Looking at an amp for an Si

[Hasdrubal]

Hello all,

My experience level with car audio isn't too great, so please bear with me if I have some dumb questions. Seems like most of the upgrade threads I've found are either adding a sub to the stock system, or going all out and adding multiple amps with full speaker replacement. I'm wondering if there's a middle ground, using something like this JBL 8 channel-

https://www.crutchfield.com/p_109DSP4086/JBL-DSP4086.html?tp=78740

Supposedly it will take multiple speaker level inputs and let you mix them, then assign outputs as needed (this is a new thing to me since the last time I did any research). So if I splice into the factory harness and run the front four channels and the sub as inputs, it seems like I can run front mids, front tweeters, and rear components (off the front combined signal) as output. I've asked Crutchfield if the last two channels can be combined for a sub output, if they can't I'm not sure what to do with them.

That's not much power for any of the channels compared to some of the systems I've seen while searching here and elsewhere, but I'm not chasing max volume and I don't need a huge amount of bass. I'd like a little more bass than the factory system provides, and I don't like how the rears are meant to provide so little sound compared to the fronts. I'll probably end up with new speakers all around too, but trying to figure out where to start right now.

So general question, does this sound feasible?

Edit- I've read about the front center channel being used for other functions, wondering if I can just leave that running off the factory amp?

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[Hasdrubal]

I guess this is an example of opening my mouth too early, Crutchfield says this amp doesn't have any bridging ability. But while searching around I realized I'd never heard of dual coil subwoofers before, so I think I'll be able to use the last two channels to a single sub in the factory location.

I did find a fair amount of information here about the rear deck flexing and reinforcement/sound deadening, so I think it ought to be manageable.

About the only problem I can imagine is if the channels from the amp end up with a noticeable delay compared to the center speaker. I don't know enough about amps to judge how likely this is, but if it happens, then I think running the fronts as components using crossovers would free up two channels and enable running the center through the amp as well.

 

[Hasdrubal]

Update, I decided to just try it and it works great. The amp sits under the passenger seat, I ended up running the center channel through it just in case the DSP added noticable lag. I have front tweeters and mids mixed, running to front and rear on each side. So no front/rear fader control unless I hook up a laptop and change the settings, but I like having the volume balanced anyway.

Using some JBL component speakers on the rear deck, going to install the same in the front but already have the crossovers in so I don't need separate channels out of the amp. The sub I got didn't even come close to fitting, so for now I have a front door speaker mixed with the factory sub signal and run through the amp back to the factory unit.

It sounds way better than it did before, almost enough to leave it... but with an unused sub in the garage and an unused channel on the amp, it seems like a waste. Thinking about making a fiberglass box to tuck in the right side of the trunk. There's more than enough room, and the software lets me link pairs of channels together. So, if I run one channel to each of the voice coils, there shouldn't be any interference.

I did not take any photos, but there's already reference photos enough on other threads here for what I've done so far. If the custom box happens, will definitely post photos of that.

My knowledge level is not yet high enough to mess with the equalizer, but there's a pretty sophisticated one in the software.

 

[Hasdrubal]

Probably last update in case anyone looks at this amp in the future-

After working great for weeks, there was no sound at all. Replacing the battery for an unrelated problem made it work again for a few days, and then no sound again. Troubleshooting led to replacing the JBL amp, which worked great for a day and then went back to no sound. Today I put all the wiring back to stock and every channel works through the factory amp.

I have a hard time thinking I got two defective amps in a row. Why the things would work for a while and then just stop, and for the first one start and stop again is beyond me. Maybe the DSP doesn't like something about the signal?

Either way, unless you're pretty good at car audio and can figure it out, my experience says to avoid this one.

 

[bikejog]

So you connect the factory amp speaker outputs directly to the amp? I remember there were talks of the amp would shut down any channel that's unloaded. So you must connect a load, like a 1K ohm resistor (not sure of the exact value), across each of the speaker + and - output pairs. Search around in this forum for the recommended resistor value though.

 

[Hasdrubal]

That makes some amount of sense, but then why would it work for days and then suddenly stop? And not even stop in the middle of playing, like if it had detected a condition outside of normal and shut down as a precaution, but working until the car is shut down and then not work at all the next day? I even started and stopped the engine several times after installing the replacement JBL, a few minutes on each time, and no issues. Then come back the next morning and no sound again.

I'm not saying you're wrong, though. Are you suggesting that the JBL amp doesn't look like any load at all to the factory amp, like I had just unplugged it?

 

[shoegazer]

My hunch is that your factory head unit went into protection mode. Changing your battery allowed it to reset. But the final amp stages shut down again. As mentioned above; that usually happens due to an issue with the speaker loads or impedance (Ohms).

I see that the inputs have a selector switch for three different levels. If that wasn't set properly to make all channels of the factory amp 'see' an acceptable 'load'; it could switch into protection mode.

And, sometimes, simply running a higher volume level can move the threshold from acceptable into danger. It's all about heat and how hard the amp is running. If you have a mismatch, but are listening to Kenny G.; you may get away with it at low volumes. But more dynamic programs at higher volumes (with a mismatch) will make your factory amp nope right out of there.

These LOCs are kind of quick and dirty (and noisy) solutions to these new car stereos that do not offer line out connections. They are already forcing a square peg through a round hole. So if everything isn't perfect with the setup; it can get weird quickly.

Glad your factory stuff wasn't damaged.

 

[bikejog]

What @shoegazer said. Maybe you turn the volume up one day and down the other? Try run it a few days with your computer speakers also connected and see if it stays working.

Also check this

Depends on the input impedance of your amp. Unfortunately, JBL didn't publish that. Nominal input impedance for low level (RCA level) input is in the order of 30K ohms. I think high level input probably in the area of 10K (not sure about this one).

So assuming the factory is 40W/CH (wishful thinking. probably much less). @4Ohm, it will be 13V. Rounding it up to 15V

E = I * R, 15V / 10K (input impedance) = 1.5 mA.

So the amp is only seeing 1.5 mA load. That's probably not enough.

You need to load balance three things when picking the load resistor

1. It must be no more than 10% of the JBL's input impedance to avoid upsetting the input frequency response.
2. It must be as small in resistance as possible in order to draw enough current to not trip the factory amp's unloaded protection.
3. It should be as high in resistance as possible so you won't end up with a 10W resistor.

 

[bikejog]

Yup, this is the datasheet for the factory amplifier

https://www.st.com/resource/en/datasheet/tda7569blv.pdf

Page 12. I doubt the JBL's input impedance is less than 100 Ohm. Looks like you need to put a 85 ohm or less resistor across the speaker outputs. I would pick a 75 ohm 5W (10W to be safer if you can spare and have the space). Try stay away from wirewound resistors.

 

[kostas]

Totally agree with the guys from my personal experience. The OEM amp may stop working in the absence of speakers in arbitrary moment!
At 75Ohm, 5W is more than enough.
Also, one resistor is enough to keep the whole amp chip alive. Thus, you only need one resistor for the fronts.

 

[Hasdrubal]

Interesting, thanks everyone for the input. Got some resistors ordered, but just to make sure I'm getting this right-

The resistor goes across from one leg to the other, like from the positive lead to the negative lead before either goes to the JBL? In parallel rather than in series?

One resistor is enough for the whole amp, or one per set of channels, like one for the fronts, one for the rears, one center, one sub?

 

[kostas]

I connected a resistor at the front right speaker out. Since then it stopped turning off.
So, you can try one resistor for every AMP IC that is left without any load. It could be enough.

 

[Hasdrubal]

Update-

I hooked everything back up the way it was, and this time put a resistor in place of each of the rear speakers on the harness coming out of the amp. The ones I had were 30ohm 10w, which obviously isn't the spec you guys are suggesting, but that's what I had handy.

So far two drives of about 30-40 minutes, with an hour in between, and the sound is back. Not ready to call it solved, considering how long it worked before giving me the initial problem, but very promising. Ordered another pack of resistors (the 75ohm suggested above) just in case.

My thanks to everyone, there's no way I would have figured any of this out on my own.

 

[kostas]

This is good news!
One more remark though..
In the unlikely event that you still face AMP turn off, don't freak out.
Although some people use the 75 Ohm resistor (I used 47 for the dashboard and 39 for the fronts) and it works, consider the actual info of the chip manufacturer.

Using between 45 and 85 Ohm is not guaranteed that it's regarded as normal load!
In that rare case, just go for <45 Ohm resistor.

 

[Mac_Daddy]

Hey all, so I had a question in regards to load. Im planning on installing the Audiocontrol D5.1300 amp/dsp combo and was wondering if the ‘LGD (Load generating devices)’ that audiocontrol makes are of the same concept. Assuming that the load threshold for normal operation for high level signal is between 0.5ohm - 45ohms, does that mean I would be only needing one ‘LGD’ thats rated for 40 ohms at say a signal thats been tapped at the right front speaker or does a resistor have to be placed at every speaker not connected to the factory amp? All help is truly appreciated! Thanks in advance.

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