Author Topic: Things that go hummmmmmm... (Read 307 times)

ronl513 · « **on:** April 14, 2005, 09:45:14 AM »

Just bought an F1X preamp. I tried to take the full range output of the Alembic and patch it into the power amp in input on my Ampeg B2R head. What I got was a very loud 60 cycle hum. My intention was to use the F1X as my sole preamp bypassing the preamp section on my Ampeg which is supposed to happen when you use the power amp in. As it is, the only way I can get a quiet input is to plug in through the effects loop return or the main input which puts me through the Ampeg's preamp--not the best setup. I think what's happening at the power amp in is probably an impedance mismatch. Anybody know how to fix this?

gare · « **Reply #1 on:** April 14, 2005, 10:17:19 AM »

Hi Ron..and welcome.
I dont have a F1X, so I'm no help there. But others will jump in shortly.
Where are the gain and/or master controls on the Ampeg set ? On many amps the gain has to be zero'd out and the master controls the output. I've managed to overlook that a few times myself...doooooh !
Anyway..welcome and good luck with the problem.
Gary

ronl513 · « **Reply #2 on:** April 14, 2005, 10:31:39 AM »

I tried every setting. I left the gain and master all the way down, pulled off the eq, and everything else I could change. I twisted knobs, changed cords and pushed all the buttons. Finally when I changed the input to the effects return or the main input, the hum stopped.

811952 · « **Reply #3 on:** April 14, 2005, 11:54:00 AM »

You might try lifting the ground and/or isolating the chassis from the rack rails, in case it's a ground loop issue...

John

ronl513 · « **Reply #4 on:** April 14, 2005, 12:34:52 PM »

Thanks for your responses. I'm honored to be part of the club. I've been waiting for a chance to obtain an Alembic preamp for a couple of decades. I've either had the money and lacked the opportunity or didn't have the cash when a unit was available. At any rate, I have one now. Yes, I did separate the unit, but I'm also thinking that mounting both units to the same rack should prevent any ground incongruities. Add in the fact that the hum stopped when I used the other inputs, and it almost eliminates a grounding problem. That's why I'm looking at impedance as the cause.

mica · « **Reply #5 on:** April 14, 2005, 12:59:41 PM »

I sent this to your Helpdesk inquiry from this morning, and post the response here as well for the benefit of other readers:

The outut impedance of the F-1X is low enough (

ronl513 · « **Reply #6 on:** April 20, 2005, 11:07:39 AM »

I have lifted the ground on the F1X and the hummmmmm has stopped. Wow what a wonderful sound this preamp delivers! As it turns out, a ground loop is the issue, but in order to lift the ground, I had to resort to the old 3 to 2 AC plug adapter on the F1X. This remedy can cause other problems such as a hightened potential for a shock hazard and radio signal interference. Yes, it's a remedy, but it seems like a seat of the pants way to resolve an issue involving some high quality components. Does anyone know of a better and safer way to resolve the resulant hum of ground loop?

effclef · « **Reply #7 on:** April 21, 2005, 08:07:21 AM »

Does anything in your rack have two-prong plugs without one prong being larger? The large prong should be going to AC-return, the small prong to AC-hot. If you have something with an unkeyed two prong cord, try flipping it over in the socket. Also, get an AC outlet tester and make sure what you are plugging into is wired correctly.

It may be worth diagnosing further and/or sending the F-1X back to Alembic for a checkout.

Maybe someone modified the wiring inside?

EffClef

David Houck · « **Reply #8 on:** April 21, 2005, 08:32:50 AM »

Have you tried the F-1X with a different power amp?

xlrogue6 · « **Reply #9 on:** April 21, 2005, 08:47:30 AM »

I suspect your loop is due to the combination of 2 different chassis grounds and signal ground. You could probably get around it by using a cable between the F1X and the B2 that has the ground lifted at one end, but that's just a different route to the same solution you got by using the 3 to 2 adaptor on the AC cable. Even when using the lift, the F1X is still grounded through the signal cable (and the rack rails, if it's in a rack with the B2). I doubt there's anything wrong with your preamp.

ronl513 · « **Reply #10 on:** April 22, 2005, 04:20:25 AM »

Thanks for your suggestions and your interest. My rack has two pieces in it: the F1X and the Ampeg amp. Both units are, in my opinion, among the best of their type in the world. I think the hum is arbitrary and not an issue of quality or workmanship in either piece. I'm probably not going to add anything more to my rack for awhile. I don't use effects in the music I'm playing so the only other component I might add to my rack will be a wireless system. I haven't tried the F1X with any other amp because I have no intention of using it with anything else.
I'm a blue collar bass player who for years played through a Peavey MarkIII head and a 1-15 bottom. Whenever ground incongruity became a problem, I simply flipped the polarity on the head's three position power toggle switch. Neither the F1X or the Ampeg have a polarity switch thus my situation with the ground loop. I've found a couple of products that claim to be able to execute a ground lift without causing the shock hazard or the radio interference that a primitive ground lift does. I'm going to try one of them soon and I'll let you know how it turns out. Again, thanks to all.