What do I need to do to match impedance properly when taking a line signal out of an interface to feed the input of a guitar amp?
 
Scenario:
E.Guitar -> to INST input on interface -> ITB effects -> Ext Insert -> guitar amp input -> guitar amp DI -> interface
 
 

You don't need to do anything to match impedances per se. But guitar levels are a little quieter than line level so you may need to reduce the output level of your interface ~6dB more than usual.

To do it properly you may need a Re-Amp box of some kind. Here is some info on them:
http://www.reamp.com/faq.html
 
And also a product from Radial:
http://www.radialeng.com/prormp.php
 
A good re-amp box does several things in one.
 
It drops the signal down to a level that a guitar amp would expect. (eg +4 dBu down to something like -20 dBu which is significant!) It often gives you a fine volume level control adjustment which you will need. It provides and earth isolation between interface and guitar amp. You may get buzz or hum connecting the interface earth direct to the guitar amp earth.
 
The re-amp box output also simulates the high impedance output from a guitar and this can effect the sound and the way the guitar amp interacts tone wise to the incoming signal. Some go further and even simulate the equivalent circuit of an electric guitar.
 
It depends on how much you want to do it and how often for you to buy the re-amp box. For a one-off you could try using a passive DI in reverse. It may work OK. Connect the 600 ohm balanced end to the interface output and the high impedance input side to the guitar amp.  The transformer inside the DI may give you the earth isolation you require.
 
But the transformer inside a passive DI is much smaller and it might saturate slightly being used in reverse and getting a high level line output signal hitting it. OTOH it may work quite well for the experiment too.
 
Re-amping is a good concept and gives you a lot of control over the re-amped sound which is all good.
 
 
 
 
 
 
 
 

Thanks -
 
I've done a bit of reamping ITB with sims, and even some using the USB port and reamping features of the IRT Studio.  The latter is a real pain though because of ASIO single device limits in Windows ....
 
Thanks again.

 
new, read about this in this month's TAPE OP
 
sounds very intriguing

Jeff gave some good places to look but forum user tlw will hopefully come by. He has been an endlessly useful resource on such matters for me and can only assume he's a guitar (and general stringed instrument) player who's been wrangling with this stuff for decades.
 
Based on his (and others) input I've learned there are lots of factors to take into consideration from output to the guitar > any DI type devices to go to either mixers/interfaces to bring up the output level of the guit (which is also dependent on the type of pickups involved) > into whatever recording platform (the DAW) and handling it there (like using the External Insert plugin in Sonar or other methods of routing back out and in again) > out from the DAW > into the device that's gonna give you your initial "reamp" signal (essentially taking the recorded track back down to an Instrument level signal to mimic the output of your guitar) > into the amp or guitar FX unit or guitar pedal(s) or whatever > back into the DAW via mics or amp line outputs or mixer or whatever (and whether you are doing it using an FX send/return/insert type setup or recording the results straight into a new track or whatever).
 
Brain bustin' shiz fo sho...
 
You have however said you want this...
 
E.Guitar -> to INST input on interface -> ITB effects -> Ext Insert -> guitar amp input -> guitar amp DI -> interface
 
I've added a bunch of crazy thoughts and notes based on all my chaotic pontifications on the matter. I have indeed spent a LOT of time thinking on this and have been enacted many of the actions I describe/allude to.
 
"E.Guitar (which I'm assuming means an electric guitar) ->
 
to INST input on interface (the Inst input on many interfaces ain't nearly as good for guitar as they claim to be... they do not have the input impedance a guitar preamp section does so you may want to consider that... especially if you are going to get some kind of guitar DI that can do reamping which may help at this stage as well)->
 
ITB effects (I would probably avoid going nuts on effects inside the DAW until AFTER you have reamped it... could you describe more what you intend to do at this stage)->
 
Ext Insert -> (This is where you need the "reamping" device to appear to bring the Line output level down to a regular guitar output level that can be properly process by a guitar amp)
 
guitar amp input -> guitar amp DI (okay... but if you have the guitar amp DI can I ask why you aren't using it to do your initial recording in the first place? You could use a splitter to send to your guitar amp AND the interface Inst input and get the best of both worlds in one shot... no reamping needed and you can always reamp the DI signal afterward anyway)->
 
interface (at this point your reamping device will probably be outputting a Line Level signal so disengage the Instrument/Hi Z stuff... or, and I would personally prefer this, send it through a mixer first so you can trim/pad/EQ the signal a bit before it hits the DAW).
 
 
What devices do you have currently available? Like what kind of amp do you have (and what cnnections does it have)? What interface? What (if any) kind of mixer do you have? Do you have any fancy pedals or pedalboards that can act as splitters?
 
I'm assuming you do not currently have a reamping device which will be the missing peice of the puzzle for i/o and the Radial stuff is definitely something to research (and there is lots of informational stuff online to help figure out the whole process as well... which may make you realize you don't actually NEED to reamp... like if you can acheive an input splitter type setup... for example like I can with my Boss TU-2 tuner pedal).
 
I have been personally looking at an ART Tube MP/C unit because it is capable of reamping but serves a bunch of other purposes like adding tube drive and external compression to mic, instrument and line input signals. It's a cheapo unit and there are much better options that are more specific (like the Radial stuff) for the various tasks but it depends on your needs and budget.
 
That's a big ole, unproofread dump of chaotic info to absorb but you're smart so hopefully it can help you do the same type of research I've been doing over the past couple years and come to the best conclusions for your own needs.
 
Cheers.
 
Edit: and batsbrew is here now too... who is actually the one who pointed me to the ART stuff and has a lot of knowledge on such things as well.

Yes, E.Guitar is electric.  In a nutshell, I want to DI my real amps.  This one is a sweet amp (Laney IRT Studio), with DI capability built in.  In the process, I want to use VST effects in the proper places, along these lines

 
I will end up using the effects loop for most stuff, so line will be fine.  I'm just experimenting with the front input.
I have done quite a bit post DI as far as delays, reverbs and such go.  Sounds good to me, but I need to get those things into the loop to compare results - the loop being the simple case.
 
I've just finished comparing line vs reverse DI box for the guitar input, and I honestly can't hear a lot of difference.
 
I have a proper DI box and 2 LINE/INST input on my interface.  I've compared results a lot, and find the INST inputs to be on par with the box.
 
Thanks for the feedback.  Next step is effects loop ... tomorrow.
 
This is the first time I've done this using the Ext Insert function.  It works VERY well - I like it!
 
 

batsbrew

 
new, read about this in this month's TAPE OP
 
sounds very intriguing

Looks interesting.  Like the original Reamp box.  I'll check it out

BobF
Yes, E.Guitar is electric.  In a nutshell, I want to DI my real amps.  This one is a sweet amp (Laney IRT Studio), with DI capability built in.  In the process, I want to use VST effects in the proper places, along these lines

 
I will end up using the effects loop for most stuff, so line will be fine.  I'm just experimenting with the front input.
I have done quite a bit post DI as far as delays, reverbs and such go.  Sounds good to me, but I need to get those things into the loop to compare results - the loop being the simple case.
 
I've just finished comparing line vs reverse DI box for the guitar input, and I honestly can't hear a lot of difference.
 
I have a proper DI box and 2 LINE/INST input on my interface.  I've compared results a lot, and find the INST inputs to be on par with the box.
 
Thanks for the feedback.  Next step is effects loop ... tomorrow.
 
This is the first time I've done this using the Ext Insert function.  It works VERY well - I like it!
 
 

Don't quote me on this but if the amp has a "Line In" somewhere that would be a way to tap the amp without worrying about impedance (I think). Line in of course meaning the "line" level being output by your interface. If you need to access the actual guitar input (where you would normally plug your guitar into) then you gotta stomp the line output from the interface back down to an instrument level output.
 
That's really the only thing that needs to happen. If you can get something in between the interface line output and the guitar amp to mimic the output of a guitar then you can just pretend like your recorded guit track is a live guitar and do whatever the frack you would do with... well a regular guitar track.
 
Perhaps that amp can actually function as an insert effect anyway if it has the right connections (like a rack unit would). Then you could just treat it like you would any external gear/rack FX and use Sonar's External insert thingie to tap it.
 
Check out the manual of the amp.
 
The usual Send/Returns won't do though. That is sending out to a unit and accepting the return signal. You want something on the amp that accepts the signal (line level from the interface) then sends it out again (line level to the interface).

Actually, most of the time the effects loop should do fine.  That's where the majority of the effects I normally use "belong".  The normal case goes like this:
 
E.Guitar -> AMP -> AMP/Send -> interface -> ||| [DAW effects] -> AMP/Return -> AMP/DI -> interface -> DAW
 
If the signal is recorded at |||, then reamping of effects/power section/cab/spkrs/mic can be done without sending the signal back out again.
 
Of course the ultimate flexibility is to go straight in and record a completely dry guitar signal ... but what led me here is trying to decide if I should be using my real amps instead of buying more sims.
 
So many toyz, so little time ...