For more than a decade, I’ve gotten so much valuable info from members on this site, as a guest. Thought I would return the favor on a subject I have much expertise and help everyone enjoy their music at a quality worthy of these magnificent machines without compromising any of its build.

When I got my SL550 R231, I was stunned at the mediocre sound system. Now, I’ve been an audiophile since the mid ‘70s and even worked in a hi-end stereo & electronics chain as a baseball player in college. Considering this extraordinary HK system, German engineers have it set-up to use about 1/4 of its capabilities. Despite my efforts, I was unable to figure out how to safely make changes through the engineering mode. But I finally figured out a way to, somewhat, bring it to life without adding amps and speakers or any other potentially damaging procedures. Keep in mind the HK system most of us have has comparable THD (distortion) levels to most avant-garde level home systems, 600 clean watts of power and surprisingly accurate speakers only hindered by their crossovers. Here’s the simple, cost effective fix:

First, follow the YouTube vid the Brit fellow did on how to access the engineering mode and activate the fader. It’s clear and easy, just be careful not to mess with other features as they might cause heartache. This upgrade does make a difference but the midrange signals and definition is still overpowered by the low end signal “overkill” produced by the two subs in the firewall and the boosted upper range signal that’s missing detail/separation. Now if you listen to pop or hard rock you could be content stopping here. Just tune and crank it!

Second, use a wired HD music source—don’t bother with Bluetooth…please…or the apple connection in the glovebox between the seats even if your old apple device is loaded with your music in wav. file format (only advantage with this is the navigation ability via the controls of the system). I use Amazon HD music and connect my phone (basically, my tuner/preamp) via the aux input using an inline DAC between it and the input receptacle. I tried at least a dozen different units and found none to be as good as either of these two 32 bit 384khz DAC units sold on Amazon:



https://www.amazon.com/Lossless-Adapter-CX31993-Headphone-Compatible/dp/B0B5L5ZQBN



The kicker that makes all the difference is this inexpensive device also sold on Amazon:





It is cheap chinese, but with a few modifications and logical tuning, it can be adjusted to be surprisingly good. There is no getting around the fact that equalizers add distortion. They basically “stretch” the music signal at various frequencies. Automobile equalizers are almost always coupled with amplification—the quality of the amplifier is everything with these. With this unit, you can minimize distortion using the main output or volume control. This volume control also indicates there is a small, weak, dirty (distortion) amp that will kick in at a certain volume level—the device is considered to be a headphone “amplifier.” I found if I keep this volume control at 2.25 notch or so (indicator at approximately 9:00 on the dial) the distortion is practically inaudible and you’ll be predominantly using the HK system’s extremely clean amp for volume.

Feedback and hum— Open the unit (screws) and apply a full layer of polymide tape across the entire solder side of the circuit board. I believe polymide tape can also be found on Amazon.

I put this EQ unit in the glovebox between the seats. It powers up through the USB port—no hassle. Be sure to use the best patch cords and shortest lengths that make sense.

You can adjust to your own ears, but I suggest starting with each frequency dial at that same 9:00 position and add from there. The reason is less stretching of the signal and minimal distortion.

With this EQ, you will be able to correct the system’s weakness. You can fine tune and increase the midrange signals and refine the detail to a very impressive, quality sound signature…which is very satisfying in the R231’s intimate cockpit.