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.

Thank you. So, it sounds like if it has the PC7 board in there, it replaced what was there originally for that channel, and I'll need to find another board to revert it back to the original condition it was in as a DH-200, correct?

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.

There is no PC7 board, just the regulat PC6 boards like my DH-200 has. It looks like it's wired the same also, the only difference I see is the input jacks are like those you'd find to plug full size headset type plug into, not phono jacks, so the also don't have the 2.2 ohm resistor attached to them like my phono jack inputs on my DH200. I can say this much, the chassis and faceplate for rack mount, both have silk screened DH-300 on them so it may have started life as a true DH-300 and maybe someone has already reversed it and removed to PC7 board....? I'll have to get a plug like these and get soem signal into it and see what it does.

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.

Yes, both inputs are going right up to pins 1&2 on the PC6 boards. If I convert back to regular phono jacks on the inputs I imagine there must have been some good reason for the 2.2 ohm resistor on the right input jack of my similar DH-200, between it's (-) and ground...? There isn't one on the left channel, nor was there supposed to be at least looking at my original wiring diagram. Thanks for all your help!

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!

Im going to show some of technical ignorance here, but are you referring to the big caps, 10,000 uF that are connected via the bridge rectifier? I think I can pick up a set from QA-Con , I think that's the supplier on eBay for about 50 bucks. They also sell full kits of all the smaller caps as well. What's the story I hear about "conditioning" amps that haven't been used for a long time with a variac or other type set-up? Thanks again! Off to radio shack to get some phono-kack bulkhead mounts to take out these headset jack type input jacks.

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 )

Wow, almost sorry I asked, now I can't plead ingnorance. Will have to see if I can locate someone with one to lend me. I have a plain vanilla DVM I use so I don't think that it will fill the requirement for "True RMS" readings.

On another note, I went and got some adapters to convert the 1/4" mono input jacks to allow them to accept my phone(RCA) jacks from my preamp. The newly received DH300 works but it has considerable hum through both channels (at zero volume) as soon as I plug the input leads from the preamp into the back of the Hafler. Both channels do work, so it's set up for stereo, although I have not done anything with the 2.2K ohm resistors I picked up, knowing my standard DH200 has that across the ground leg and chassis ground on the right channel. Could that have anything to do with the hum, or is it that I have some floating ground or something with these 1/4 mono plug inputs that I think are isolated from chassis ground.

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).

thx, I screwed up and am pretty sure I grabbed some 2.2K ohm resistors at Rat Shack instead of true 2.2ohm, will need to correct that, and report back to you.

So far I tried the new DH300 and tried reversing the power plu polarity and that helped considerably, but there's still hum as soon as I plug the input cables from the pre-amp into the Hafler. I have not yet tried to ground the Hafler chassis with a jumper or anything back to the pre-amp. I'm still using the 1/4 plugs with adapters that allow me to connect my RCA phono plugs into the DH300 and the 1/4" jacks appear to have plastic mounts isolating them from the chassis. Any thoughts?

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!