Can I plug a laptop into a guitar amp?

April 23, 2006 4:36 PM

Can I plug my laptop into a guitar amp without damaging the laptop? I'm using a male eigth inch to male quarter inch cable between my laptop's integrated soundcard and a fender amp's second input. I tried this briefly and it worked, though the sound quality is what you'd expect of such a setup. I then plugged a guitar into the same amp. Everything still worked, but upon removing the laptop, the guitar got noticably louder, leading me to worry about what kind of potential I was putting across the soundcard's output from the guitar. The soundcard does not seem to be at all damaged.
posted by phrontist to Technology (9 answers total)

wait ... you're putting the sound from the sound card into the "second input" of the amp? ... i don't see how that would harm anything

i've tried putting two guitars into an amp before ... the signals are bound to interfere with one another to some degree ... your guitar is going to sound louder by itself than it will with another signal source to share with
posted by pyramid termite at 4:45 PM on April 23, 2006

Response by poster: Sorry, what I said above was unclear.

The laptop wasn't playing anything. The guitar was. When the plug was removed from the laptop, the guitar got louder. So just the connection effects the volume of the guitar.
posted by phrontist at 4:47 PM on April 23, 2006

my guess would be that if the second input was a guitar instead of a laptop, you'd still see the same thing happen, even if no one was playing the second guitar

i think it might trigger something in the amp that causes only so many watts to be assigned to each output, dampening the signal
posted by pyramid termite at 4:53 PM on April 23, 2006

What pyramid termite said is accurate.

But I'm having a hard time figuring out what you were trying to accomplish with this? If the laptop wasn't producing any sounds to be amplified, why was it plugged in to the amp's input in the first place?
posted by peewee at 4:57 PM on April 23, 2006

Which Fender amp do you have? Because the answer will lie in the schematic for its input circuitry.
posted by flabdablet at 4:59 PM on April 23, 2006

Often inputs are summed. When two inputs are connected, but only one is actiuve, the resulting output will sound softer than if the active input were connected alone.
posted by pmbuko at 6:50 PM on April 23, 2006

... this is because one source is driving the other source's input, which has impedance that eats up some of the signal. It's possible to damage your soundcard this way, but not very likely really -- a guitar is a pretty low-level signal. Still, a mixer is recommended.
posted by kindall at 7:02 PM on April 23, 2006

The real question is whether the amp actually has two inputs, or only one input wired to two parallel jacks.

If it's the latter, then you're seriously risking toasting your sound card when the guitar is also plugged in, because the two are directly wired to each other with nothing in between.

If it's truly a separate input, i.e. it connects to different circuitry inside the amp than the other jack, then I don't see any particular problem.

But my guess would be that it is not a second input, that it is actually just a parallel jack, and that using it the way you are is not a good idea.
posted by Steven C. Den Beste at 9:34 PM on April 23, 2006

No, it really depends on which Fender amp you have.

An electric guitar can't blow out a sound card; its output is measured in micro-amperes. But because they are feeding into a high-impedance circuit, they are often measured in ohms. In short, they can't put out enough current for your soundcard to notice it.
posted by ikkyu2 at 9:56 PM on April 23, 2006