The Mercedes-Benz GLK350 (X204) is a vehicle known for its blend of luxury and practicality. If you’re the proud owner of a pre-facelift model, like our 2012 GLK350, you appreciate its size and utility. However, one area where the base model often falls short is the audio system. Our “Audio 20” system was noticeably lacking, suffering from distortion that initially led us to suspect the amplifier. A closer inspection of the factory door speakers during a window repair revealed the culprit: basic paper cone speakers simply incapable of delivering high-fidelity sound.
This article, the first in a series, details the initial steps we took to significantly improve the audio quality in our Mercedes Glk350. We’ll explore upgrading the audio input and replacing the front door speakers, laying the groundwork for a richer and more enjoyable listening experience in your GLK350.
Modernizing the Audio Input in Your GLK350
Living in an apartment at the time limited our ability to undertake extensive car modifications. Our initial focus was on improving the audio input to the factory “Audio 20” head unit. This system, common in many Mercedes models of this era, offered several input options: Bluetooth SBC, iPod connector (the older 30-pin type), USB drive, and a Line In (Aux) jack.
While Bluetooth, iPod, and USB playback are functional, they represent older technology. The Line In (Aux) input, however, presented an opportunity to connect a modern, high-quality audio source. Conveniently located within the glove compartment alongside a USB port, these connections are perfect for a discreet Bluetooth receiver.
Our goal was to find a USB-powered Bluetooth receiver that supported advanced codecs like AAC and aptX HD for superior audio streaming from a smartphone. The market is full of such devices, but we needed one that would power on automatically with the car’s system, eliminating the need for manual activation. The BluDento BLT-HD “desktop” Bluetooth receiver proved to be an ideal solution. Its minimalist design, lack of buttons, and excellent analog output were appealing. Crucially, it powers on automatically when it receives USB power, meaning it activates every time the Audio 20 system is turned on. This creates a seamless, almost factory-integrated Bluetooth streaming experience in our Mercedes GLK350.
One minor caveat with the BluDento BLT-HD is its relatively strong line output (2.1 Vrms). If the Bluetooth volume on the phone is maxed out, it can cause clipping at the Audio 20’s Line In input. However, by keeping the phone’s Bluetooth volume at a reasonable level, the audio quality remained excellent and distortion-free. This simple addition drastically improved the input quality to our GLK350’s audio system, setting the stage for further enhancements.
Upgrading the Front Door Speakers for Enhanced Sound
After moving and gaining access to a proper workspace, we began researching speaker upgrades for the Mercedes GLK350. We discovered that Mercedes offered three tiers of audio systems for the X204 GLK and W204 C-Class: the basic “Audio 20” (which we had), a mid-range system, and the premium Harman-Kardon Logic 7 system. The Harman-Kardon system, boasting 9 speakers and a spare-wheel-mounted subwoofer, was clearly designed for superior audio performance. We reasoned that the HKL7 system must utilize higher quality speakers – no amount of digital signal processing can truly compensate for fundamentally poor speaker drivers.
Research on enthusiast forums like MBWorld and YouTube videos indicated that the front door speakers were relatively straightforward to replace. The rear door speakers, however, were more complex to access, often leading to less-than-ideal modification methods. Therefore, we decided to initially focus on upgrading the front door speakers in our GLK350, with plans to address the rear speakers later if needed.
The front door speaker system in the GLK350 consists of a woofer in the lower door panel and a tweeter in the mirror triangle trim. The part numbers for the Harman Kardon Logic 7 speakers are: A2048202102 for the woofer and A2128201002 for the tweeter. We sourced a pair of used HKL7 tweeters from eBay, as these components are generally robust and less prone to damage. However, eBay offerings for the woofers often showed visible damage, so we opted to purchase new HKL7 woofers from a Mercedes-Benz dealer to ensure pristine condition.
A direct comparison of the stock “Audio 20” speakers and the Harman Kardon Logic 7 components immediately reveals the difference in quality. The basic tweeter is a simple compression driver with a waveguide and a protective harness. This harness design is likely to cause sound reflections, leading to undesirable comb filtering effects. In contrast, the HKL7 tweeter is a hard dome design with a simple protective grid, a more acoustically transparent and higher-performing design.
Similarly, the Harman Kardon Logic 7 woofer demonstrates superior construction compared to the basic speaker. The HKL7 woofer features an aluminum cone with a suspension system, while still maintaining a relatively lightweight design suitable for car audio applications. The basic woofer, as previously noted, is a simple paper cone, inherently less rigid and prone to distortion, especially at higher volumes.
Replacing the front door tweeters in the GLK350 is incredibly simple – no tools are required. The mirror triangle trim pops off easily, allowing access to disconnect the old tweeter and plug in the new HKL7 tweeter. The housings and connectors are identical, making it a truly plug-and-play upgrade. Replacing the woofers is slightly more involved, requiring door panel removal and dealing with rivets. However, with basic tools and patience, it’s still a manageable DIY project. The most frustrating part of the woofer replacement was the door panel retainer clips (part A0009917940). These clips are over-engineered and difficult to remove without damage, likely requiring a specialized tool. We recommend purchasing a spare set of these clips from a dealer beforehand, allowing you to confidently remove the door panel without excessive frustration.
During the speaker replacement, we uncovered another cost-saving measure in the base “Audio 20” system that negatively impacts audio quality: the complete absence of crossovers for the front door speakers. In the basic configuration, the audio signal from the head unit is simply split at the door connector, sending the full frequency range to both the tweeter and woofer. Presumably, the Harman Kardon Logic 7 system utilizes separate amplification channels and active crossovers to properly manage the frequency ranges for each driver, enabling time alignment and optimized sound reproduction. The omission of even a basic passive crossover in the “Audio 20” system is a significant compromise in audio fidelity.
Implementing Passive Crossovers for Improved Speaker Integration
Even with just the upgraded Harman Kardon Logic 7 front door speakers, the audio quality in our Mercedes GLK350 was noticeably improved compared to the original “basic” system. However, the lack of crossovers remained a concern. Without a crossover, the woofer and tweeter are forced to reproduce overlapping frequencies, leading to potential interference, distortion, and an imbalanced soundstage. We decided to address this limitation by adding passive crossovers.
The challenge with passive crossovers is that they are designed to work in conjunction with specific speaker drivers, taking into account their impedance and inductance. Using a crossover designed for different speakers may result in suboptimal crossover frequencies and attenuation characteristics. Furthermore, car audio components need to withstand the harsh automotive environment, including vibrations and temperature fluctuations.
We discovered AudioFrog, a company specializing in high-fidelity car audio drivers and passive crossovers. After reviewing the specifications of their drivers and comparing them to the Harman Kardon Logic 7 speakers, we decided to experiment with the AudioFrog GSC610C passive crossover. This crossover is designed for a 2-way component speaker system, and we hoped it would be a good match for the HKL7 speakers in our GLK350.
To better understand the characteristics of the Harman Kardon Logic 7 speakers, we took impedance measurements of both the tweeter and woofer using a QuantAsylum QA401 analyzer and QA460 amplifier. Here are the impedance curves for the HKL7 tweeter:
And the HKL7 woofer:
The measurements revealed that the HKL7 woofer has a nominal impedance closer to 2 ohms, while the AudioFrog GSC610C crossover is designed for 4-ohm speakers. To assess how the GSC610C crossover would interact with the HKL7 speakers, we built a makeshift speaker enclosure using readily available plumbing parts – a practical solution for testing without modifying the car itself.
Using this test setup, we measured the electrical output of the GSC610C crossover when connected to the HKL7 speakers. Here are the resulting frequency response curves:
The GSC610C crossover offers three high-pass filter settings for the tweeter, visible in the right portion of the graph. The low-pass filter for the woofer exhibits a gentle slope of approximately 1.2 dB per octave, resulting in only -3 dB of attenuation at the crossover point. However, it’s important to consider the acoustic attenuation provided by the woofer’s location low in the door, near the floor.
We also measured the acoustic output of the drivers with the crossover in place, using a microphone positioned close to each driver in our test enclosure. This revealed the acoustic impact of the crossover on the speaker system:
Note: The low-frequency roll-off and the peak at 230 Hz in the woofer response are artifacts of our test enclosure. When installed in the car door, the woofer’s response extends smoothly down to 40 Hz. The top lines in the graph represent the direct output of the drivers without the crossover. As evident, the woofer’s output extends up to 7 kHz (likely in breakup mode due to its 6-inch size) with a resonance around 4 kHz (indicated by the arrow). The low-pass filter effectively attenuates this resonance. For the tweeter, the crossover provides a smoother high-frequency roll-off. Overall, the use of the AudioFrog GSC610C crossover demonstrably reduces frequency overlap and potential interference between the woofer and tweeter.
Based on these positive preliminary measurements, we decided to proceed with installing the AudioFrog GSC610C passive crossovers in our Mercedes GLK350. The installation process and the final tuning of the system will be detailed in the second part of this article. Stay tuned to learn how we completed the audio upgrade and achieved a significantly enhanced listening experience in our Mercedes GLK350.
