Meridian Explorer: A Case Study of the Effects of Output Impedance on Headphone Listening
Meridian Explorer ($299)
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.
Fortunately, Meridian had already received feedback from early units delivered to dealers and distributors, and had got wind of the problem when driving the low impedance headphones commonly used in portable applications. An engineering change was put into motion. Unfortunately, the wheels were already turning and the product introduction continued on schedule with the original product going into production. From what I could gather, a few thousand Explorers were shipped before the running change could be made. The first run of modified product has just finished production (about March 13th 2013), and is currently in shipment to dealers. I'm told the Explorer is selling very well, and dealers have little stock on-hand from previous production runs. I expect it would be safe to place an order now from any retailer that is currently out of stock, and am fairly certain that the unit received will include the changes.
I received the revised version a few days ago, and measured the output impedance:
With 0.1988Vrms out open circuit, I get 0.1922Vrms with a 150 Ohm Load for a calculated 4.99 Ohm Output Impedance. With 0.1988Vrms out open circuit, I get 0.1723Vrms with a 32 Ohm Load for a calculated 4.92 Ohm Output Impedance.
Bottom line: The revised Meridian Explorer now has a about a 5 Ohm output impedance.
What's the big deal? Let me explain...
Output Impedance, Frequency Response, and Damping Factor
In the illustration above, the circuit to the left models the prior Explorer with a 50 Ohm output impedance, the circuit on the right shows the current version. The resistor shown as Rzout is the output impedance of the amplifier. The word 'impedance' indicates both resistive and reactive (capacitive and inductive) elements and is not actually a simple resistor, but for the moment we'll just think of it like a resistor inside the amp and in series with the output. The Rzload is the voice coil of the headphone. I used the Sennheiser Momentum in my listening tests, which has a 22 Ohm impedance, but I've shown a 25 Ohm load just to make the math easy. With an output impedance of 50 Ohms into a 25 Ohm load, 2/3 of the voltage (and power) in the amp is being lost in driving through the output impedance of the amp, with only 1/3 the power being delivered to the load. Power is lost inside amplifiers all the time, so inneficiency isn't really the problem here. Let's move on to the diagram on the right.
The circuit to the right models the new Explorer with 5 Ohms output impedance. Now the ratio of output impedance to load is much different. In this case only 1/6th the voltage is lost in the amp, and much more voltage is available for driving the load, so this is a more efficient configuration.
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.
Effects on Frequency Response with Headphone Impedance Changes
Let's look at the impedance curve for some headphones.
Sennheiser HD 600
The purple trace in the graph above is the impedance curve of the Sennheiser HD 600. At 1kHz you can see it's just above 300 Ohms, but the primary driver resonance at 100Hz causes the impedance to rise to 550 Ohms. So at 1kHz the HD 600 looks like a 300 Ohm load, but at 100Hz it looks like a 550 Ohm load. That means that the HD 600 will get a little more drive voltage at 100Hz than at 1kHz, and will be somewhat louder at that frequency dependent on the output impedance of the amp.
In the case of the original Explorer with 50 Ohms output impedance with 1Vrms drive, the headphones will get 0.86Vrms at 1kHz, and 0.92Vrms at 100Hz. That calculates out to a 0.58dB increase at 100Hz. The point here is that the impedance curve of the headphones interacts with the output impedance of the amp causing a frequency response change of the headphones. But because these are fairly high impedance headphones the change doesn't amount to much. This is one of the reasons why it's important to use high impedance headphones with high output impedance amps (like OTL tube amps).
Let's look at another pair of headphones.
Here's a pair of planar magnetic headphones (Audeze LCD-2). One of the unique characteristics of planar magnetic cans is that they are purely resistive in nature and the impedance doesn't change at all with frequency. Even though these cans have a low impedance and would strongly interact with the high output impedance of the early Explorer, the impedance doesn't change so there will be no frequency response change due to the high output impedance of the amp. The poor damping factor will have its effect on the sound though, we'll get to that in a bit.
Here's the impedance plot of the Audeo PFE balanced armature in-ear headphone. Again, the purple line shows the impedance plotted against frequency. It starts out as a 32 Ohm headphone, but as frequency rises, so does its impedance. At 3kHz it has a driver resonance bump to 50 Ohms, and at 20kHz its impedance has risen to 80 Ohms. With the high 50 Ohm impedance of the original Explorer, and the low overall impedance of these cans, we can expect a significant coloration as the high output impedance interacts with the widely swinging impedance curve of these headphones.
In the plot above I measured the frequency response of the Audeo PFE with both the old and new versions of the Meridian Explorer. I then plotted the difference (blue) between the two FR plots in dB on the left hand scale. I also plotted the impedance curve of the Audeo PFE on the right hand scale. Because of the step size of the digital volume control of the Explorer I couldn't exactly match the two levels and ended up with about a -1dB difference at the low frequencies. So the difference reference is at the -1dB line.
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.
Frequency Response Summary
If you have one of the early Meridian Explorers you will find some headphones more colored than they normally would be due to the interaction between the impedance changes with frequency and the output impedance of the amp. Generally speaking, higher impedance headphones and headphones with very flat impedance curves will suffer less coloration due to the 50 Ohm output impedance of the early Explorer.
There's more to this story though, let's talk about Damping Factor...