Hardwiring s.e.x. 2.1 for 8 ohms and balanced output?

J

jrebman

New member
I know this is probably a long shot, but has anybody here wired their s.e.x. 2.1 for 8 ohms and balanced outputs?  If so, I'd love to know what the connections are for this.

I plan to build my kit with a mini 4-pin xlr in place of the tRS connector as the headphone cable I will be ordering will be a mini xlr, and then I'll make the necessary adaptors for TRS and 3.5 mm cans, but I want to run my HE-400s and HD-800s balanced.

Thans for any help,

-- Jim
 
Hello Jim,

You need only wire the output transformer for 8 ohm operation, then connect the transformer outputs to the 4-pin XLR. 

The convention for the 4-pin XLR is as follows:

Pin 1 - L+
Pin 2 - L-
Pin 3 - R+
Pin 4 - R-

Next, acquire some low-ish impedance resistors (around 100 ohms) and connect one to ground from each of those pins.  Note - you may find that part of the operation easier if you connect these resistors to the ground tab on the XLR jack itself.

-PB
 
Hi Todd,

Thanks -- I'll take a look though right now my inclination is to use just one balanced connector and make adaptors for other types, both balanced and unbalanced.  This way I can demo the amp wiith anything that comes along and not have a top plate full of holes and connectors :-).

The other nice thing about adaptors is that if I need to attenuate certain sensitive headphones, I can build the attenuation network directly into the adaptor.

-- Jim


 
Caucasian Blackplate said:
Hello Jim,

You need only wire the output transformer for 8 ohm operation, then connect the transformer outputs to the 4-pin XLR. 

The convention for the 4-pin XLR is as follows:

Pin 1 - L+
Pin 2 - L-
Pin 3 - R+
Pin 4 - R-

Next, acquire some low-ish impedance resistors (around 100 ohms) and connect one to ground from each of those pins.  Note - you may find that part of the operation easier if you connect these resistors to the ground tab on the XLR jack itself.

-PB
Click to expand...

Just confirming, does this mean 4 x 100 ohm resistors?

Also, what's the reason for needing these?
 
The output transformer secondary must be grounded for safety reasons. For balanced operation, this pseudo center tap is a convenient way to do so - the transformer itself has a center tap when wired for 16 or 32 ohms, but not for 4 or 8 ohms.

FWIW, all this is incorporated into the impedance switch board - though that's overkill for what you are planning here.
 
Loquah the impedance board is worth having if you use different pairs of headphones, i switch mine fairly regularly depending on what i'm using.
 
Grados or any 32ohm high sensitivity headphone.  The HD650's i use with 8ohms balanced, haven't done much testing with the HE-500's so i dont know if 4 or 8 will be best, i have also made up a balanced cable to try too.
 
Loquah said:
Caucasian Blackplate said:
Hello Jim,

You need only wire the output transformer for 8 ohm operation, then connect the transformer outputs to the 4-pin XLR. 

The convention for the 4-pin XLR is as follows:

Pin 1 - L+
Pin 2 - L-
Pin 3 - R+
Pin 4 - R-

Next, acquire some low-ish impedance resistors (around 100 ohms) and connect one to ground from each of those pins.  Note - you may find that part of the operation easier if you connect these resistors to the ground tab on the XLR jack itself.

-PB
Click to expand...

Just confirming, does this mean 4 x 100 ohm resistors?

Also, what's the reason for needing these?
Click to expand...

Sorry, still trying to confirm - do I need resistors on + and - connections or just on the -ve?

Mark, in relation to your post, my understanding is that the S.E.X. outputs higher power on the 32ohm setting and potentially has a higher noise floor. That's why I'm figuring to stick to a hard-wired 8 ohm balanced setup because if it can comfortably drive my HE-500s then there's no need to go to the 16 / 32 ohm outputs. Am I missing something?
 
Thanks Mark.

My limited understanding of how to read schematics leaves me in the unfortunate position of still not knowing whether the resistors are required on +ve and -ve or just on the -ve. I'll look at the impedance kit I have here for my friend's kit and see it the "real thing" makes more sense to me than the schematic.
 
LOL thats where i started looking at the PCB's and couldn't make head nor tale of it.  It was only when Paul explained how the two windings are joined together in parallel for 4 & 8 ohms, or in series for 16 & 32 ohms that it all started to make more sense.  So when in parallel there is a resistor from each winding joined together to create a center point which is grounded, and the other end of each winding becomes your + and - terminal for balanced operation.  Dont ask me to work out which terminal on the transformer is which, that would require a few diagrams and lots of head scratching.

Now you see why i fitted the impedance switch board with an XLR socket, changes are just a flick of a switch away, bit like the mainline.
 
Yeah. I hear you!

I'm reluctant to spend $80 plus shipping for switches that I will never, ever use (i.e. once they're set to 8 ohm and balanced) so would like to just wire it once and be done with it.

Reading the post above from PB, it certainly sounds like a resistor needs to connect each of the 4 XLR terminals to earth (or the body of the XLR socket in this case)
 
Makes sense if you never need to switch it over again or use unbalanced headphones.  Are you adding a socket or just going to connect via the speaker taps?
 
Based on some discussion on a new thread I created about enlarging the hole in the plate, I think I'll stick with an adapter lead from the speaker taps.
 
mcandmar said:
...  Don't ask me to work out which terminal on the transformer is which, that would require a few diagrams and lots of head scratching....
Click to expand...
Funny, thanks! You have no idea how much head scratching and how many diagrams it took before I could put that simple PC board together - and I designed the #$%^ transformer just so we could do this dance!

For 4 or 8 ohms on the SEX amp, the windings are in parallel and there is no center tap. Balanced lines is short for balanced-impedance, meaning the impedance to ground of each side is the same. So you need an equal resistor from each side to ground.
 
Paul Joppa said:
mcandmar said:
...  Don't ask me to work out which terminal on the transformer is which, that would require a few diagrams and lots of head scratching....
Click to expand...
Funny, thanks! You have no idea how much head scratching and how many diagrams it took before I could put that simple PC board together - and I designed the #$%^ transformer just so we could do this dance!

For 4 or 8 ohms on the SEX amp, the windings are in parallel and there is no center tap. Balanced lines is short for balanced-impedance, meaning the impedance to ground of each side is the same. So you need an equal resistor from each side to ground.
Click to expand...

Sorry to keep asking this, Paul, but does that mean a 100 ohm resistor (for example) from each of the -ve pins to the chassis?
 
Yes, that's exactly what I meant by "an equal resistor from each side to ground" - assuming you also have a 100 ohm resistor from the positive to chassis ground.
 
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