I get asked that question a lot. Possibly because in the past four years, I’ve auditioned probably 100 preamps and have produced two preamp comparison projects, featuring multiple sources through dozens of preamps (3D Pre CD in 2000 and the Preamp Summit DVD to be released in 2005). Not only do I spend a lot of time comparing preamps, I extensively researched how to compare preamps by talking to preamp designers and considering different testing methodologies. But let’s consider four ways to compare preamps: Cheesy, Easy, Obsessive, and Efficient.
Plug a mic into each preamp consecutively at similar gain settings and then run the signal through a limiter to match up the levels.Believe it or not, I first came across this setup in a press release for a console manufacturer. But if you’re truly interested in accurately comparing the sonic signatures of different preamps, this isn’t the way to do it.
One of the most important criteria for conducting valid preamp comparisons is level matching. Unless the levels are matched to within 0.25dB, preferably to 0.1dB, the judgments that you make about the sound of each will be based on differing levels instead of the different sonic characteristics. Absolute level matching, which this procedure eschews, is critical for honest results.
Advantage: The hardest part is hauling all the preamps in from the car.
Disadvantage: It’s almost meaningless.
Plug the same mic into several preamps at the same time, by using Y cables or a transformer splitter. Bring the preamp outputs back to the faders on the console to match up the gain, then A/B all the preamps while using the exact same performance.
I asked Paul Wolff, formerly of API and now designing Tonelux recording gear (www.tonelux.com), whether using a transformer mic splitter or a Y would degrade the signal. “Everything degrades everything, but to what extent? The differences are typically that the transformer will lose a few dB and sound a slight bit darker, but not much. The parallel (Y) will lose a few dB and sound a little thinner, but not much. I’ve never had problems multing a mic to two preamps.
“When I want to compare something and get to the differences quickly. I mult the mic to both preamps. The mic will change tone, but it will be the same change in both preamps. Then, I bring them into a good console, flip one out of phase, then null them. The sound that you hear is the difference between the two preamps. If there’s perfect cancellation, then you have a match. If there’s a slight thump, then one mic pre is a little richer in the low end, or one is a bit thin. To find out, engage the channel EQ and re-null the channel. (You may have to switch both EQs in if you can’t get a null, which should also tell you that you have the wrong console!) With one channel set flat, adjust the other’s frequency and boost/cut until you get a better null. ‰
“Now remove the EQ, listen to them side-by-side, and see if you can hear the differences you found with the nulling. If you can’t hear the differences, then your ears aren’t trained enough. Actually, this is a good training exercise to improve your hearing. Now, if you could hear the difference, pick the one that you prefer and go make some money.”
Advantages: Each preamp sees an identical performance. It’s quick and easy to set up.
Disadvantage: Doesn’t work well for more than two preamps.
Set up a single signal path, with high-quality cable used for all preamps, skipping all patch panels and bays. Accurately calibrate all preamps to within 0.1dB using precision test equipment, and then have the talent sing through each preamp and record the results for comparison.
This is the technique we used for the 3D Pre CD. After talking with dozens of preamp designers, I felt this was the fairest and most accurate way to do the comparisons. In a discussion with Dave Hill of Cranesong (www.cranesong.com), he confirmed that the interface between the mic and the preamp is critical. Multing a mic to several preamps won’t give you the most accurate results. Preamps are typically designed to work with one mic, not ganged to several other preamps.
So for my tests, the mic, the distance of singer to mic (which was measured for every performance), the cabling, and every aspect of the recording chain was absolutely identical for each recording. The only thing that changed was the mic preamp.
For level calibration, we used a 1kHz tone piped through an Auratone on a mic stand as the sound source. The test equipment was accurate to 0.001dB. For our testing setup, there were only two variables. The preamp and the performance. We got around the performance issue by hiring one of Nashville’s finest studio singers, Marabeth Jordan. Her performances were so consistent that many people didn’t realize that each of the 33 performances were different.
Advantages: Truly accurate level presentation, identical signal path, what you hear is precisely the difference between the preamps.
Disadvantages: Time consuming, demanding, impractical for anything other than very intense comparisons.
A “real-world” method for comparing multiple preamps. It’s fairly fast and unobtrusive to the session.
For most sessions, the level of accuracy and calibration described above isn’t an option, and may not be necessary. So this is the technique I most often use: Have everything set up before the artist arrives — powered up and ready to go with all output cabling and patching in place. Have one mic cable run to the stack of preamps and each preamp coming back into an input on the console or DAW. (You can check with a tone and make sure each of the ins is identical in level.) Have the singer sing the song through one time and set the gain. Then dismiss the singer for a few minutes while you calibrate the other preamps. Since many preamps don’t have fine trim controls, you may need to fine tune their outputs with the faders. Using a steady-state source, white noise, a stationary buzzer, or a radio tuned to inter-station noise, set each preamp so that the level on the console output meters is consistent. Analog meters work well because of their fine resolution. If you have digital meters, you can set them for peak-hold. Note: The accuracy of your calibration can’t exceed the accuracy of your meters. So if their max resolution is 2dB, you may have to match the final level by ear.
Once the levels are matched, have the singer sing the same section of the song as many times as you have preamps. I suggest a short section so as to not wear them out. If the dynamics or range are greatly different between verse and chorus, use as much of the song as you need to adequately represent their voice during the song. Since the levels are already calibrated, you should be able to run through six preamps in as little as five minutes, assuming a 30-second audition on each. Most producers will consider the extra 5 or 10 minutes this takes as a worthwhile investment, since it will ensure the best vocal sound; plus many singers get an emotional boost knowing that someone cares enough to take the time and go for the best.
Advantages: Fast, easy to do in a real-world session.
Disadvantage: Not quite as precise as the Obsessive method.
Consider all these auditioning scenarios and see which works best for you. Preamps in the studio are like paint colors to an artist. It’s hard to ever have enough. They’re a major part of any recording and it’s worth the time and effort to match to the mic and the performance. So happy preamp hunting. One word of caution from someone who’s done this many times before: The more you try, the more you���ll buy.