How deadly is transformer hummm...

There is a sense that the amp is on if you listen very close to the transformers, but any hum is at such a low lever as to be virtually inaudible unless maybe you listened to it with a heat resisting stethoscope. Note that relative to any other amp I've owned, either transistor or tube, there is nearly zero noise even if you put your ear on a driver...the the Firebottle is by far the quietest amp I've ever owned. 

@jea48 This two-story single family dwelling was constructed in 1986. I have not checked the mains voltage. I imagine this requires a volt meter, which I do not possess. I see a number of them for sale at the Home Depot, which is the closest such facility to my home. Is there a special type I should be using for this test? 

Hum is my number one bugaboo. I can not stand it .. Often when asking a seller if there is any hum in the unit and being really specific that Im referring to the amp itself they will respond with --oh, its very quiet from the speakers. thereupon now comes a lengthy explanation from me.
I have never held onto anything long that had hum. It often can be in the power supply design but also being directly coming from a transformer Ive been told that it is the plates vibrating. On new transformer in a new unit like the one you got I cant imagine -either p.s. design or cheap transformers .. Im not a tech so cant identify exactly but pass along my experience. I had a custom 2a3 SET built once and it had a hum and the builder used a recycled power transformer ---how old was that thing ? I sent it back,, and he didnt do a thing about it. I sold it. Really had good sound but the hum was so obvious it could be heard from 5 feet away even with music on low. Send it back to Had or at least complain. ruins the experience.

I had a 80's  Sony STJ75 tuner that is well known how black the noise floor is but it had a hum ...  Sent it to ASL   Absolute Sound Labs and he actually had the transformer re-potted .  fixed the hum.     

@wolf_garcia My wife thinks I'm being too sensitive about the hum, which doesn't show up at the speakers, as I've already described. Given the sellers description of the "quietest amp he's ever owned" I figured it should be literally dead silent, but now I'm hearing that there may be some small sound issuing from some of these amps, and perhaps the old circuitry of my dwelling is going to place the amp under different forces/conditions than what the seller experienced. As I've also described, it's definitely not impacting the sound, for the system is presenting, along with the speakers, the most beautiful music I've heard in this space. My main concern is longterm difficulty and having just picked up the amp I wanted to address it right away. 

Would this do the job?

https://www.homedepot.com/p/Commercial-Electric-110-220-Volt-AC-DC-Voltage-Tester-VT-8900R/202520892...

As long as your home AC is within reasonable spec I dont think it would be the cause of tranny hum. The type of hum from home AC would likely be most evident at the speaker. Although good to check your AC voltage although that tool is just to check the circuit and wont give you an actual  reading.  I suspect that the 110 volt will light up even if your at 120 or so--probably even at 130 ...because at times that is what home ac is.   Thats what it was at my previous home and I had
utility company come over and even he had to comment . well the AC is a bit high here and he went to the sub station to lower it for the neighborhood I guess.

Its in your amp for any number of reasons. As Atmasphere mentioned be sure to check the bolt down for the PT and put rubber washer in there as well if you can.

I want to update this thread, bring it full-circle, so to speak.

I was able to finally develop a conversation with Dennis Had, who built the amp. He asked for a picture of the amp, which has been sitting on a thick slab small side-table type of stand, or pseudo-stand... He then asked me to put the amp on my kitchen counter and plug it in... he said the Inspired design is stable, doesn't need speakers, so I didn't have to haul any other equipment into the kitchen. 

The amp was noticeably more quiet in the kitchen. 

He then suggested I buy some sound coat, or similar product, claimed that the wood stand acted as a wood resonator, much like a piano. 

This is great news, because it's a wonderful piece of equipment and I've got some new open baffle speakers on order, which should complement the amp nicely. The speakers are based on the F-15 Lii Audio driver and boast 97db efficiency.

This will be a single-driver setup - I'm thirsting for a vast soundstage!

Thank you all, for participating in this discussion... 

Also if the amp only has 50 hrs on it it's possible the hum may go away to some extent with further run-in.  My Line Magnetic amp hummed slightly at first but after 60 hrs or so it pretty much went away except during the initial 10 minutes of start up.  Once it gets warm and presumably the transformers expand a bit everything becomes dead quiet.

The OP asks how deadly is transformer hum. With only a little bit of exaggeration, I feel transformer hum might have significantly lowered risks to my life:


I have a class A tube amp. A little bit of transformer noise when up close, but nothing to annoy or out of the ordinary IME. Earlier this year, I moved out of the new-ish apartment building I was living in when I bought the amp into an old house. Now the transformers were humming loudly. I tried new tubes I had in reserve. I tried tightening the transformer mounting bolts. I tried a couple of DC blockers. No improvement. I checked voltage — consistently within a half volt of 123v at different times and at different outlets. 

A clue that the house wiring might be an issue came from noticing that the equally-annoying hum from the speakers was reduced after switching off all other circuits (but still no reduction in mechanical transformer hum).

I gave a call to the owner of a local audio shop from whom I had bought speakers and sources. To his credit, he referred me to his electrician, rather than different amps or power treatments. Long story short, the electrician replaced the corroded meter box and exterior wire, the ancient and miswired electrical panel, the grounding rod (with two), all of the outlets and switches (removing a bunch of dimmer switches), and some old interior wiring (where it appeared that high current draw had nearly started a fire). In the course of that, he removed some weird junction boxes in between the meter box and before the electrical panel, fixed phase miswiring and shorts in a few outlets I had not yet used, and redistributed load at the electrical panel. 

The result? Not exactly silence, but the same faint mechanical hum as when I bought the amp, and equally low noise at the speakers. 

So, despite what you are told by some (@cakyol), mechanical transformer hum is not always because of mechanical issues. An amp that works fine at the factory bench can misbehave when fed AC with loads (literally) of harmonics on the line. I would recommend trying all the nearly-free options I tried above. If you can, I would highly recommend following @jea48’s advice to see how your amp behaves connected to a couple of different mains power sources. (A techie’s would be ideal if you don’t mind paying the inspection fee, but a friend’s in a newer house in a different neighborhood would also be useful.) 


How did the hum improve my odds of survival? About a week after the electrical work was done, we had several nights of intense thunderstorms and strike lightening. I sure felt more secure knowing that I had new grounding rods and a whole-house surge protector, and that several wiring faults had been rectified — which I probably would not have had done were it not for going all out to fix the transformer hum. 
Best of luck in your quest for (good) silence.