In early February 2013 Meridian released it's new USB powered DAC/headphone amp with much fanfare. By the end of the month, Head-Fi.org member Purrin had found its performance in listening tests underwhelming and had measured the output impedance of the Explorer at about 48 Ohms. This is not good.
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The important thing to observe here is that changing the ratio of load impedance to output impedance changes the voltage at the load. Lower the output impedance and more voltage is available at the load. But the converse is true as well: If you raise the resistance of the load, it will get more voltage. That's very important to know because most headphones have impedances that change with frequency. Let's talk about that.
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We can see that with the 50 Ohm output impedance of the old Explorer, we have a fairly strong effect on frequency response due to the interaction of the headphone impedance curve and the high output impedance of the amp. At the 3kHz impedance bump the Audeo PFE was 1.5dB louder with the old vs. new Explorer, at 20kHz it's nearly 4dB louder. This is a fairly significant change in frequency response and would be easily heard as the PFE being substantially brighter sounding on the old Explorer.
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The ratio of the load's impedance to the amp's output impedance is called the "damping factor." In the case of the old Explorer's 50 Ohm output impedance and the Sennheiser HD 600's 300 Ohm impedance we get a damping factor of 300:50, or 6:1. While there's no hard and fast line in the sand here, it is commonly accepted wisdom that a damping factor greater than 8:1 is desirable for tight control. Bottom line: the 50 Ohm output impedance of the old Explorer is to high even for high impedance cans like the Sennheiser HD 600.
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From the plot above, you can see that the old unit results in a more curved top than the new unit. The wave shape is better controlled due to the better damping factor of the new unit.
It doesn't look like much, but boy, it makes a difference in sound quality!
