DIY - HAES 4 Channel Amp Gain Boost Mod
 

I have read a lot of posts asking what can be done to improve the power output from the stock HAES 4 channel amplifiers. I have a touch screen Pioneer head unit with a 6 speaker setup in my 2001 Boxster. The head unit drives the rear speakers directly and the pre-amp outputs are wired directly to the stock amp in the front trunk which drive the dash and door speakers. Before modification, the amp output power was very low and the dash and door speakers did not contribute much to the overall sound volume in the car. It was pretty terrible.

I really didn't like the idea of going to an aftermarket amp so I decided to reverse engineer the stock amp to see what could be done. (I'm a EE by trade so these kinds of things are second nature to me) It turns out that the stock amp has a pre-amp input stage that is set up as 2 differential to single-ended converters. Both FL and RL channels are combined into a single Left channel and the same goes for the Right channel. Then they get split back up again before they go the power amp stage. I've sketched out a rough schematic of what's going on with the pre-amp in figure 1. I didn't bother to trace out the schematic completely because I just wanted to know enough to see where I could adjust the gain.

The most straightforward way to increase the pre-amp gain is to lower the input resistance value in the pre-amp stage. (You could also increase the value of the feedback resistors but that would increase the overall amp noise, which is not good)There are 4 resistors in the input circuit that have a stock value of 33 k ohms that need to be changed. (See figure 2) In general, increasing the output power amp level by 10 dB (factor of 10) doubles the perceived volume so I reduced the 4 resistor values to 3.3 k ohms. These resistors are 0805 size and can be purchased at digikey.com or mouser.com for a few dollars including shipping. You can adjust the resistor values to between, say 3 k ohm to 5 k ohm to match the output of your specific head unit. If you aren't comfortable soldering small components I suggest you ask for help from someone who is experienced at soldering. The circuit board is delicate and you can easily damage the component pads if you heat it up too much.

The stock amp actually sounds pretty decent after this mod and doesn't distort at the max levels needed to comfortably listen to music while driving with the top down. And note that I am driving the door woofers with a full bandwidth signal.

I suspect the same technique can be used for the 6 channel amp which I believe uses the exact same circuit board as the 4 channel amp but with more circuitry populated. Just find the first series resistor after the input DC blocking capacitors (big blue box shaped components) on channels 5 and 6 and replace those with the lower valued resistors.

Hopefully this is of some help to those of you who aren't happy with your stock amp.


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What is the wattage rating of the 0805s?

Also, how did you get the non-filtered signal to the woofers in the doors?

Ah, the good 'ol 0 ohm resistor trick...AND TL074s!This is like undergrad senior design all over again.

Might those caps you have '?' on be bypass caps? What are their values? It would make sense for the differential setup.

Great write up and if I had an amp, I would do that mod.

It's pretty easy to find a 6X40 amp from a 996, preferably a cabrio--plugs right in with a little removal of connector plastic tabs. Then run rear speakers from there.

0805 resistors would normally be 1/8 W.

To get the unfiltered signal to the door speakers I just set the head unit pre-amp outputs to full range rather than sub out. I won't swear that the door speakers are operating at full range but I definitely can hear some mid range and highs out of them. You can 'scope them easily enough if you have an oscilloscope and a test CD that does a sweep tone and find out what the real response is. I might do that at some point.

Those input components to ground had a 'j' indicator on them that I didn't recognize. They might be high pass filter caps but the overall frequency response would be dependent on the source impedance of the pre-amp outputs, which can potentially vary. I could have taken them out and put them on an LCR meter to find out what they are but I decided that it wasn't worth the effort.

Just saw this...my kind of mod! Very nice work! :cool:

6x 40w amp
 

Hey thats a great write up and appreciated, i know I'm a few years late on seeing but just got a boxster and replaced the 4 ch it came with a 6 ch from a 996 and installed a customer rear speaker box.
Only issue is i got a very cheap Coupe amp and the rears are not as load as the front channels and Cabrio amps are getting harder and dearer to find.

Could i do the same thing to increase the rears only or do all 6 by replacing the resistors ! thinking its worth a go as i got the amp for £35 so no real loss.
do the resitors need to match the watts of the unit ? which is 40watt output ?

From reading posts i believe porsche used lower values own the coupe amp vs the cabrio.

thank again for the write up.

On my 2004, the rears are 3" 6 Ohm HAES speakers. they are wired in series so thus I think they sum to 12 ohms on each side. If you replace each of them with 3.5" 4 Ohm speakers, you should get MUCH better sound/volume without having to modify the pre-amp.

Thanks for that info and Nice Anniversary Model.

I have custom fitted 2 x 4x6" speakers into the storage box that are driven by each channel separately. The issue i believe is with the coupe amp vs the cabriolet that the rears are lower powered in the coupe. Hence i want to beef up the rear channel but seeing this thread boosting the entire system would be nice and keeping it OEM..

Im running the PCM1 satnav headunit and want to keep it original otherwise i would go Alpine 7" headunit and custom 6 challenge amp.
Just need to run a bit more research on replacing these resistors and what type i need to be safe and maybe give it a try.

Mazman, like your idea so ordered some to do. Received today, man these are small! Going to have to buy a really small tip soldering iron just for it. Will see how it works out. Thanks,

Bookmarking this one. Nice info

Very interesting information. I just got a 2003 Boxster for my wife and have been fixing it up.

The stock audio system sounds awful. I do have the amp in front of the battery and it is feeding the dash speakers.

2 days ago I was replacing the driver door window regulator and found that there are speakers in the door. They do not appear to be working, but I was impressed with the molded plastic case. I bench tested the speaker and it sounds pretty good.

Now I have to find out why it is not working. I will check the wiring and the frunk mounted amp.

Any ideas or suggestions??