borland

My thinking on this topic for my 90' S4 was ..... I had a aftermarket head unit with plenty of power to drive the mid and tweeters of my ten component speaker system. So, all I needed was an external 2 channel amplifier to drive the two remaining 6-1/2" rear woofers.

I found a compact or mini Sony 2-channel amplifier (around 200 watts) that fit in the factory location. Sony no longer makes that model, but there are a couple new compact amps selling on eBay now with decent specifications that would probably work in that same space. Search eBay for "mini amplifier' under 'car electronics'.

SQLGuy

After actual measurements, here are my conclusions:

1. The smallest current production 4-channel amp I've found is the Sony XM-4S (9 x 7.75 x 1.75 inches). It would, technically, fit under the passenger seat, if mounting tabs were added (probably by welding) to the tops of the seat rails (the parts of the rails that move with the seat). However, with the seat all the way down, the passenger's weight would probably rest at least partially on the amp. Not a perfect solution. A car without rear AC might have a bit more clearance to mount this directly to the floor under the passenger seat. With rear AC, as in mine, there are an AC hose and a solenoid taking up space in this location.

2. Alpine makes a unit, the KTP-445, that's designed as a heat unit booster. It takes only speaker-level input, and runs 45W / channel x 4 channels RMS output (not peak). I'm pretty sure that it's small enough to fit where the stock amp used to be. This would be a good upgrade from pretty much any head unit, but would need the wiring harness chopped and adapted (it's designed to plug in directly to newer Alpine head units.) I would also probably want to open it up and convert it for line level input, which should be doable.

I think I'm going to try the Alpine. I'm going to check whether my local Alpine dealer has one they can pull out of the box so I can verify fit under the PS cover.

Edit: While typing this, it occurred to me that I might be able, with L-shaped mounting tabs, to mount the Sony upside-down from the passenger seat rails, and make everything fit. Need to think about this some more....

Cheers,
Paul

SeanR

If you can find one, I picked this up off E-bay a few years ago. Fits perfect and sounds great.

SQLGuy

Cool... but 2-channel. I'm looking for a 4-channel.

Thanks,
Paul

borland

Paul,

Why 4 channel? Most newer aftermarket head units can put out at least 200 watts of power. On a compact car like the 928, you can't reasonably use much more than that. What you probably want is not more total power, but better quality sound at lower power output.

I'm using the same amp that Sean is showing in my 90' S4 with 10 speaker component system. Those amps are no longer made and hard to find. My head unit drives 4 channels, and the external amp drives another 2 channels. So my system is a 6 channel system. If I needed a sub, then I'd require an additional amp for the 7th channel.

How many channels does you speaker system require? Are you using component speakers with external crossovers like the factory used, or 3-way speakers with built in crossovers?

SQLGuy

This is something I want to do in phases.

The head unit, like most head units, uses amplifier modules that run off single-ended 12V. Which means that, while they are specced at 52W per channel, they're really about 10W per channel RMS at reasonable distortion.

The amp I'm trying to replace is a JL Audio 300/4. It's a 4-channel amp driving the doors from one pair of channels, and the B-pillars/cargo speakers in parallel from the other pair of channels (these are not the original B-pillar or cargo speakers). So, to replace it directly, I need a 4-channel that's 2 Ohm stable for the rear channels. I don't really want to go back to the 10W/channel of the head unit... both because of the lower power / higher distortion, and because of the amount of rewiring that would be required.

If I were to drop a 2-channel in where the stock amp originally went, I'd have to rewire the cargo speakers in series with the B-pillars, or add a crossover between them, then wire that setup in parallel with the doors - and lose fade capability.

Eventually, I want to upgrade the door speakers to a better mid-woofer/tweeter pair (currently have the old tweeters left for decoration, and Pioneer coaxials in the mid-woofer position). I also want to upgrade the B-pillars and cargo area to the later setup with tweeters on the B-pillars, which, to do correctly, requires either another pair of channels for the cargo area, or a 2 Ohm stable amp to run the B-pillars and cargo in parallel.

Cheers,
Paul

jmrjames

I don't believe that you can mount an amp "upside down" because that will affect the heat dissipation of the amp.

borland

So, your driving multi-way speakers with extra crossovers from only one 4 channel amp? That's a mess.

Rewiring should not be that big a deal.

SQLGuy

Extra crossovers and one 4-channel amp is how most 928's left the factory.

Re-wiring to run power from the head unit IS extra work and gains me only the benefit of using the high-distortion low-power built-in amplifier modules.

SQLGuy

I doubt it. These amps are generally built using the heat sinks as the majority of the chassis. People mount them sideways, vertically, and horizontally. When you mount them sideways, one side of the heat sink is on the bottom, but they work fine. Upside-down won't be any worse, and the amp still has overtemp protection in case it does overheat for some reason.

Keep in mind, that there would be air on all sides of the amp in the setup I'm thinking of.

jmrjames

My work in progress. Three crossovers below with amp and crossover above. Installed in spare tire space.

I have limited tools so I had to catch a beaver from the pond to chew up the wood for me.

jmrjames

From my JL Audio manual

SQLGuy

Since they don't elaborate on why they strongly discourage it, my guess would be they're talking about mounting it upside-down against a surface... where there would be no air flow over the heat sink.

In my setup the top, bottom, and sides would all be open to air.

Your setup looks cool. I thought about something like that, but didn't want to give up the compressor or spare. How does it sound so far?

BTW, are you installing a fan to allow for airflow in your installation?

M. Requin

Another place to put an amp in in the passenger parcel tray, but it is difficult to find something that will fit. I used a 4 channel Blaupunkt GTA-4 but it has been discontinued. You can check out the installation at https://rennlist.com/forums/928-foru...p-project.html .

jmrjames

I agree that if I remove the spare I will get a flat. The spare is in the storage location to prevent getting a flat. A smaller compressor or the original can fit somewhere else.

I have about 5/8" clear above the amp and hope that the area is large enough to not require a fan. Also, this is a class D amp which puts out less heat than an A/B amp. There is room and available power if a fan is required.

I saw that and it was a great idea. Problem for me is there is no tray in the 1991.

Before my 1991 was stripped I believe that it was a factory 6 channel amp with four crossovers.


Like a lot of work. I am still waiting for parts and just finished the crossover / amp set up this morning.