How to best compare mic preamps?

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I want to practically compare two mic preamps and I've been thinking about how to best do this. I don't need a "scientific" comparison as such, but I'd like the test to be somewhat detailed and trustworthy anyway. I don't think the difference between the amps is all that great so I'm afraid it'll be easy to get fooled by variations in performance, such as distance to mic (proximity effect), mic angle, volume of voice and by difference in metering and input/output level on the amps. (I'd use the same mic for comparing the amps obviously!)

How would you guys go about this? Would you e.g. record something prerecorded from a speaker rather than new performances every time? Would you use different types of sound sources, and maybe multiple recordings with different types of mics?

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I would (and I have) use two matched microphones or a stereo microphone (two capsules in one body) with adjustable capsules, recording once, then switching between the outputs to each preamp and recording again. Don't use any "standard" axis setup. Put one right above the other, as close as possible, pointing to the same horizontal AND vertical direction. If you have a small sound source that you want to place very close to the mics (less than ~15 cm), then make sure its position in the Y axis is between both mics.

It can depend on the mic a lot, but in general it's better not to record from a very close distance, because wind hitting each mic differently, and sound waves coming from so close, can make an audible difference. If you use a stereo preamp with matched channels, then with not very close recordings, you'll most probably find that the recording sounds identical with both mics, because small height difference has no audible effect, and the difference between the matched mics is insignificant.

Although one recording session would probably be enough, you could provide two recording sessions (one with switched mic into each preamp). This is as pedantic as it gets. Mic splitters always degrade the signal, often significantly. Finding a really high quality splitter, which can handle mic impedances properly and doesn't "color" the sound, seems almost impossible, so I wouldn't bother with that. I think it's better to get a hold of matched mics for the test than trusting a mic splitter.
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I would maybe choose the playback through speakers and re-record the same mike put into each preamp. Then it's exactly the same spot mike is - be careful not to touch one mm even.

But take some time to really match peak levels being the same at playback, it does not take more than 1dB for them to sound different just because listening level. Our ears work that way giving different frequency response depending on level. Higher level tend to sound better.

And different material, just voice, classical music and something modern.

Then you got it all on different tracks in a daw and can switch and compare as you like.
You can slide one track 2s forward, so when you switch to solo the other you get the same section and it's much easier to hear differenses.

And you can loop a section to really listen into just that part and listen for anomalies.

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I think you really have to ask yourself "to what end" are you doing this comparison? If it's not scientific or some other reason for a detailed comparison, then you really should consider if it's worth the effort, or how conclusive of an answer you expect to find. A/B comparisons have limited utility IMO - do you really need to qualify or quantify the minutiae of differences between two preamps and, if you do, is that going to help you decide on a preference any more than just using each one separately would? After all, there's nothing inherently wrong with being a little subjective when choosing mics or preamps. There are some intangibles that go into selecting preamps, mics (and combinations of the two) and that's why we don't usually base that decision solely on things like frequency response charts. Then again, maybe you do have a good reason for this comparison and I'm out in left field here, but that's my 2 cents.
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Dunno the best method. Unless you have a very good recording room, if one mic is even an inch or two different location in the room, the difference in acoustics may be more significant than the difference in preamp sound.

I doubt that there is an overall "best preamp". Microphone A might sound best with preamp X, wheras mic B might sound best with preamp Y. I suspect some of that variation may have to do with the way the electrical characteristics of each mic interacts with the electrical characteristics of the preamps.

For instance, sometimes it may be found that transformer mics may work a little smoother into transformer preamps, and sometimes transformerless mics may work a little better with transformerless preamps.

When I've tried to compare mics and/or preamps in the past, sometimes would wear headphones and set up a bunch of mics a couple of feet apart from each other, then sing into the various mics. Play hand percussion or guitar into the verious mics (which can show rather obvious differences in transient handling).

Just go experimentally between the different setups trying to discover the sonic differences in the headphones.

Hardly scientific, but a way to get some idea of what might work best.

Noise and distortion are deal-breakers for me, but some people apparently like certain distortions and don't mind a little noise.

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Thanks a lot for the input guys, this is hugely helpful! Loved the tips so far!

The reason I want to do this comparison btw, is that I have access to one good condenser mic (for my voice anyway) and two (actually three, counting the console) different mic preamps. I'm about to put a lot of time and effort into recording vocals in an upcoming project and I don't want to learn during or after the project that I made the wrong choice in choosing the preamp. I want to feel confident I'm recording with best available quality from start. Knowing how easy it is to get the ears fooled when doing AB-tests, I thought I'd start by gaining some wisdom.

Also, I thought the topic itself was quite interesting. After all, the mic & preamp is the very start of the signal chain and it's a golden rule to always start with best possible quality and never count on processing tools for improvements. Doing AB-tests of signal processors is often easier, isn't it, running the exact same audio through them, matching levels and settings and switching back and forth? I think comparing mics and preamps requires more thought and preparation.

Again, thanks for helping! :tu:

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Record a drum machine through both. You'll be able to tell apart the punch and it's a consistent variable.

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A "clinical scientific" low noise transformerless discrete preamp, maybe even such preamps mass-produced into nice mixers, coupled with a quality transformerless FET small diaphragm mic, might give the "highest quality" result, if quality is defined as low noise, flat frequency response, good transient response and wide dynamic range.

However that may or may not equate to what the artist or engineer's ears expect out of "good sound".

A perfect accurate pristine reproduction of a bad vocalist might not be appreciated. :) It is as much art and taste as it is science.

I don't record much lately, but back in the day tended to find "good sound" out of sopranos most difficult to capture, and "good sound" out of bass and baritone singers easiest to capture.

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Bang your head against each of them five times. The one that doesn't instantly crumble into a thin paneer of dust and grease is the one. Legit advice.

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