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Old 18th September 2005, 05:30 PM   #1
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Default Stupid n00b RCA question

Hi again all...I'm tapping into factory RCA outs from the head unit for my new amp, which are unfortunately in a harness instead of a plug (for OEM woofer amps). The factory wiring is woofer +, woofer -, woofer shield. Obviously the + goes to the center conductor, but where do the - and/or shield go?
thanks in advance,
Nat
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Old 18th September 2005, 05:43 PM   #2
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Default Re: Stupid n00b RCA question

Quote:
Originally posted by GhettoSQ
Hi again all...I'm tapping into factory RCA outs from the head unit for my new amp, which are unfortunately in a harness instead of a plug (for OEM woofer amps). The factory wiring is woofer +, woofer -, woofer shield. Obviously the + goes to the center conductor, but where do the - and/or shield go?
thanks in advance,
Nat
If you had a pair of shielded TP(twisted-pair) or differential, you would tie the ground to the outer conductor. With regular RCAs, you can just tie the ground to the shield and the shield to the shield.
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Old 18th September 2005, 06:14 PM   #3
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I made my own, TP. So I should just wire both the shield and the negative to the plug shield?
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Old 18th September 2005, 11:38 PM   #4
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I should clarify: I made something like this:
Click the image to open in full size.
The factory wiring uses pre-outs from the HU but they are just wires that run into a harness rather than RCAs. I tapped into the + and - for my cables, and it works but A) it's very noisy (cd changer sounds amplified, bad squelch on turn-off) and B) I think the voltage is lower than it should be since I have to turn the gain all the way up to match the other speakers. I also connected the shield wires to the RCA plug shields, but it made no difference.
Thanks again
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Old 19th September 2005, 12:38 AM   #5
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Quote:
Originally posted by GhettoSQ
I should clarify: I made something like this:
Click the image to open in full size.
The factory wiring uses pre-outs from the HU but they are just wires that run into a harness rather than RCAs. I tapped into the + and - for my cables, and it works but A) it's very noisy (cd changer sounds amplified, bad squelch on turn-off) and B) I think the voltage is lower than it should be since I have to turn the gain all the way up to match the other speakers. I also connected the shield wires to the RCA plug shields, but it made no difference.
Thanks again
Make sure that both the HU and the CD changer have local grounds. since you have noise in your system, I would bet that you have a ground-loop. They may have been using the shield to ground your equipment before.
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Old 19th September 2005, 12:49 AM   #6
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Hmmmm...the cd changer is in the HU. I always thought ground loops involved alternator noise, but this is the player reading discs heard thru the speaker. I think the pre-outs are definitely not grounded/shielded right, but I'm not sure how to go about it. I'm only hearing the noise thru the woofers (only thing connected to the amp), so I doubt it's the whole HU that's not grounded, but maybe...
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Old 19th September 2005, 01:08 AM   #7
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If you use a ground loop isolator (on the preamp side of the amp) or a Line Output Converter on the speaker side of the amp, you will break the ground loop.

Ground loop isolators have transformers that pass the AC but break the DC path that's giving you the noise. Most all LOCs have transformers and also break the ground loop. If you get an LOC that has 'ground' wires (often brown), you will have to experiment with their ground connection point. Some amplifiers can operate with the ground wires left unconnected but some amps oscillate or have other problems if they are not grounded.
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Old 19th September 2005, 02:56 AM   #8
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Quote:
Originally posted by GhettoSQ
Hmmmm...the cd changer is in the HU. I always thought ground loops involved alternator noise, but this is the player reading discs heard thru the speaker. I think the pre-outs are definitely not grounded/shielded right, but I'm not sure how to go about it. I'm only hearing the noise thru the woofers (only thing connected to the amp), so I doubt it's the whole HU that's not grounded, but maybe...
So are your woofers amplified separately? If so, that is where the ground-loop is happening.

Ground-loops are caused by dissimilar potentials between grounds. This causes current to flow through alternate paths. Usually this happens with amplifiers that have poor ground connections and are grounded through the RCA shields.
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Old 19th September 2005, 10:24 AM   #9
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Thanks guys. I guess I will try the isolator first. I remember reading on Perry's site that ground loops can occur when the cable is too long -- mine (from a kit) is 3ft. and I read that 2ft. or less is better. I mounted the amp up front where the old ones were, and rn the ground out thru the firewall to a ground point in the engine bay (pre-sanded, yay). If that's the problem, will an isolator help or should I ground elsewhere?

So, the other thing is the gain setting -- if the RCAs are fine, is it just that a factory HU uses such low-vltage pre-outs that I do in fact need to turn the gain all the way up?

thanks again!
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Old 19th September 2005, 11:56 AM   #10
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Be careful what you do with the speaker negative from the woofer amp, it's probably not at ground potential -- from what I understand, a LOT of car audio gear runs in bridge mode, due to the fact that it's hard to get decent supply voltages. (Disregard this pargraph if this is a question about the inputs).

If this were MY car, the first thing I'd try is running the woofer through shielded twisted pair -- + to RCA tip, - to RCA outside, shield connected at a one end and left hanging at the other. The hanging end can be connected back to the car's ground through a small ceramic non-polarized capacitor (100pF?) to kill radio noise.

As for turn-on squelch, is it coming from the starter, or does it happen any time the equipment is turned on? If the latter, it's either a mis-wiring problem or an equipment design problem IMHO. See if it has turn-on faders which might disguise the issue..

Wes
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