Subwoofer hookup

[johnvb]

A year ago I posted this topic on the old board, dealing with hooking up a pro crossover to my SEX amp:

http://www.audioasylum.com/forums/bottlehead/messages/13/134964.html

I instead picked up a pair of Costco cheapo subs w/plate amps and high level inputs, had other plans for the passive subs.

I would now like to use the passive subs for this system, but I need clarification of the cable configuration, which would be SEX binding post outputs to XLR inputs, of a Behringer DCX2496.

Using the standard pin designations of pin #1 as ground, pin #2 as positive and pin #3 as negative, do I use pins #2 ( SEX +) and #1 (SEX -), or pins #2 (SEX +) and #3 (SEX -)?

Thanks, John

Another question; if I want to upgrade to the upcoming Stereomour amp (which I do :^) ), would the above setup work there also?

 

[Paul Joppa]

For the SEX amp, I would connect both the shield (pin 1) and the negative (pin 3) to the grounded binding post, with pin 2 to the "hot" binding post.

If you want to be really really anal about it you can connect a 3-ohm resistor between the negative binding post and the wire that goes to pin 3. The output impedance of the SEX amp is about 3 ohms, and this will create an equal impedance to ground, i.e. a balanced line. It won't make much real difference; the importance of source impedance balance is small when the source impedance is much smaller than the (balanced) load impedance, i.e. the DCX2496 input impedance. But it would be a fun exercise to implement the standard. Note that voltage balance is not important to the functioning of a balanced line, it's all about impedance balance.

The same arrangement will work equally well with the Stereomour, but its output transformer is more flexible and could be wired with a center-tapped output in the 8 ohm and 16 ohm configurations.

 

[johnvb]

LOL, IMO, being anal is synonymous with being an audiophile. So does the resistor just go in series with the negative line, then the ground is tied at the binding post with the resistor, or do I tie both neg. and grd. together, then series that with the resistor to the binding post?...sounds like another anal question :^)

 

[Paul Joppa]

The first one.

Pin 1 is the shield, not "ground" which is an indistinct term anyhow. The shield should connect to the chassis rather than signal ground (if they are distinct). The chassis shields the internal electronics from external electric fields, just as the cable shield protects the internal twisted pair from external electrical fields. It's the twist of the wires in the cable that cancels out any magnetic fields.

There are some nice white papers that can be downloaded from the Jensen Transformers web site, most of them by Bill Whitlock. If you have any interest in how and why shielded twisted pair balanced lines are the gold standard for audio interconnection, read some of them. Naturally they will tout the advantages of transformers in this connection, but the science and engineering is first class.

 

[johnvb]

Thanks again Paul. I''m a big fan of Jensen, own a few of their Isomax units, even installed a few of their JT-11 trannies in some ss stuff I have. Just like Bottlehead, they're a first class act.

 

[johnvb]

One last newbie question, what would be the wattage/type of the resistor needed? One of those big ceramic jobbers or not (it would be nice if it was small enough to hide in the XLR shell).

 

[Paul Joppa]

It could be very tiny. If the Behringer input impedance is 10K and the SEX peak output is 4v, the peak current would be 0.4mA. The power dissipated by a 3-ohm resistor at 0.4mA is half a microwatt. I don't think you can buy a resistor with that small a rating!

 

[johnvb]

Just wanted to report back of success, for future reference (and of course another question). I ended up cutting a couple of Mogami "Gold Studio" XLR cables I had lying around, used one end of each cable to go from the SEX to the DCX. Installed the extra resistors in the XLR shells on the load side. Then soldered a TS connector on the other cut cables to go from the DCX to a Nano Patch Plus volume control, which is a budget passive that appears to be adequate for sub duty. Lastly made a TS x RCA cable (as short as possible) out of Belden coaxial for the Nano to the power amp (Aragon 8008ST). Works great. The SEX is not overloading the DCX; in fact where I usually run my levels, I could use even more gain. Thanks to Paul for your help.

But now, more questions... I want to try my Magnepan MMGs in this system, fully modded (new frames and x-overs). Obviously the SEX won't drive these; I have an Aragon 8002 for that. And now that I'm a big time Bottlehead fan, I'll want to eventually add the Foreplay pre. But the funds for that won't be there 'till the end of the year, and if the maggies don't work out, I'll then go back to Hi-Eff and the Stereomour route instead.

So now that I know the SEX will work as a "preamp of sorts", I would like to slum it for a while until I can afford the Foreplay. I had already tried to drive the Aragon directly with the SEX, it didn't like it. One of the channels dropped out, luckily only temporarily. So is there a way I can pad down the output on the signal going from the SEX straight to the MMG's amp (leaving the DCX hooked up as is)? Or was it operator error, the direct connection should of worked? I