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
grundig OP
8 posts
 
The saga continues
HERE
grundig OP
8 posts
 
OK... So I got a $12 Kenwood preamp (no kidding) and even though it was more modern and quiet, the sound was not as good as the Hafler DH-101. Don't ask me to explain it but the Hafler just had a better sound. At any rate I think I want to keep the Hafler but it has a hum in the right channel that you can hear at low volume levels which is where I do most of my listening. It is only in the right channel. Also there is some hiss with the volume all the way down. Additionally the sound will not turn all the way down like maybe a bad pot or something. The balance was off which is part of the volume control but loosening a hex bolt and repositioning the knob took care of that problem. I have found some places on the internet that will go through the unit for $150 to $300 but I hate to pay somebody that much just to replace a resistor or capacitor.

My question... Does anybody have any suggestions on what I might look for or do to eliminate this problem? Is this a common problem with an easy fix? I could probably replace a resistor or a wire but even that would be at the far limits of my expertise. I always worry about getting electrocuted by a capacitor or something. Now I did open the case up the other day just to see if there was something obvious like a loose wire or something but didn't see anything. The unit did have a glass type fuse by the power supply which looked like it had a resistor inside of it. Never seen that before. So if any of you have any suggestions I would greatly appreciate it. I already tried the previously mentioned suggestions. (Thanks RWWEAR & DGAYLIN)
Thanks in advance for any help or advice you might have.
rodman99999
6,160 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
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