Cables

Cables are easy to compare – simply plug and unplug line level RCA, XLR, and 1/4-inch phone cables to swap them. Loudspeaker cables are also easy to compare, though it helps if they have banana plugs at both ends to allow switching more quickly than connecting stripped wire ends to binding posts.

Converters

Comparing sound cards and converters isn’t difficult, and it doesn’t matter if both converters are connected to the same computer or not. All that matters is recording the same source through both into suitable software.

To compare the A/D sections, split the output of your source – whether a mic preamp or media player – using a Y adapter to send the same signal into both converters at once. It helps if at least one converter has an input volume control, otherwise insert the passive volume control shown in Figure 1, after the splitter, into whichever converter records hotter.

Even though you’ll match the playback volumes when listening critically later, I suggest recording at similar levels too. Most modern converters are highly linear over their entire range, but a record level around –6 dBFS through both converters guarantees a clean capture.

Of course, you could also record through both at –20 or even –30 to see how the converters compare when recording at lower levels. I suggest recording a 1 kHz sine wave from a smartphone or media player into both converters too, before the music starts. This helps in setting the record levels during the test and matching the playback levels later. (A 1 kHz Wave file that loops seamlessly is available free from my website by clicking here.)

Comparing playback through the D/A part of a converter is even easier, though again it helps if one or both have output volume controls. Adding the 1 kHz tone to the start of the music before making the audition recordings means the tone will stay attached to the music throughout the process. Then it’s easy to match levels using a voltmeter or a recorder with an analog VU meter as noted earlier.

Preamps

Microphone preamps are surprisingly difficult to compare because it requires more than using a Y splitter to send one mic to both units. That can be done, but the increased load on the mic from driving two preamps in parallel is different than when only one is connected, which could change the sound. Some preamps have a variable input impedance that further confounds matters.

The mic’s output could be split using a special transformer having two secondary windings, but that too might change the sound more than the preamp differences. Therefore, a better method is a re-amp setup that lets you create identical sound sources repeatedly.

Re-amp is short for re-amplify, where a recorded source is amplified and played through a loudspeaker to be recorded a second time. Rather than recording live performances that will surely vary, record one performance to create a new source, then play that through a quality full-range loudspeaker. Unless you have access to a professional studio with serious isolation between rooms, it’s necessary to record each preamp’s output anyway to compare later in a DAW program through loudspeakers as described earlier.

Figure 3 shows a re-amp setup in my home studio using a JBL 4430 monitor and DPA 4090 microphone two feet away pointed directly at the horn tweeter. A loudspeaker playing music isn’t the same as an original recording, but that doesn’t matter. The source simply is what it is, and a clean loudspeaker source is as valid as any other to audition tonal differences between preamps.

Before using a re-amp setup to compare preamps, it’s necessary to set both preamps to the same amount of gain by playing a test signal through the loudspeaker.

The noise level and sound of a preamp can vary slightly with different amounts of gain, so this ensures a fair comparison. Since the loudspeaker and mic won’t move while matching levels, either pink noise or a 1 kHz sine wave can be used as the test signal.

Start by placing the mic a few feet in front of the loudspeaker, at high-frequency driver height and close enough to avoid picking up too much room ambience. The best type of mic is a small diaphragm omni condenser because they tend to have the flattest and most extended response. Otherwise, just use the best mic you own or can borrow.

Next, connect the mic to one preamp and play the test signal through the loudspeaker at a reasonable volume, i.e., 80 dB SPL. The gain amount needed depends on the mic’s sensitivity, so adjust the gain for a reasonable record level. Then connect the mic to the other preamp while the test signal continues to play through the loudspeaker and set that preamp to record onto a different track with the same amount of gain. As long as both preamps are recorded at the same reasonable level, the final level matching can be done later.

Finally, play the test signal and then the music or other source through the loudspeaker while recording the microphone through one preamp. Then switch to the other preamp and record the same test signal and music source again. Now there are A and B tracks in your DAW that can be level-matched and played at random for the blind test.