A U D I O... P A P E R


 FEB 2003
by Steve Deckert


While this information may in part be aplicable to other tube amplifiers, it is intended only to help people who own our Zen Triode amplifiers diagnose and indentify the actual cause of hum problems in their systems.

You might find some irony in the fact that I would do a paper on solving hum problems when our amplifiers are known to have one of the quietest noise floors in the industy, but the assumption that the amp is always the cause is generally incorrect.

Hum is a 60 cycle (50 cycles in other countries) tone that lives in the noise floor of all audio gear or when problematic, becomes the noise floor.  Hum can be measured at the output of any amplifier with a decent AC volt/ohm meter.  The proceedure is simple; Just hook your speakers up to the amp, remove the input cables from the amp, turn the gain knob (if it has one) all the way down.  With your meter connected across one channels output jacks (with the speaker hooked up at the same time) your meter should slowly drop to below 2 milivolts AC.  (All of our amplifiers fall below this value.)  

This value has been determined acceptable because to hear any hum on a 90 dB speaker you have to put your ear on the dustcap.  On a 100 dB speaker you may hear it as far away as 2 feet in a quiet room.  The goal is simple, once you're in your listening chair you should hear nothing.  

There are two types of hum problems.  The first is a low level background hum that you can notice when the music is not playing.  The second is an objectionable hum that makes trying to listen to music basically not an option.  

The first type of hum problem is not considered a problem if the amp sounds so good when music plays that you simply don't care.  Every manufacturer gets to guess where this threshold is because it's subjective to every listener.  High power amplifiers coupled to power hungry speakers (the norm in todays homes) make this threshold easy to beat but take a high efficiency speaker and suddenly any hum or noise that was there is now 10 dB louder!

The second type of hum problem is a mechanical failure of some componant or wire to do it's job.  This can be anywhere from the source to the loudspeakers, although the problem typically stops at the amplifier.  The first objective is to find out if the hum is being amplified by the amplifier, or if it is coming from the amplifier itself.  This is easy to do, just unhook the single cables feeding the input of your amplifier.  This breaks the signal path at the point of the amplifier.  If the amplifier stops humming, you know the problem is before that point.  If the amplifier still hums, there is a problem with the amplifier.




STEP 1 - Remove the interconnects from your amplifier so that the only thing hooked to it is your loudspeakers.  Turn the volume control (if it has one) all the way down.  Turn on the amp and let it warm up for a minute.  

I hear hum from both channels - If you hear hum from both channels it could be caused from a bad input tube.  Turn the amp off and remove the input tube.  On the SE84C, SE84CS and SV83M this is a single tube located farthest towards the front of the amplifier.  On the SE34I and TORII this is the small tube located farthest towards the rear. (Note: Only the TORII-C has an input tube, the TORII-A and TORII-B do not). Turn the amp on without the input tube.  If the hum stops, try a different input tube.  If the hum continues with different input tubes, and if your amplifier has a rectifier tube, try replacing the rectifier tube.  If the hum continues there is a problem with the amplifier.

I hear hum from only one channel - Hearing hum from only one channel usually means that you have a bad tube.  Try the same proceedure above with input tubes.  If that doesn't solve the problem, try reversing the output tubes by swaping the left and right tubes.  If the hum is now on the other speaker, replace that output tube.

It is important to have clean tube pins.  While this usually won't cause an amp to hum, it can add to the noise.  A good tuner cleaner/lubricant works well for this if used sparingly. Connectors are equally important. Use it on both the input and output jacks also.  

If you have determined the hum is not coming from the amplifier, the most likely causes are as follows:

  • Intermittent interconnects that have a broken or cracked ground path.
  • Faulty input jacks on the amp, or on your source or preamp.
  • Plugging your source and or preamp in a different outlet than your amplifier.
  • Using two to three prong adapters to lift the ground.
  • Running interconnects parallel with power cords.


Decware is a trademark of High Fidelity Engineering Co.
Copyright 1996 1997 1998 1999 2000 2001 2002 2003 2004  2005 2006 2007 2008 by Steve Deckert