by GP49 Mon Mar 31, 2014 1:34 pm
It works for theoretical specsmanship but it doesn't work well for fine sound. Parallelling the two channels results in mixing of the feedback-correction signals (derived at the output transformer secondary) between the two channels. Unless the two channels are exactly identical (they never are), this means that the correction offered by the feedback will be non-optimum for each channel...meaning that instead of reducing distortion, it may well increase it.
Additionally, the Stereo 70's power supply will not sustain two 35-watt channels at once (if you doubt that, read the FTC-mandated specifications that were published in the early 1970s). Driving both channels simultaneously with the same strong low bass signal (which is the only place you would need 70 watts) will result in power supply voltages going down, limiting the power output which is the opposite of what you are trying to achieve.
The only PROPER way to parallel two channels is to invert the phase in one, and drive the loudspeaker across the positive speaker outputs. This is called "bridging." It keeps the feedback signals separated but requires additional active circuitry: a phase inverter which must be so good that it will do no harm.
As for the VTA ST-120 the power supply not be an issue if sufficient storage capacity is provided. But if it takes its feedback signal from the secondary circuit of the output transformer, as in the Dynaco Stereo 70, I cannot see any difference in the behavior of its feedback circuits when the two channels are paralleled.
