Thursday, April 6, 2017

T-Cable for Output Level Measurements and Surprise from Benchmark

When performing headphone amplifier comparisons (actually, any audio-related comparisons), matching output levels is of a paramount importance. Louder sounding equipment always perceived as sounding "better" (unless it is clipping because it has exceeded its capabilities). And human ears are amazingly sensitive to volume levels, even a bit of difference in them may affect our judgements.

That means, before starting any comparisons of headphone amps "by ear" make sure that they have been set up correctly. Two tools that are helpful for this job are: good "true RMS" multimeter, and a special cable that has open contacts for attaching probes (unless one is OK with partially disassembling the amplifier or headphones to reach their contact plates).

That's why I decided to make a simple pass-through 1/4" TRS T-Cable with an outlet where multimeter cables can be connected to. This is how is supposed to be used:

This is how an assembled cable looks like:

After finishing the cable, I decided to test it with my headphone amps. First I tried with SPL Phonitor Mini and AKG K550 headphones. I've connected the T-cable in between, and started playing a 1 kHz sine tone—a simple wave, so the multimeter doesn't have any problem measuring the output level. As I expected, the output level was increasing or decreasing with my volume adjustments, and levels of the left and right channels matched pretty closely (within 1%).

The next was Benchmark DAC1 HDR, and here I've got a big surprise—the levels of the left and right channels were pretty much off from each other—as much as 16%. Something that I wasn't expecting from this piece of equipment. I listened to this sine wave myself, and indeed I noticed that it was shifted to the right, and the amount of shifting was changing as I was adjusting the volume.

I've searched on the web, and found this old thread on ProSoundWeb forum describing exactly the same problem I have, and the conclusion there was that the left / right balance for headphones on DAC1 only holds at a certain output level. This seems pretty strange to me, especially combined with the fact that Benchmark has a remote control. So they put a motorized volume pot in this amp, but couldn't make it to preserve balance across the volume control range?

Having figured out this sad fact, I decided to adjust the balance on the Benchmark. Thankfully, it has a trimpot for that. Here is how my setup was looking like:

The trimpot on this model is easy to find. What it seems to do is adjusting the level of the left channel. After I balanced the channel levels for a level of about 100 mV RMS, I've found that it actually only holds in this region. As soon as you move the volume slider by a couple of marks, the sound is getting slightly out of balance again. Not great, but at least I'm now aware of this issue.

For me, the conclusion is never trust the brands, and always check everything with tools before jumping into any comparisons.

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