Allen&Heath GS3000 - Manual (Page 15)

GS3000 U SER G UIDE 15
M
IC
/ G
UITAR
V
ALVE
I
NPUT
Two high quality patchable SVT (symmetrical valve technology) valve mic/line pre-amps are included in
the master section. They have been designed to give a pleasing rich warm sound with a highly expressive
high frequency response and subtle compression/soft clipping when driven with high signal levels. A
valve ‘drive’ control, varies the amount of signal being amplified by the valve and a 3 colour LED indicator
illuminates green when the signal level is greater than –15dB; yellow, when the signal level is greater
than 0dB and red, when the signal level is greater than +15dB.
In mic/line amplifier mode, the double triode valve is configured differentially ('balanced' mode) in the
circuit so that one triode is amplifying the positive half-cycle of the signal and the other triode is amplifying
the negative. The advantages of this symmetrical design is that it maximises the characteristics of the
valve, slightly compressing and then soft clipping both half-cycles equally, giving a more natural
sounding, high gain, very low noise pre-amp. The 'soft clipping' characteristic ensures that only low order
harmonics are produced when the amplifier is overdriven with a high transient peak signal.
With the push of a button the valve is configured as a guitar input stage, the valve is switched to be 'single
ended' ('unbalanced' mode) which means that in true guitar amp fashion the signal is not balanced and
the non-linear overdrive characteristics of the valve are utilised. This means that the positive half cycle of
the guitar signal will distort differently to the negative half cycle when overdriven or 'cranked up to eleven'
giving lovely rich second harmonic distortion sought after by guitarists.
The GTR EQ is tailored specifically to the frequency output of most guitar pickups and can be used to
overdrive the valve at a certain frequency.
The input source, either Mic, Line or Guitar is plugged directly into the valve stage.
+48V – selects phantom power to the XLR input.
LINE – selects the LINE jack input on the rear connector panel and adjusts the input
preamp for line level sensitivity. In the up position, the XLR MIC input is selected. Both
these inputs are balanced.
GAIN – Use this control to adjust the input sensitivity to match the connected source
(e.g. mic, line or guitar) to the console operating level (0dBu). Press the PFL switch to
check that the signal reads an average ‘0’ on the meters.
GUITAR MODE – selects the GUITAR input on the rear connector panel. This also
configures the input for a very high impedance (Hi Z) suitable for direct connection of a
guitar. A direct injection (DI) unit is not required.
EQ – This EQ only affects the signal when guitar mode is selected. It is tuned to guitar
sound characteristics and is pre-valve so that the valve can be driven by the EQ. The
swept mid frequency band has a peak/dip (bell shaped) response, which means that the
maximum boost or cut occurs at the selected (centre) frequency. The centre frequency
can be swept over a wide range (120Hz to 6kHz) and can boost or cut by up to +/-
10dB. The centre flat position is detented for quick resetting.




10 0

 


   
0
- 10 + 10
120Hz 6kHz
1kHz
510Hz


(GTR)





- 10
MIC
- 5
40  60 10
40 20
30
20
50
0
10
Guitar mode EQ frequency response
Guitar mode EQ No EQ, HI-CUT filter in