Film-Tech Forum ARCHIVE: Sound Rack Wiring

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Posted by Barry Floyd (Member # 385) on 07-17-2003, 09:20 AM:

Our sound rack at the Drive-In will have a SMART MODII B-DIT processor for the film sound, a CD player,Tape Deck,and microphones running thru a small rack mount mixer, and an FM transmitter for the final output to the cars in the field.

My question is, the non-sync "inputs" on the back of the SMART unit are screw post terminals, whereas the output from the mixer is two RCA jacks. How do I transition between these two and still get a clean signal?

Also, the "outputs" from the SMART processor to the transmitter are also screw post terminals, but the inputs to the transmitter are two XLR jacks. How will I connect these two?

Posted by Adam Fraser (Member # 1074) on 07-17-2003, 11:34 AM:

To go from RCA to screw post there are two wires in each RCA cable, a positive and a common. Just either remove the other end of a RCA cable and put connectors on it or pick up a pre made one. The signals are the same.
As far as going from XLR to screw post I think you may need to get a converter from XLR to non-XLR as the signals are not the same, I know you can get them at Radio Shack but Im sure there are much better suppliers out there for them.

Posted by Scott Norwood (Member # 30) on 07-17-2003, 01:17 PM:

Does your Smart box have balanced or unbalanced inputs and outputs? IE are there two or three screw-terminal connections per channel? Dolby uses unbalanced line inputs and outputs (this is true of the CP45/50/55/65/200/500; I haven't seen the 650 up close.

RCA connectors are used for unbalanced lines only; XLR connectors are usually used for balanced lines and can be wired with either pin 2 or pin 3 "hot" (pin 1 is always ground). Once we know what type of inputs and outputs you have on the Smart box, someone can tell you how to connect everything.

Posted by Pete Lawrence (Member # 130) on 07-17-2003, 03:18 PM:

People generally make connecting balanced to unbalanced lines more difficult than it has to be. Your two wire source ground (or earth as Dolby likes to call it) connects to the XLR pin 1 and pin 3. Your signal lead (the center wire in the cable) connects to XLR pin 2. You only need a single conductor shielded cable.

This of course removes the advantages of balanced wiring and you may have a hum problem if the wire run is more than a few feet, but it generally works. Also, this applies equally to both input and output circuits.

A more expensive way to interface balanced to unbalanced is with a transformer if you have a hum/ground loop problem, but it's usually not necessary.

Posted by Bruce Hansen (Member # 281) on 07-17-2003, 03:48 PM:

In the days when balanced inputs and outputs were done with transformers (inside the equipment), you would connect pins 1 and 3 as the shield and use pin 2 as the hot; but today most equipment uses IC balancing. On the output of the device that uses XLR's: if you connect pin 3 to pin 1 you are shorting the output of the IC feeding pin 3 to ground. This is a no no, and can cause problems such as distortion, and cross talk. I like to use pin 1 as the shield, pin 2 as the hot, and connect a 1K 1/8 watt resistor batween pins 1 and 3. If you are doing this as a perment install, on the output of the device using XLR's, use pin 1 as the shield, and pin 2 as the hot, do not connect pin 3. On the input to the device using XLR's, you can connect pins 1 and 3 together, and use that as the shield, and use pin 2 as the hot. Some people will also use pin 3 as the shield, and pin 2 as the hot on the input to a device using XLR's. Some equipment manuals, today, have diagrams that show you how to do all this.

Another problem is that RCA connectors normanly have about -10 db on them, XLR's normanly have +4 db on them. There are units made that will convert properly between XLR's and RCA's, both balancing, and unbalancing properly, and changing the levels.

Posted by Phil Hill (Member # 371) on 07-17-2003, 04:04 PM:

Stop screwing around and just do it right! Why take a chance with hum and/or distortion, etc?

RDL makes a universal, inexpensive, 2-channel, screw-terminal, bal/unbal & unbal/bal "converter". I think model is STA1 ...wait...

Here's the link:

http://www.rdlnet.com/sta1.htm

>>> Phil

Posted by Jason Burroughs (Member # 68) on 07-17-2003, 05:38 PM:

On the Non-Sync question:
The MODII B appears to use unbalanced inputs, terminals for Music Left and Music Right and Ground for each.

cut the ends off a pair of RCA cables exposing the wire inside and the "shield". Strip about 1/4" of the wire. The shield is ground and the wire is your "hot" wire. Match up left and right, adjust levels as necessary, and you're ready to go.

Posted by Jack Ondracek (Member # 1466) on 07-17-2003, 05:46 PM:

quote:
The MODII B appears to use unbalanced inputs, terminals for Music Left and Music Right and Ground for each.

Yep. Everything's unbalanced & low-level on that unit (no pun intended).

Posted by Pete Lawrence (Member # 130) on 07-17-2003, 06:30 PM:

Here is Rane Corporation's in depth discussion of inter-connecting balanced and unbalanced systems. http://www.rane.com/note110.html

Here is their discussion on grounding with a section on connecting the XLR pin 3 directly to ground. http://www.rane.com/note151.html

Posted by Oscar Neundorfer (Member # 458) on 07-18-2003, 07:04 AM:

Barry,

For the non-sync inputs, obtain a standard dual (stereo style)rca plug patch cable and cut the plugs off one end. This will leave rca plugs on one end of the cable. On the other end where you cut off the plugs, separate the two cables (these are usually joined like zip cord), and strip the wires outer insulation back about an inch and a half. You will see the outer shield wire wrapped around the inner insulated wire. Unwrap the shield wire and twist the individual strands together. This will become the ground connection. Now strip back the inner insulation to expose the inner conductor. This wire will become the left or right non-sync input connection. You may connect these wires directly under the screw terminals, or you may choose to use spade lugs and solder the wires to the spade lugs which will then go under the screw teminals. Either way is acceptable although the spade lugs may be a little more robust. The wires in these cables are usually rather small and somewhat fragile. It may be a good idea to tie wrap the cable at some point to relieve any tension on the connections themselves.

Regarding the outputs to the transmitter, you will just have to make these cables yourself. Get the proper xlr connectors and some 2 conductor plus shield cable. Connect the shield and one of the conductors to pin 3 of the xlr, and then jumper pin 1 to pin 3. You will have pins 1 and 3 and the shield and one conductor all tied together. Now connect the other conductor to pin 2 of the xlr.

PLEASE NOTE: MOST manufacturers use pin 2 of the xlr as the hot or non-inverting input. If your transmitter uses pin 3 as hot, you may need to reverse the pin designations in the above instructions. If it is a truly balanced input, then it probably will not matter anyway whether you use pin 2 or pin 3 as the input. Pin1 is always ground.

At the processor end, cut off the shield (no connection) and connect the conductor which went to pin 3 of the xlr to the ground terminal. Connect the conductor which went to pin 2 of the xlr to the hot output terminal. You will need one cable for each channel (left and right).

This should normally work fine. However, there may be some reason the transmitter needs to have a truly balanced input (the SMART unit's output is unbalanced). But usually this arrangement works OK.