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jamlam
05-19-2016, 09:00 PM
Does anyone have a simple pre-amp or buffer circuit to allow the input of an electric guitar signal to the Audio Shield line-in?

adrian
05-19-2016, 10:19 PM
Have a look at this page (http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp2.htm#schematic).... its not super simple (about 10 real cheap parts plus some perf board and a battery), but I reckon its the way to go.... think of it as a ensuring there is no weak link in your guitar signal chain.

There are lots of cheap good mixers with a Hi Z inupt (and line out obviously) ... you could probably get one for as little as $50!! might even have a usb interface for recording at that price!

my personal favourite is the mackie blackjack .... pretty good DI, line out, and very good usb implementation (even works on linux!) They run around $100 on a good day, and are portable-as.

you could just use an opamp as a buffer. there are about 1000 versions of that circuit on the interweb

Theremingenieur
05-19-2016, 11:48 PM
A cheaper and simpler alternative would be to connect the guitar to the audio shield's microphone input. Normally, that shouldn't work, but it finally does. Theory and practice...

adrian
05-20-2016, 12:00 AM
A cheaper and simpler alternative would be to connect the guitar to the audio shield's microphone input. Normally, that shouldn't work, but it finally does. Theory and practice...

Well there you go .... I hooked my guitar up to the mic in on my tapedeck once (upon a time), and that worked too ... théorie et la pratique

Stendall
05-20-2016, 12:41 PM
I've been using for a time my guitar with the line-in input of the teensy audio board (tuner sketch) with no problem at all.
Gives plenty of signal with all pickups. Singles and humbucker.
Maybe the problem it's the line-in sensitivity.
audioShield.lineInLevel(15);

Boosting up the gain on the SGTL5000 will add some noise, but much less than adding a preamp stage badly designed or poorly shielded.

adrian
05-21-2016, 12:03 AM
@Stendall

Hmm

Over the years I have put my guitar signal in a lot of "bad places" it should not have been ... :p

looking at the sgtl chip spec sheet, it looks like the line in is 100Kohm... so I think you are probably right. putting a (say) 20Kohm guitar pickup into a 100Kohm audio chip line will do quite nicely ... maybe for really hi Z inputs there will be a loading effect that will degrade the signal processed by the audio chip ... maybe its 'minimal' ... is it worth buffering it? will a bad buffer make things worse? I suppose that is the practical question to be answered, but theoretically, I doubt it will make much difference ... stick it in the line in, I say!

gain is a whole different consideration ... I recommend one of these (with mullards)7210....

jamlam
05-21-2016, 01:09 AM
I tried building a couple powered pre-amp/buffers using a JFET and op-amp to try and match impedance etc, but have not had much luck in getting a good signal. Running the guitar output directly into the mic-in and fiddling with the gain gives a surprisingly good result. Will have to experiment with the line-in gain as mentioned. Thanks for the suggestions!

Stendall
05-22-2016, 06:35 AM
I tried building a couple powered pre-amp/buffers using a JFET and op-amp to try and match impedance etc, but have not had much luck in getting a good signal. Running the guitar output directly into the mic-in and fiddling with the gain gives a surprisingly good result. Will have to experiment with the line-in gain as mentioned. Thanks for the suggestions!

Try the line-in input.
The datasheet shows that SGTL5000 has 29K input impedance on line-in while the mic-in has 2.9K.
Line-in it's match closely the guitar impedance and will give better results.

adrian
05-22-2016, 07:28 AM
Try the line-in input.
The datasheet shows that SGTL5000 has 29K input impedance on line-in while the mic-in has 2.9K.
Line-in it's match closely the guitar impedance and will give better results.


Yes this...

Somehow when I read the datasheet I saw 100kOhms, ....now when I read it, it says 29kOhms (parallel universe??).... 29kOhms is on the **theoretical** low side for a guitar? but better than the mic input. The DI I use for bass at home is 450kOhms.

EBK
05-24-2016, 08:53 PM
Does anyone have a simple pre-amp or buffer circuit to allow the input of an electric guitar signal to the Audio Shield line-in?

I've been workinong this issue as well. What I've decided to try is this: http://www.geofex.com/circuits/Adjusticator.gif plus a simple limiter made out of 4 diodes to keep the line in signal within 2.8Vp-p. It's more complicated than connecting the guitar straight to the mic input, but should, in theory, be cleaner.

PaulStoffregen
05-24-2016, 09:29 PM
Somehow when I read the datasheet I saw 100kOhms, ....now when I read it, it says 29kOhms (parallel universe??)

I believe an older version of the datasheet did spec 100k. I recall seeing an errata notice that it was actually lower and they were updating the datasheet.

The SGTL5000's mic preamp doesn't have a great noise floor. It works, but you'll get less noise with a high quality preamp circuit and the line-in pins.