I'm struggling to find the answer to a really simple query: will connecting my guitar to my Marshall amplifier, selecting the undistorted channel and then connecting the amp's line output to my soundcard give me the same result as connecting my guitar to a DI box or guitar preamp, then plugging the output into my soundcard? I'm trying to connect my electric guitar to my PC so that I can use my amp-modelling software.
Editor In Chief Paul White replies: In theory there's no reason not to do the former of your two options, as long as you can adjust the level of your amplifier to stop it overloading the soundcard input. However, the sound you hear won't be the same as that of the guitar cabinet, as the speakers filter and colour the sound in a very obvious way. Clean sounds may be fine but overdriven sounds tend to be thin and buzzy with too much high end if you simply DI them. However, there are several solutions, the first being to feed the line output from the guitar amplifier into your PC via a line-level speaker-simulator box such as the Hughes and Kettner Red Box, which we briefly reviewed in SOS November 2000. Note, though, that if your amp is a tube model you shouldn't run it without either the speakers or a dummy load attached, as you could blow the output stage.
Another alternative is to DI the sound from your amp into the soundcard as you suggested, but then use your software guitar-amp simulator with just the speaker-cab simulation section switched on. This should get you back to somewhere near the miked sound of the amp. Of course, using your amp-modelling software you could do away with your Marshall altogether when recording, and instead use a cheap active DI box to match the impedance. You can pick these up from around £20. Cheaper still, if you have any guitar pedals you can use these between the soundcard and the guitar, though pedals that feature a true mechanical bypass won't act as impedance matchers in their bypass position.
Having mentioned some of the the lower-budget options, I'll skip to the higher end. If you really want to capture the sound of your Marshall without terrorising the neighbourhood, a combined power soak and speaker simulator is your best bet. The most convincing one we've tried so far is the Motherload from Sequis. You can read our review in SOS July 2005 (www.soundonsound.com/sos/jul05/articles/sequis.htm). It's not cheap but it does a fabulous job.