Power supply hum transmitted to loudspeakers

Audiobrian 1-20-12017
Al, I do have a question for you. I have both a VAC 300.1a and a Ren Sig 2A Preamplifier. I have noticed no pertinent electrical buzz from either but both do have power transformer hum, not particularly disturbing, but noticeable in a very quiet room. Kevin has suggested different stands that might not resonate but I am still unsure why potted transformers are quite so audible. (truly not disturbing while playing music through Kevin’s most musical components)

I can’t really shed any light on that, Audiobrian. And my Renaissance 70/70 is of course a very different design than your 300.1a.  FWIW, though, my amp makes some very slight mechanical noises during the first 25 seconds or so after turn-on, at which point B+ is applied to the tubes, but it is subsequently totally silent in terms of mechanical noise.

The amp is placed atop a 4 inch Mapleshade Isolation Platform, which in turn rests on a set of Mapleshade Isoblocks.

Good comment by Peter, as always. In my case, I have reason to believe that I have better than average AC power quality.

Regards,
-- Al

Thanks very much, Ralph.

Other than the Sophia Electric 6SN7’s discussed in the thread that I linked to, which caused significant hum in my VAC 70/70 and in another member’s 30/30, the only small signal tubes I’ve used in the amp have been 1950’s/1960’s vintage tubes from various manufacturers. Currently two Sylvania 6SN7GTBs and two GE 6SN7GTBs. Some of those I’ve tried have had unmeasurable leakage, while all of the others (aside from the Sophia’s) had VERY minimal leakage. All of them (aside from the Sophia’s) produced similar amounts of hum/buzz. I don’t know if the amp uses AC or DC on the filaments of those tubes.

BTW, a few months after I posted in that other thread, shortly before the one-year warranty I purchased for the Sophia’s expired, I had them replaced under the warranty with a brand new quartet. The hum level with the replacements was ok for a few months, although a bit worse than with the vintage tubes. Eventually, though, their hum level degraded to the same level as with the first set, and their leakage measured poorly (it was not all that great even at the outset), so I retired them to my tube collection.

In any event, I’ve been very pleased with the sonics of the amp, and when VAC replaced various parts about 3 years ago (followed by a couple of days of burn-in and some listening tests) Kevin Hayes said that I should be good to go for about another 20 years. So I’ll leave well enough alone. BTW, the main reason I sent the amp to them was that one of its numerous power transformers shorted out. They wound up replacing all of the transformers (as well as some capacitors), I believe with ones they use in more recent models, because the original transformers were no longer available. They had to make some mechanical changes internally in order to accomplish that, but it worked out very nicely.

Thanks again. Best regards,
-- Al

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
  

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
 

I agree with Cleeds. To make a meaningful test of an amplifier's intrinsic noise level shorting plugs should be connected to its inputs.

Many choices of RCA shorting plugs can be found at eBay, by entering "RCA shorting plugs" into its search box.

Regards,
-- Al