Digital Design M3c protects when relays click

These DD amps seems to fail in strange ways. This one has me scratching my head.

If I apply 14.5 volts the amp will go into protection as soon as the relays engage.

If I bring the amp up at 10 volts and slowly bring the voltage up to 14.5 volts the amplifier stay up and stable.

I was able to check all of the voltages and drives this way and everything looks good in the output section and in the power supply. The amp produces 125 volts rail at 14.5 volts input.

I don’t understand why bringing the amplifier up at 14.5 volts trips the protection circuit but bringing it up on low voltage and running the voltage up does not.

This amp has some 5 leg predrivers in front of the output driver card. They’ve been masked off as well as the ICs on the driver card.

Anyone have any ideas?



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I didn’t know there was such an animal. Learned something new every day.

I don’t see any switches to choose an input voltage so I’m assuming this is a 14.5 volt setup.

I’m thinking I have a transistor electrically leaking somewhere, or a bad resistor. What are your thoughts.

Any idea what those 5 legged drivers are for the output mosfets? Or the through hole IC is that is mounted on the daughter board for the output section.

The daughter board for the power supply is just a basic power supply board found in hundreds of amps, American bass and whatnot with a KA7500 PWM IC. No worries there.