13
CIRCUIT THEORY
ANALOG CIRCUITS / SIGNAL PATH
The SR40•8 signal path begins with hybrid (discrete/op-amp) mic/line preamps. After the
mic/line preamp, the circuitry is largely op-amp based. The function of audio op-amp
“building blocks” should be self-evident from the schematics (If not, you need to fold up
this manual, put the screws back in the mixer, and take it to someone who knows
electronics!). This section will explain some of the circuits and architecture unique to the
SR40•8.
VLZ
ARCHITECTURE
To keep thermal (Johnson) noise to a minimum, Mackie mixers employ VLZ architecture.
In VLZ, or “Very Low Impedance”, architecture internal impedances are kept as low as
practical in as many places as possible. Resistor values are scaled down by a factor of 3
to 4.
NJM4560 and NJM2068 op-amps are used to drive the lower internal impedances. These
devices’ outputs are capable of sinking more current than comparable devices.
It is critical that these op-amps not be substituted. Using NE5532 or TL0xx op-amps will
compromise performance.
(Yes, there are some costly devices that outperform the 4560 on paper, but in optimized
VLZ circuitry, they will offer little to no improvement. Besides, do you REALLY want to
replace 600+ SMD op-amps AND void the warranty?!!)
