Power supply hum transmitted to loudspeakers

.. the faint buzz when your ears are within about 3 inches of the mid-range drivers or the tweeters is perfectly "normal".  

Back to the hum/woofer... I submit that at worst, you should hear a faint whisper of air when your ear is essentially right on top of the woofer and the amp is on. But a hum, at 10 inches?

I'm very much open to debate on this as in my opinion this simply should not be the case

@almarg
Thanks for that correction!

Al, even at 3 inches with your speakers I would not expect any buzz (maybe some hiss, but not buzz).

Nor hum at 10 inches.

One thing that can cause hum in amplifiers employing single-ended circuits is when the grid circuit of a tube is grounded independently of the cathode circuit. IME the two should use the same circuit (the same wire) to ground. In this way the differential effect of the grid and cathode circuits can prevent noise on the ground connection from being picked up. This is especially important if you run an AC heater circuit on the tubes!

Also, if you do have AC heaters, if the tubes show any leakage at all on the leakage test on a tube tester, they will introduce buzz. We've found that the Russian small signal triodes tend to have problems in this area, so if you are running EH, Sovtek or the like it might be a good idea to change them out and see if the problem is still there!

With your fine OTLs and the sensitive loudspeakers you often use, can you hear any noise/hiss 1-2 ft from your loudspeakers at idle? My VAC amps and my AudioNote Kit Interstage Monoblocks both have low level buzz audible 1 ft from my KEF Ref 3's, not particularly sensitive loudspeakers.

I need to nail down some variables here. The speakers I have at home are 98 db; at shows they are either 98 or 107 db.

With 98 db I have to have my head by the mouth of the mid-range horn to hear anything. What I hear is a gentle hiss. With the 107 db speakers the hiss is more apparent- so we developed a little 'jumper plug' that replaces a tube in the voltage amplifier of our amps. With the plug installed where the tube was we knock out about 12 db of gain and then the hiss is back down to an acceptable level.

When designing an amplifier or preamp the challenge is that the end user might have a speaker of only 86 db (which is bordering on criminal IMO, due to the vast amount of power needed to make that work in most rooms, at least at the levels I like to play...), and at the other end of the spectrum is 107db. That's a range of 30db, which is 1000:1 in amplifier power (IOW, to do what an amp does on a speaker of 86 db, one need have 1/1000th the power to do it on a speaker of 106db)! Obviously this is a trick to make an amplifier of the same gain work on a range of speakers like that.

For this reason amp manufacturers tend to make amps of more or less gain depending on the speaker that might be used with their amps, which has a lot to do with how much power the amp can make. If a really high powered amp (+500 watts) the gain is going to be 30-36 db (there are some outliers; the Hurricane amp has 50db!); for a really low power amp that will only see use on speakers of +100db, the gain might be only 12-15 db. We typically set our gain around 25db, which is fairly reasonable to work on speakers from about 87db up to about 101db or so.

Keep in mind that on a really high efficiency speaker, its unlikely that you won't hear some sort of hiss or buzz coming from the electronics. It can get to be a bit of a challenge to design for speakers like that, even if your amp is only making a few watts.

So to answer your question with all that in mind, it sounds like you have a problem somewhere- with KEFs I would expect that you would have to lay your ear directly on the tweeter before you could hear any noise floor at all.

have had their power supply upgraded, but still has a small amount of buzz at the loudspeakers, not audible while playing music beautifully.(with and without shorting plugs). I've learned to ignore it and realize the amplifier was meant to play music, not to necessarily be dead silent at idle.

Its worth getting to the bottom of little buzzes like this- you will notice the system sounding better. The reason is, depending on how the buzz originates, it can cause intermodulations. Its worth it to get them gone!