Author Topic: Alembic F2B shielding (Read 419 times)

ken · « **on:** February 04, 2006, 05:35:40 AM »

I just a got a used Crest CA9 power amp (to use with my F2B) and it can't be within a foot (literally) of my old Alembic F2B preamp without emitting a subtle hum that gets louder as I turn up the power amp up. (I didn't have this problem with my QSC PLX power amp.) If I turn off the F2B or move it to the side of the CA-9, the hum dissapears...and the tone is awesome. I guess they won't be sharing a rack together anytime soon, but it's a bit inconvenient. Not too big of a deal since the CA9 deserves it's own 2-space rack due to its weight. Maybe this is just a bedroom volume issue....I don't know.

Anybody have any thoughts on how best to shield the F2B? Or, do I just get longer cables and put the CA9 on it's side next to the cab or something? (Is it OK to be on it's side?) Is this an issue with other tube pre+lead sled power amps?

Thanks,
Ken

David Houck · « **Reply #1 on:** February 04, 2006, 06:46:13 AM »

Ken; I don't think this is a normal issue. Unfortunatley this is not something I'm good at; but there are others here in the forum that should be able to help. My first suggestion is to try different cables to make sure this isn't a cable issue. Second, if possible, try a different preamp and see if the same thing happens. Is the power cable for the Crest hard wired into the chassis or is it plugged in? And you're having this problem when neither unit is in a rack? Are both units plugged in to the same power strip? Try plugging in to separate strips. Does this problem happen in other locations; i.e. another room in the house? (i.e. is it sitting near a TV?)

ken · « **Reply #2 on:** February 04, 2006, 08:25:23 AM »

Hi Dave, the F2B's the only preamp I have at the moment. The Crest cord is hardwired. Each unit is plugged into a different plug from the same outlet.

Once again, the problem dissipated when moving the F2B away from the Crest about a foot or so, so I'd automatically expect that this is happening because the F2B is emitting something. (The same low hum is audible when the power amp volume is turned all the way down as well.) Thanks!

Ken

P.S. Hope your F2B is working out for you!

David Houck · « **Reply #3 on:** February 04, 2006, 03:33:17 PM »

Ken; I still haven't taken it to the amp doctor yet, so I haven't been using it. Hopefully, I'll get a chance to take it in sometime this week.

dela217 · « **Reply #4 on:** February 08, 2006, 07:50:18 AM »

I am not sure about the newer F2B's but on my old F2, there is a hum cancelling trim pot inside. It is worth a look and see. On mine, if you open the cover it is located toward the right side of the unit near the capacitors.

Michael

David Houck · « **Reply #5 on:** February 08, 2006, 08:11:52 AM »

Michael; there are pictures of the inside of my F-2B here. Is it the blue wheel on top of the white box?

dela217 · « **Reply #6 on:** February 08, 2006, 08:30:42 AM »

I don't remember what the inside of my F2 looks like, but that does ring a bell. I was having a hum problem with my F2 when I first got it. The problem was exactly what Ken was describing. I opened it, and gave that a twist, and my hum went away. There was also some other issues that this unit had, and that was taken care of too. The other problems that this unit had was that the BASS controls were bypassed making the MID controls act like the Bass knob. Since the controls are interactive, it gave strange results. Once I connected those back and dialed out the hum, the unit sounded GREAT. A previous owner of this unit drilled a hole in the top of the pre amp so that the wheel could be accessed without opening it up.

Michael

dela217 · « **Reply #7 on:** February 08, 2006, 08:39:19 AM »

Here is a link to the pic of mine. Check out the hole in the top. Does it put in in the same neighborhood as the blue wheel?

http://club.alembic.com/Images/395/9282.html#POST17419

David Houck · « **Reply #8 on:** February 08, 2006, 08:45:44 AM »

They are very close. On mine, it's behind the volume control on channel two; and on yours, it's behind and between the channel two volume and bass controls.

mica · « **Reply #9 on:** February 08, 2006, 05:17:10 PM »

Ken,

Here's a little bit of troubleshooting I'd like you to try.

-Put your F-2B on top of your power amp or wherever you actually want it to be, but do not connect the power cord
-Plug an audio cable from the F-2B to your poweramp
-Set the level for your poweramp where you usually like it
-Now listen - is the hum there? If not,

-Plug the AC for the F-2B, but leave the power switch off.
-Now listen - is the hum there? If not,

-Then try it with the volume control for the F-2B all the way down but powered on.
-Now listen - is the hum there? If not,

-Then try turning up the F-2B volume control to where you usually like it.
-Now listen - is the hum there?

Does your old F-2B have a modular line cord? If not, what city is listed on the back of the unit?

Note that there are some certain power amps (like the Crown DC300) that create LARGE hum fields from the power transformers that preamps like to pick up when they are close by.

ken · « **Reply #10 on:** February 13, 2006, 11:18:39 AM »

Thanks, Mica. Sorry it's taken me a while to get back...I've been out of town. I will check this out tonight and post answers. I have tried some of these combinations already though.

First, I did find that the hum goes away when the Crest CA power amp is stacked ABOVE the 2 space rack holding the power conditioner and F-2B. (I've put the Crest and F-2B in there own 2 space racks for convenience and better portability. The Crest is heavy!)

When the F-2B is stacked above Crest, the hum only exists when the F-2B is powered on and the audio cables are plugged in from the F-2B to the power amp. The hum comes from the speakers and exists when the volume on power amp is up and F-2B volume is all the way down. The hum is louder when the F-2B volume is up. I think you are right...the preamp is possibly picking up something from the power amp. I never had this problem with my QSC poweramp.

BTW, if I didn't already mention it, I have an old red-knob version and I have already tried the hum trim-pot to no avail.

Not sure what a modular line cord is...

Let's say that I have a workaround for now, but it would be nice to have them in the same rack someday.

Thanks for all the input!

Ken

ken · « **Reply #11 on:** February 14, 2006, 08:17:25 PM »

Spoke with a longtime Crest dealer and tech today. He confirmed that the Crest tends to emit a strong magnetic field and that they don't always play nice with others in a rack. The F-2B must be picking this up. Maybe one of the tubes is slightly microphonic as well. Regardless, Crest on top, the problem seems removed. Stange, but workable.