I would argue the furnace is really well-grounded and so when at work all that circuit's parasitic currents go to ground, relieving the amp from those nasties while that happens. Try to use a dedicated circuit for your audio and make sure it is efficiently grounded. I may be wrong, so better ask an expert.

That sounds correct, from my general troubleshooting.
I'm been thinking of buying a Topaz isolation transformer, not quite sure that would fix the problem though.

I've been using a Tripp-Lite Isolation Transformer (500 Watts) for years now, and I find it indispensable. Yet, although it is very useful in filtering noises coming from the mains, I'm not sure if it will work as a "humming blocker". It essentially isolates the mains from your gear through the transformer's secondary terminals. The issue is if the humming originates in your audio system's circuit or elsewhere. If it is the latter, it will work. BTW, I've never experienced any audible humming.

You wouldn't, if you use the amp as intended with speakers. Which is what I do 95% of the time. I made myself a speaker taps to 4pin xlr adapter, so I could use the FB with my hd800.

I've actually talked to Dennis about this at some length in the past. He said it's because of the space limitations, he had to wire the right channel by the PSU, thus creating the hum. Again, can only be heard if using headphones, which are many times more sensitive than speakers.

I also own his headamp design iha1, but unfortunately while great, doesn't recreate the magic of the FB. I already have word he's working on a new head-amp though, which I'll be getting to replace the iha1.

At the end of the road, the FB hum is my issue as I'm not using the amp as intended. More than anything, I just found it curious that load on the house circuit removed the hum.. still got me thinking how to replicate that...

I actually already own a Triplitt isolator and a double conversion UPS as well, neither of those fixes the hum. I've thought about trying to rewire the right channel not close to the PSU, but Dennis epoxied all the wires in the amp, so eh. It really is whatever, as the FB is running my Omega Alnico speakers 95% of the time. Just gotta wait for that new headmap design.

Like I said, if the hum originates in your audio circuit the isolation transformer won't fix it.

Classical music dynamics
 

A question to those of you enjoying classical music, especially large orchestral selections. There are evident limitations with flea-powered amps, like our otherwise excellent Inspire units, when playing those intense but certainly essential "tutti" passages.

My question relates to what are some of your suggestions to alleviate those limitations, be it the use of larger tubes, specific rectifiers or even speaker alternatives. There may not be much more we can do, but I'm curious to know what do you guys suggest.

The "oomph" factor required when representing climaxes in this genre is an essential cue in the composer's musical message. This consideration has made me consider SS alternative amplification, but I resist loosing the traits of my Inspire-based system, even when playing classical music. Thanks for any feedback.

Using my higher powered Inspire KT88, the 17 wpc version w/20w output transformers, with KT120 output tubes and GZ34 rectifier, thus maximizing output power, into my 91dB speakers. I find that this combo falls short on classical music at higher volume levels. There just isn't enough power to faithfully reproduce cresendos, deep organ notes (a la Bach), tympani drums, etc.

I think the only path is to have truly efficient speakers. And I mean truly efficient. Many speakers are sold today with efficiency ratings of 95-97dB and yet are really 91-92dB, essentially the same as mine, which truly met their 91dB spec. This include speakers from Zu and DeVore. Even the expensive DeVore O/96 measures at 91dB.

I love listening to artists like Dianna Krall, Holly Cole, Bob Dylan, Norah Jones, CSNY, Sade, and more on my system. They sound so natural. The sound is mesmerizing. The Inspire is incredible on such material. But not so on classical or hard rock.

Do you use that same tube configuration for the less demanding vocal/acoustic selections? I wonder if it would be worth it to get into KT-120's, but as you pointed out, these would probably won't do the trick either. My speakers are theoretically 98 dB/W/M but I suspect these are more like 95 dB/W/M given their impedance distribution.

The idea of much higher sensitivity speakers (i.e., 104 dB/W/M, or so) has intrigued me for some time, although these unavoidably incorporate some type of horn driver design and the affordable horn-type speakers tend not to be that impressive. It's either that or go back to sand power at lower SQ levels.

I do use my KT120/GZ34 combo for all types of music, however I sometimes sub in other output tubes - KT77, KT88, KT66, 6L6GC, 6CA7. Also sub in a 5U4GB from time to time. I haven't yet decided what my favorite combos are, and for what types of music. Narrowing it down. Just got some Gold Lion KT77s, so now I've further muddied up my choices. Had a pair of JJ KT77 before this. I do like the GL KT88s a lot.

Wish my speakers were 95dB. That would make my 17 wpc Inspire sound the equivalent of a 40 wpc amp with my 91dB speakers. I suspect you have a 12 wpc Inspire, that is the standard model. Still a 12wpc with a 95dB speaker will be louder than my 17wpc - 91dB combo.

As discussed here several times, you can get more out of these amps if you use a powered subwoofer. Both of my Inspire amps struggle to produce loud deep bass, going into clipping well before they clip in the midrange. When I filter off sub-80Hz frequencies from my amp, it runs like it is twice as powerful. I've been surprised how loud it can play in that configuration. Still, it is really taxed by classical music cresendos. When played at concert levels, those can need a couple hundred watts on peaks - unless one has very efficient speakers.

Some of those big Klipschs measure at around 104dB. That means they need 1/20th the power that my 91dB speakers require. Down side is that you then have to listen to Klipschs all the time. :)

My HO amp sings with my supposed 91db speakers, helped along by 2 REL subs…no filters on the amp…with plenty of dynamic volume level sent to my earballs 8 feet or so away. This is good news, and I know about concert level sound as I've mixed live concerts professionally for decades (often at disturbingly loud levels…but clean…ya know?). Also, since the HO showed up I've been tube rolling the bejeezus out of it and have decided that for my rig the Gold Lion KT88s, Amperex 6SN7GTB, and Shuguang 274B are the winning combo…KT77s don't have the sizzle I demand (DEMAND!) but they're JJs and not GLs so maybe that makes a difference.