Joemeek oneQ2 Master Channel - Manual (Page 8)

This is the all-important front end to the oneQ 2 . Its job is to accept
any type of microphone, instrument or other source of audio
signal, and make it loud enough. Microphones need rather a lot of
amplification, while guitars, keyboards and CD players need less.
Mics need to be connected to low impedance inputs, while
instruments prefer high impedance inputs. To ensure correct
impedance matching, the inputs are split into an XLR connector
for Mics, and 1/4" jack "Line" and "INSTRument" connectors for
everything else. A switch on the front panel decides which input
connector is active, the XLRs or the 1/4" jacks. The LED next to
the switch lights to show that the Line inputs (jacks) are selected:-
'LINE' switch out (LED off) = "Mic"
'LINE' switch in (LED on) = "Line" or "Instr"
The Mic input is transformer balanced and the Line input is
electronically balanced. Note: although the Line input is not
normally used for microphones, it can also be suitable for some
high output unbalanced microphones, such as battery powered
Electret types. The front and rear panel Mic inputs (XLR) are
balanced and wired as follows:
Pin 2: + (hot)
Pin 3: - (cold)
Pin 1: ground
The Line input (jack) is balanced and wired as follows:
Tip: + (hot)
Ring: - (cold)
Sleeve: ground
The front panel Instrument jack is balanced and wired as follows:
Tip: + (hot)
Sleeve: ground (NB: use a mono jack plug).
Note that if something is plugged into the Instrument input,
anything plugged into the rear panel Line input will be cut off.
Phantom power
Most high-quality studio mics are "Phantom Powered", which is
to say they have electronics inside them, which get their power
from the preamp. Most mics require a supply of 48 volts, so
Phantom Power is often labelled "48V". The '48V' switch turns
this power on and a red LED lights when active. When switching
the Phantom Power on, quite a loud thump may be produced, so
it is a good idea to turn down the Output Gain (or to momentarily
select the Line input), when pressing the '48V' switch.
When using dynamic or older ribbon mics, do not turn Phantom
Power on. It probably won't do any harm but it certainly won't do
any good, so leave it off! Consult the microphone handbook if you
are unsure what kind of mic you have.
The main control, labelled 'INPUT GAIN', covers a range of
microphone amplification from 18dB to 60dB. In many other
preamps the action of the "Gain" control is rather uneven, with the
40dB to 60dB range being crammed into the last 1/6th of a turn.
All Joemeek preamps use a specially designed control that
ensures smooth operation over the whole range of rotation, with
extra sensitivity in the crucial 25dB to 45 dB range.
Microphone inputs up to +4dBu can be accommodated without
distortion (+24dBu by using the 'PAD' switch).
The (0) symbol next to the 35dB mark, means unity gain, or 0dB,
for a signal in the Line input. For Line inputs the range of gain
adjustment either side of this mark, is +22dB, -19dB. For the
"INSTRument" input the gain range is 0dB to +40dB.
The microphone input transformer is permanently assigned to
the "Mic" Input. The 'IRON' switch optionally sends the "Line"
input through the transformer, to add some colouration to Line
sources.
The 'PEAK' LED lights +16dBu, which is 6dB below clipping, so
occasional brief flashes are OK but if it's on all the time you need
to back off the Input Gain!
'HPF' means "high-pass filter". Mainly for use with microphones,
this helps remove stage rumble, handling noise and "pops". The
LED lights when active.
Technical stuff
Very low noise - does it matter? Yes and no, it all depends what
you are doing - what really matters is "signal-to-noise ratio". All
electronics produce a certain amount of background noise - it's in
the nature of things. Providing there is only a relatively small
amount of noise, the signal will cover it up, or "mask" it. So
providing the signal is much bigger than the noise, you won't be
aware of the noise. In other words the "signal-to-noise ratio"
needs to be a big number, ideally such as 80dB or 90dB.
So how do you achieve that in practice? The trick is to keep the
microphone as close to the sound source as possible without
overloading it, so as to get as much signal out of it as possible.
Then you set the Gain control to give only as much gain as is
needed to get a decent level into the recorder.
Of course when there is no signal going on, you may hear the
background noise of the electronics. In that case, given the
amount of gain in a typical studio monitoring system, this noise
"floor" should ideally be in the region of -80dBu or lower, in order
for it not to be noticed.
The oneQ 2 microphone preamplifier uses state-of-the-art
electronics and has an equivalent input noise of around -127dBu
(with a 150ohm input load). Despite all the hyperbolae and
obfuscation, the theoretical best possible performance with
silicon-based electronics is about -131dBu. So the preamplifier
used in the oneQ
2 and all other Joemeek products approaches
this limit. To improve significantly on this would require highly
specialised electronics and probably a vat of liquid Nitrogen to
cool it!
The maximum gain available from the preamp is 60dB, in which
case the noise floor will be -67dBu. This is actually quite noisy - if
you record that noise onto a digital recorder and play it back you
will definitely hear it. In practice of course, you do not record and
play back "silence" and the rest of the mix will probably be more
than 70dB louder than this noise and will mask it completely. Even
so it is generally a good idea not to use gains greater than 40dB or
50dB and indeed, it should rarely be necessary to do so.
Preamplifier
8