Dave Navone, one of the premier voices in Autosound sent me suprising results as they relate to aftermarket vs. oem head units. I talked to him about my factory delco/bose unit in my Escalade and the fact that I wanted to keep it. He agreed:


Autosound 2000 Test Lab Report on OEM vs. Aftermarket Head Units


Car Audio in the 21st Century
By David Navone


The 1990s brought us OEM systems that are integrated into steering wheels, air conditioning systems, warning lights, factory anti-theft systems, etc. Not only that, but the features and benefits of some OEM systems cannot be easily duplicated in the aftermarket. For instance GM’s Driver 1 or Driver 2 preset preferences and steering wheel controls make swapping head units a difficult decision. Also many OEM in-dash decks can play a CD and a cassette. Upgrading such a deck can be a really tough decision. In fact, many OEM head units are being left in the vehicle and aftermarket system designers are interfacing components to the factory deck. There are shops that lament the fact that they can’t upgrade the factory deck, but with many new vehicles, the OEM deck will probably remain in the car until it hits the wrecking yard.

A2TL Tests New OEM Decks

But is replacing a modern deck with an aftermarket deck really an “upgrade?” We’re nearly into 2001 and our Car Sound Forum’s have been alive with questions about interfacing new OEM decks with aftermarket components. Questions about “sacrificing quality by using an OEM head” have prodded our Autosound 2000 Test Labs into examining the electrical differences between aftermarket decks and OEM decks.
For our tests we chose a new Visteon OEM deck, a new Delco OEM deck and a mid-line priced Japanese deck. (We recently reviewed this popular deck in Car Sound). The Visteon deck utilizes a 6-CD changer (CD-6) and the Delco deck is the 4185. We tested the OEM decks with a high quality interface attached in the same manner they would be used in a typical OEM upgrade. We chose Soundgate LOCHVA OEM interface devices for the tests because Rob Putman at Soundgate is our AutoMedia OEM Interface Editor. The tests were our standard specification measuring programs in the Audio Precision System II.
As we all realize, an important spec is the Maximum Undistorted Signal Level, which was 2.5 volts in the Japanese deck, 3.4 volts in the Visteon deck and 3.9 volts in the Delco deck. These readings were taken just under clipping and notice that the distortion figures are 1% or less. Distortion of less than 2% on musical program material is impossible to hear so the 1% spec is excellent.
The useable dynamic range is the difference between the noise floor and the maximum undistorted output. The Japanese deck yielded a specification of 93 dB, the Visteon deck came in at 82 dB and the Delco deck measured 93 dB. The maximum theoretical dynamic range for a 16-bit CD is 96 dB. This means that these specifications are excellent – especially for a car audio component that must contend with road – wind – vehicle noise.
The last specification we measured was the output or source impedance. The Japanese deck measured a respectable 300 ohms. Both the Visteon and Delco decks had very low output impedance measurements (less than one ohm) because of their speaker level outputs, but with the OEM interface devices installed, the specs were a little over 1000 ohms for each deck. Is this a bit high for car audio?

Output Impedance Specs

In Noise Troubleshooting 101, we learned that a high output impedance could be a source for noise. The output impedance can be thought of as a resistance (actually impedance) in series with the signal. For those readers who hate Ohm’s Law, we have can say, “The highest voltage will be developed across the highest resistance.” This means that a high source impedance could allow a higher noise level to be produced across that resistance. This last statement is also the key to the potential problem for noise.
Since the output impedance of the speaker leads of the OEM decks themselves is certainly very low, there will be very low noise levels produced on the signal path leading to the OEM interface devices. The only possibility for noise appears at the OEM interface device and on the output of the OEM interface device. This means that the speaker level signal should be routed close to the input of the aftermarket amplifier. (We recommend Unshielded Twisted Pair signal cables for this task.) Then the OEM interface device should be installed at the input of the aftermarket amplifier. If the installation is done correctly, a source impedance of 1000 ohms over a short path of 6” or so will not present a problem.

Conclusions

The specifications of the OEM decks, connected to the OEM interface devices, proved to be as good as a mid priced aftermarket deck. It’s important to correctly install the OEM interface devices, but once in place, the electrical differences are certainly not noticeable.

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