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THEORY OF OPERATION
The US Audio Mix 6 has four mic or line input channels and two stereo aux line inputs. All of the
XLR and 1/4” input and output jacks, except aux 1, are active electronically balanced circuits. The
The output section of the Mix 6 was designed to accommodate any configuration, line or mic level
input sections of channels 1-4 on the Mix 6 use the relatively new Analog Devices 2017 integrated
balanced or unbalanced XLR and balanced or unbalanced 1/4” connectors. Each channel's output
circuit, which was designed to yield high amounts of gain with the lowest possible noise. This IC,
jacks are individually buffered so any combination of the outputs, at any impedance, will not affect
along with careful component selection, a toroidal power transformer, and true star grounding on a
the other outputs. With the Mic/Line output switch in the Mic position, the XLR's output is reduced
2 ounce copper double sided circuit board, enable the Mix 6 to achieve a working signal to noise
by 20db (this switch does not affect the 1/4” jacks). The Mix 6 uses an H-Pad resistor network at
ratio (S/N) of 92db with 22dB of headroom at unity gain. The Mix 6 holds a working S/N ratio of
the XLR to maintain the S/N ratio, regardless of the position of the Mic/l.ine switch. For the XLR
85db at 40db of gain and can provide up to 76dB of gain, still maintaining 10 to 15db of headroom.
outputs, if ground lifting of pin 1 is desired, jumpers JMP 6 and JMP 7 inside the unit can be clipped
With the mic/line switch is in the mic position, the 2017 is factory set at 36db of gain by an internal
off. Then, in Line mode pin 1 of the XLR is completely lifted and in Mic mode the XLR's pin 1 is
resistor. This 36db plus the two sections of 20 dB available from each of the channel volumes and
ground isolated and referenced to pins 2 and 3. If unbalanced operation is desired from the XLR
master volume offer a total gain of 76db. For applications which need even more, the Mix 6 can be
outputs, pin 2 is hot, pin 1 is ground, and pin 3 should be left UNCONNECTED.
ordered or modified to obtain up to 50dB of gain from the 2017 via a simple resistor change.
LED dB meters provided on the Mix 6 are precisely calibrated at 3dB increments and span a range
All the gain controls on the Mix 6 (Channel gain, Aux and Master) have a range of -60 to +20dbm.
of -15 to +l2dBm measured unbalanced at the 1/4” output. The output of the XLR, with the switch
With the input and output switches in the Line position and with both the channel and master
in Line mode, is 6db greater, measured balanced.
volume controls at the same position, unity gain is at the 9 O'clock knob position, 10dB gain at 12
O'clock, and 15dB gain at 3 O'clock. The 60db of attenuation in the off position allows the Mix 6 to
adequately turn off line level signals. Maximum signal level through the Mix 6 at clipping is
The limiter section of the Mix 6 is pre master fader, allowing versatility in selection of output
+28dBm balanced and +22dBm unbalanced. The sonic figures apply to both the input and the
levels while the peak limiter is active. The limiter is true stereo, with separate detectors for each
output and individual clip LEDs are provided which illuminate at -3dB below actual clipping. The
side, which engage at 0dB and limit up to 15dB. The left and right LEDs on the front panel
OL (Over Load) circuit senses the signal level in 2 places (input diff amp and after channel gain) to
illuminate when the limiter is working and get brighter as limiting increases. With a quick attack
ensure foolproof visual indication of distortion.
time and moderate ratio the limiter is ideal for open mic situations where unexpected level changes
can occur.
Channels 1-4 have a pan control. The center detented, dual element, audio taper pot provides a
wide range of stereo positioning for the input signal. The center detent allows quick centering of
The Mix 6 has a headphone monitoring section consisting of a source select switch, a volume
the signal. Also included on channels 1-4 is a low frequency rolloff switch for elimination of sub
control, and a 1/4” TRS headphone jack. The headphone circuit is stereo and drives any
frequencies such as mic handling or wind noise. The cutoff frequency is 120Hz with a 24dB per
headphones with 20 Ohms or greater impedance. The volume control has a gain range of -60 to
octave slope.
+20db to accommodate a wide range of audio levels. The select switch allows listening to either
the prefader stereo master outputs or the postfader signal coming from the aux 2 inputs.
Aux inputs one and two are true stereo line level inputs. Aux input 1 is via unbalanced RCA jacks.
Aux input 2 is accessed through left and right balanced 1/4” TRS jacks, which can be used
The Mix 6 utilizes a dual primary toroidal power transformer which is configured with internal
balanced or unbalanced. Aux 2 has a unique main/off switch which routes the input signal to the
jumpers for 120 VAC 60Hz or 230 VAC 50Hz operation. There is an internal fuse on the hot side of
left and right stereo busses when activated. This feature allows aux 2 to be disconnected from the
the AC cord, and the power switch makes and breaks both the hot and neutral legs of the AC cord. A
main output and in combination with the headphone main/mon switch, routes the signal to only the
ground lift switch is provided which can disconnect the audio circuit ground from the AC chassis
headphones for cueing or monitoring. The Mix 6 aux inputs will interface easily with most all
ground.
types of consumer or professional equipment.