Hafler DH-220 Buzz

[theepenguin]

I have two Hafler DH-220 amplifiers. Currently being used in a bi-amp stereo system. Sound great, have no real interest to make them sound better, so don't mention that, unless it's cheap. My issue is that they buzz, most of the time its clearly audible at lower volumes, really annoying. I was thinking that maybe someone here could help me with this issue?

Another issue is that one of the amps, on the left side, gets reallyyyy hot. I know these amps run at a higher temp. (~108 F according to an infrared thermometer) The left side of the one amp is running a solid 120~130 F. That amp also buzzes louder than the other (the actual amp itself not what comes through the speakers).

I did not build these myself, but I am fairly decent with a soldering iron, and as long as the schematic is clear, replacing things is not an impossibility.

Thanks in advance...

[kurtbauer]

Your heat issue is likely to be a bias issue. I can dig out the bias spec for you if you need it. Sounds like these are standard units, right?

Is the buzz coming through the speakers? Or is it from the chassis? Or both? Is it a new phenomenon?

[d3imlay]

If it's a mechanical buzz It's almost certainly the transformer. Try tightening the bolts that hold the transformer together and then the bolts that mount it in the chassis.

[jaycee]

Could also be worn out power supply capacitors causing the buzz. Unless it's been done by you or prior to you owning them, they could do with an overhaul of such parts. It'd be worth the expense to replace all the electrolytic capacitors at this stage.

[theepenguin]

Kurtbauer, if you could dig out that spec for me that would be great! (and maybe let me know what you mean by that, not the super amplifier know it all. but if you have an apple computer question, ill be glad to help ) The amps themselves have always buzzed, but not audible unless you stick your head right up to it. The one with the heat issue buzzes worse (as far as actual amp). But the buzz that I have an issue with is the one that comes through the speakers. They both buzz through the speakers, but considering one amp drives tweeters and the other drives midranges, the tweeters is obviously more noticeable.

Jaycee, as far as replacing stuff, I'm up for that as well. As far as I know, nothing has been replaced since assembly, ages ago. I would love it if you wouldn't mind telling me what exactly might be replaced and where I could get those parts.

Thanks guys, I am going to try tightening the transformer later as well. I'm glad I found this forum, take pride in your knowledge!

[jaycee]

http://www.hafler.com/techsupport/pd...20_amp_man.pdf has the full schematic. These were also sold as build-it-yourself kits!

If they're electrolytic capacitors, replace them. All of them. After 20 odd years, they are due for it. Use good quality capacitors - Panasonic's are pretty good.

The hardest one will be the two large caps in the power supply. Pretty big cans. You will probably have to get 80V or 100V parts. It's also worth getting some good polyester caps, say 470nF 100V, and soldering them across the large caps terminals in addition.

The manual above also gives the details on how to re-bias the amp. It would be worth doing this for both amps in case someone in the past decided to fiddle with them. If youre finding it hard to set the bias, it might be worth replacing the bias trimpots.

There's a few hafler mod threads on here that talk about the DH-200 I think - the DH-220 is very similar, so advice there also applies.

[kurtbauer]

The bias setting on the standard DH-200 and DH-220 is about 250mA.

To measure it, here's what you do...

0. BE CAREFUL. You can be seriously injured by the voltage inside the the unit.
1. Turn the amp off and unplug it from the mains.
2. Disconnect input and speaker connections.
3. Remove cover and don't lose screws -- that's my job. Don't separate the amp boards/heatsiinks from the chassis. You can do what you need to do with them in place.
4. Before you touch anything inside, check the voltage on the big can caps in the center and make sure they are at 0V. If you're unsure about how to do this STOP. Don't proceed before being clear on what to do.
5. Locate the two dual fuse clips on he floor of the chassis. One side will be for the positive rails of both amp boards and the other will be for the negative rails.
6. On one of the fuse clips - it doesn't matter which one - pop out of the fuse.
7. Connect the leads of your DVM or whatever across the clip ends. Be sure that the leads are oriented to measure current (Amps) on your meter. Also make sure that your meter can take half an amp or so just to be safe.
8. Follow the wire from the fuse you've removed the amp board and locate the bias adjustment pot. You adjust it by getting your tweaker (screwdriver) straight down into the slot it the center.
9. Plug the amp to power and turn it on.
10. Adjust the pot to so that the meter reads 250mA. The let it run for 5 minutes or so and readjust it.
11. Power down the amp and disconnect the power. Wait for the big caps to discharge before sticking your hands in there.
12. Now repeat same procedure for the other side by replacing the first fuse and work on the other side of the clip.

Once you've got the bias adjusted, you should check the DC offset on the speaker terminals.
1. With all of the fuses in place, power up the amp.
2. Measure the voltage from the positive speaker terminal to bar between the two can caps (this is ground). Ideally this should be 0 and be less than 125mV.
If you've got excessive DC offset, then the input stage of the amp board has drifted over time and needs to be fixed. It's doable without breaking the bank, I think.

For more info about the amps you can get the manual from here.

If you were to search on "Hafler DH-200" you'd find this megathread. It's a little obnoxious to slog through but there's a LOT of info in here about what you can do with the amps from the simple to the esoteric.

Hope this helps.

kurt

[kurtbauer]

To follow up on jaycee's thoughts...

John Hillig at Music Concepts near St. Louis sells his LC-200 Capacitors that will work in a DH-220 if you decide to take the plunge. He recommends bypassing the caps with 47µF/100V Blackgate in parallel with a 6800Ω/2W resistor. I did this in my DH-200 along with his amp boards and it's pretty great.

As for the DC offset that I mentioned, I had forgotten that the DH-220 has provision to adjust that too.

Cheers.

k

[d3imlay]

Years ago I had a DH200 that was oscillating because of an open electrolytic on the power supply. An oscilloscope would tell you that. If it was oscillating above the audio range you would probably have blown tweeters.

When the amp is turned off it will continue to operate a short time with the stored energy in the caps. with no input signal, does the buzz stop immediately when the power is turned off?

[theepenguin]

Tinkered for a couple hours this afternoon, and got everything! I reset the biases on both amps the troubled on was at 500mA (there goes a couple years off the lifespan). The DC offsets all appear to be good as well. The amps have been playing for about half an hour now and all temps appear to be the same (98-104). Thanks guys so much! Especially Kurt, those directions helped. I have the manual/assembly instructions for these things, it would have taken me a little longer to figure out what they wanted me to do.

Real pleased guys! Glad I found such a helpful community.