Power supply hum transmitted to loudspeakers

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.

@normie57, it could be the quality of the AC coming into your home from the grid. Even though you have a dedicated line from your circuit box, it is not completely isolated from noise. 

Normie, FWIW my speakers are rated at 98 db/1 watt/1 meter.  The gain of my amp isn't specified, nor is its sensitivity (gain can usually be calculated to a good approximation from the combination of sensitivity and maximum power capability specs), but I believe its gain is fairly high (in part because I am using the zero feedback setting of its feedback select switch).

I can hear a faint buzz when my ear is within about 3 inches of the mid-range drivers or the tweeters.  I can hear a faint hum when my ear is within about 10 inches of the woofers.

Ralph (Atmasphere), thanks for your characteristically informative post.  If I may offer a correction to a slight oversight in your arithmetic:

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.

Should be:

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 21db, which is 126:1 in amplifier power (IOW, to do what an amp does on a speaker of 86 db, one need have 1/100th 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.

Best regards,
-- Al
 

@almarg 

you already know I have the utmost respect for your "all things audio" knowledge, so I submit the following response to your last post with the utmost respect...

also, I am aware of your Daedalus Ulysses speakers and to my understanding they do not have their own built-in amp, which is relevant to my response....

assuming you have normal human hearing, I would argue you should NOT hear a faint hum when your ears are within about 10 inches of the woofers....

.. 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.

Thanks for your nice comments, Hal.  It's possible that the slight hum is related to the fact that my VAC Renaissance 70/70 MkIII amp is, I believe, a bit more than 15 years old.  Although VAC worked it over about three years ago, and replaced some capacitors among other things.  I'll mention also that the hum is apparently not the result of a ground loop, as I had done some experiments a while back to rule out that possibility.

Also, it seems that hum levels produced by this amp, and also its smaller brother the Renaissance 30/30, are particularly sensitive to heater-to-cathode leakage that may be present in its small signal tubes.  See this thread.  Although the tubes I'm presently using measure fine in that respect, on my Hickok 800A tube tester.   

In any event, it's safe to say that with speakers having more typical efficiencies of say 90 db the 10 inch distance at which the hum is audible would be substantially less.

Best regards,
-- Al