Is it safe to drive a 4 ohm load with a Nak TA 4a

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.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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.
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
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
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
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