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Post subject: Hum on B-DEC Line Out
Posted: Wed Apr 01, 2009 9:23 pm
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Aspiring Musician
Aspiring Musician

Joined: Thu Aug 09, 2007 4:04 pm
Posts: 388
Location: Lititz, PA
I thought I'd have some real fun with my B-DEC by connecting the line out to my Bassman 250 amp. I figured it would be cool to get all those B-DEC effects and tones with the bigger amp. However, when I did, it outputs a very noticeable hum. It's not super loud. Some may consider it acceptable but I can't say that I do.

On the B-DEC, if I turn the volume all the way up, I don't hear any hum coming out of its speaker. It kind of seems like the hum is strictly associated with the line out.

I considered the possibility that I might have a ground loop problem so I attached an audio isolation transformer to the line out signal to isolate the grounds between the B-DEC and the amp. There was no difference in the level of hum.

My teacher has a B-DEC, too, so I borrowed his to see if there was any difference. His B-DEC had the exact same level of hum.

Has anybody else noticed this? Does somebody have a schematic for a B-DEC? I'd like to figure out what is going on.


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Post subject:
Posted: Wed Apr 01, 2009 9:40 pm
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Aspiring Musician
Aspiring Musician

Joined: Thu Aug 09, 2007 4:04 pm
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Location: Lititz, PA
Let me add a little more information.

This line out hum occurs even with no instrument plugged in.

I just noticed that there is no hum on the headphone jack. This may be my workaround but it still stinks that the line out isn't quite right.

The headphone jack has no hum. I get just a tiny bit of hiss when I crank up the gain. This I consider completely reasonable.


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Posted: Thu Apr 02, 2009 8:56 am
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Aspiring Musician
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Joined: Mon Jul 09, 2007 10:59 am
Posts: 457
Just guessing here...and you've probably tried these already:

1. Try a different cable between amps

2. Try plugging the two amps into power outlets on different circuits in your house - outlets controlled by different breaker switches in your breaker box. This may be impractical for daily use because it might require you to run an extension cord to another room but it's worth a try. If that's the problem, at least you'll know and can decide whether to live with the hum, use the headphone jack, etc.

Perhaps your audio isolation transformer does the same thing as using different circuits but it's worth a try anyway.

3. Where do you have the B-DEC plugged into the Bassman 250?

If you have it plugged into the regular input (where you'd plug in a bass), try plugging it into the return of the effects loop or the RCA aux input jacks, if the Bassman 250 has them. If I were using the headphone jack as a line out (which I do on my G-DEC 15), I'd run it to the RCA inputs as my first choice.


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Posted: Thu Apr 02, 2009 10:37 am
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Aspiring Musician
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Joined: Thu Aug 09, 2007 4:04 pm
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Location: Lititz, PA
I'm plugged into the effects return of the Bassman 250. I've also tried going through a preamp with no change in the hum level.

I'm an electrical engineer and I'm pretty convinced that this isn't an external ground loop problem. To me, it seems like poor power supply regulation or a ground loop within the B-DEC. That's why I was wondering if someone had a schematic.


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Posted: Thu Apr 02, 2009 11:28 am
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Aspiring Musician
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Joined: Mon Jul 09, 2007 10:59 am
Posts: 457
warnergt wrote:
I'm an electrical engineer...


Heh. That'll teach me to make dumb guesses in front on an EE.

I'm sure you checked here:

http://www.fender.com/support/amplifier_schematics.php

They have G-DEC and G-DEC 30 but no B-DEC listed.

Maybe call Fender and ask for one?

Obviously a different company, but Carvin lists schematics on their website just like Fender does. I needed a schematic a while back for an older Carvin amp that wasn't listed on their website. I called and got a tech guy who emailed me a full-size schematic in about five minutes.

Perhaps you'll have the same luck with Fender? After all, it's a current model. You know someone there has one somewhere.

Good luck.


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Posted: Sat Jan 23, 2010 12:46 pm
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Hobbyist
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Joined: Sat Mar 29, 2008 5:20 am
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Has anyone figured out a fix for this? My B-Dec has this same problem. I love using it for practicing but I could never use the line out without solving this problem.


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Posted: Sat Jan 23, 2010 4:24 pm
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Aspiring Musician
Aspiring Musician

Joined: Thu Aug 09, 2007 4:04 pm
Posts: 388
Location: Lititz, PA
I'm now kind of inclined to believe that the B-DEC simply doesn't have a high signal to noise ratio. It just wasn't designed to be played at more than 30 watts. It's not high end audio; it's a practice tool.

It's still a great tool. It just won't do some of the things I was hoping to make it do.


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Posted: Mon Feb 08, 2010 12:34 pm
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Amateur
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Joined: Sun Jul 22, 2007 10:50 am
Posts: 102
I use two solutions to get around this problem:

Use a ground loop isolator from Radio Shack

Disconnect the plug from the Speaker out jack and run a line from there to your Bassman 250


A third alternative is the one you already tried, which is to run a line from the headphone jack to the input jack on the Bassman.


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