Hafler P505


Has anyone come across Hafler P505? I looked at the Hafler's webpage but didn't find any manual. Not much talk on the internet about it as well. I am wondering if anyone here knows about this amplifier. Claims to have same power specs as DH-500. It has quarter inch inputs on the back and level controls. I think they are balanced inputs. Any help would be greatly appreciated.

This is my first post at DIY Audio, so if I have posted this at a wrong place please let me know.

The circuitry seem identical...of course the 505 was marketed to the pro market with the 1/4 inputs, level pots and I think extra fuse protection, not absolutely sure about the latter.
The P505 manual list it as being net 48lbs. vs 45 for the DH500. Also channel separation: 65dB vs 60. @1Khz.
I have one, but haven't played it in quite some time...I do remember that it didn't sound as clean as the DH500.
Thanks a lot for your reply. You mentioned P505 not being as clean as DH-500. I am about to acquire one for just purr stereo listening at home. Do you think this is a good choice? I ask because you mentioned having one. Also do you have the manual for it, would it be possible for you to share it?

Thanks again...

Hope everyone had a wonderful Christmas.

Well I picked up the P505. I also bought a DH-110 from ebay.

(1) Connected speakers to the amplifier, turned on and there is a audible hum (more from tweeter and midrange than woofer). The amplifier has a three prong plug with the ground prong chopped off. There is a switch on the back, one side says "float" the other side says "chassis". The switch is currently on "chassis" side. I tried the "float" side but it did not help with the hum, I also flipped the ac chord but that didn't help either.

(2) Second I connected the preamp to the amplifier, the hum remains at the same level. But when I turn the "tone" switch on the DH-110, the hum gets amplified. I also noticed when I turn the volume to maximum the noise shifts from one side to other side (some kind of phase shifting) the hum level doesn't rise a whole lot.

Does anyone has any ideas? The 1/4" connectors are screwed on a fiber washer so they don't make contact with the chassis. There is a level control on the back which is essentially a potentiometer blocking the incoming signal. No circuit, op amp etc.

Please help me in getting rid of this noise. To make things worse the fan is a loud one as well... I don't know what to do with the fan either. Help is greatly appreciated. I will be looking into it more, will try to post some pictures if that helps.

AcidJazz said:
I could send you the page or two that is different from those included in the DH500 manual. BUt it is so minor, dealing with the wiring of the balanced 1/4 jacks.
Good luck, hope the amp you are picking up is in good condition.


Thank you very much for your offer, I am still trying to understand the very basics of this amp. I posted the problems with the amp, as you will read. Amplifier looks good, just a hum as I mentioned in my post...

d3imlay said:
One channel or two? are there level controls? If so do they make any difference?

Both channels have the high frequency hum, but the left channel has low channel as well. There are level controls on the back. High freq. doesn't change with changing the level pots on the back but the low freq. does go away with the the pot turn to lowest on the left channel. When I say high and low frequency it is relative to each other. I don't know what is the frequency of the hum. I did some tests which I have posted... please take a look...

djk said:
Using an ohm meter, check for continuity between the black speaker binding posts and the ground on the 1/4" jacks. Early 500s have a resistor here, if it burns up from a ground loop the amp will hum.

I measured the resistance and it seems like it is shorted to ground, visually I did not find any resistor as well... so the hum continues. I also cleaned the power cord and input connectors but no change. I don't understand why pressing the tone control button on DH-110 increases the hum... maybe it is another issue with the DH-110 but to check that I have to get another amp... Hmmmmm.
Some Measurements...

Hi everyone,

I did open the amp and the two power cap are the original ones I think, I have attached the picture. I noticed that on the negative rail cap the pressure valve is released (?) by the looks of it. The orange rubber is popped out. The positive rail cap looked okay as you can see in the attached images. I also checked the ripple with an o'scope and found it to be 180mV on positive side and 160mV on negative side. Is this okay? Are the caps busted which is causing hum.... Also I noticed that the caps discharge within a minute of turning off the amp. There is not bleeding resistor. Isn't this quick for a 20KuF cap? This is what I could test so far.

I couldn't get to this before today... thats why delayed replies from me as well. I hope this helps... I have also attached the input connectors picture, where the level control pots can be seen for both channels...



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I think the caps are OK for now. If a cap vents I would expect to see a pin hole in the rubber and residue around it. There is probably 2 100mfd/100v caps on the driver board that can open up. As I recall there used to be a 47ohm resistor from the input ground to output that used open up.