Adding a Third Amp Under the Driver Seat

I have this old Sony 200 watt amp from my previous vehicle:

https://docs.sony.com/release/specs/XM752EQX_mksp.pdf

I was thinking about running this under the driver's side seat and using it to power my passenger door speakers.

I currently have two of these amps mounted in the back on the generic Amazon twin sub box for Tacomas:

http://www.sonicelectronix.com/item_39893_Polk-Audio-PA-D2000.2-PAD2000.2.html

One amp runs the two 10" Polk subs in the box. The other amp runs CTX 65 component speakers in the front.

There are distribution blocks on the rear of the sub box that connect the two Polk amps for Power (red) and ground (connected to bare metal by the rear seat belt).

I'm not sure how to wire the distribution blocks for the third Sony amp for the rear. Should I buy new distribution blocks to stick on the back of the sub box and run all three amps from the distribution blocks? Or should I mount distribution blocks near the third amp on the floor under the seat and cut the cables running from the box and split them from there?

Also, I think I would need a bigger fuse for the red cable going to the battery as well. What size should that be?

I'm a little new to sub/amp wiring, but have wired many head units. I just want to make sure I get it right. Thanks in advance for your feedback.

 

Let's start with the fuse and red wire question....

I know how confusing this stuff can be since there are a lot of variables. But bear with me.

The fuse by the vehicle's battery is there to protect the red wire, not the amplifiers. If the red wire is still appropriate for all three amplifiers, then you likely can keep the same fuse.

First, you need to determine if your red wire is appropriate for all three amplifiers. To do that, you need to find the total RMS of your system. Depending on your speakers, it looks like you'll be in the 1,000 to 1,200 watts RMS range. But I don't know the specs of your speakers so you need to double check your RMS. I'm assuming you have a double cab based on your "passenger doors" and "rear seatbelt" comments, so you'll probably be around 10-12 feet in length. Now that you know the length of wire and the total RMS, you can go here and figure out which gauge is appropriate for your system. If your current red wire is an appropriate gauge for all three amps, then you can keep it. If its not an appropriate gauge, you need to replace it.

Still with me? If you can use the same red wire, you will likely be able to keep the same fuse. But if you need a new fuse, the next step is finding an appropriate fuse. This is the easy part. Just remember you should pick a fuse with the capabilities of the red wire in mind. If that red wire is capable of carrying 350 amps, then the purpose of the fuse is to prevent MORE THAN 350 amps from being carried by the wire. This does not mean you NEED a 350 amp fuse, just that you must not allow more than that. I've always followed this:
8 gauge - fuse at 100A
4 gauge - fuse at 150A
2 gauge - fuse at 225A
1/0 gauge - fuse at 350A

A lot of people will say to go with a slightly smaller fuse rather than the max just to be on the safe side. For instance, if you have a 4 gauge wire, a lot of people will get a fuse between 100 and 150 amps, rather than a 150 amp fuse. Just make sure your fuse is NOT smaller than the draw of your amps. So, by now you should know if you need to replace the red wire and/or fuse. There are differing opinions/theories on some of this, so depending on who you talk to you might get a slightly different answer.

 

And now lets tackle the distribution blocks....

This stuff is purely my opinion as there's not really a science to it like there is with the amps and fuses.

If I were you, I'd get new distribution blocks that are capable of handling three amplifiers. Something like this perhaps. Just make sure that it will work with the gauges you're using.

Also, if you're going to put the distribution block more than 18 inches away from an amplifier, you need another fuse on that wire. So if the distribution block is going behind the back seat and that Sony amp is under the seat, you'll need another fuse for that wire. And now that you know how to figure out fuse sizes, you shouldn't have a problem at all.

To make it easier, you might consider a fused distribution block like this. I realize that at least one of your amplifiers is likely closer than 18 inches from the distribution block and that using this fused block is unnecessary for that particular amplifier. But its not hurting anything by having an extra fuse in there. It just makes the install faster by only having one distribution block. Make sense?

For the ground, you can run the Sony back to where the other two amplifiers ground or you can ground it somewhere closer. Just make sure its a GOOD ground. You're going to be pulling a lot of power and you don't want to mess it all up by using an OKAY ground.

I hope this helps you complete your install. Sorry, but I tend to be long winded in my posts. If you have any questions or need clarification on anything let me know.

 

I like the idea for those blocks there.

I have a non-fused Stinger block on mine, but the amps are within 12 inches of it so I don't need another fuse like you mentioned.

 

Thanks for the detailed response. RMS on the two Polk subs is 270 watts each. But the amp running them is only 250 watts split into two channels.

The RMS on the components should be about 200 watts per set/side I think there is a 100 amp fuse on 4 gauge wire running to the battery and then 4 gauge wire to the ground. The red 4 gauge wire set up was rated for up to 1000 watts. And that of course means my red wire will NOT work and that I will have to run fatter cable, likely 1/0 awg gauge.

And then there would the the additional fuse between the first two amps mounted on the sub box and the third amp under the seat. And based on your recommendation, I would need another fuse (because it will be more than 18") from the third amp.

This is getting more complicated than I imagined, not necessarily even in terms of time, but also the cost of the new wire kit and distribution blocks ($100 + -). I'm wondering if it will be worth it in the long run.

Now thinking about just running the rear door speakers off the head unit, which is a Kenwood DNX890. If I understand it correctly, the rear speakers aren't really doing anything spectacular for the sound stage.

Any other thoughts on this?

Riverdog, many thanks for helping me work through this. Again, the clarity in your post was extremely helpful. Rep coming your way.

 

I think you'll be fine with running the rears off the deck. Yeah, it would likely be better to run them off of your Sony amp, but is it worth the cost and time to install it all? That's up to you. Unless you have the stock speakers in the back. If that's the case, don't connect them to any amp. They're designed to run off the stock deck, not an external amp. The amp in your Kenwood deck is fine to use, but nearly all external amps are more likely to damage the stock speakers.

As far as the soundstage is concerned, some people don't even use rear speakers. To them it sounds better with only speakers up front. While others like to have a second set of coaxial, or even components in the back. Whatever sounds best to you is exactly what you should do. (Unless what sounds best to you is honestly terrible. Then listen to random people online like me ) Anyway, there are best practices when it comes to this stuff but whatever sounds best is what you should do.

You might consider selling your Sony amp and the other amp that is currently running your fronts in order to make some money to put towards a 4-channel amp. You'll probably still have to replace the power wire from the battery, but you won't have to mess with the distribution block or adding another inline fuse. Another benefit is you'll maintain your fade control. You can also save some money by piecing together your own amp kit. Since you know what gauge wire and fuses you need, you can buy that stuff separately and save some money. Home Depot and Lowes sell wire by the foot. They also sell fuses and the little connectors for it so you can connect it to your battery and inline fuse holder. Depending on what's in your truck right now, you might even be able to reuse the inline fuse holder. Sure it'll take a little more time on your part, but it could be worth it.

 

The four channel amp indeed seems like the way to go. Would I then run the component speakers in the front doors and the aftermarket 6.5 coaxials in the rear doors (to be purchased) off this four channel amp? That would also mean keeping one of the Polk amps to run the two 10" subs.

Piecing together an amp wiring kit seems to be cost effective as well. Thanks for that tip.

I'm not sure how much the Sony amp would yield in a sale. It's pretty old and prob not worth much. It would prob make a great paperweight in my office though.

 

You're exactly right. The four channel amp would power all the door speakers (components and coaxials) and the Polk amp would still power your subs.

Ha, I too have an amp/paperweight. Although the Sony might not yield that much, it might cover the cost of your amp kit.

 

Just for funsies, give not running the rears a shot first. If you are happy with that it saves you some cash.

Nothing but stock, but on my wife's Rav4 I usually just use the fronts and don't see a tremendous difference. Maybe just go with the fronts and 2 amps now..then when the money is there and if you're wanting more then do the rears.

 

I will most likely end up just adding sound deadening material to the rear door panels and running coaxials off the head unit for now.

 

I did a similar in my Oldsmobile Achieva..rears on the MTX amp, box off the Jensen, fronts off the HU. Never felt a need to upgrade the fronts in 4 years, and like I said if you do when the money is there great..