Play guitar along with MP3 player (on holyday so cant bring too much stuff)

[arma61]

Hi there!

I've a practice amp made with a tda7056b, I use it with my guitar, I would like also to play along with
music played from my MP3 player. How do I mix the signals together? would something like this work


       |R-channel ------/\/\/\---|R1          R3
MP3 |                                  |----------/\/\/\--------|
       |L-channel ------/\/\/\---| R2                          |
       |GND    -----------------------------|                  |
                                                       |--------o      |
Guitar |GND -----------------------------|                   |------o
         |                                                 R4           |
         |Signal -------------------------------/\/\/\------|

I suppose R1&R2 and R3&R4 should have the same value (suggestion?), shall I "protect" the MP3 player with a diode ?

Whant do you think about it

In other ways I've tried (parallel!) I only get signal sucking!

thx for help
Armando

[gez]

IMO, the best way to mix signals is with an active op-amp mixer.

[brett]

Hi
with the system that you are proposing, you need to follow it with a high impedance stage.  That's why people use op-amps (especially those with JFET input stages, like the TL07X series). However, you can also use a very simple JFET buffer.
I built something very similar as a headphone amp for my son, using: 2 x input pots (250kA), a JFET buffer/booster (10k drain resistor, 2.4k source resistor, output from the drain), and an LM388-based amp.
cheers 

[jrc4558]

Buffer at least one of the signals, ideally - both.

[Timebutt]

Would a Buff 'n Blend circuit be a good solution here or not?
If it's better to use an active op-amp mixer, does anybody have a decent schematic so I can start studying?

[gez]

The advantage of a true op-amp mixer over a passive arrangement, is that (due to the virtual earth/ground effect) you don't get interaction of all the signals on the 'input' side.  It's also relatively easy to control individual levels. 

As Constantin mentioned, buffering the guitar (if the signal is direct) is going to help matters, no matter which approach you take; though the output of the MP3 player wouldn't need buffering (it'll be low impedance).

[arma61]

thx to all for help, I got it! I need an active mixer, any suggestion for an easy⁴!!! one to build with few components just 2 INs and 1 OUT; I'm not looking at an HiFi solution!.
Armando

[gez]

thx to all for help, I got it! I need an active mixer, any suggestion for an easy⁴!!! one to build with few components just 2 INs and 1 OUT; I'm not looking at an HiFi solution!.
Armando

Try googling 'opamp mixer' or something similar.

All were talking about here is an op-amp in inverting mode.  Every input gets its own input resistor.  If you want to control overall volume, make the feedback resistor a pot which shorts out (min volume).  Equal value feedback and input resistors give you unity gain.  Make the guitar's input resistor smaller (or use a variable pot), and you'll give it some boost; make sure you use large enough input caps to allow all frequencies, though.

If you want individual control over each input, then you need pots to ground before the DC blocking caps of each input: wiper connects to cap, one lug to ground the other to your guitar/mp3 output.  If you use a dual amp, then one half can buffer the guitar signal to prevent loading.  Alternatively, it could be used to invert one channel of the mp3 player if phase issues are a problem (you're mixing the stereo outs to mono).

[gez]

Haven't read thru it (so can't vouch for accuracy of info), but the schemo (scroll down) looks standard:

http://colomar.com/Shavano/intro_opamp2.html

Intro info here:

http://colomar.com/Shavano/intro_opamp.html

Edit: the input caps are the wrong way round on the inputs (use single supply, not split).  Probably better to use NP anyway.  Also, 10k isn't 'high impedance' for a guitar (though it is for your mp3 player), so use the other half of a dual as a non inverting buffer.

[gez]

PS  If you use a NE5532 op-amp, it can drive phones (no need to take the amp on holiday).