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Focusrite Octopre LE microphone preamplifier - introduction

Focusrite Octopre LE microphone preamplifier – introduction

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The Focusrite Octopre LE is clearly designed as a front end for a digital audio workstation. In this context, eight channels is just enough, although lacking that extra one you would need for an under-snare mic on a five-piece drum kit. Still, 9-channel mic pres are thin on the ground, and you could always buy two units to supply sixteen channels of input.

Before the days of digital audio workstations, we used to use devices called 'mixing consoles'. If you remember, mixing consoles came with twenty-four or so channels, each of which was fitted with a microphone preamplifier. Mic pres were not considered anything special in those days!

Any decent microphone preamplifier should be able to accept line-level inputs too, and the Focusrite Octopre certainly is. The mic inputs are round the back on eight XLR connectors, the line level inputs are on jacks on the front panel.

This layout works for me. I have the mic cables permanently connected at the back, and whenever I want to connect a line level source, I simply plug in and the mic connection is overridden. Very convenient.

It is common also to want to connect an instrument such as an electric guitar. Guitars have weak and feeble outputs compared to line-level sources and needs special 'high impedance' inputs that don't drain much current.

Two of the Focusrite Octopre's channels have 'inst' switches that allow electric guitars to be connected without the need for a DI box. I guess that the reason that only two channels are so equipped is due to panel space restrictions. And who would want to record eight guitars simultaneously anyway?

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The common features of all channels are the 'level' control calibrated from 0 to 10. Odd this, because it is really a gain control and should be calibrated in decibels. The difference between the terms 'gain' and 'level' is subtle. Gain involves an increase in signal voltage, a level control such as a mixing console fader reduces the voltage. (Where you see a fader with calibrations up to +10 dB, there is a 10 dB gain stage before the fader itself, and the zero position of the fader reduces the signal level by 10 dB again).

The range of gain of the Focusrite Octopre is actually +13 dB to +60 dB. It is true to say that other mic pres have a wider range of gain. At the low end you would have to ask the question whether an operatic soprano singing at close range into a high output mic might clip the output of the Octopre. But you would probably be sensible enough to use a mic with a pad. At the high end, if you need more than 60 dB of gain then the sound source must be exceedingly quiet or distant, and you would probably end up amplifying noise generated by the mic, or ambient noise from the room.

Each channel is very sensibly provided with a low frequency cut filter with a cut off frequency of 120 Hz and a slope of 12 dB/octave. You can adopt the philosophy of not switching them in until you encounter a problem with low frequencies, such as an overenthusiastic performer occasionally kicking the mic stand in a live recording. Or you can leave them switched in for all sources except bass instruments, which cuts low frequency clutter.

One feature that some might describe as an omission is that only channels 1 and 2 have phase buttons. So if you find yourself with a phase problem on any of channels 3 to 8, you would have to reconfigure. However, having one out-of-phase signal might be considered unlucky; two would signify carelessness; three or more should set alarm bells ringing.

Continued in other parts of this series…

David Mellor

Introduction to Audio Recording

Introduction to Audio Recording

You can't learn this type of info anywhere but here! Pro audio engineers share deep secrets about how they choose microphones for common recording situations.

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David Mellor

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