My '93 SC4 has the Nak system and I am upgrading the sub and will run it with an Xtant power amp. The Xtant 202M has a number of signal input voltage settings which range from 100mV to 17V, so I can run it with the speaker outputs of the stock power amp - but I'd rather not - I would rather get the sub section of the Nak amp out of the loop completely.

Which raises two questions:

1. What is the subwoofer signal output voltage of the Nak head unit?

2. Does the head unit provide a low-pass signal to the sub portion of the Nak amp, or is the low-pass crossover in the amp itself?

Thanks for any help you can provide!

 

Hello?

Isn't there anybody who reads the Electronics forum who knows the answers to these questions? The Lexus manuals don't cover any of these issues.


Does anybody have any ideas about where I might find this information?

 

I wish I could speak in detail about YOUR Nak amp, but it unfortunately is very different from mine. Fortunately for you, I think the older Nak stuff is better quality than the newer, especially the amp/sub. They hadn't had a chance to cheapen it yet, and they wanted to make a definitive statement that it was a good system.
Generally though, the amp is the place where the crossover is. You'll typically have full range front/rear going to the amp, and from there it is split into doors vs. sub.
Many here have used the sub output successfully, running it into a new sub amp thru a line level converter (LLC). Yeah, while it would be better to bypass the factory amp's sub portion, you may be looking at some significant hacking inside the amp. And your Xtant amp needs to be 2-ch with a lowpass for the sub if you're going to do this. If you choose to take the sub output that you already have available, it will be cleaner than what you now hear because it will only have to drive a high impedance, low-current amp input, instead of a dynamic, high current subwoofer load. The signal output of the sub section will therefore remain much cleaner since it doesn't have much of a load on it. This is the main reason why most people have reported good results when adding a new amp to the sub leads. You do lose control over where the crossover point is, though, since it is preset by the factory amp.
The speaker output levels to the sub may be higher than the 17V that your input can handle, due to the jacked-up supply voltage of the amp's output stage. You can try using an LLC (Audiolink PowerlinkII recommended) between the sub leads and your Xtant input, then adjust the input gain accordingly.

Anyway, hopefully someone who has schematics and has directly been thru this will reply. Good luck.

 

engin_ear, thanks very much for your response. It's good to know that folks have had good luck using the speaker-level outputs of sub amps to power the inputs of upgraded amps.

The wiring diagram (I spent the $$ for the factory manuals, good investment) shows separate sub signal outputs from the head unit to the Nak power amp. I am hoping that this means the signal is low-pass from the head unit - if the crossover is in the amp, why would there be sub signal leads from the head unit when there are no sub controls in the head unit? It seems to me if the sub channel is low-passed in the amp, then the mono sub channel would be derived in the amp as well, and there would be no sub signal leads from the head unit. But I have no way of verifying this, since the Lexus wiring diagrams contain no internal schematics, and no signal specs.

The Xtant's input gain can be set to 5 levels (-20 to +20 dB) with a jumper, so I will experiment with both sets of signals and see what sounds best.

Thanks for your help!

 

strongsail:
It certainly sounds like the signal is lowpassed in the headunit, based on what you're saying. Does the schematic show one set (+/-) of sub wires, or two? (two would be for left/right, where the signals are combined into mono in the amp, but not lowpassed)

It sounds like that you could connect into those sub outputs from the headunit and run them into your Xtant. You would need to splice in shielded preamp wires to connect to your amp inputs, and maintain the integrity of the shield and its ground. If you go a very short distance (inches), you may be able to get by with no shield (I did this with my Nak headunit's main L/R outputs with excellent results because the signals were higher than line-level). Either way, you can solder RCAs to the spliced-in leads and go into your amp. The wires in the harnesses are usually 20 gauge minimum, and some are 18 for preamp stuff. 16 gauge is usually reserved for the speaker output stuff.

The low-risk way of doing it (as far as cutting into wire harnesses) is to buy an LLC for $30 and attach to the amp sub output and see what you get (or even set the input gain all the way down and attach directly, without the LLC). You may be pleasantly surprised.

 

The head unit has just one pair of sub output signal leads (one plus, one minus), and I notice that it has ground and "sgnd" leads, which probably means floating ground, which should reduce the chance of noise in the patched lead from the harness to the new amp - but I will try the sub amp speaker outputs first, hopefully won't need the LLC since the Xtant manual says the unit's balanced line input circuit will "...accept a speaker level input signal." I will try both sources, and if I use the low-level input from the head unit, I will use shielded cable. I will also pull a new power lead - the manual says awg#8, but I may add another amp later so I will pull #4 if it will fit, #6 if it won't.

I hope to do the install this weekend, will report back with the results...

 

"SGND" is the "shield ground". It is a separate signal ground, and it has to do with how the grounds are tied inside the headunit. "GND" is the common chassis ground (for power, etc.). The shield is very often only tied to ground at one end, usually the source (headunit) end, so if you are adding anything to the cable, be sure to maintain the tie to ground in the same place - in other words, tie the shield of the added section to the existing shield only, and not to a new ground point.

Good luck with the install.