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Old 21st March 2015, 08:01 PM   #1
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Default Hafler Pro1200: Newb Needs Help

This is my first post here,

I have an older Hafler Pro1200 that I use to power a pair of KRK 7000 studio monitors. I've noticed since I bought it off of eBay that it has a large power on and power off thump. I've used a speaker switcher to disconnect the monitors from the amp to sidestep the issue, but I'm getting tired of this. Also, I notice that it has some nasty low level distortion on high frequencies (sounds like a ring modulator mixed at 50% on anything 5khz and up).

I've had soldering experience with cables and a couple of diy studio mic pre kits, but that's about it. I think it might be DC offset due to the old caps, but want to ask here before screwing things up. Here's a link to the manual with schematic:
http://www.hafler.com/pdf/archive/PRO1200_amp_man.pdf

Thanks for any help,

Wes
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Old 21st March 2015, 08:22 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by wesarvin View Post
I have an older Hafler Pro1200. I think it might be DC offset due to the old caps,
It's easy enough to check for output voltage offset, it should be well under 0.1V.
They don't specify how to adjust the output current bias, though.

Last edited by rayma; 21st March 2015 at 08:30 PM.
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Old 21st March 2015, 08:40 PM   #3
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After being on for 10 min, I got 50mv from channel A and 80mv from channel B. I believe channel B was the side where distortion was most apparent.

Last edited by wesarvin; 21st March 2015 at 08:57 PM.
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Old 21st March 2015, 08:57 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by wesarvin View Post
After being on for 10 min, I got 50mv from channel A and 80mv from channel B. I believe channel B was the side where distortion was most apparent.
These offset values are probably not excessive for this amp. Some of the electrolytic capacitors may need replacement anyway.
The bias current, if too low, could be a problem as well. This is adjusted by replacing one of the DC supply fuses with a mA meter
and adjusting the pot for a certain current value after thermal stabilization. They don't specify the current in the manual, though.

Last edited by rayma; 21st March 2015 at 09:06 PM.
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Old 21st March 2015, 08:57 PM   #5
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Oh,

Upon opening the cover, I do notice 4 mini trims on the board. Maybe one of those?
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Old 21st March 2015, 09:00 PM   #6
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Originally Posted by rayma View Post
These values are probably not excessive for this amp. Some of the electrolytic capacitors may need replacement anyway, though.
The bias current, if too low, could be a problem as well.
Would you recommend just looking for the same spec on mouser and go ahead and replace all the electrolytics? The 6800uf ones are only about $5 a piece.
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