Adding Line Level inputs AND Guitar Ins???

Can anyone think of any good examples of (opamp based) schematics that have the ability to plug in 1/4" line inputs OR
guitar inputs?

I am building a box and am  trying to think of the best way to do this and am not sure if it should be with a switch or with 2 separate inputs.

for guitar I usually would have a cap, and a 1m resistor to ground and a high gain.

for Line, I usually have a different size cap and less gain.

any advice highly appreciated!

thanks

What are you trying to do?  Are you making a mixer or a buffer?  Are you planning to balance these signals electronically or with a transformer?  Are you adding gain or making a simple buffer?  Fixed gain or variable?

If you want to make say a DI box where you might plug in your guitar or a line input then it doesn't matter if you are running at unity gain.  The impedance for both the guitar and line input can be left at 1meg.  If you want them to be the same volume (roughly) no matter what you plug in then you could pad the line input with resistors.  Line level +4 or -10 are typically fine with 10k and up.  Guitars should be 1meg.  But line is fine going into 1meg impedance input - it's just not needed because the impedance of most line devices is low and under 1k.  There's that 1:10 rule I believe (the input impedance should be at least 10x the amount of the output impedance device).

Many mixing boards simply pad the line input with resistors and share the same gain structure as the microphone preamp.  People who are more knowledgeable than I can tell you the potential noise problems with front end pads.  I believe it is gain (and any potential balanced to unbalanced scheme) that is more problematic with combining guitar inputs and line inputs as the same device.

I would suggest for a lot of DIY guitar circuits and even circuit boards to look at "Projects" under www.generalguitargadgets.com

Maybe the bigger question is are the line inputs balanced or unbalanced?

If you are trying to do a balanced signal OR guitar signal (but the option for both) - you have some more things to take into account.  If you have a balanced line signal and want to simply run it unbalanced into your device then you need to know how the signal was balanced to devise a debalancing scheme.  Some electronically balanced outputs would not like their ring grounded (pin 3) and could cause some problems.  Transformers don't care and impedance balanced outputs (where there isn't a symmetrical out of polarity second output) also are not affect (I think).

I hope in my mild understanding of this stuff that that was helpful.

CC

this has been done for years
even Ampeg had accordian inputs on the front of their guitar amps,
this, in my opinion, is immediate grounds for incaceration,


look at the G9 preamp, there is exactly what you are talkin bout, a di or mic in,

you need to adjust levels and impedance
the rca and guitar world are miles apart
unless you use a Les Paul recording.


which is a guitar, not a record. 

sorry i wasnt clear enough

I have been building stompboxes for a little while. and I have been building filter circuits and modular synths for longer.

now i would like to be able to build a box that will sound good with my guitar OR the INS and OUTS of my MOTU Daw interface which has unbalanced 1/4" ins and outs.


I know what the requirements are for Guitar In and out.

I don't have such a good handle on what preamps and DAW interfaces like, say, A MOTU want in their inputs.

A guitar wants 1m impedance. and a different size input cap than would work with LINE.
so if a guitar wants HIGH, a line wants LOW. correct? and the cap on the IN is for DC blocking. correct?

so from there, I assume you also want a low impedance output and another DC blocking cap.

is that it?

thanks so much for all the help

the output impedance of an electric guitar is 5k-15k ohms (DCR) depending on the pickup. that output wants to see a high input impedance. usually 1 megohm or higher. the unbalanced inputs of a MOTU daw are probably 10k or 20k ohm input impedance. any source with an impedance below 10k will drive those inputs properly. line-level keyboards are usually around 10k output impedance. those should drive the daw input properly. the unbalanced outputs of a MOTU daw are probably about 100 ohms output impedance but the spec sheet will say 600 ohm maximum load on those outputs. this doesn't answer your question yet. i need more coffee first. DW.