Accessory power for amplifier

[Matt@EDC]

Hello Wizards of Electrickery

With the airbag module in the bin, I have the perfect little spot for the stealth installation of a diddly Pioneer GM-D1004 amp to bolster the headunit I’m fitting in my sedan build with a true 4x45Wrms potential. The car is getting complimentary Pioneer A-series components up front and G-series 2-way coaxials in the rear.



I’ve drilled and tapped the SRS/ABS module bracket to carry the amp securely, bundled the speaker harness rather than cutting it all 3” long (may still happen for ultra neatness 😬) and used a terminal block for serviceability of speaker wires.

Location is pretty perfect as it keeps speaker wire runs to a minimum (just passenger front to go through dash) and it’s right next to the ground points and C302 accessory plug.

The little amp requires a 15A fused power source. A lot of installations seem to run these off the cigarette lighter (which I’m deleting) supply, however that’s fused at 30A in the E30. While locating this fuse, I found it shares that Fuse 28 with the power antenna (also deleting) which connects to pin ‘Z’ in the C302 accessory socket. Bingo!

So I can use the connector from a spare antenna harness to terminate the 12v supply for the amp and plug it straight into the antenna pin in the C302, then replace Fuse 28 with a 15A. The power wire on the amp harness is 14awg, and so is the lighter wiring. The antenna wire looks to be 16awg or 18awg and significantly longer than either, though one must presume capable of carrying 30A otherwise Fuse 28 would offer no protection? All the wire gauge charts if referenced suggest so.

Anyway, hope that’s of interest.

[bmwman91]

I would definitely not use the antenna power wire for this. This is not the same as how mains wiring is governed under the NEC...yeah the fuse is 30A and you might think that all wires in there would be "the right size" for it as such, but it's not really the intent. A 18ga wire can carry 30A without burning up, but you will see hefty voltage drops across it which will in turn make the amp's distortion performance take a dump (even 15A is asking a lot).

I realize you want to make a simple and clean install, but if you want to not totally compromise the system's performance then you would run a dedicated power line to the +12V battery terminal (with its own dedicated fuse in-line and as close to the battery as possible), and maybe also a dedicated ground. I cannot recall which ground lugs are up under there...under no circumstances would I use one on any of the little brackets in there, and even the steering column is a little questionable to me. If your battery is up under the hood then things are easier, and you could run 6ga (8ga OK but that's about the limit) power+ground with 15A peak loads and see ~1% voltage drop versus the 13.8V supply. If the battery is in the trunk, 6ga for one wire and 4ga if doing both.

[Matt@EDC]

bmwman91 thanks for the insight! Electrical is far from my strong suit, aside from the mechanical aspect of putting things together.

Hopefully it’s clear that the antenna power is being accessed at the accessory connector under the steering column, not in the trunk? So there’s maybe 2ft max run between there and the fuse box. Running it through a calculator, 15A at 13.8v over 2ft of 18awg renders approx 0.47V drop. If it turns out to be 16awg, it’s 0.3v. Does that sound reasonable to you? And that’s maxing out the amp, which is unlikely through a quartet of 5.25” speakers.

Additionally, I would ground to one of the 2 ground posts next to the steering column, where the rest of the vehicle harnesses are terminated, about 6” away from the amp.

My main concern is safety in the event of a short. If the antenna is fused at 30A, then surely the wire can carry 30A before melting, even if only for a short time, otherwise the fuse would not blow? Hence stepping down to 15A fused circuit, the wire should have a lot more headroom?

Pioneer direct users to tap into the headunit power, however this is only fused at 7.5A in the E30. The 12v constant in the harness supplied with the Pioneer headunit is same wire gauge as the antenna power supply.

For DIYers, the GM-D1004 amplifier is not only small and powerful but it’s easy to install. Simply connect power and ground at the radio and get more installation flexibility using hi-level or low-level inputs with a signal sensing remote turn on so you can use it in combination with a factory or aftermarket headunit.

However, as above, the 12v constant on the amp harness is 14awg, a little thicker. The power supply to the cigarette lighter is also 14awg and a well used power source for these little micro amps. However, seems daft to run 3ft of power wire across the dash to pick up on a power supply that comes right back across to where I started. More opportunity for pinched wires. Calculated voltage drop for this longer 6ft run in 14awg wire at 0.6v.

4/6/8awg all seem excessive for a dinky little Class-FD that is only fused at 15A and suggested to run off radio harness power?

[bmwman91]

Yeah, it's probably fine to pop a 15A fuse in place of that 30A fuse if nothing else is using the branch, use the empty antenna power pin on the accessory connector and ground on the big lug where the other stuff is grounded. From a safety perspective, you should be just fine. Even leaving that 30A fuse in there is probably not an issue. If/when any kind of major short does happen to occur, it will easily be pulling well over 100A and will blow that fuse almost instantly. The recommended 15A fuse is based on a thought process like, "the amplifier is rated for up to a 15A load under normal operation, and anything exceeding that is the result of a fault."

[Matt@EDC]

bmwman91 Thanks again

I have experience installing larger amps with 8 < 0awg, so this guy is certainly a little different! All the same concepts apply, as you say, just a case of picking up on an appropriate existing power source (as intended by Pioneer). I’m sure people power these off all sorts of random +12v sources under the dash, though I’ve seen only a couple of forum threads around the internet 🤷♂️ They are pretty well sold out everywhere, so they’ve been consumed! Maybe just by professional installers doing stealth installs, as your average ICE consumer is probably after something with more power and a bit showy.

Anyway, speaker wires to run next…