guitar fx mixer not working

[john lan]

Hello i am trying to make this mixer

http://music-electronics-forum.com/attachments/4780d1237323928-opamp-s3.gif

as a part of a pedal that will allow me to mix sounds from two multifx.
when I have the battery disconnected it works ,sound comes thru and all pots work
when I add the battery the volume drops a lot and you can barely hear it
the battery is 9 volt.
I have connected circuit ground and tl072 ground to battery ground.
caps direction is correct.
I used different tl072 with the same result
is the schematic ok?

[duck_arse]

hello and welcome, john.

that circuit implies a positive and a negative supply for the op-amp. otherwise, you need a method of re-referencing or biasing the opamp to a point midway between the 0V and the 9V.

possibly the simplest fixxe is to add another 9V battery, connect the new battery (+) to the points marked ground (the little triangle pointing down) and the new battery (-) to the opamp -V pin (pin 4 on the tl072).

[slacker]

Here's a similar mixer designed to run off a single supply like a battery or typical stompbox power supply.

http://www.muzique.com/schem/mixer.gif

If you rewire yours so that the + input of the opamp is connected the same way then it should work.

[PRR]

> mix sounds from two multifx

For *two* sources in typical stage setups, this will probably work very well, is a lot simpler, no power.



I've diddled the input impedance to better suit typical stage toys. However it will work, for most situations, if everything is 10K.

[john lan]

thank you all
i like 'simple' because it usually works...

so  what could I add to the end of PRR circuit to get some volume back
not too many components would be good

[alexanderbrevig]

I would add a capacitor to block out potential DC from the signal. If one device is broken it can quickly cause more damage if you do not protect from DC.
If you want to add gain then the inverting configuration shown in your original schematic is close to minimal in terms of parts count. The only think you need to add is a bias.
You do that by raising the + side to half of Vcc (4.5V) so as to 'bias' the virtual ground up, enabling the opamp to go above and below the virtual ground point. This DC offset is then removed with the capacitor that is placed on the opamp output. How do you raise the + side to halv Vcc? A simple voltage divider!
Instead of going directly to ground, you go to a point that is wired with a resistor to ground, and a resistor to Vcc (for instance 10k, make sure they're the same value).

Here is a version hacked together with a 4.5V bias.

10K output impedance may be OK but I'll leave that for you to figure out.

[john lan]

yes! It worked !

the signal is a bit hot but the pot takes care of that
I did cheat(lazy) by using the first schematic and adding only the voltage divider
thanks a lot!


I will be posting the rest of the pedal in some other thread for others can find it easily
this circuit  is a very simple way to change sounds but keep the reverb and delay ''tails''.something surely done before... but i could not find something on the net.

[ashcat_lt]

10K output impedance may be OK but I'll leave that for you to figure out.

The Out-Z of what you've drawn is significantly less than 10K.  It would be best to have a series resistance of some sort after the opamp.  1K is a pretty common value.  That 10K will be barely big enough then.

[PRR]

> Here is a version hacked together with a 4.5V bias.

Input caps polarity should be the other way (fairly serious).

One cap after the Y should be fine (unless the pedalboard is littered with leaky pedals). Saves a few cents.

That cap(s) does not have to be as big as 10uFd (but smaller caps may not be cheaper).

The 4.5V bias will have "all" (well, half) of any crap on the power supply, will inject this right into the signal. It is customary to hang a cap (at least 10uFd) on this point to cut the crap. (So you do need the 'extra' cap I suggested you take off the mixer Y.)

Output impedance is dang near zero. (Plus 10uFd.) This is fine for short wires. On the chance that we may drive a long wire to a remote point, it is usual to add a few hundred Ohms in series with the cap, so the opamp won't be too offended by line capacitance and other strange stuff.

[john lan]

why should the caps be the other way around ?
what is the theory behind this 



can i only post link of images not upload them?

[ashcat_lt]

why should the caps be the other way around ?
what is the theory behind this 

Because the opamp is pushing its side of that cap up to nominal 4.5V and the other side is (hopefully, assuming our input devices are playing nice) sitting at 0V.

can i only post link of images not upload them?

Pretty much, yes.  You have to upload them somewhere else on the web (photobucket is relatively easy) and then reference that web address in the img tag.