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I recently resurrected an old Hafler DH-200.
When I tried it in my office system, there was no obvious fault but it didn’t sound as good as the power section of my NAD 3020.
I also noticed that it did not seem to run quite as warm as another DH-200 I tried a while back. From memory, I think that the other one sounded smoother.This raises a few questions
1) Is it realistic to expect the DH 200 to sound better than the 3020 when operating within the 3020’s power capability?
2) What is the proper bias current setting for the DH-200?
3) What is the range of bias current settings that is OK?
4) Is there somewhere I can download a free copy of the DH-200 manual? (I have the circuit diagram and I have a DH 220 manual)Thanks for any help that you can offer.
Regards
Follow Ups:
Try this:
Thanks for the link.
I searched when I bought the amp and I'm not sure how I missed that link. Is the site new?I now have a copy of the Instruction manual but unfortunately it does not indicate the bias current. It just says return the unit to the factory for adjustments! The DH-220 instructions say set bias at 275mA and the output transistors appear to be the same. Does this mean that the bias current would be similar?. On the other hand since the DH 200 gets less power from the same transistors, perhaps it is a bit more heavily biased? I'm guessing!
Thanks again
Those kind of FETs need 100mA per device, plus about 50mA for the front end card.That means about 250mA is correct for the DH200, 220, 350mA for the XL280, DH500, and 450mA for the XL600.
The DH200 pre-dates capacitor no-how that the DH220 has.
Add 0.1µF film caps in parallel with the 10µF electrolytic input coupling cap, and another 0.1µF film in parallel with the 470µF electrolytic in the feedback loop. Also add a 47µF 100V electrolytic in parallel with the 10,000µF 75V main filter caps.
Total of four 0.1µF film needed, 50V polypropylene best, and two 47µF 100V. I use Bennic non-polar electrolytics here, you can't hook them up wrong and they need to be better than the garden variety electrolytic.
This is about $10 of parts from Madisound or Digi-Key.
Thanks for the info.Once I realized I could get info directly from Hafler, I phoned them. Their figure for the DH 200 matches yours at 250mA but they recomend 275mA for the DH-220. I'm not sure why the difference. It sounds as though I could run the DH-220 at 275mA also and get slightly better sound but a bit more heat.
Thanks also for the capacitor suggestions. What difference will they make in the sound?
regards
"Thanks also for the capacitor suggestions. What difference will they make in the sound?"A little cleaner, more detail in the highs, a more open quality. Bass gets a little tighter with the 47µF added.
I've reset the bias to 250mA (It was at 145mA)
It now sounds a lot better and is really quite listenable.
Compared to my Sugden A28B, the DH-200 has more authority in the bass but doesn't match it the initial attack of a sharp note, detail in the highs, cleanliness in the mid range or openness.
It sounds as though the capacitor mods you suggest will be just the thing to improve it in all these areas.PS 1 It has a very slight directly radiated transformer hum which most people might not notice but I'm real fussy. I'm going to see if I can do anything about it and would welcome any suggestions.
PS 2 A friend tells me he had one that he succesfully ran with the bias current cranked right up to about 500 mA. Apparently it sounded a lot better but the heat sinks were almost too hot to touch! Any comments?
I keep a fan on my DH-200 since I seem to have a bias creep on one channel.Make sure your transformer is tight to the chassis.
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