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

[Jim R.]

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

[Paul Birkeland]

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

[Jim R.]

PB,

Thanks -- that seems simple enough! :-)

Thanks much,

Jim

[Todd R]

Jim,
I have the impedance boards installed, but you might be interested in what I did with mine.
I can run TRS or 4 pin XLR http://www.bottlehead.com/smf/index.php/topic,3133.msg27985.html#msg27985

[Jim R.]

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

[Loquah]

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

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

Also, what's the reason for needing these?

[Paul Joppa]

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.

[mcandmar]

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.

[Loquah]

What headphones do you use >8 ohms for, Mark?

[mcandmar]

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]

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

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

Also, what's the reason for needing these?

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?

[mcandmar]

Yeah that's pretty much it.  I have never used the 16 or 32 ohms options, only 4, 8, balanced and unbalanced.

As for the resistors, all i can tell you is the kit uses 60.4ohm, i'm not even going to try and get my head around that layout again. Its all in here http://bottlehead.com/smf/index.php?topic=5043.msg48467#msg48467 , i added the schematic Paul posted into the initial post since it got lost.

[Loquah]

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.

[mcandmar]

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.

[Loquah]

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)