Table of Contents
Advertisement
and goes through C12 and R15 to U1B's inverting input at pin 6. The right
channel signal is buffered and exits at pin 7.
J2 and J3 can also be used as unbalanced inputs by simply connecting an
unbalanced input to the connector tip and the ground to the ring and sleeve.
The signal will now be applied to the '-' input of the op amps and the op amp
'+' inputs will be tied to ground. This is handy if you have inputs that use
separate left and right cables and mono 3.5mm plugs. Also, when using the
UAM4SYS in mono mode only the right channel, J2, input is used for mono
operation. Again this can then be used either balanced or unbalanced.
From the output of the input buffer the left and right audio signals go to two
different places. First they go to the line output buffer input headers H1 and
H2. They also go to the tone, volume, balance control IC, U3.
Tone/Volume/Balance
U3, an LM1034, provides tone, volume, balance, and loudness control for both
channels of the UAM4SYS. The IC supplies a DC reference voltage for the
bass, treble, balance, and volume controls. These controls are adjusted by
the user and provide individual DC levels to the IC where they set appropriate
levels of each function. Because DC levels are used to control the various
functions the scratchy noise produced by the controls in many audio
components is not a problem in the UAM4SYS.
C15 and C21 set the treble operation points. C19 and C20 set the bass
operation points. C18 and C16 are bypass capacitors for the IC. C6, C7, C8,
and C26 act as filters for the DC control lines. R17, R18, R19, and R20 (sheet
3) are the control potentiometers. R5, R6, R8, and R13 provide isolation
between the control pots and the IC. If you wish to learn more about how the
LM1034 actually works and how component values were selected do an
internet search on National Semiconductor and download the data sheet.
The unprocessed audio enters U3 pins 2 and 19 from the input buffers
through C9 and C4 which are coupling capacitors. The audio is then
processed according to the settings of the control potentiometers and fed to
the right channel output at pin 8 and left channel output at pin 13. From here
the processed audio goes three places the line output buffer headers H1 & H2
(sheet 1), headphone amp (sheet 2), and power amp (sheet 4).
Loudness
The loudness function is turned on when U3 pin 7 is connected to U3 pin 12.
It is turned off when U3 pin 7 is connected to U3 pin 17. Loudness is
controlled by front panel switch S4 (sheet 3) which controls U5A and U6 as
described below.
UAM4SYS • 6
Advertisement
Table of Contents
