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Articles for Audiophiles by Steve Deckert





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

Hum is a 60 cycle (50 cycles in non-US countries) tone that lives in the noise floor of all audio gear. The question is how loud is the hum. Is it a benign background sound of a glowing tube amplifier that you have to really try to hear, or is it easily heard from the listening chair. Have you become neurotic about it because you think it means the amp can't sound good, or that the hum and the sound quality are somehow related? Do you obsess over it with you ear inches from the woofer of your loudspeakers?

If the hum can't be easily heard from the listening chair, you can rest easy that it has no effect on the sound quality of the amplifier. In fact, when an amplifier is designed the amount of AC ripple in the power supply that gets through as hum is determined by the desired dynamics. If we use higher resistance or more filtering in the power supply to reduce hum, the dynamics get softer. So as a designer you pick your target efficiency of loudspeaker in this case 100dB and listen to music with the amplifier while you play with these values and basically see how dynamic you can get it without hearing any hum.

This value has been determined acceptable because to hear any hum on a 90 dB speaker you have to put your ear on the dust cap. 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. So far we have been focusing on the the first.

The first type of hum problem is not considered a problem if it's barely detectable 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 today's 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 component or wire to do its 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 or it's tubes.


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 procedure above with input tubes. If that doesn't solve the problem, try reversing the output tubes by swapping 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. For tube pins alone, a pencil eraser works very well.

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. In this case you will hear the hum stop when you start bending the interconnect cables around.

Faulty input jacks on the amp, or on your source or preamp. Could be loose causing poor ground contact creating a ground loop.

Plugging your source and or preamp in a different outlet than your amplifier. This can cause a ground loop because the resistance to ground from the different outlets may vary.

Using two to three prong adapters to lift the ground. This will turn the chassis of the amplifier into an antenna and it will become noisy and hum as it pics up all the magnetic waves in your room.

Running interconnects parallel with power cords. This is especially true with un-shielded cables.

NOTE: in 2023 an online troubleshooter was installed on the contacts page. It can help you diagnose hum problems.


Articles are (C) by Steve Deckert / DECWARE High Fidelity Engineering Co.

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