Adding multiple outputs on a DJ mixer project

[bcmoore]

Cheers guys,

I started a DJ mixer project a while back, I'm basically cloning the infamous Bozak DJ mixer. I'm a complete novice, so excuse me if this sounds like a silly question.

At this point I have a summing circuit that sums up 5 (stereo) channels, and then I have 2 output amps connected to the output of that summing circuit:
-1 for master house output (going to the main soundsystem)
-1 for booth output (going to the DJ booth monitoring speakers)
(and then there's an aditional output amp for the headphones to cue tracks but that's connected to a selector switch which allows you to pick one of the 5 channels on the mixer and listen to it in the heaphones)

I have one more output I would like to add to this mixer setup, one for tape/recording. So, my question is, where would be the best place to add this output? Could I just split the master house output in two and put an extra output for tape/recording there? Would I need to add a buffer circuit? Or, should I add an extra output on the summing output (before the output amps)?

Thanks in advance!

 

[indianajo]

No expert help in 15 hours.
How many outputs a "summing circuit" can drive has to do with the current drive available, internal resistance, and the current hogging aspects of the loads.
Op amps have a "output current" spec and a voltage droop table with various amounts of voltage droop with different ohms loads.
In general, another op amp, as a tape loop driver, and the couple of inches of wire over to the other op amp, do not hog very much current.
OTOH cables tend to have capacitance of 120 pf/foot or more, so cables to various destinations tend to have their own dedicated op amp drivers in most pro mixers.
So usually another op amp inches from the summing amp won't change anything much. Putting the tape loop out after the master out driver usually won't change much, either - unless you're using noisy op amps like 741's or 4558's or something else unsuitably noisy.
Some german did a nice study on analog input of which popularly priced op amps had a lot of performance for the price, but I can't find his name today. In dual in line he came up with NE5532, NJM2068, NJM4580, MC33078, LM4562 and maybe another. Peavey also uses NJM4560. 4580 is very useful to drive long cables with lots of current. I've heard the 33078, 4560, & 4580 in equipment I own, and approve. I did not approve of 4558 and took them out. 4558 does seem to work okay at very low gains, but hisses like a boiler at 50x.
Impedance of a passive load is a resistance analog, and you can look the formula up. Something like 1/sqrt(resistance^2 +2*pi*freq*capacitance^2 + 1/2*pi*inductance^2) but don't quote me. Capacitance in farads and inductance in henrys. Once you have the impedance Z of your load, I=V/Z

 

[rayma]

I have one more output I would like to add to this mixer setup

Can you post the schematic?

 

[bcmoore]

Thanks for the input thus far.

@indianajo The circuitry is entirely discrete, so no opamps here.

@rayma I never actually drafted out a final schematic of the final version of my clone, but it's very identical to the bozak as per these images:

http://www.wavemusic.com/community/attachment.php?attachmentid=894&d=1147750819

and

http://www.wavemusic.com/community/attachment.php?attachmentid=895&d=1147750859

So basically, splitting the outputs on the master output amp would have less desirable implications? Also, to make it clear, this would NOT be a send return loop, just another output.

 

[indianajo]

Thanks for the input thus far.

@indianajo The circuitry is entirely discrete, so no opamps here.


So basically, splitting the outputs on the master output amp would have less desirable implications? Also, to make it clear, this would NOT be a send return loop, just another output.

You can get weird reflections and interactions with two cables attached to one set of driver transistors: whether they are in an op amp package, or out in the open.
So you need two sets of driver transistors for two different cables. Whether the pre-driver driving one set of outputs has enough current to drive another set of output transistors, is an engineering problem involving the input impedance of the driven transistors and the current available from the pre-driver transistors. A couple of inches of wire an inch or more apart from predriver to driver set have capacitance of tens of picofarads and can usually be ignored at audio frequencies. It is only when you bundle up wires in a cable close to each other that the capacitance rises a whole lot and has to be accounted for so it can be fed with current.