscillation under and condition of use.

Specifications:

Power Output: 35 watts continuous, 80 watts peak each channel.

Frequency Response: ±.5 dB from 10 cps to 40 kc.

Power Response: 20 cps to 20 kc within 1 dB of 35 watts at less than
1% distortion.

Intermodulation Distortion: Less than 1% at 35 watts (normally about
.5%). Less than .05% at 1 watt.

Hum and Noise: Inaudible; better than 90 dB below rated power.

Sensitivity: 1.3 volts rms input for 35 watts out.

Output Impedances: 4, 8, and 16 ohms each channel.

Damping Factor: 15.

Minimum Channel Separation: 55 dB

Power Consumption: 190 watts 50/60 cps

Preamp Provision: Two power takeoffs with independent heater windings
to eliminate interaction. Each 6.3 volts 1 amp, 350 volts dc 10 ma. Dynakit
preamplifiers plug in directly without modification.

Tube Complement: EL-34 (4), 7199 (2), GZ-34.

Size: 13" by 9-1/2" by 6-1/2" high.

Weight: 32 pounds.

Finish: Chassis and bottom plate bright nickel plated. Cover finished
in charcoal brown vinyl paint.

Designed by:

Ed Laurent, Bob Tucker

Year Introduced:

1959

Price:

$99.95 kit
$129.95 assembled

Comments:

The classic 35 watt per channel stereo tube amplifier; sales continued from
its introduction in 1959 until mid-1990. Channels could be paralleled for use
as a single-channel 70-watt amplifier. Almost as desirable a collector's item
as the Mk. III, and for the same reasons. Designed by Ed Laurent with
assistance from Bob Tucker, while David Hafler was out of town on a business
trip. Thanks to Ned Carlson from Triode
Electronics for the scans of the classic reviews!

click here to see the 1959 review of the Stereo 70 in
Audiomagazine.

click here to see the 1959 review of the Stereo 70 in Stereo
Reviewmagazine.

This page created and maintained by Greg Dunn.

Copyright © 2000 Greg Dunn