Film-Tech Forum ARCHIVE: Wiring up a stereo cell

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Posted by Matthew Taylor (Member # 2310) on 05-15-2004, 03:58 PM:

I am attempting to wire up a stereo cell into my portable projector. At the moment it is mono and has an XLR connector to hook it up to the amps. I would like, if possible, to still use that XLR connector and the 2 core+shield cable that I have. The problem is if I do that, I will have to use the shield as a 'common' and the two conductors in my cable as the Left and Right signals. This will leave me without a shield to connect at the pre-amp end. Would this cause hum?

Posted by Sam D. Chavez (Member # 1841) on 05-15-2004, 07:32 PM:

Using the shield as the solar cell common will almost certanly add hum and noise to the signal. You should switch to a 4 pin XLR.

Posted by John Anastasio (Member # 607) on 05-15-2004, 08:18 PM:

Sam's right....it's going to hum like crazy. XLR-4's are readily available from any electronics or A/V supplier. 4-conductor shielded cable can be found at any home improvement warehouse for about 30p per foot.

Posted by Frank Angel (Member # 248) on 05-15-2004, 09:29 PM:

Or go the other route -- come out of the soundhead with two standard 3 pin XLRs and use two standard, shielded single pair audio cables to the processor. That's what we've done in all our theatres. Mark you cables clearly.

Posted by Michael Schaffer (Member # 1204) on 05-16-2004, 01:25 AM:

Correct me if I am wrong, but don`t have most solar cells unbalanced outputs anyway with just L/R/G? I had this problem recently when installing a reverse scan reader - there were only three connections available on the sound head even though a 2-pair shielded cable went from there to the processor. The L and R of the solar cell was connected to L+/R+, the solar G was coneected to L-/R- tied together, the shield was connected only at the processor`s G. There was no noticeable hum, but it didn`t work out for the reverse scan reader. Since the cable din`t reach to the soundhead terminals, and I didn`t want to splice them inside the projector, I ended up pulling all new wiring to the processor.

Posted by Peter Hall (Member # 695) on 05-16-2004, 10:09 AM:

We always use 5 pin XLRs, retaining the pin config for mono, eg 1 screen (n/c at projector) 2 L+ 3 L- (common with 5 R- if wiring to a cell) 4 R+ 5 R-. This way if you ever upgrade to a basement reader which continues the screen straight to the preamp all wiring is in place. We'd rather install SRD heads however at the moment we are having delays getting hold of the best SRD readers on the market - thus we have the time to participate in forums like this ! I guess they are worth the wait !! (Subtle nudge Mr C !!)

Posted by Sam D. Chavez (Member # 1841) on 05-16-2004, 11:35 AM:

Peter,

I also only have time for these forums because I'm juggling which of 150 orders to fill first. Judt kidding!

I agree the 5 pin approach is better for future upgrades.

Posted by Demetris Thoupis (Member # 833) on 05-17-2004, 01:23 AM:

Why bother with all the technical connections
Use Connecting BLOCKS!!!!!! Or even better some scotch tape and don't even bother to solder it. That will give you a perfect FULL Humming sound!!
D

Posted by Gordon McLeod (Member # 33) on 02-23-2005, 02:16 PM:

Michael they are balanced in the sense that the common of the cell (black wire) isn't ground referenced
So L+ is balanced in respect to the common black wire and R+ is balanced to the common black wire of the cell

Posted by Randy Stankey (Member # 64) on 02-23-2005, 02:36 PM:

If I saw an XLR connection on a piece of equipment I would assume that it is a balanced output. (Or input, as the case may be.)

Since a plain solar cell is not balanced, if you plugged it into a sound system that was expecting a balanced input, you would get all kinds of noise and just plain crappy sound to boot.

My sound board (at work) doesn't really care if the inputs are balanced. It's better if they are but the system will work at a minimal level. Just make damn sure you keep the polarity straight! Pin #2 is HOT in an XLR connector. Pin #3 is the SHIELD. Don't get those wires crossed! Some sound systems have "phantom power" That means there is 48 volts going through the line. (Used to power condenser mics, etc.) Wire the thing up the wrong way and you could turn your equipment into quite an efficient smoke generator!

I would build a small preamp with balanced outputs which has two 3-pin XLR connectors and embed that in the projector. If there's only room for one XLR I'd make it a 4-pin then solder up a "Y" adaptor that ended up with two 3-pin XLRs.

If you are not going to make the outputs balanced I would not use XLR connector(s). Make it a 1/4 inch stereo phono plug or something. That way people won't acidentally connect things up the wrong way.

Posted by Gordon McLeod (Member # 33) on 02-23-2005, 07:04 PM:

Randy said "Since a plain solar cell is not balanced, if you plugged it into a sound system that was expecting a balanced input, you would get all kinds of noise and just plain crappy sound to boot."
I would like to know who says a solar cell isn't balanced and how is it ground referneced since the silicon ship is mounted on an insulator there is no signal that is ground referenced

Posted by Randy Stankey (Member # 64) on 02-23-2005, 07:38 PM:

It may not have a reference to ground but the black wire is common between the two cells. You don't have common mode rejection between the "hot" and "cold" wires because the signal isn't equal and opposite. Do you?

I thought that was the main reason for using balanced lines.
Is it not?

Posted by Scott Norwood (Member # 30) on 02-23-2005, 08:16 PM:

By "connecting blocks," do you mean "terminal strips"? If so, what's wrong with using them? The Dolby manual indicates that this is how one should connect the cell to the shielded cable that runs to the processor. Soldered connections (with heat-shrink tubing) seem to be more popular, but I'm unclear as to what is wrong with terminal strips (if anything).

Posted by Gordon McLeod (Member # 33) on 02-23-2005, 08:45 PM:

Actually it is it also has a very high CMR which is why it has to be connected to the preamp in a balanced fashion

Posted by Randy Stankey (Member # 64) on 02-23-2005, 09:41 PM:

So, assuming it isn't otherwise grounded, a solar cell is a naturally balanced system?

I'll have to remember that!

Posted by Steve Guttag (Member # 268) on 02-23-2005, 11:36 PM:

Randy, you need to remember several things, actually.

First Pin 1 on an XLR is signal ground, not pin 3. Even Bozos that make Pin 2 "-" still keep Pin 1 ground.

Next, until a signal is referenced, it is a differential signal and thus has good CMRR. Techncially it is not "balanced" since that implies a reference that is balanced about the reference.

A solar cell, that is not referenced to ground may be wired in differential fashion to achieve the benefits normally assoicated with a balanced input. Note, Dolby has always provided a + and - terminal for the cell input "E" was always just for the shield.

Other, lesser designs had one wire the cell in an unbalanced fashion by connecting one of the leads of the solar cell to ground. These systems are normally hallmarked by the snaps and pops in the sound as each piece of apparatus is turned on (either manually or via automation).

XLR or TRS connectors normally imply differential that is often also balanced. Thus pin-1 is ground (signal). There is also the often forgotten terminal on XLRs that is the chassis ground (the odd funky tab that is often connected to the release button on an XLR). Shield wires would normally benefit by connecting to chassis rather than signal ground so a good drain is established. Then again, in many systems, signal and chassis ground are one and the same. In fact, if you read the various RANE notes, they connect chassis ground and pin-1 on all of their XLR connectors.

Steve

Posted by Randy Stankey (Member # 64) on 02-24-2005, 01:19 AM:

I knew about the XLR pins. I've soldered up many of them.
What you mentioned about Rane grounding Pin #1 rang a bell, although I didn't remember it, per se.

What I still have to remember is about how balanced lines work vs. unbalanced. I know about not having a reference to ground and about common mode rejection. About Pin #1 being signal ground and/or chassis ground sounds counter intuitive. If balanced lines are good because they don't have a reference, then why ground them and give them one? Unless that reference is held precicely at earth ground, thus making the reference 0.

I need to hit the books some more!

Posted by Steve Guttag (Member # 268) on 02-24-2005, 09:02 AM:

Randy,

Think about it like this....something can not be Balanced or Unbalanced until there is a reference to balance or unbalance to!

Until there is a reference, you merely have a potential difference or a differential output...not the same but similar to balanced. Some would also call this "floating."

By definition, a "balanced" line is balanced about its reference (normally ground). An "unbalanced" line is unbalanced with respect to its reference (again normally ground).

Ground makes a great noise drain is also quite available. It can be quite safe and sooner or later something in the system will be referenced to it. So, if everyone can't agree to the reference when interconnecting systems, you will create problems. So ground it is.

Steve