R 309 L & R were cooked. It connects between the +55 and -55 supply voltages with a zd301. It is known as one of the failure points in these old Nakamichi receivers. They produce a lot of current and drive my Dahlquist DQ 10's better than any amp I have owned. I have read that they can drive a 4 ohm load safely, but I am afraid because they produce a lot of current into an 8 ohm load.
It is both. The circuit, the power supply, the heatsinking... all has to be capable of allowing the amplifier to deliver safely into 4 ohms.
But lets get real... there is a world of difference between bench testing a power amplifier and perhaps running it at maximum output (although that isn't the worst case scenario for heat dissipation) for 30 or 60 minutes into a test load and playing music loudly into a speaker.
With music you might be putting only a few volts across the speaker, and then only in the loud bits. Power testing with a sinewave and its closer to 20 volts rms assuming 100 watts into 4 ohms.
Although I'm not familiar with the amp, I would bet it would be just fine with a nominal 4 ohm speaker.
But lets get real... there is a world of difference between bench testing a power amplifier and perhaps running it at maximum output (although that isn't the worst case scenario for heat dissipation) for 30 or 60 minutes into a test load and playing music loudly into a speaker.
With music you might be putting only a few volts across the speaker, and then only in the loud bits. Power testing with a sinewave and its closer to 20 volts rms assuming 100 watts into 4 ohms.
Although I'm not familiar with the amp, I would bet it would be just fine with a nominal 4 ohm speaker.
That resistor/zener combo is used to set the base voltage for the input and VAS CCSs. 4 or 8 Ohms makes no difference in that part of the circuit. By the way it's 47K in the TA4. In the TA3 that resistor is dissipating at least a 1/2 Watt at all times, probably why it was cooked, probably should have been a 1 Watt in first place. It would even be worse if the line voltage was high. Those damn bean counters!!!!!
Craig
Craig
The resistor value is going to depend on the rail voltages.
I've worked on a couple of the Nak receivers, don't remember which ones though. Here in the shop I don't get to do much "critical listening", I listen for problems and that's about it. Not a fan of receivers anyway so I'm prejudiced right from the start.
Craig
I've worked on a couple of the Nak receivers, don't remember which ones though. Here in the shop I don't get to do much "critical listening", I listen for problems and that's about it. Not a fan of receivers anyway so I'm prejudiced right from the start.
Craig
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