Film-Tech Forum ARCHIVE: sound system

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Posted by Bill Hall (Member # 10520) on 10-22-2018, 09:37 PM:

HI I have a Smart MN 586 6 channel booth monitor conected to a CP 45 processor. At the moment I have been using a Scott stereo amplifier. To connect the amplifier I put a connector onto the XLR plug and wired it to a RCA type plug to use with the amplifier. I used the ground pin number 3 and the live I hooked up to pin number 2. I do get sound but does not sound very good. It has a hum to it and sometimes sounds scratchy . I did have the shield connected to the ground at the amp and the sound was worse . I was wondering if anyone may have some suggestions to improve the sound so as to enjoy the movie I have on the platter.Many Thanks

Posted by Dave Macaulay (Member # 813) on 10-23-2018, 08:48 AM:

Check the amp documentation. As a general rule, current XLR usage for audio uses pin 1 as shield/ground, 2 and 3 for signal with 2 "hot" for phasing.
I recently replaced an amplifier that used pin 3 for shield, the sound with a new amp using pin 1 for shield was pretty much as you describe.
To connect an RCA CABLE to an XLR, jumper the shield pin to the "-" pin and attach the shield wire to them. For pin 1 shield connection, that is jumper pins 1 & 3 with the RCA cable shield connected, RCA cable inner wire to pin 2. You must use both + & - connections for an amp with balanced input, implied by having an XLR input connector.

Posted by Sean McKinnon (Member # 612) on 10-24-2018, 11:12 AM:

I always prefer to use a balanced to unbalanced transformer when converting signal types like this. By simply eliminating the "-" signal phase you still usually have an impedance mismatch.

Posted by Bill Hall (Member # 10520) on 10-28-2018, 07:45 PM:

HI I checked my connections and the appear to be fine. I still have the hum so I am not sure where to look next. Does the processor and amp need to have a ground wire between them ? I am waiting for an additional amp to arrive from Florida and have it wired for XLR plugs.
Many thanks to the people that have given me very good advice on my platter and projector as I have it working now and nice to see a picture on the screen now

thank you

Posted by Dave Macaulay (Member # 813) on 11-02-2018, 08:46 AM:

I would install a ground loop isolation transformer in the signal. We use a lot of them! Unbalanced sources connecting to balanced inputs are prone to picking up hum from stray current between the source and amplifier, the isolation device eliminates that.
Maybe "The Source" has some amongst the toys, maybe a music store, or you can get them on Amazon etc.

Posted by Bill Hall (Member # 10520) on 11-10-2018, 07:51 PM:

I installed a ground loop isolation unit and the sound was a little better but I only put it on two channels . I hope to have two other amplifiers to complete the system this week so will hook them up to see how the results go. I have an idea that I may have to have isolation transformers on the other 4 channels.Once the sound part is fixed up then watching a movie will be much better .Thank you all for your help in my sound problem