hum from speakers

[pyonc]

Hi friends,

I've got a vintage JBL SG520 preamp in good working condition, but the only issue is the very audible hum from speakers when I hook it up to a power amp. Strange thing is when I hook up a different preamp to the same power amp, I don't hear any audible hum. So, it looks like SG520, not the speakers, has an issue here. Not sure if its' caused by ground hum or some outdated parts (faulty filter capacitors). Before I take it to the tech, I'd like to ask for your opinion first.

[HCSGuy]

Considering you are dealing with vintage equipment, some level of hum is probably unavoidable - I remember hearing hiss and hum in my father’s systems as a kid, and they were all considered high end for the time. However, the hum should not be audible over quiet passages of music and should be barely audible at a reasonable seating distance (15ft maybe?).

As far as testing, I would first disconnect everything from the preamp except power, and put together a few adaptors to let you plug a pair of headphones into the preamp outputs. You shouldn’t have to turn up the volume to hear if the hum is still there, but if you do, keep it low. If the hum is still there, have a tech look at the preamp and give you his judgement. If the hum is gone, go to step 2:

2. Connect your amp, but with no sources connected to the preamp. Does it hum? If so, does the amp have a 2 prong or 3 prong power plug? If it hums, and the amp is 3 prong, you can try testing with a ground removing cheater plug like you get at a hardware store, but this is temporary only - not safe to leave it this way. If it does not hum with the cheater plug in, you can go to 3 and start connecting sources, and also shopping for a different amp - sometimes you just get an amp that doesn’t work well with your preamp.

3. Sources - are any of them grounded? This could be a satellite receiver, cable box, or anything with a wired ethernet jack that you have connected. If you’re a vintage nut, this is not likely, but if so, you will need to explore ways to ground your audio system to the house ground - you can try attaching a wire from your preamp’s chassis to the safety ground of your power outlet and see if it helps. I have not had a problem with modern sources (CD players, etc) causing hum as they are usually 2 prong plugs that are not grounded, but let us know what sources you have and how they are connected.

This is a start; sorry for the shotgun approach, but I’m interested to hear what you find on these.

[pyonc]

Considering you are dealing with vintage equipment, some level of hum is probably unavoidable - I remember hearing hiss and hum in my father’s systems as a kid, and they were all considered high end for the time. However, the hum should not be audible over quiet passages of music and should be barely audible at a reasonable seating distance (15ft maybe?).

As far as testing, I would first disconnect everything from the preamp except power, and put together a few adaptors to let you plug a pair of headphones into the preamp outputs. You shouldn’t have to turn up the volume to hear if the hum is still there, but if you do, keep it low. If the hum is still there, have a tech look at the preamp and give you his judgement. If the hum is gone, go to step 2:

2. Connect your amp, but with no sources connected to the preamp. Does it hum? If so, does the amp have a 2 prong or 3 prong power plug? If it hums, and the amp is 3 prong, you can try testing with a ground removing cheater plug like you get at a hardware store, but this is temporary only - not safe to leave it this way. If it does not hum with the cheater plug in, you can go to 3 and start connecting sources, and also shopping for a different amp - sometimes you just get an amp that doesn’t work well with your preamp.

3. Sources - are any of them grounded? This could be a satellite receiver, cable box, or anything with a wired ethernet jack that you have connected. If you’re a vintage nut, this is not likely, but if so, you will need to explore ways to ground your audio system to the house ground - you can try attaching a wire from your preamp’s chassis to the safety ground of your power outlet and see if it helps. I have not had a problem with modern sources (CD players, etc) causing hum as they are usually 2 prong plugs that are not grounded, but let us know what sources you have and how they are connected.

This is a start; sorry for the shotgun approach, but I’m interested to hear what you find on these.

Thanks for your kind and detailed reply.
I tested two highly efficient JBL speakers (L101 & Olympus) with this preamp, but the hum was pretty much audible even with the volume all the way down. Googling on this, it looks like some faulty filter capacitors are to blame for this very audible hum. Not sure, though.
Anyway I took it to a tech shop for checkup and possible service.