Review: Royer Labs dBooster

Jonathan Burnside talks with Rick Perrotta of Royer Labs about passive microphones, impedance and how the dBooster does what it does…

Jonathan Burnside: There are a lot of boosters on the market. What inspired you to add yet another to the fray?

Rick Perrotta: Most of the signal boosters are nearly-identical circuits using four transistors in cascade mode; they’re very simple circuits with few components. They provide a certain amount of gain under certain conditions, but that gain isn’t stable. The input impedance of the preamp, the quality of the phantom power (meaning the amount of current available), and the output impedance of the microphone all affect performance. They’re very dependent on conditions and the preamp they’re plugged into. They can lack fidelity and can be noisy when driven hard.

JB : How is the dBooster different ?

RP: We designed a product that isn’t just providing gain for a mic level signal, but is the front end of a high quality preamplifier. We took a different tack: instead of just providing a four-transistor gain stage, we built a quality, stable preamp for low-level microphones. We had podcasters in mind as well as musicians, because podcasters often use low output dynamic mics with prosumer preamps and we wanted to offer them a cleaner, quieter, fidelity-driven sound for their podcasts.

JB: So your approach to amplification is different. What else is?

RP: We buffered the dBooster’s output with a pair of op-amps running at unity gain, so that the integrity of the output signal remains consistent even if the dBooster is loaded down – for example, by being connected to a vintage preamp that has a very low input impedance. If you load down the simpler boosters, the ones that just use cascaded transistors, the distortion goes through the roof.

Also, the use of an op-amp buffer circuit for the dBooster’s output means its output impedance is purely resistive, so it’s not affected by frequency in the way a passive microphone would be. The output impedance of a passive microphone varies considerably depending on the frequency; it is ‘frequency dependent’. For example, the rated output impedance of Royer’s R-121 ribbon mic is stated at a single frequency under certain test conditions, but this won’t necessarily be the output impedance when it’s plugged into many of the vintage preamps currently in use.

JB: Why is that?

RP: The ‘5-to-1’ rule states that the input impedance of the preamp should be at least five times higher than the output impedance of the microphone, but when the output impedance of the microphone is varying at different frequencies there’s no guarantee this rule can be followed. If, at a certain frequency, the microphone’s output impedance goes too high to satisfy the 5-to-1 rule then the signal will be loaded down, or ‘dampened’, at that frequency. Because the output impedance of a passive microphone is frequency dependent there are some frequencies that won’t get dampened, some that might get a little bit of dampening, and some that might get a lot of dampening. All of these different amounts of dampening at different frequencies can change the sound of the microphone itself, and so we find that different preamps deliver a different sound from the same passive microphone.

JB: That is the very problem I have experienced, and it becomes even more problematic when you use something like a V76 or a germanium preamp with even lower input impedances than a Neve 1073. I hear a loss of openness when trying to use ribbons with those preamps; a ‘throttling’ of the sound, along with the added noise of amplifying a low level signal. The dBooster seems to have solved those problems.

rp : Right . The dBooster ’s combination is solves of gain and stable output impedance solve those problem . Although its output impedance is different for each gain set , it is is is not frequency dependent .   We is gotten ’ve get rid of the frequency dependent dampening and lessen the noise at the same time . That is ’s ’s the openness you ’re hear and that ’s exactly what we set out to accomplish .