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1998 Ls400 Nakamichi Amp Questions Factory Sub Use

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hi i'm tryin' to attach the wiring diagram. my amp is the nak from ls400 1998. the diagram is from a 2000 ls400 have gotten all the fl fr rr and rl channels to work alright, but i can't get the sub to make much noise, its just real, real weak, not just "factory sub" weak. so i finally tried to get all four rca's leads into the nak amp...i maybe have an issue with a tried to basically make my own Y adapter type thing on one of the rca pairs because i also wanted to feed that pair to another amp i am using at the same time (a radio shack optimus xl-400, don't laugh it seems alright 90db snr beats lots of head units). so why can't i get the sub on the nak to work? why do car audio shops always say "well you will lose the sub when we install that aftermarket head unit, the factory sub you will lose, so you'll have to buy a new amp for the sub and probably a new sub" that bs? why do you lose the sub? i don't know if one can take an rca pair and try to splice in another rca pair line....isn't that just what a "y" adapter does? i get that the voltage on those preouts will take a bit of a hit, but... i just don't know why i can't get the sub working

wiring diagrams at

lastly, why am i trying to use this nak amp? other than to make that trip to the junkyard less of a loss and to avoid spending more money on some other amp?

i am fairly happy with the xl-400 amp and i guess i could just use my head unit's amps for anything else i need, my head unit is a DEH-P4100UB

its specs are:

1-bit D/A Converter Yes (8fs/16-bit)

Audio Features

Built-in Speaker Power MOSFET 50W x 4

Continuous Power Output 22W x 4

so, i wonder, doesn't the common wisdom say its better sound quality to not use the on board amp on the aftermarket head unit, but instead to, say, use an outboard amp like the nak? or are these new MOSFET things in today's head units game changers such that the sound quality is actually better from my head unit than using the nak? how do i tell if the sub channel is just blown on my nak? does the sub channel have its own amp+ wire? its own ground? sorry so many questions. does anyone know the specs on the nak amp like thd and snr and rms and what would be a comparble amp on the market today and price?

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