Hi,

I'm the new owner of a 1992 Lexus SC400. Couldnt be happier. The car came with a busted NAKAMICHI amplifier. Sadly these are 200+. I know that the fuse blowing is a common problem but my fuse looks ok. Is it possible to just put in a regular pioneer amp? I just would like some audio from my speakers for now. Any ideas? Thank you!!

 

I would check for blown stock speakers. They were generic cheaply made speakers search DIY rear/front speaker threads. That as well could be the problem.

 

Hmm, will do. Basically as soon as I turn on the power button for radio cd etc, there is like a popping sound that comes out of the speakers.

 

I did a DIY on my rear speakers search under my club name.

 

I've been digging into the Nakamichi system this week. If it is the amp that's bad, I don't think replacing it with a Pioneer one will work too well. The Pioneer system has an amp in the trunk for the 4 channels, plus a small sub amp mounted to the parcel shelf alongside it. The Nakamichi setup does away with that separate sub amp.

I haven't followed the wiring yet to see how compatible that is, but I know from Raine's "aftermarket head unit" thread that they do differ.

I replaced my Nakamachi door speakers with Infiniti 5.25" 2-ways and the sound quality went down a bit (much better mids and lows, significantly worse highs). I then replaced the Nak rear speakers with Pioneer 6.25" 3-ways and the sound quality went up dramatically. On the old car, that same setup seemed pretty balanced front-back, having replaced them one at a time in similar fashion. That leads me to believe that the strength of the Nak system is the component tweeters (and the sub), and that the rest of the 4" speakers are pretty run of the mill.

 

You can't just swap in the pioneer amp, they're different.

The Nakamichi system powers all 5 channels from the single amp, both of the Pioneer systems just power 4 channels from the main amp and the sub is its own amp. Now, you can't just easily wire in a new 5 channel aftermarket amp and call it good, nor any combination that gets you 5 channels. The sub channel on the Nakamichi system isn't its own wire, it's multiplexed in some way, but it's not documented and it doesn't seem like anyone has figured out how to get that signal out. So, to change the amp you'd need to change the head unit, too. Which means at least wiring a signal from the deck for the sub and new power for the amp(s). The Nakamichi sub is 5 ohms, so factor that in for how much power you'll need (you'll get about 80% of the 4 ohm rated power, the Nakamichi sub is about 120 watts.)

All that is going to cost over $200 for anything decent, so if you don't care to upgrade you're best off replacing the amp with a stock replacement.

Now, the stock amp can die for any multiple of reasons, but if you can hear it "pop" on, it's probably the amp itself or the head unit that's died since it's at least getting power. I'm not sure how to tell you an easy way to test which one it is without pulling a lot apart. But, mine (98 Nakamichi) doesn't otherwise make noticeable noise when it powers on.

 

I have never tried this, but a possible solution to having all channels running with an aftermarket amp may be to use a 5-channel amp and split the rear channel (or front) head unit output RCA with a Y-connector into the 5th amp channel. As long as the amp has a low-pass filter, I don't see why that wouldn't work.

 

I think the High Pass filter for the rear speakers is in the head unit. Please correct me if I'm wrong. If it is, you won't get much to your sub from the stock head unit.

When I bought my car it had an aftermarket head unit, but the stock amp. It powered the 4 regular speakers, but not the sub. It would, however, appear to send plenty of bass signals through the amp to the main speakers when the high-pass wasn't enabled on my head unit, which would distort trying to re-produce. Leads me to believe the crossovers are in the head unit and the amp just amplifies what's sent to it, just the sub channel appears to be multiplexed in some way, or not labeled correctly in the wiring diagrams. I was trying to achieve the opposite, feed the sub signal in to the stock amp, but I tried searching for how it's done in general to no avail.

 

I am not sure how the factory Nak units work, but if the factory amp is putting out a sub output, it should be getting a full range signal from somewhere. This would lead me to believe that the crossover is occurring at the factory amp area and not from the head unit. Of course, I am not 100% sure about this but I don't see how the factory amp would be able to generate low frequencies if the crossover happens at the head unit signal output.

 

If you just feed full range signals from a regular source (aftermarket head unit) in to the stock amp, sub output doesn't happen. I don't know how it gets there, but it doesn't seem to happen via "regular" signals over the 4 channels listed in the wiring diagram. There's something else going on.

 

I am replacing my old Nak with a new HU using the metra harness. With the 4 rcas plugged in to the front and rear outs, I get plenty of sound, but no sub. If I take the rear rcas and plug them into the sub out, I get raging bass but absolute garbage from the rears as far as mids and highs go (this leads me to believe that the Nak sub amp is in fact getting and processing a signal. Leads me to believe that it's a HPF causing the problem. This is very similar to a thread I found today referencing the LS400 Nak system. The poster in that thread said he got his sub working by turning the HPF down in his aftermarket HU. I plan on testing the theory tomorrow.

 

Yeah, it doesn't seem to work that way in mine. Before I installed an aftermarket sub amp, turning off the High Pass Filter (well, not turning it on, it's not on by default on my head unit) just gave me lots of distortion as the rears try to deal with bass (they're real bad at it.)

Hearing that plugging in just the sub out of a deck gives you bass is interesting, but do you mean from the sub, or just the rear speakers?

I just bit the bullet and ran a new signal cable and power for a new (well, new to the car) aftermarket amp. The biggest pain was getting the power through the front firewall, otherwise it was just fairly normal car stereo install stuff.

 

Indeed, that sounds like the filters are in the head unit. I would lean toward wiring diagram errors to explain the lack of sub output. I can't imagine them running 4 channels of audio and then going to the effort to multiplex 5th channel over an analog audio frequency signal. Digging around a bit, I found this diagram:



This was labelled as a '93 SC400 diagram. Looks like D1/D5 labeled as RU+ and RU- would be the subwoofer output signals, if this is indeed the correct illustration.

I would expect the sub signal to be summed after the fader, so that the sub volume will track appropriately when the fader is adjusted.

Important:
I would treat these signals as balanced, especially since D5 is referred to as RU- instead of a signal gnd. The easiest way to handle this is to use a ground loop isolator or signal transformer to isolate the amp input from the head unit. Radio Shack used to have small inexpensive transformers appropriate for such.

Unfortunately my car has a Pioneer system, so I can only look at it on paper.

 

pogoism9 above mentioned that when he plugged in the rear RCA to his aftermarket HU sub-out connection, the woofer and rear speakers functioned (although not filtered correctly). Wouldn't that pretty much mean that the rear channel going into the Nak amp powers both the sub and rear speakers?

 

I read his post to say his aftermarket sub out causes the rear speaker to function as subs, but didn't see any mention of the sub itself producing output. This would lead you to believe the rear speaker amp channels are unfiltered full-range, and would most likely be the case. I could be misinterpreting his statements.

I never ran across any 80's-90's factory system that was all that complicated. If the 2 Nak rear head unit outputs indeed feed the rear speakers and sub, the crossover must be inside the amp.

If the amp were to have a low-slope hi-pass filter on the rear channels, and you were to feed it a low-pass signal, plenty of that signal could still make it through to the speakers and they would sound like garbage.