Adding amp to head unit w/o Low-Levels

I've been getting conflicting advice regarding the best way to deal with adding a Kenwood KRC-8452 amp to the head unit in my '05 Prius (which lacks both a 12v on and low-level outs.)

Although I've purchased a PAC-OEM2 step down converter (with 12-v on generator), I was told today by an experienced install shop that it's better (to prevent engine noise) to just use the high level outs of the Prius head unit to get the audio signals into the amp .

Swapping out the head unit of the Prius isn't an option as that would lose both touchscreen and steering wheel control over the audio.

The amp will be installed in the back, and others have told me the opposite: that since I'm sending the audio signals all the way to the back of the car that doing so using RCA cables is a better way to prevent engine noise.

(continuing post above)  One reason I'm sort of hoping the high level outs is the way to go is that that would enable installing the PAC-OEM2 in the back with the amp instead of in the dash.  I had planned on installing it the dash so that the audio from the head unit could be sent back to the amp in the back via RCA cables (as I already stated in the last post, I had been told sending audio to the back of the car using RCA cables would lessen the chance of engine noise.) 

But if I don't need to send the audio over RCA I can install the PAC-OEM2 in the back and that way I wouldn't have to worry about engine noise being generated by running the PAC-OEM2's 12-v on to the amp along side audio cables.  (Even though that same install guy assured me that I need not worry about a 12v-on line causing engine noise.)   The PAC-OEM2 generates a 12v-on to the amp only when it senses audio present on whichever speaker wire its sensing wire has tapped into.

You can find some answers by searching this forum for LOC in message body.  But here's a couple points: 

• Using an aftermarket LOC is superior to using the built-in converter in the amp.  Use the OEM-2.
• The 12V trigger in the LOC may give intermittent results, sometimes turning the amp on and sometimes not.  If you have trigger-on problems then run a 20 gauge wire from the amp turn-on to a 12 volt switched supply somewhere in the car.
• Any noise pickup would be between the deck and the LOC.  You can use regular speaker wires but twist the + and - around each other rather tightly (1.5 to 2 twists per inch) to cancel out noise.  Or you can buy twisted speaker wire.
• You can place the LOC in any location that is convenient for you to get to.  If in the rear next to the amp, twisted wires will carry the signal back to it from the deck.  Isolate these and any other pre-amp signal wires from nearby power wires....they should not be allowed to run parallel to each other.
• Don't worry about noise unless you get it.

Use the crossovers supplied with your new speakers.  They can be placed anywhere you wish to put them.  Run regular speaker wire to the new speakers.  The signal wires after the amplifier do not need to be twisted.

Thanks Stevdart - those are very helpful tips.

I will do that LOC search but can you tell why you (and even my amp's manufacturer's tech guy if I remember correctly) believe the PAC-OEM2 is going to do a better job of converting high level to low level?   It seems it would be the other way around - there's so much more room in the amp and whatever electronics are needed could be custom balanced for that amp.

Whatever the reason, it's good new because I'm looking for reasons to go with the PAC-OEM2 instead of high levels directly in to the amp.  From our discussions on PriusChat, it seems the gain controls on the PAC-OEM2 are the best way to solve an increase in touchscreen beep problem that occurs in a Prius after adding an amp (beeps are routed through the audio system even when the radio is off). 

Regarding engine noise - I would take your advice (only worry about it if it happens) but it's a consideration that might (but hopefully won't) prevent me from placing the PAC-OEM (LOC) where I really want it: near the amp where I can still get at it to fine tune the beep volume after install. 

But since you say the only place to worry about picking up engine noise will be between the head unit and the LOC, placing the LOC in the dash minimizes the amount of wiring exposed to engine noise pickup.  Is that because audio downstream of the LOC will be sent by RCA cables, or is it because the audio is at that point low-level?  But I should wire under the assumption there won't be any if I understand you and it sounds like using your twisted speaker wire to send high level audio from the head unit to the LOC in the back near the amp is the way to go.

Any advice regarding wires from the amp to the crossovers to the speakers?  If you're absolutely sure I don't need to worry about engine noise with those it would be easier to run the right speaker wires up the right side of the car and the left side up the left side.  

But if there IS any chance of engine noise, I could run all the speaker wires up the left side and get the right wires to the right side by sending them through the dash area. 

To help answer that question, here is what's going up the right side of the car:

1.  PAC-OEM2 12v on (but ONLY if I place the PAC in the dash and hopefully I won't be doing that.)  Or possibly the 20 gauge 12-v on you mentioned if the PAC one isn't reliable and I can't find one at the back of the car near the amp.

2.  Composite XM cable - all the wires necessary to power and send signal from the special Toyota Prius XM receiver (a Solaris part but integrates just as well into the Prius touchscreen - see PriusXM.com, way better than any other XM solution for the Prius!) up to the touchscreen.

3.  XM antenna (from the XM receiver in the back with the amp up to the antenna I will (hopefully only temporarily) be placing on the dash.)

The aux battery is in the back of the Prius where I'll be putting the amp so no amp power line (other than maybe that 12-v on) will have to be run up either side.

I suppose I could run speaker wires both ways, start out using ones up the right side and switch to the ones up the left side and through the dash if I have any engine noise problems?  Hate to run wire through the dash area though if I don't have to.  Thanks again for your help.

RCA wiring consists of twisted-pair wire.  That's the key here.  Twisted speaker wire costs less than RCA's.  The LOC should be where you can easily get to it, which is why I don't like behind the deck.  You will want access for awhile until you get it all tweaked. 

A converter built into an amp is a freebie - just knowing that, you don't have to disect the why's and wherefore's;  a high quality aftermarket converter is of higher quality.  You will have better frequency response and adjustability with it.  Installers will hide the LOC behind the deck because then that will be one less thing the client can mess with after the install.  But when your doing this for yourself you want access to all your equipment.

Signal wires will only pick up interference from adjacent parallel power wires, so if you take the precautions I talked about you needn't worry about noise.

I've seen decent prices on 10 gauge and 18 gauge (from same seller) and also on 16 from another   Thought I might try 10 gauge for everything except for the tweeters where I thought 18 would be enough.  It's not just the less expense but worried about too many 10 gauge might be hard to get into my front door.  Otherwise I assume bigger wire the better? 

But since the wires out of the head unit are likely smaller than even 16, probably can't use a butt connector so any advice how to connect them? 

Curious though: you mentioned no worry about engine noise from amp to speakers because they are amplified.  But aren't lines from head unit to LOC also amplified?  They are high level outs after all.

Thanks.

16 gauge speaker wire is all the bigger you would need.  You would have to be running one thousand watts into 2 ohms to need to go to 14 gauge speaker wire.

Solder and tape all connections.  Two different gauges can be soldered together, no problem.

There is a difference there.  The signal from the amplifier is not going to another amplifier to be amplified again.  The signal from the head unit is amplified, yes, but is going to be amplified more when it gets to the amplifier.  That classifies it as a pre-amp signal, and pre-amp signal wires can pick up interferering noise and that noise will be amplified.