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Active Bass burnt out Preamp?

Baker69

Baker69

Mar 24, 2008
2,048
1,149
UK
#1
Hi guys,

Could I have burnt out the input stage of my Line 6 LD150 by continually plugging by Bass with Active Circuitry into the Passive Input Socket instead of using the Active Input Socket?

The amp repair guy said it looks like someone has put a powered signal into it like from the speaker-out socket on another amp but I know this isn't the case.

I know a lot of people do use the Passive Socket as it is a bit warmer as Active input sockets seem to suck the tone out of a bass, and even the dealer I bought the amp from suggested using the Passive socket as a preference.

Is an Active Bass capable of doing such damage? I saw the circuit board and it had burnt out about 3 transistors and the board itself was scorched.

I'm using the Active socket now, but i do miss the sound I used to get through the Passive socket, but I don't fancy paying another £100 to get it repaired again!

Can anyone throw any light on this?
 
#5
As you have said, I agree that in theory an Active Bass shouldn't have caused the damage as I can't imagine it putting out anywhere near that sort of damaging power, but the engineer who repaired it checked the whole amp out and it was faultless other than the input stage and said it was definitely damaged by whatever was put in the input socket and unless someone has used the amp without my knowledge the only thing that has been plugged in there is my bass.

He did go on to say that Line 6 gear doesn't have the same 'limiting' circuitry that will cut the signal down like on other bass amps and that I should never use the Passive Socket on Line 6 amps with an Active bass.
I don't know whether this is true or not. A lot of people on TB have said that they use the passive sockets on their amps instead of the active socket as it just sounds better!

Anyway, I'm never going to find out for sure what caused the damage, I just wondered if there was the remotest chance it could have been my bass that did it.
 
#7
A lot of amps don't have anything in the front end that would limit signal coming in, other than a resistor that limits current a little. I concur with all the others, it did not get burned out from having an Active bass plugged into it.

However, I can think of several ways high voltage can get injected into the Input.

-Plugging an amp's "Speaker out" into the input, as others have mentioned.
-If the "Line Out" of your amp was plugged back into the input, perhaps?
-Plugging in a defective effects unit that ran off of AC voltage into the input
-Using a defective Direct Box, powered by 48V phantom power from a mixer, also plugged into the input
-Using a Y-cable to connect the input of this amp to the input of another amp...and the other amp had a "hot chassis"

Bottom line, your amp's problems didn't occur from using an active bass.
 
#8
The situation was that the amp was left in church and in theory anyone could have had access to it during an 11 day period and experimented with it, although after much effort have been unable to find anyone who would have needed to have used it during that period, and to date certainly no one has owned up to having used it (or borrowed it).

It was interesting that Nashvillebill commented on DI boxes as the church does use them and I have stopped one of the sound guys connecting one to my amp on setting up on a couple of occasions as I usually use the Direct out on the amp. I know the desk has phantom power as well as the drum mics all run on that.

Perhaps someone used the amp using a DI box with phantom power running to it then?
 
#11
dmusic148 said:
Ask him exactly which parts failed, and how they relate to each other, on the schematic or at least in a block diagram level of detail. That would help us solve the crime.
Click to expand...

Will do, however I do have a photo I could post if that is of any help? From memory it was a row of 3 transistors I think, but the circuit board was blackened so it was difficult to see, I might phone the engineer tomorrow and ask him. I know he had to rebuild the board as Line 6 were unable to supply a replacement board as in their words "the amp is now obsolete and past it's serviceable life"! So much for their support on a 4 or 5 year old amp!

I've read that how XLR cables are wired they are normally not an issue as they 'cancel' each other out, but I'm not sure whether this is foolproof? There's an article I've just been reading about how a Phantom powered DI Box burnt out a keyboard.
 
#12
Suppose the sound guy whips out his DI box. He plugs the bass into the DI box's input. He plugs the DI box into the mixing board with the XLR and turns on phantom power. Now he plugs an unbalanced 1/4 cable from the DI box into your amp's input.

The XLR to the board would be fine as far as the board and DI box is concerned. If the DI box is not functioning correctly, though, it could put the 48V phantom power onto the 1/4 instrument cable....poof, there goes your amp's input.
 
#14
I don't know about the el-cheapo imports, and I've seen "homemade" DI boxes in other peoples rigs.

edit:

The main thing we can now get from this, though, is that we no longer have a case of an amp that was burned out "just" by having an active bass plugged into it. We now know the amp was left in unknown circumstances for several days, and anything after that is pure speculation. Who knows, maybe the DI box was/is just fine...but some dufus plugged another amp's speaker out into this amp's input for "more power!" Or maybe it was Aliens.

Whatever it was, we can say with certainty that merely plugging an active bass into the amp did not cook it.
 
#15
Sometimes there are catastrophic circuit failures. Things can happen without any help from the user. If you were plugged in at the time, try your bass in another amp to ensure that it wasn't damaged.

£100 ($167 US) is a lot of money to repair the input stage on an amp. They must have changed a board.
 
#19
beans-on-toast said:
Sometimes there are catastrophic circuit failures. Things can happen without any help from the user. If you were plugged in at the time, try your bass in another amp to ensure that it wasn't damaged.

£100 ($167 US) is a lot of money to repair the input stage on an amp. They must have changed a board.
Click to expand...

Yeah, the repair guy said Line 6 didn't have or wouldn't sell him a replacement circuit board as they said the amp is obsolete, so he tells me he had to basically re build the existing board with some guidance from Line 6 UK Technical Support Team. The length of time it took is reflected in the price, however they do charge a standard minimum rate of £60 for whatever repair they do.
 
#20
nashvillebill said:
Suppose the sound guy whips out his DI box. He plugs the bass into the DI box's input. He plugs the DI box into the mixing board with the XLR and turns on phantom power. Now he plugs an unbalanced 1/4 cable from the DI box into your amp's input.

The XLR to the board would be fine as far as the board and DI box is concerned. If the DI box is not functioning correctly, though, it could put the 48V phantom power onto the 1/4 instrument cable....poof, there goes your amp's input.
Click to expand...

I did notice when I played there next time but using another bass amp while mine was being repaired that the XLR cable from the multicore that normally goes to the bass amp did not work so there is a fault in the cable wiring or it too burnt out, and they had to use another cable.

I did check the desk since and noticed that all the phantom power lights are illuminated on there, so they are on all the while by the looks of things.
 

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