Hi all,
I am trying to build a simple circuit that will take three stereo lines, and split them into three stereo lines and one mono line. It's a little difficult to explain so i have included a picture:

however my problem is, that simply wiring it like that will create problems of backflow and the three lines not being seperate when they come out again as 3 lines. I said it was confusing...

Anyway, any help or questions about explanation welcomed, thanks!

 

Hi,

You'll need to use several op amp buffers to isolate your lines (prevent "backflow"). Use an audio rated op amp like an OPA134/2134 and no one will know they're there.

 

So there is no way of doing it passively? Also its been ages since i've used an opamp... so how would i use one as a buffer?

 

Go to the E-book, Volume 3, chapter 8, negative feedback. The first illo is of an op amp connected as a buffer.

For the mono channel, use another op amp set up as an inverter. Feed the left and right channel inputs through resistors, say 20K. Use another 20K as feedback on the op amp.

 

Thanks for your replies, I have quickly drawn up this diagram. I have left out the opamp power supplies for simplicity (i should be able to power them all from a 9v right?) also all of the negative connections (black lines in my original drawing) are wired to the ground on the inverter, will that be ok? So basically I'm just checking with you guys to see if I have made any silly mistakes:

Finally I am in the UK and can't find those ICs you spoke of, I only really have the choice of whats on this page: link (very long URL)

Thanks!

 

Yes that is the exact circuit.
As the opamp I think that NE5532 will do the job.

Just don´t forget to have the ground in the middle of supply voltage, use another buffer connected to two resistors of the same value, one connected to + and one to -, buffering the middle voltage.

 

Sorry, could you explain that a bit further kubeek?
Also I will only need one 9v cell to run the whole thing right?

 

Look at this circuit, R1 and R2 should have the same value, suggest 22k.
You need just one battery or 9v source for this circuit, but if you plan to use batteries, you should look for some other opamp with lower consumtion, NE5532 needs cca 10mA, and you will need four of them. That is 40mA total, and if a standard 9v battrery has like 250mAh, then it will last for 250/40=6.25 hours.

if you have any questions feel free to ask.

 

Cool, thanks. So my final circuit should look something like this then?

Any corrections or comments? Also I'm wondering is this circuit is going to keep my original 3 lines isolated? I am wondering if it would be better to take the original line outs from before the feedback connection on the opamps then have the mono channel be connected after the feed back connecter?
Thanks!

 

the three lines will be still isolated, so it doesn´t matter. But it is probably better choice to take the lines out before the opamp, because then the three lines will pass through the circuit even with no battery.

 

Hi, I'm having some problems working out how to power this baby. I plan to power it from the mains, using an adaptor to give me 9V DC. I'm wondering, how am I going to get the required -9v from a single power source? Any suggestions much appreciated obviously i can then get the +/- 4.5 using a potential divider, but really there is a cloud of confusion over it atm, if anyone would like to edit my last circuit diagram to include all the power lines, rather than just power points, that would be much appreciated also

 

If you are using double opamps, then you just connect all chips +voltage together and to the + voltage you have, and the -voltage to the other end of your supply. You just use the supply you have instead of the battery.

 

Cool, so the power supply will deliver +9v on the positive and -9v on the negative as opposed to 0v. So then I will need the other final opamp to get the 0v ground.

 

No, the "power supply opamp" just creates a voltage in the middle of the supply you yonnect it to, no matter what voltage it is.

So if you connect a 9V battery, or use some 9vDC wall adapter, it will still work, and have + an - 4.5V supply when reffered to the ground, which is the output of the opamp.