Hafler DH300 Monoblock forever?

If the jacks are isolated from the chassis, try grounding one, just to see what happens, Be certain to ground the terminal that contacts with the barrel of the 1/4" plug and not the tip. Only connect that channel, with your pre. Should be a simple test to perform. Check both channels and see if the wires from the hot terminal(contacting the plug's tip), are connected to terminal #1 of the PC-6 boards. Are all your power cables and interconnects, dressed well away from one another, with none running parallel?

I tried the exercise and grounded one (-) of one input channel. It didn't make any audible diff. Here's what I found tonight. The hum will emit from the speakers once either one is conncted to the amp WITHOUT any pre-amp input plugged in. Then when I plug the right hand channel input in, it gets somewhat worse as soon as I touch the jack tip at all as it enters the jack, even before the positive tip seats down to it's clip, and when I plug the left channel input jack in it get even worse. With no input cables plugged in, and no speakers connected, I show .3 volts DC at the right channel posts and .5 volts DC on the left channel posts. I took a look at wire routing inside and both pin #1 feeds are going to the positive of the input jacks from the PC6 boards. It does look like perhaps the wire routing might be able to be improved, it's wired electrically the same as my DH200, but they ran the #6 outputs wires up to the bridge between the large caps and then back down to the output posts, more like a star config. My DH200 runs them (per recommened Hafler diagram) from the PC6 board to the positive output post, and then there's another jumper that runs from there up to the solid wire bridge between the caps. Woudl you think any of this routing difference could be creating some induced hum on the output channels?

Whatever differences you find, between the quiet 200, and your new project: duplicate them. It would appear that the amp has been modified by someone that shouldn't have. Follow the DH-200 manual, and rebuild the amp to it's proper configuration. I would install RCAs at the inputs, and eliminate the 1/4" jacks, before proceeding(personally). Be certain to route any wires, according to the manual. Keep in mind; if you haven't changed the filter caps yet, they are 30 years old.

OK, thx again, will go that route and let you know how I make out!