3.5mm \ RCA to XLR issues - AVS Forum
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post #1 of 7 Old 10-12-2013, 07:07 AM - Thread Starter
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Hi.  Sorry for such a noobish 1st post but I'm stuck!

 

We have a custom built amp / pa system for our building with speakers in the ceiling.

Cables are wired into the walls to XLR sockets for microphones.

We want to take an audio output from either a laptop or dvd player to the amp using these xlr cables.

 

I have setup the following adapters-

 

Laptop with 3.5mm headphone output converted to twin RCA (future proof for DVD player with direct RCA out)

Twin RCA to 1/4" stereo jack

1/4" stereo socket to XLR plug

XLR wall socket to amp

 

 

The sound is very washed out and any vocal audio (like what dolby pro logic would put as center) is missing.  If I unplug either channel leaving just one, the missing vocal appears but everything is very tinny (no bass).

 

I have since been reading up on XLR and my guess is that a stereo signal is being forced down the XLR cable (positive left on hot, positive right on cold, ground on ground), and the amp is cancelling out any duplicate information between the two as noise (the 'mono' narration is lost, but the 'stereo ambience' of the music is kept) when it '180 \ converts' the cold to negative.

 

But when I send a mono signal the sound is half volume, very weak and tinny.  Is this because now although there is no duplicate sound, the lack of any sound on one of the wires is not enough?

 

If I connect directly into the amps 1/4" input the sound is perfect.  But that would mean a new 25 meter unbalanced cable run which is not really possible and will probably have noise anyway.

 

Will sending a correctly balanced mono signal down the XLR cable solve my problems?  Would this be achieved by swapping the 1/4" stereo to XLR adapter to a 1/4" mono to XLR adapter? Or is the signal too weak regardless?

 

I would then have to lose one channel but I'd rather have better sound quality (it's a mono amp anyway).

 

Thanks very much for your help, I've lots of theory to learn!

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post #2 of 7 Old 10-12-2013, 03:47 PM
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XLR to RCA http://www.monoprice.com/products/product.asp?seq=1&format=2&p_id=4778&CAWELAID=1329450500&catargetid=320013720000011014&cadevice=c&cagpspn=pla&gclid=COSV5rCvkroCFSgS7AodLkwAYQ

XLR to 3.5mm http://www.sweetwater.com/c873--Balanced_Cables_1_8_to_XLR

just a couple of sources....many more out there in internet land.
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post #3 of 7 Old 10-12-2013, 04:32 PM - Thread Starter
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Thanks but I already have a similar adapter. Trying to solve the problem I'm facing...
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post #4 of 7 Old 10-12-2013, 06:24 PM
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The problem is that you are SEVERELY overloading the mic input. Mic inputs are designed for a very low signal level. You are feeding a signal level about 20 times hotter than the input is designed for.

You need to use a "direct box" from a store like Guitar Center or Monoprice to match the signal levels and impedance and to passive combine the left and right channels of your source into a mixed L+R balanced mic level signal.

The best choice of a solution would be a miniature mixer with a balanced mic output as it will have a gain stage and tone controls (probably).

Also the "hot" and cold' conductors on the XLR do NOT equate to left and right. Pins 2&3 in the XLR are the balanced line for a microphone or balanced line level signal.
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post #5 of 7 Old 10-13-2013, 02:27 AM - Thread Starter
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Thanks very much the direct box looks right.

I know the hot and cold aren't left and right, but the adapter I bought is a stereo 1/4" to xlr and instead of combining l&r into a balanced signal, it seems to be sending left down hot and right down cold.

In a proper balanced signal, hot and cold are identical, but cold is out of phase. If any noise is picked up, it will be identical on both hot and cold without cold being out of phase.

The amp can then remove any noise as it shows up the same on hot and cold, instead of being out of phase on cold.

But as my signal isn't balanced and is being sent as stereo instead, any mono sounds such as vocal which are identical on both left and right are being removed.

Why else would I be getting this karaoke effect? Or have I not understood this properly?
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post #6 of 7 Old 10-13-2013, 08:23 AM
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Quote:
Originally Posted by dale12345 View Post

Thanks very much the direct box looks right.

I know the hot and cold aren't left and right, but the adapter I bought is a stereo 1/4" to xlr and instead of combining l&r into a balanced signal, it seems to be sending left down hot and right down cold.

In a proper balanced signal, hot and cold are identical, but cold is out of phase. If any noise is picked up, it will be identical on both hot and cold without cold being out of phase.

The amp can then remove any noise as it shows up the same on hot and cold, instead of being out of phase on cold.

But as my signal isn't balanced and is being sent as stereo instead, any mono sounds such as vocal which are identical on both left and right are being removed.

Why else would I be getting this karaoke effect? Or have I not understood this properly?

You're sending left and right signals into a differential amplifier, it's cancelling out the common information...effectively left minus right.
You need to mix left and right together, then either send it down the balanced line as an unbalanced signal, or balance it, and attenuate it (since it's a mic preamp) first.

....which is what Gizmologist said yesterday .
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post #7 of 7 Old 10-14-2013, 03:12 AM - Thread Starter
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Thanks for the replies everyone.
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