How Good is that Sound ? Part One – The Plan

If you are into high quality audio, as I am, you probably rate your ears as the ultimate test equipment for your sound rig.

Still as a confirmed geek, I like to measure things… it’s in my geek DNA.

So this post is about benchmarking the performance of a sound system, or in my case, benchmarking my newly-built solid-state headphone amplifier — essentially an M3 design — against my small collection of valve amps – a heavily modified Melos SHA-1 GOLD and a Musical Fidelity X-CAN V3 with mods by Parts Connexion prior to my purchase of it.

My plan is to use my computer to run a set of high-end audio tests on each amplifier and then use the results to compare the amps. With any luck, the measurements will put some quantification on the sound differences I can hear.

I need a few key pieces. The set up I’m going to use  is to run Right Mark Audio Analyzer software on my PC driving my M-Audio Audiophile sound card with my sound system attached on the back-end as a loop from the outputs of the sound card back into its inputs.

The Right Mark Audio Analyzer (RMAA) provides an open source set of tests that cover such useful measurements as frequency response, noise level, cross talk, and total harmonic distortion, in fact, all the usual suspects that you would see if you cracked open an issue of Stereophile or your favourite quality Hi-Fi mag. Not only does RMAA provide these cool measurements, but there is a community of people who have measured various sound components over the years. So not only can I potentially benchmark my particular components against themselves, but I can compare to other peoples rigs too. More power of the Web in action.

Right Mark was devised predominantly to measure the quality of sound cards. So I’ll need a sound card. Here I’m going to be using an M-Audio Audiophile sound card. I chose the card simply because I have one, and because I’ve seen others such as Ti Kan (see below) use the same card for the same purpose. It does have some very decent specs especially in the area of noise – which is  a large part of why I own it.  I’d previously blogged about this card on my EEE PC blog so I’ve some familiarity with it. Some of the interesting specs are:

Dynamic Range, Inputs: 109dB (A-weighted)
Dynamic Range, Outputs: 107dB (A-weighted)
THD (at 0dBFS): -92dB (typical)
Frequency Response: 22Hz – 22kHz, -0.4,-0.4dB

Of course, the sound card itself does introduce some “colouring” into the measurement chain and I’m using the USB version so potentially I will need to be careful with drivers to ensure that latency does not become a problem.  I’ll have to perform a loopback test to understand the degree of colouring. At the very least, I will know if any of my amps has better specs than the sound card itself.

No project of mine would be complete without some home brew. I don’t intend to simply connect up the components using standard connectors, but I need to introduce some loading along the way to put the amplifiers under some duress – as they would be in actual use. So for that I’m going to to design a small circuit to simulate the load of some typical headphones that  I use.

The next post I will look at the design of this set-up.

Thanks to Ti Kan (AMB Labs) for the inspiration for this (and for his continued devotion to online DIY community).

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