As has been suggested, by careful design and the introduction of Toshiba's brilliant power transistors that are still legend and cloned by no less than On-Semi, after 15 years obsolescence.
Don't let general design principles and that DIY obsession to adjust anything that can be, overshadow the evidence of both measured and observed performance. Quad's feedforward current dumper amplifiers, for example, also need very little if any bias for similarly clean crossover behaviour. As you already observed, the sound was good regardless of whatever generalised predictions are made. Stick with reality
Hi there. I have taken the HA-6 to my local hifi shop; The technician has replaced 2 diodes and more than 5 resistors which were off target value.
He has tested the power supply filtering capacitors, all of which were within tolerance. Seems as though the power supply voltages were steady and correct as well.
What we did not address is the signal path, or phono EQ section.. Not sure how close the circuit is running to original spec. My tech seems to feel that it is best to leave the linestage and phono section alone since they look clean on an analog oscilloscope with inverse-RIAA signals applied(?)
To me, the amp sounds the same even after replaced a bunch of parts. It is from the early eighties, so it is rather old however.
We ran sine waves at 10Hz from a USB DAC into the auxiliary input of the Hitachi, and set the output power level @ 75Watts both sides driven. Running into large dummy load resistors, we did this to test the thermal protector circuit (at the factory prescribed bias levels)..the heatsink for the finals got extremely hot to the touch, and it clicked off after 22 minutes.
I do not normally perform 'torture' testing of gear, but we had the equipment to do so.
Before buttoning up the unit, we cleaned the bias potentiometers and monitored the levels for 2 hours at random intervals, no signal.
I think it shall perform well for at least a few years now.
He has tested the power supply filtering capacitors, all of which were within tolerance. Seems as though the power supply voltages were steady and correct as well.
What we did not address is the signal path, or phono EQ section.. Not sure how close the circuit is running to original spec. My tech seems to feel that it is best to leave the linestage and phono section alone since they look clean on an analog oscilloscope with inverse-RIAA signals applied(?)
To me, the amp sounds the same even after replaced a bunch of parts. It is from the early eighties, so it is rather old however.
We ran sine waves at 10Hz from a USB DAC into the auxiliary input of the Hitachi, and set the output power level @ 75Watts both sides driven. Running into large dummy load resistors, we did this to test the thermal protector circuit (at the factory prescribed bias levels)..the heatsink for the finals got extremely hot to the touch, and it clicked off after 22 minutes.
I do not normally perform 'torture' testing of gear, but we had the equipment to do so.
Before buttoning up the unit, we cleaned the bias potentiometers and monitored the levels for 2 hours at random intervals, no signal.
I think it shall perform well for at least a few years now.
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