In Part 1 of this article, I outlined how to conduct useful listening tests. Now we’ll look at testing specifics, beginning with software.
The easiest type of listening test to set up is comparing two music files in a multitrack digital audio workstation (DAW) program. Put the files on separate tracks and then link the track solo (or mute) buttons while they’re in alternate states, so clicking either button instantly toggles between tracks A and B.
These are just some of the things that can be compared using DAW software:
• Recordings made through different converters and sound cards
• Lossy compression versus an original Wave file
• EQ and other plugin types when all parameters are set the same
• A hardware emulation plugin to a recording of the same source sent through the original hardware
• Different dither algorithms or even dither versus no dither (truncation)
• The same Wave file played from different hard drives
• The recorded output of different preamps (described below)
• Assessing whether sample rate conversion affects the sound audibly
• And, assessing the degradation of one converter via multiple record generations using a loopback connection
In most of these cases, the files being compared will be the same volume. But if you’re comparing different converters or multiple loopback generations, you may need to adjust the level of one track.
All DAWs can vary the volume in 1 dB steps, but that’s not good enough for matching levels. If a DAW doesn’t have a fine enough resolution to match levels to 0.1 dB, the free Sonalksis Free G plugin fosters setting the volume in increments as small as 0.01 dB.
Human hearing memory is poor so it’s important to switch between the two sources as quickly as possible. This is easy in a DAW program where you can instantly alternate playback from one track to another.
But when comparing gear such as preamps or power amplifiers, the output cables – and possibly the input cables – must be switched, which takes longer. Some people believe it’s better to listen for a few minutes and then switch; I disagree because our limited hearing memory makes comparing more difficult.
But if that’s their belief, let them do as they please, even if it means they have less chance of being correct! Anyone who insists they can hear such an obvious difference should be able to tell A from B no matter how long they listen or how long it takes to switch.
When comparing hardware, cables can either be changed manually or via a suitable switch. Passive switchers aren’t expensive, or you can make your own. However, levels must also be matched, which is more difficult if neither device has a volume control. Many power amplifiers don’t include a continuous sensitivity control, so there must be a way to attenuate the level into whichever amp is louder.
Figure 1 shows how to wire a potentiometer to create a simple passive (no added noise or distortion) level control that can optionally be put into a small box with connectors. A dual control could be used to adjust both channels for stereo, but that’s not recommended.
First, it’s easier and more direct to compare music coming from only one loudspeaker because it minimizes slight acoustic imaging changes as you move your head. Just as important, the left-right channel tracking accuracy of any dual level control will surely be off by more than 0.1 dB at most levels.
For line level devices, switching input wires can usually be avoided by simply splitting one signal to feed two devices using a Y adapter, for example, to compare power amplifiers. But the loudspeaker outputs still need to be switched so only one amplifier is connected to the loudspeaker at a time. A/B loudspeaker switches are available commercially or you can make your own for even less cost.
The DPDT switch in Figure 2 accepts the plus and minus outputs from two separate power amps, sending one or the other to a single loudspeaker. DPDT stands for double pole (switches both the plus and minus wires) double throw (sends to either of two possible amplifier sources).
Aside from hardware that adds intentional color, you’ll probably be listening to compare which device sounds clearer and with a correct tonal balance. So it’s important to keep signal levels reasonable to be certain peaks aren’t being clipped even a little, because that obviously affects audio quality. The best way to monitor levels for clipping is with an oscilloscope connected to the device output, but few people have one available.
Unlike loudspeakers, most electronic gear is very clean right up to the point of hard clipping. So you could increase the input level until you just barely hear slight distortion, then back down by 6 dB to be sure you’re within the device’s linear region.