xlr/balanced Input

[STURMJ]

I set my prebee up without the xlr output.  I am hoping to configure one of the inputs as xlr, for my DAC.  I know that to get a true balanced input I need a transformer.  Can I just  copy the output configuration in the manual to the input and get reasonable results? Or would it be better to just direct wire the input without the caps and resistors, untill I get the transformer?

[Paul Birkeland]

Generally, you can wire the - pin to ground on the XLR jack if you'd like to use an XLR cable in an unbalanced configuration.

If you'd like to be the first one to install the balanced intput transformers, let us know and we can walk you through it.

-PB

[STURMJ]

Im in for installing xformer,  what would you reccomend?  I've looked at some and decided I didn't know enough to make a choice.

[Paul Birkeland]

The Cinemag CMLI-15/15B is probably not a bad place to start, be sure to specify the single 3/8" stud for mounting.

The XLR jacks that you'll need will just be Neutrik D-shell jacks, Parts Express has reasonable prices on these.

Other than that, you'll want some extra resistance across the output of these transformers to load them properly (try 30K).

-PB

[STURMJ]

how about a Hammond 140UEX (10k / 10K, 1:1 ) impedance is a little lower then the cinemag (15K) but I can get the Hammonds easily.  If lower impedance is preferred I can also get 140TEX (1K/1K 1:1) 
thanks for the input!

[Paul Birkeland]

The holes in the chassis are designed for a Cinemag, Jensen, and Sowter model.  It actually never occurred to us to even include Hammond on the list.

While you could use the 10K Hammond, you would end up spending more money and drilling more holes in the chassis, with the disadvantage that there is no internal shield.

Also, having the transformer mounted through a single hole allows one to twist the transformer while you're listening to it, which should allow you to find the sweet spot where no magnetic hum is coupled (ymmv, this may or may not be an issue).

So, to use the Hammonds, you will need to poke all the wires through the hole in the chassis, twist the transformer around until it's quiet, mark the holes, uninstall the transformer, machine the holes, then reinstall. 

For loading the secondary, I'd try a 12K resistor.

-PB

[HF9]

Are two of the Cinemag/Sowter's needed or just one?

[Paul Birkeland]

You will need a pair of transformers.

-PB

[STURMJ]

I should have them by the weekend!

[HF9]

I got this info from David Geren from Cinemag for any interested parties:

The CMLI-15/15B in standard format is $52.78.  The threaded bushing option adds on $2.50.  UPS ground will be $XX.  Send me a copy of the schematic if you want me to see about upgrades.  If it is a balanced circuit topology, I have something new that I designed a few months ago that might be interesting.

I know the Bottlehead schematics are proprietary, but I was hoping one of the gents here (PB?) could provide David the info he needs - cinemag[at]att[dot]net - so he can offer Bottleheads the best option.

[Doc B.]

The circuit is not balanced. That's why you need the transformer, to create a balanced input. I'm just sayin'.

[HF9]

Got it, nevermind then.

[STURMJ]

I got the stuff!  my only question is the load resistor you suggested.  Does the 14k resistor go (I presume in serial) with the inbound wires, going in the transformer? Or on the wires coming out of it, or both. (my guess is the wires coming out of the trans. but I want to be sure)
Thanks again

[Paul Birkeland]

I got the stuff!  my only question is the load resistor you suggested.  Does the 14k resistor go (I presume in serial) with the inbound wires, going in the transformer? Or on the wires coming out of it, or both. (my guess is the wires coming out of the trans. but I want to be sure)
Thanks again

Which transformers did you end up with?

I would put the load resistor on the secondary winding of each input transformer.

-PB

[STURMJ]

I got the Hammond 140uex 1000k 1:1. I made a nice mounting platform for them. I'll have that done this weekend. And I got some of the nice Dale resistors (14k).