Hafler DH300 Monoblock forever?

Hafler made a bridging kit for the DH-200. You can find the manual here: ( http://www.hafler.com/techsupport/pdf/DH-202_PC-7_bridgeKitForDH-200.pdf ) Your amp probably has this installed. If you can recognize the circuit(the PC-7 board), inside your amp: reverse the install procedure, per the manual.

No! Both active component boards(PC-6) of the original DH-200 will still be in place. The PC-7A(bridging circuit) is a separate, much small board, that should be located/mounted on the rear of the right heatsink, it's components facing to the rear of the amp.

Take a look at the twisted wire pairs from the input jacks. If they are each connected directly to terminals 1 & 2 of both PC-6 boards, you should be good to go(in stereo). If it's bridged for mono operation; only the left input will be connected, and the two positive speaker terminals will be the output.

You are correct, concerning the 2.2ohm resistor(R45/pn 133022). There was only one provided, in the kit. The DH-200 schematic shows that on the ground of the left input. Given the age of your amps(30yrs+), you might consider obtaining some new power supply filter caps. Though yours may still work; I promise you will notice a marked difference in performance, with new ones. ( http://www.digikey.com/product-detail/en/CGS103U075V3C/338-1244-ND/953292 )Happy listening!

Yes- those are the ones. Be careful from whom you buy the caps. Believe it of not; aluminum electrolytic capacitors have a shelf-life. re: the variac- It's good to bring the voltage up slowly(and, "reform" them), when electrolytics haven't been powered for a long time. Some good info here: ( http://www.allegrosound.com/index21.html )

R45 should be a 2.2 Ohm, 1/2W resistor. I would install that(if the 300 has none) on the right input, and see if that helps. But first: The AC plug on your amp should be ungrounded and non-polarized. Reverse(flip) it in the wall socket, before you do anything else. Then try connecting the preamp again. It's possible that the phase is reversed between the two units. If the RCAs that you bought at Rat Shack ground to the chassis: you'll have to find a way to float the right input, so it grounds through R45(if you're still humming).

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?

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.