Schematic for Behringer RX1602 mixer

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mav_kbd

Member
Joined
Apr 26, 2010
Messages
16
Hello, I'm planning to modify my Behringer RX1602 mixer, simply I want to change the unbalanced main outputs to balanced.

The matter is that I'm not able to find schematic for this mixer, I tried to contact Behringer but they told me that can't release schematics to customers.

Anyone have idea how can I get them?

Thanks
 
They normally use SM (surface mount) for their products and although the output connectors will likely be through hole mounted, there most likely to be only 2-pole for unbalanced, whereas you would need them to be 3-pole for balanced.

Apart from using the schematic to find the power rails, you would still need to fabricate some kind of daughter board with the balancing circuit on it then feed it to output jacks/XLRs for a balanced output.  I've made mods to small soundcraft and mackie mixers, but the manufacturers always go with the cheapest options, only putting 2-pole jacks where needed and packing the circuit board as dense as possible.

Much simpler solution would be to buy an unbalanced-to-balanced converter (like the Sonifex and Alice units) and use it externally. Or alternatively, a quick search will reveal that some members here have already designed and made balancing boards and sell the PCBs on the white market.

At least that way, if the Behringer mixer dies and becomes uneconomical to fix, then you've still got a balancing box spare for your next mixer.
 
I have reason to believe that Behringer do not release schematics for any of their products without a signature on a non-disclosure agreement.

Try the search engine of your choice, you might get lucky, but I'd doubt it.

I'd go with an external balancing box of some sort. There are passive options available consisting of a box with connectors on the outside and a transformer (or two) inside. I have found ARTcessories DTI (Dual Transformer Isolator) quite satisfactory. http://www.artproaudio.com/products.asp?type=90&cat=13&id=106

C
 
Thanks.

Actually I have already built a board using two Burr Brown DRV134, and it works fine but I have not tested yet on my mixer. And I have also a passive DI box http://www.palmergear.com/pan04.shtml, but frequently I need a line signal instead of a microphone signal, so it is better to make this mod and then no more problems.

However, I opened the mixer and I found the 15V bus connected to all V4580 amplifiers (every amplifier drives a balanced stereo input channel), but I have to unsolder the main outs connectors because they have 5 pin each despite unbalanced signal and are connected to both sides of PCB. Also there are some resistor and in some way the mono aux output is connected to main outputs bus (there is also a condenser that seem to be connected on aux bus only, but I'm not sure...). Now I'm able to find the pins connected to ground but I don't recognize where are the two output signals.

Just a shot of the schematic would be very useful...
 
It is good to see a schematic - and this reveals that the mixer has TRS jacks for outputs - and the outputs are impedance balanced. This technique works very well - so you don't need to do a modification at all!


 
Yes, the output connectors are TRS, but the signal is unbalanced anyway, as reported on user manual.

I use this mixer only live, as sub-mixer for keyboard and rack modules. So I need long cables for master outputs like 8-10 meters and is raccomended to use balanced cables to avoid interferences.

This is the only reason...
 
Andy Peters said:
mav_kbd said:
Yes, the output connectors are TRS, but the signal is unbalanced anyway, as reported on user manual.

The schematic indicates balanced outputs, regardless of what the manual says ...

-a
Where can you see the balanced output ?
(The 120 ohm resistor from ring to ground is NOT a balancing circuit)

analogguru
 
You've definitely been lucky that the output jacks are 3-pole, and those 120R resistors look like your ideal way of getting your balancing daughter board connected - I'd pull them and connect wires, one each from the unbalanced L&R feeds from the op-amps (where it says X30-X27 pins 2 & 3 for L & R) to your daughter board inputs, and then wires from the daughter board balanced outputs straight to the jack sockets where the 120R resistors were. Power shouldn't be an issue since you've already found that too.
 
solderspongebob said:
You've definitely been lucky that the output jacks are 3-pole, and those 120R resistors look like your ideal way of getting your balancing daughter board connected - I'd pull them and connect wires, one each from the unbalanced L&R feeds from the op-amps (where it says X30-X27 pins 2 & 3 for L & R) to your daughter board inputs, and then wires from the daughter board balanced outputs straight to the jack sockets where the 120R resistors were. Power shouldn't be an issue since you've already found that too.

Yes, I was thinking to proceeded exactly in this way. I will remove all 120R SMD resistors and I'll put them on the new daughter board, however I have to unsolder the TRS connectors because I have to modify the routing of the ground pin. Then I'll put them back in their place, but without ground pin.
 
analogguru said:
Andy Peters said:
mav_kbd said:
Yes, the output connectors are TRS, but the signal is unbalanced anyway, as reported on user manual.

The schematic indicates balanced outputs, regardless of what the manual says ...

-a
Where can you see the balanced output ?
(The 120 ohm resistor from ring to ground is NOT a balancing circuit)

analogguru

Dear guru... Yes it is... It is called "impedance balanced", and meets the functional requirements for a balanced circuit, input or output, namely that both + and - circuit paths exhibit the same impedance to ground. That added resistor is about the biggest marketing bang for a penny resistor ever, first touted in marketing by Mackie IIRC.

Don't ASSume one needs dual polarity outputs to receive the benefits of balanced interfaces. There can be additional benefit from differential outputs for crosstalk reduction and 6 dB more combined peak signal swing, to increase the S part of S/N, but for noise rejection balanced impedance sends working with a true balanced receiver will get you there cleanly.

Note: not all differential inputs are balanced. The common one opamp differential input despite having + and - inputs exhibits a different impedance to ground at both inputs, so common mode noise can be picked up differently in both. It typically takes 2 or 3 opamps to make a proper balanced input. 

JR

PS: I am commenting on the schematic posted,,, I don't know or care what is in an actual Behringer mixer. They have been "inspired" by other company's designers who use that approach so that schematic looks possible.
 
I'm not sure that I would call cables of 8 - 10 metres very long - so before you start work on the mixer modification - have you actually tried to use it as you would on stage? If so - did you experience any problems?

As I originally said - and others have confirmed - impedance balanced working is an effective technique, so make up a TRS jack plug cable and connect up!



 
AudioJohn said:
I'm not sure that I would call cables of 8 - 10 metres very long - so before you start work on the mixer modification - have you actually tried to use it as you would on stage? If so - did you experience any problems?

No, actually I never tried with unbalanced cables on stage, I used a passive DI box.
However when I talk with most of PA engineers, they prefer balanced cables for distances over 5 meters. Specially in small stages where audio cables are often very close to 230V power cables.
 
If your gear uses 3 circuit connectors (TRS, XLR, etc) and any reasonable circuitry, you will generally get benefit from using 3 circuit wiring.

At a minimum using 3 circuit wiring with simple differential inputs will keep the - audio feed from being contaminated by shield currents, with balanced inputs and outputs, signal integrity is even better. 

JR
 
Thanks Mr. Roberts for taking the time for your explanations.

JohnRoberts said:
Dear guru... Yes it is... It is called "impedance balanced", and meets the functional requirements for a balanced circuit, input or output, namely that both + and - circuit paths exhibit the same impedance to ground. That added resistor is about the biggest marketing bang for a penny resistor ever, first touted in marketing by Mackie IIRC.
I am familiar with that circuit arrangement, but I think the benefits on CMRR of a true balanced line can´t be achieved with it.  Therefore I prefer to call it "ground decoupling" which was not really "invented" by Mackie and done since the 60´s as you can see here in this WSW (siemens) console where this technique already has been used:
Resistor to the ground line:
http://www.wsw.cz/01WSW_MONO_IN/WSW_SCHEMA_IN_MONO.JPG
looking into a transformer:
http://www.wsw.cz/03WSW_GRUPPE/WSW_SCHEMA_GRUPPE.JPG

Don't assume one needs dual polarity outputs to receive the benefits of balanced interfaces. There can be additional benefit from differential outputs for crosstalk reduction and 6 dB more combined peak signal swing, to increase the S part of S/N, but for noise rejection balanced impedance sends working with a true balanced receiver will get you there cleanly.

Note: not all differential inputs are balanced. The common one opamp differential input despite having + and - inputs exhibits a different impedance to ground at both inputs, so common mode noise can be picked up differently in both. It typically takes 2 or 3 opamps to make a proper balanced input.
I think the (one opamp) differential line input of the Amek BC III should take care enough of this problem and give reasonable results.
 
analogguru
 

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analogguru said:
Thanks Mr. Roberts for taking the time for your explanations.

JohnRoberts said:
Dear guru... Yes it is... It is called "impedance balanced", and meets the functional requirements for a balanced circuit, input or output, namely that both + and - circuit paths exhibit the same impedance to ground. That added resistor is about the biggest marketing bang for a penny resistor ever, first touted in marketing by Mackie IIRC.
I am familiar with that circuit arrangement, but I think the benefits on CMRR of a true balanced line can´t be achieved with it.  Therefore I prefer to call it "ground decoupling" which was not really "invented" by Mackie and done since the 60´s as you can see here in this WSW (siemens) console where this technique already has been used:
Resistor to the ground line:
I didn't say Mackie invented it, they were the first to hype it as blah-blah balanced...

You can call it whatever you want but it is balanced impedance to ground so technically and functionally balanced.

Strictly speaking any differential input circuit will reject common mode voltages present at it's two inputs, but if the impedances are not equal to ground, external noise will not be induced equally in both so CMRR will be degraded.


Don't assume one needs dual polarity outputs to receive the benefits of balanced interfaces. There can be additional benefit from differential outputs for crosstalk reduction and 6 dB more combined peak signal swing, to increase the S part of S/N, but for noise rejection balanced impedance sends working with a true balanced receiver will get you there cleanly.

Note: not all differential inputs are balanced. The common one opamp differential input despite having + and - inputs exhibits a different impedance to ground at both inputs, so common mode noise can be picked up differently in both. It typically takes 2 or 3 opamps to make a proper balanced input.
I think the (one opamp) differential line input of the Amek BC III should take care enough of this problem and give reasonable results.
 
analogguru

That differential does not present an equal impedance to ground in both inputs. 

It will probably work adequately for modest length, properly shielded, low impedance feeds, but the only common mode it it rejecting is differences between the local and distant signal grounds, not any common mode noise picked up in between.

JR
 
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