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In Reply to: RE: No posted by drlowmu on December 02, 2016 at 15:53:29
Jeff,
what AC level of hum do you have across a 8 ohm load with inputs shorted using a DVM ?
Cheers Johno
Follow Ups:
Hi Johno,
After ten minute warm up this morning, about 11.5 to 13 mVAC , inputs shorted, at speaker terminals, using Fluke 8060A DMM. JJ 2A3-40, AC heating.
I have 515B ALTEC woofers, horn loaded in VOTTs, in a small living room.
Had to put my head IN the horn section, while standing at the speaker, to hear it.
Jeff
3mV noise from my previous 845 SET amps were loud enough to hear from 6ft away on my 100dB+ 8ohm nominal load speakers at night.
My current amps have about 0.5mV or less which quieted down my speakers at night.
Something's odd with your numbers and your speaker efficiency.
.
.
.Thou shall not stand where I type for I carry a bottle of Certified Audiophile Air and a Pure Silver Whip.
I have only had this amp for a couple of days, only been on five hours for listening. and so, it could be a measurement problem. I am not too worried about this at the moment, it does not seem to bother me right now, and I have other things to work on.
I will re-look this over in January. As best as I recall, I had 4 or 5 mVAC on my 45 amp, and it was audible to me in October, 2016.
I do not want to throw out the baby with the bath water. Its easy to build a DC supply for a 2A3, and get low numbers, but AC heating will typically sound better.
Jeff Medwin
I think Tre has nailed it down, His speakers present a "suck out" at or around those freq.
Jeff I think its time to get your large fultons back and build a higher power SE amplifier
Lawrence
There is something wrong with the numbers.
11.5 to 13 mVAC is going to be loud on a 515 no matter what type of box it's in.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I am quite puzzled by this. I just tested with 98dB speakers, and 12 mV ac or so as a background hum is almost intolerable to my ears. There must indeed surely be some mistake in the measurements Jeff reported. I cannot believe anyone could put up with that level of background hum in a supposedly high fidelity amplifier.Even 1 or 2 mV is still enough to be irritating, I think. There is really no excuse for having any perceptible level of hum in an amplifier. And claims that killing the hum necessarily kills the music are really just hogwash. If an amplifier needs added mains hum in order to make it sound good, then there is something fundamentally wrong with its design or implementation.
Chris
Edits: 12/04/16 12/04/16 12/05/16 12/05/16
is it a 120Hz or 60Hz?
I have 0.25mVrms @ 100Hz (in EU) on my latest 801A direct coupled amplifier ... just finished yesterday (well ... I am missing the definitive 4pin sockets actually). I love direct coupled amps
:-)
in my fathers amp I think little bit of both 60 and 120 cycle.
no direct couple for me as I am trying to preserve "dynamic transconductance" and this can only be done with plate choke/grid choke RC couple system this combined with lower power supply DCR works is what I am doing
Lawrence
Jeff, something must be off here as I can hear on my fathers less eff altec 98db watt/mtr speakers 5mv hum! and this is a few feet away.
so at 12mv it should be very loud with your higher eff horns.
what am i missing here??
Lawrence
I agree, the hum should be loud even if the 515 was in a bass reflex box (which it, kind of, is).My point is, I don't think the horn has any effect at 60Hz and very little (if any) at 120Hz.
"The A-7 horn is a 110Hz exponential with a half size mouth according to Bruce Edgar and the few FR plots I have shows it unloads at 175Hz."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 12/03/16 12/03/16
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