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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