2. amps-preamps
  3. 1233992640

Newbie Troubleshooting Question


I am new to using a separate amp and preamp so I apologize in advance if this question has an obvious answer. I recently purchased an ADCOM GFA-555 amp and a Hafler DH-101 preamp. Both are older units and work but... When the volume is turned all the way down, there is a slight hum in the right speaker that I can hear up to about 10 feet away but the left channel seems fine. There is also a very small amount of hiss coming from both speakers if you hold your ear within a foot of the speaker and the volume is all the way down. Using the balance control on the preamp to cut off the right or left channel does not make the hum go away. I read a little about ground loops and tried inverting the plug on the preamp and even moving it to a different outlet but it didn't seem to make any difference. I also tried switching cables from/to the amp/preamp. I also tried using a different inputs on the preamp. I did not try moving/switching speaker wires/channels.

MY QUESTION.....
Is there a simple method for me to determine whether the problem is in the amp or in the preamp? I'm hoping it's the preamp because I don't have much invested in it and can probably still get my money back on it.

Any suggestions will be greatly appreciated.

One other very minor issue which I'm pretty sure is the preamp. When you turn the volume all the way down the music is still audible. Not in a bleeding or feedback kind of way but just like the volume POT is not adjusted right. Again this is a minor issue I can live with but if it is a simple, idiot proof adjustment, I might be tempted to make it.

Thanks again for any input...
grundig
rodman99999
6,132 posts
 
The capacitors in the unit are past the 20 year old mark by a very comfortable margin(going on 30). No doubt- Ripe for replacement. I'd start with the power supply caps. Were you provided with a manual when you bought the unit? If not- Buy one on eBay. That will give you a complete list of parts, and the full construction instructions that originally came with the piece. The circuit board was pre-assembled, but the parts layout, and schematic are included in the manual. Then too- A few people offered power upgraded power supplies for the 101. Some are still available. Here's something on eBay that may interest you: (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260359603612&ssPageName=MERCOSI_VI_ROSI_PR4_PCN_BIX&refitem=260350575646&itemcount=4&refwidgetloc=closed_view_item&refwidgettype=osi_widget&_trksid=p284.m185&_trkparms=algo%3DSI%26its%3DI%26itu%3DUCI%26otn%3D4%26ps%3D42) BTW: The S/N ratio was high enough that no hiss should be audible, especially with the volume turned down. I was the Hafler dealer in Orlando/Winter Park, FL for a few years, and used the 101(modded) in my demo system back in the day(early 80's). No noise at all would have tolerated in my audition room. RE: The power supply fuse- That's just what some slo-blo fuses look like. RE: Getting electrocuted by a cap- The voltages in a SS pre-amp are too small to worry about. If you can solder, you can handle the DH-101 upgrade. It would be a great way to get involved in the DIY electronics/modding hobby(It's highly addictive).
grundig OP
8 posts
 
Thanks for the advice Rodman. By the way I used to live in Orlando before I moved to the Dallas area. I had seen that kit and another on that website. Do you think this kit would eliminate the hiss and the hum? I'm thinking the hum is from a loose ground related to the right channel, but what I know about electronic circuitry wouldn't fill a thimble. Gosh I hate being so stupid about something that I know is ultimately very simple. I would probably end up soldering one of those parts in backwards and frying my preamp, amp and setting the house on fire. There's an electronic shop down the way from me... Maybe I could get one of those guys to put those pieces in for me. I don't think it would take more than about 20 minutes to do.

I actually have the assembly/owner's manual I found somewhere on the internet. Anybody wanting a copy can send me a message and I'll email you a copy.
grundig OP
8 posts
 
P.S. Free Download of Hafler Manuals Here:
http://www.hafler.com/techsupport/index.asp?ID=3
grundig OP
8 posts
 
Well I opened the preamp up, inspected what I could, moved wiggled and jiggled and the hum dropped about 90% which I can live with for now. I hooked up the Kenwood preamp I paid $12 for the other day thinking the sound difference wasn't as vast as I had imagined and could give the Kenwood a second chance. Guess what? I was wrong! It's amazing how much difference in sound there is between the two. I swear it's like night and day. The Kenwood is quieter at low volumes but the sound difference is drastically worse than the Hafler. It's hard for me to understand that there is as much difference in sound quality as there is because I'm certain that the Hafler is not in 100% working order. Maybe I should send the Hafler and have it completely rebuilt. Are newer designs markedly better sounding than a 23 year old Hafler that is in good working order. Maybe I should just plunk down the cash for a new preamp and forget about worrying about worn out capacitors etc. Hmmm....
rwwear
2,867 posts
 
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