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Discussion Starter #1
A while back I was posting about the bass signal falling off from the stock HU, trying to figure out if they were doing something tricky to get more volume from less equipment. My system includes a sub and amp using high level inputs (speaker level). Turns out that as I suspected, at higher volumes, the stock headunit cannot supply enough current to a loaded channel, and when avail. current drops, voltage drops... when voltage drops, signal to the sub's amp drops, and bass level drops.

Currently the amp is getting signal from the rear speakers in the car, which are faded 75% out, so very little load is placed on those channels. This helped out a lot. However, with newly upgraded speakers in sealed enclusures, it's becoming painfully obvious how little power the stock deck puts out. So, for anyone else building this kinda system (keep the stock deck, and fix up the rest), here's the master plan.

Completely unload the deck. In my system I will only be using the front channels, but if you want all 4, go nuts. Place a 8ohm or greater resister in each of the two front channels, and then run that signal back to the amp(s) in parallel. The idea is this, the deck knows when the rear channels are not loaded, and cuts output to them, I suspect the same is true for the front. So, it must see a load (even a small one, I have yet to test and see just how small I can get away with) in order to output the signal. If we use a higher ohm resister in place of the 4ohm speaker, we reduce the load on the deck, but still fool it into thinking that there is a speaker there to drive. Less load = better signal to the amps.

I plan to finish my install with the 2nd amp, and configuring the system as above.... provided I can get off my ass and get it done.... I'll post on how it goes...

So, long story short, there's no trickery going on inside the single disc stock HU, just a lack of juice.
 

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The problem with putting 8 ohm resistors accross any channel used or not, is it will consume precious power. The high level inputs on an amp are more like 100 ohms or more (practically no load) to the head unit. The only way to get more gain to the sub amp is to put amps on all 4 speaker outs on the head unit. Then it will have all the voltage you need to drive the sub and the other amps.

As far as "trickery" going on in the head unit, there HAS to be limiting going on in the HU's amps or it would just clip and distort when the juice ran out. On mine, it gracefully just lowers the gains internally. It is possible to trick the HU for a second by turning the volume up all the way very fast. It briefly DOES distort, then the limiters catch up and it comes back down to clean but lower sound.

BTW, you never indicated which factory HU you have. I have the Blaupunkt/Ford mp3 and I am using the low level sub out signal in the yellow connector. I don't get less bass as much as an overall level reduction when the HU runs out of juice to the main speakers. My setup is the stock HU with upgraded coax speakers in the doors and no extra amplification. The sub uses an old Alphasonik 60 watt rms mono amp.

I hope I understood your plan correctly.
 

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Discussion Starter #3
I have the single CD, base model Blaupunkt HU. My head unit has the "normalize" function too, although I run that turned off. The normalize will give you that loudness right away, then dropping off too.... I can't remember which, but the higher end Focus head units have a "clip" feature, that does like you say, reduces volume if the amp starts to clip. My low end one just tries to do whatever it asks you.

About the 8ohm resistors in place of the speaker, the idea there is to use atleast 8ohm, the higher the resistance the better. Like you say, more resistance = less current draw = cleaner signal to the amp. I'm not sure what you mean about "The only way to get more gain to the sub amp is to put amps on all 4 speaker outs on the head unit. Then it will have all the voltage you need to drive the sub and the other amps." Even if you run the 4 channels of the head unit's amp to 4 individual amplifiers, you have the same problems as with 1 amp right?
 

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htrfocus said:
I'm not sure what you mean about "The only way to get more gain to the sub amp is to put amps on all 4 speaker outs on the head unit. Then it will have all the voltage you need to drive the sub and the other amps." Even if you run the 4 channels of the head unit's amp to 4 individual amplifiers, you have the same problems as with 1 amp right?
What I meant was that if you had an amp with 4 or 5 channels and used the high-level inputs, the HU would now see the high resistance from the amps and not the speaker loads. With the gains set properly, you now shouldn't have the problem of the bass reduction because the HU can easily supply enough voltage to drive the amps.

I am now realizing that you may not want to install any more amps and are trying to use the speaker outs to drive the sub amp. I would agree that if you use higher ohm resistors INSTEAD of the rear speakers you should get an improvement. You of course lose the output of the speakers you disconnected.
 

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Discussion Starter #5
Ok, I understand what you're saying now about the deck having plenty of power for the amps. True, at 100ohms the power requirements are fractions of a watt, but I suspect the deck may deactivate those channels, seeing them as not connected (close to infinite resistance). I will deffinatly try without any load though! (Just the amp as a load)
 

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htrfocus said:
I suspect the deck may deactivate those channels, seeing them as not connected (close to infinite resistance). (Just the amp as a load)
In my experience, I don't think this will happen. I've never heard of any amp doing this or even having a reason to. With no load on the speaker outs, unused channels are effectively disabled automatically. Voltage is still present but is of no consequence to the amp until it has a load to make the current flow. Adding a circuit to disable the unloaded channels would just add complexity and cost and even possibly add distortion.
 

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Discussion Starter #7
I can't think of a good reason to do it either, but when I was working on the stereo, while I had the rear channels disconnected, the fade option had been removed from the options menu.
 
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