Adding balanced aux sends

[conleycd]

I have tried to see what is done to create aux sends for a channel in a mixer but I can't find any schematics really detailing what is done.

I'm thinking it would probably require a unity gain opamp buffer into an audio distribution network of opamps (however many sends are desired).  The individual output of each opamp would be attenuated (level of the aux send) and the electronically balanced or transformer balanced.

Am I right with this?

Let know.  Thanks.

CC

 

[abbey road d enfer]

You need pots that extract signal from the channel, which are consequently sent to a mix bus, this mix bus being fed to a summing amp, and then the signal from this summing amp being fed to a balancing stage (maybe with a master fader inserted). You don't seem familiar with these structural elements of a mixer. Only when you are comfortable enough with the role of these elements, you may undertake the task of actually building them.

 

[conleycd]

Hmm...  I'm quite familiar with the structure or how to use a mixer.  I've worked on tons of boards - both low grade - and also a Neve and SSL 9000 console.

I know how a mixer works.  But I'm planning on building an active summer, with aux sends (to allow me to do parallel processing), and then combining those signals together for monitoring and final output.

I'm interested in the build structure of the sends from the channel (not the return of them) or in any other aspect of the mixer.

Most channel strips have a mic preamp (signal becomes unbalanced [for processing] buffered and converted to low impedance), eq, and then aux sends that are pre or post eq.

To create aux sends from the channel (I suspect - which is my question) would then take that unbalanced signal pass it through a unity gain buffer opamp and then split the signal however many times using either transformer or an opamp.  At the end of the opamp would be a pot (?10k) which would then electronically (or with a transformer) be balanced and appear on an output jack.

So a block diagram would be:

low impedance unbalanced signal --> Unity Gain Opamp --->  Opamp Unity Gain (Aux 1)  ----> Potentiometer----> Balancing scheme-->Output
                                                                                        --->  Opamp Unity Gain (Aux 2)  ----> Potentiometer----> Balancing scheme-->Output
                                                                                        --->  Opamp Unity Gain (Aux 2)  ----> Potentiometer----> Balancing scheme-->Output
                                                                                        --->  Opamp Unity Gain (Aux 2)  ----> Potentiometer----> Balancing scheme-->Output

So that is my real question if it wasn't initially clear.

Can people confirm if I'm on the right track?  Thanks.

 

[abbey road d enfer]

You say you want AUX sends. By definition, an AUX send is derived from several channels, mixed into a bus, and then outputted.
So the block dgm would be:
low impedance unbalanced signal --> Potentiometer----> Aux1 Bus---->Aux 1 Summing amp----> Balancing scheme-->Aux 1 Output
                                                  Potentiometer----> Aux2 Bus---->Aux 2 Summing amp----> Balancing scheme-->Aux 2 Output
                                                  Potentiometer----> Aux3 Bus---->Aux 3 Summing amp----> Balancing scheme-->Aux 3 Output
You don't need a cascade of unity gain buffers if you have a low-impedance signal available (unless you want 16 auxes)

Now, according to "I'm planning on building an active summer, with aux sends (to allow me to do parallel processing), and then combining those signals together for monitoring and final output.", maybe what you want is having multiple outputs of the summing buses. In that case the block dgm would be:
Inputs----> Summing bus---->First (Dry) summing amp---->Second (Wet) bus---->Second (Wet) summing amp-->Balancing scheme-->Main output
                                                                                Balancing scheme-->Aux 1 Output
                                                                                Balancing scheme-->Aux 2 Output
And your FX returns must be routed to the Second summing bus. If you return them to the first bus, you will create an infinite loop.

 

[cuelist]

Hmm...  I'm quite familiar with the structure or how to use a mixer.  I've worked on tons of boards - both low grade - and also a Neve and SSL 9000 console.

Knowing how to use and operate and understanding "what makes it tick" inside are two different things. I would not know where to start using a Neve or SSL console (I'm an ex  live sound guy) but I have very good understanding of mixing console basics.

In 99% of all consoles, the aux sends are quite simply a log law pot between the signal (unbalanced) and 0V (signal ground). The wiper of the pot is connected to an aux buss via a resistor (typically 10k in value). All channels and their aux sends feed this buss via their 'summing resistors" (the 10k guy) and somewhere in the console sits a summing amp that creates a low impedance signal suitable for the outside world. Most probably this signal is again fed to a log law potentiometer and from then on to an output buffer. This enables an aux send master level control.

If a large number of sends are to be derived from a channel then yes probably there will one or more unity gain buffers to ensure that the main signal flow isn't affected by the loading of the aux send pots and summing resistors. In rare cases, balanced busses are used and in some even rarer cases, very low value summing resistors are used, necessitating buffers (my late friend Barry Porter used 3k6 for his Cadac designs).

Details of console signal flow can be found on most console makers websites. Some even provide schematics for their "legacy" consoles, Allen & Heath and DDA have plenty of information. They're not Neve or SSL but the principles are exactly the same.

See: http://www.allen-heath.com/veterans  and http://www.ddaconsoles.com/

Best of luck!


C.

 

[conleycd]

Thanks both of you.

Abbey Road... I should have been more clear in my first post.  I'm not creating an aux from multiple channels (as in a typical mixing board) just from 2 channels (stereo) - so in essence really just one channel.  I think that is where I confused you.  Most people would think as you did that more than one channel is sent to a bus for summing.

So both of you figure that isolating each aux send with an opamp because the signal is low impedance already would be redundant and unnecessary?  I should be happy with a resistor isolation and potentiometer and then balance the signal before hitting the output jack.

Can anyone speak to any advantage or disadvantage to having each aux send isolated with an opamp rather than a resistor?  Would an opamp increase noise potential?

Thanks.

CC

 

[shabtek]

paralel procesing--?where do you return the parallel signal after 'tis split off from the main pair?

api line output comes to mind; 2 or 3 secondaries from output xfmr can feed multiple destinations

 

[cuelist]

... you figure that isolating each aux send with an opamp because the signal is low impedance already would be redundant and unnecessary?  I should be happy with a resistor isolation and potentiometer and then balance the signal before hitting the output jack.

In a word, yes.

I don't quite understand what "isolation" you are looking for. Adding an opamp or any other type of buffer will not help. The noise contribution will be negligible, noise in any console is set by the mic pre primarily and the summing amps.

I think you may be "overthinking" this, solving a problem that does not exist in the first place.

 

[sahib]

When extracting a signal you only buffer to prevent loading. So, you check your schematic if this is the case. Otherwise we can all talk about buffering or not buffering until  the cows come home. Rest is pretty well explained. There are a huge wealth of schematics on the links that cuellist gave. One picture says one thousand words. In this case a schematic replaces the picture.