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Username: ronl513

Post Number: 1
Registered: 4-2005
Posted on Thursday, April 14, 2005 - 9:45 am:   Edit Post

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?
Intermediate Member
Username: gare

Post Number: 180
Registered: 11-2003
Posted on Thursday, April 14, 2005 - 10:17 am:   Edit Post

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.
Username: ronl513

Post Number: 2
Registered: 4-2005
Posted on Thursday, April 14, 2005 - 10:31 am:   Edit Post

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.
Advanced Member
Username: 811952

Post Number: 388
Registered: 10-2003
Posted on Thursday, April 14, 2005 - 11:54 am:   Edit Post

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

Username: ronl513

Post Number: 3
Registered: 4-2005
Posted on Thursday, April 14, 2005 - 12:34 pm:   Edit Post

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.
Username: mica

Post Number: 2379
Registered: 6-2000
Posted on Thursday, April 14, 2005 - 12:59 pm:   Edit Post

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 (<5K) to drive any power amp, so we don't think there is an impedance issue in this case.

To help troubleshoot the problem, we ask you to connect the Full Range output of the F-1X to the power amp in on the Ampeg, with the power switches on both units off. Now, turn the Ampeg on and see if you have the hum. Check with the AC line cord connected to the F-1X and not connected. You shouldn't hear any hum with this cabling whether the line cord is
connected or not.

Then, if that is silent, then turn the F-1X on, but have the volume control at 0 on the F-1X, and again there should be no hum.

Let us know the results of these trials and it may give us a clue as to what is causing the problem.
Username: ronl513

Post Number: 7
Registered: 4-2005
Posted on Wednesday, April 20, 2005 - 11:07 am:   Edit Post

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?
Advanced Member
Username: effclef

Post Number: 324
Registered: 1-2004
Posted on Thursday, April 21, 2005 - 8:07 am:   Edit Post

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?

Username: davehouck

Post Number: 1630
Registered: 5-2002
Posted on Thursday, April 21, 2005 - 8:32 am:   Edit Post

Have you tried the F-1X with a different power amp?
Username: xlrogue6

Post Number: 94
Registered: 8-2002
Posted on Thursday, April 21, 2005 - 8:47 am:   Edit Post

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.
Username: ronl513

Post Number: 8
Registered: 4-2005
Posted on Friday, April 22, 2005 - 4:20 am:   Edit Post

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.

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