M-Audio FTUSB & VOX AD50VT Impedance problem ?

[dgegag]

Hi !

I'm a newbie in recording and I have a M-Audio Fast Track USB recording module and a VOX AD50VT. I recorded myself using a dynamic mic placed directly in from of my amp. It gived a great result but I had to put the amp at a fairly high volume. Not very convenient for the other people I live with.

I then tried to plug the Line/Phone output of my amp directly in the Guitar/Line input of my M-Audio module (Guitar/Line switch to the Line position) so that I could mute the speaker and record the amp output.

I got back to my recording program (Soundforge 8.0), created a new file at 48000, 24-bit, mono, started the recording.

To got a good recording input (~ -3db) I had to put both amp volume and master to 3/4 position !

I suspect an impedance matching problem. I searched for AD50VT Line/Phone output specs but couldn't find the output impedance. I don't understand how this output can adapt its impedance with my headphone (32 ohms) and my FTUSB (20kohm). Maybe it doesn't...

What do you think ? Can someone explain me the way such type of output works ? How can I resolve this ?

Thanks

[BlackBoxDude]

Either way, your recording is going to sound like crap. The line output of the amp sends a full range signal so it will sound bright and unfocused. If you didn't get GT Player Express software with your interface, you can download it for free from the M-Audio website. At worst they can send you a CD. Then you can just record with the plug-in which will sound great. This is a much better option then recording from the line output of your amp.
The volume would need to be set pretty high on your amp to drive the input of the Fast Track though. This is because it is PREAMP level. It doesn't take much to drive your poweramp and speaker in the amp, but it does take quite a bit to send a proper line level output signal.

[dgegag]

That means that this output is completly useless :confused:

What do you mean by "a full range signal" ? I think I experience the unfocused feeling but I don't understand why. Is it a frequency issue or is it the fact that the too low input doesn't fit in the linear operationnal range of the Fast Track USB ?

[BlackBoxDude]

Yes, it is pretty much usless.
The sound you're used to hearing is the preamp feeding the poweramp feeding the speaker. This narrows the frequency range from low mids to hi mids. This is the focused sound your ear knows as the sound of your amp. When you bypass the poweramp and speaker and only listen to the preamp you hear a wider range of frequencies from very low to very high. This gives it a buzzy, tiny, unfocused sound. You'd be better off turning down the amp volume and still using the mic on the speaker. You'll have to turn up the mic pre on the Fast Track quite a bit to compensate for the low volume of the amp, but your guitar tone will sound much better. Make the amp as loud as you can without getting on peoples nerves.
Or, like I said. Use the GT Player Express VST plug-in for recording. It actually sounds pretty good.

[dgegag]

Thanks a lot, that clarifies things up !