You don't have much to loose by trying, they can reduce noise from a source external to the pre-amp. And have worked for me. But so has a good power conditioner or a dedicated line. You won't harm anything by shorting out inputs. But, unfortunately sometimes 'hum' is intrinsic to the amp or pre-amp itself (have you listened to the amp itself without anything plugged into it?).
Actually, no, I haven't listened to the amp alone. I did,however, listen to the preamp alone (with headphones, and everything else unplugged), and I still got the hum. That, for me anyways, rules everything but the preamp. Believe me when I say I've tried just about everything.
The short answer is it is worth the effort IMO, though don't expect magic.
I had some serious background noise some time ago and tried shorting plugs (home made) to try to solve it. I found it helped very little, though did seem to help some. I also found that my problem appeared to be a cumulative effect of bad ground connection (on my TT), connectors in need of cleaning, IC's that needed to be re-routed, and open connectors on my preamp.
Not for your problem. Try disconnecting cable tv line and ground and see if that makes a difference. Have you tried the preamp headphone combo at another location to see if it's still there? Take it to a friends house that doesn't have a humming system. If it hums there, something internal is defective or a poor design.
Have I got a short memory or what! LOL.
I thought you had cured your hum issue by letting your pre-amp warm up. To isolate your problem to the malfuncion of something internal or interface with something external disconnect everything from the pre-amp. Take it to a part of this house which has power outlets on a different circuit than any major power consumers, or better yet, any at all, plug it in. Listen - what do you hear? Go back every couple of hours and listen. If you hear nothing the hum is caused by something external to the pre-amp, most of which have already been sugested, or if you hear something its probable that there is something wrong inside the pre-amp, like a failing cap or something similar and needs to be diagnosed by a techie and fixed if possible.
Disconnecting the preamp, and temporarily relocating is the only thing I haven't tried.
I thought I had cured the problem, too, by leaving the preamp on, suggesting that the problem was thermal in nature. But no. One of the things that makes this problem so hard to solve, is that it's sporadic. It makes it very hard to find a hard fault that can be isolated. Every time I think I have it solved, it comes back. Maybe, if I have about 45 beers, I won't care...
If the pre-amp hums with nothing else plugged into it (except headphones), then you probably have a leaky capacitor in the power supply.
The only other thing I can think of is that you're equipment is right next to an extremely high level of EMI or RFI radiation.
This is easy to diagnose. Just set a portable AM radio next to the disconnected pre-amp and tune it off station to static. If the pre-amp and radio hum together it's not the pre-amp.
Now unplug everything from all your 120V electrical outlets and then plug in only the pre-amp and a clock radio into the same outlet. Tune the radio as above and listen for the hum. Both together means it's not the pre-amp. Just the pre-amp means it's the pre-amp.
Final final test, buy an emi/rfi filter at your local BB, Walmart, etc. Try it and see if the hum stops. If it does, keep it and call and see if you can find the cause of the interference. If it doesn't return it.
I guess it's the preamp. Just the preamp hummed using the radio test as described by Dan Myers. I like black and white tests like he describes. I can make a determination, and move on to my next step. Whatever that is. It just may be a new phono preamp, plugged into the (silent) tape inputs.
I had a serious hum problem in my system that turned out to be with my tuner and cd player. My pre-amp and tuner have two-prong plugs. My power amp has a three-prong grounded plug, so does my cd player.
I was be-fuddled about the hum because my tuner and pre-amp have different grounding schemes. Never-the-less, I floated the ground on the power-amp with a three to two prong plug adapter and now the nasty hum is gone. I had a ground-loop producing the nasty hum...
I hope this helps...
Oh, also, unless the cheap IC's that you spoke of are plastic garbage jobs that used to come with cassette decks, etc., you don't have to sacrifice them. Just get the cheap rca plugs from radio shack and solder the hot to ground. Viola, you have super cheap shorting plugs. Not any considerable amount of money invested, and they get the job done as well as any shorting plugs.
Hum can be induced by a nearby transformer from another piece of equipment or simply a power cord position or i/c position. Have you tried 'cheater plugs' for your AC power?
First, just go to RS and buy shorting plugs if you want to try them. IMO, they will do nothing, however.
What I would do. Disconnect all sources going into the preamp. If the hum is gone, it is one of them. If it remains, it is the preamp or the amp, which I doubt. Add one source as a time. When it returns, it is that device. If it seems to be the preamp or amp, disconnect the preamp from the amp. If you have shorting plugs put two on the amp. If you get no noise from the amp directly connected to the speakers, it is the preamp.
My guess is that it is a ground loop problem with having many units grounded as ULS requires. This makes for ground loops as all devices have two or more grounds; one through the interconnects to other devices.