Sonosax SX-PR prefade mod

[adamasnan]

Hi all, first post: Please forgive me, as my understanding of electronics is rudimentary, if that..

So, I have a very nice Sonosax SX-PR mixer than has been modified for post-fade direct outs by an expert. I was curious as to where and how they made the mod, so I had a look inside. Curiosity got the better of me, and so I tested whether there was another point that a pre-fade output could be drawn from, found a point, and made a simple output cable from that (making sure to be very gentle with the solder points.)

It works very nicely, and does not audibly effect the mix bus in anyway. However I am well aware that simply drawing directly from a point on the board might be bad practice without calculations (e.g. so that the next component in the circuit - or any subsequent components for that matter - may be receiving less than adequate current to perform properly ..?)

The post-fade mods were made after a couple of resistors, and have what look like chip capacitors attached between the contact of the resistor and the each wire to the output. When enquiring about a possible pre-fade output mod, the engineer mentioned that transformers should be used.  Should this really be the case?

I have the schematics (attached, hopefully it's enough to help with my dilemma), and would really appreciate some input on either what possible damage I could do with my existing pre-fade feed, or what possibly alternative I could take? Marked on the schematic are red circles, where the professional post-fade feed takes place (this might not be the case, though, I don't have the unit in front of me, and I am not really able to read the schematic!) and the blue rectangles where I took a pre-fade feed.

Just to clarify, I do not intend to use both pre and post fade together

Many thanks!!

Adam

 

[moamps]

The correct points where pre and post fader signal can be taken are shown in the attached picture.
In order to protect opamp's output from short, you can insert a simple RC network consisting of, for example, 100ohm resistor and 100uF/25V bipolar electrolytic capacitor in series between opamp's outputs and direct output connectors.
If the cables from the mixer to the recorder? are short, most probably you don't need any additional circuitry  (active or passive (transformer based) balancing stages).
Usually you can use pre and post fader signals simultaneously if you need it.

 

[adamasnan]

Amazing, thanks for your advice!

Here are some images -- First image is the mounting layout for the board, locating R12 and R32

Second (in next post) is an overview, with indications of where I might take the pre-fade feed?

Third (in post after next) is showing where the pro had used a chip cap for post fade (?).. but no 100ohm resistor (added, at least). Might it be possible for me to simply repeat this logic at the the pre fade point?

Many thanks again!!

 

[adamasnan]

Second

 

[adamasnan]

Third

 

[moamps]

Second (in next post) is an overview, with indications of where I might take the post fade feed?

If you mean pre fader then it is correct.

Third (in post after next) is showing where the pro had used a chip cap for post fade (?).. but no 100ohm resistor (added, at least). Might it be possible for me to simply repeat this logic at the the pre fade point?

Yes, but I would add a 100ohm in series with the capacitor. Check the value of this capacitor, it looks little bit too small in dimensions.

 

[adamasnan]

Great thanks again!!

I am just re-reading your original response, and I just would like to have this clarified (forgive my ignorance!):

If the cables from the mixer to the recorder? are short, most probably you don't need any additional circuitry  (active or passive (transformer based) balancing stages).

Just to ask whether you mean there is no additional circuitry needed after the 100ohm resistor>100uF/25V cap, as the output need not be balanced for a short run, or whether you mean that a direct connection at the pre-fade point without the 100ohm resistor>100uF/25V cap might be fine for a short run (btw, no longer than 50cm) ?

 

[moamps]

Just to ask whether you mean there is no additional circuitry needed after the 100ohm resistor>100uF/25V cap, as the output need not be balanced for a short run

RC circuitry, IMO,  is needed to protect opamps from short etc, regardless is the cable short or long.  If the cables are long, most probably, balanced circuitry should be implement.
I have sent you PM.