Dynaco Stereo 70 Power Amplifier
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.
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.
Ed Laurent, Bob Tucker
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.
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Copyright © 2000 Greg Dunn