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Old 12th September 2006, 08:46 AM   #1
diyAudio Member
Join Date: Aug 2002
Location: Belgium
Default tube amp subwoofer hum problem

On hooking up a REL sub via the "high" inputs to the loudspeaker output on my KT88 tube amp, I get a terrible hum.

This is not so on hooking up passive speakers (there still is some hum, but not uncomfortably so), nor is there any hum when I hookup the REL in the same way to another (SS) amp, using the same passive speakers.

Any idea on the cause and/or possible remedies ?


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Old 12th September 2006, 12:56 PM   #2
MarkER is offline MarkER  United States
diyAudio Member
Join Date: Aug 2005
Default One potential solution

I've had the same experience with hum on my subwoofer with tubes (preamp) and not with solid state. I have effectively killed the hum using Jensen Isomax transformer that is designed to eliminate RF induced hum. I don't know whether that is the cause, but it does work. There are both stereo and mono versions depending on whether your subwoofer has summed, or discrete channel inputs. See the link:
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Old 13th September 2006, 11:24 AM   #3
diyAudio Member
Join Date: Nov 2002
There is always a possibility that one might end up with hum when interfacing different components.

A cheaper way to go might be to hunt thru ebay and snag up some 15k-15k altec Lansing transformers or a 600-15k or 600-600 depending on what your going to hook up.

Generally the Jensen transformers work ok but I have seen times when they don't which is true to anything.
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Old 13th September 2006, 01:41 PM   #4
MarkER is offline MarkER  United States
diyAudio Member
Join Date: Aug 2005
Default I agree

There are combinations even in my system where the iso-max does not give much help, probably because of different resistance/impedence requirements as given in you example. I'm also using Magnequest B7 600-15K output transformers between my tubed preamp and amp to kill hum that comes in with the use of step-up transformers for a low output moving coil cartridge. The iso-max has no effect on that part of the system.

However, you can get the iso-max products at a very nice discount from Markertek ($120 versus $165 list for the stereo, $100 for the mono) and they are ready to use as is.

Unfortunatley, all transformer-based solutions require spending some money with the chance they may not work unless you are lucky enough to have tried one from a friend first to see if it does work in your specific application.
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