Do I need a preamp for recording my synth?
Short answer: ‘need’ is a strong word, but your synth tracks would certainly benefit greatly from a preamp. Simply put, adding a preamp to your synth lines will make them come through a little stronger and have more impact on the mix. Any preamp, whether solid state or tube, is designed to make your signal louder.
We have done some A/B testing with one of our favorite analog synths, the Korg Minilogue, and found that a preamp or external tube amp helps the synth to stand out. It goes without saying that you will be using a wide range of synth voices, so where and when a preamp will benefit the most will vary. Synths are used for rhythmic beats, pads, harmonic accompaniment, lead-tone lines and leads. We would make that case that, used properly, a preamp can add punch or presence to all of these applications in varying degrees, all with a positive outcome.
What does compression do for my sound? Many tube preamps include a compressor, which can be a useful feature. Compression mainly affects dynamics. In short, a compressor creates a more even level of audio by allowing the user to increase the gain/volume of the signal while the compressor actively compresses the peaks (loudest parts) of the audio avoiding clipping. For example, compression helps your lead synth part achieve consistant dynamics across the line. Moments where your notes nearly clipped will be smoothed out, and lower notes that may have been given less velocity in the performance will have a little boost.
What about simply using a tube preamp that does not have an optical compressor? Tube amps are loved for the subtle warmth they impart to the audio that goes through them. When a tube amp starts clipping it has creates peaks that ease into the clipping which actually pleasing to the ear, whereas solid state devices go into clipping more abruptly and thus sound a little harsher. Because tube circuitry commonly imparts natural compression to an audio signal, you will still get some compression simply by using a tube amp.
What? Tube amps naturally compress audio? To a degree, yes. Sometimes the power supply in a tube amp cannot deliver enough current to the power tubes to fully reproduce the louder notes, thus inducing a subtle and pleasing compression. Additionally, if your tube amp features an alnico speaker - which most vintage tube amps do - the alnico magnet itself will impart some further compression when the speaker is being driven.
What is tube ‘coloring’ that I keep hearing about? When a tube amp is overdriven, it produces a lot of second order harmonics. This phenomenon is one way in which tubes ‘color’ audio. Second order harmonics are an octave above the fundamental and they make the sound appear fuller. It should be noted that this ‘coloring’ is subtle but it is there.
How about impedance matching? You can certainly safely plug a keyboard or synth into a tube amp or preamp. Most tube amplifiers’ input impedance is in the range of 100kohm. Though many keyboard manufacturers aren’t listing their output impedances, because it is a non-issue, digital and analog keyboards and synths are in the 1kohm range. This falls nicely in the low output / high input impedance rule range.
In cases where you are using a recording preamp that has active compression, or a tube amp with natural compression, your synth signal will be more defined and impactful. Even if you are doing soft pads that will remain back in the mix it is worthwhile to try processing them with a bit of smoothing warmth that a preamp or tube amp can provide. Of course, if you are looking for a sterile, strictly digital tone - which is certainly a valid style - you can opt out of tube circuitry. In this case, you can look at a tube amp or preamp as a tool in your music making arsenal.
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