Service manual

2 Channel Mic Line Input Board (SPN815-121)
CARD DESCRIPTION
Only one board in a module pair controls and reads the front panel board via a ten way IDC
cable connected to J3. This cable carries switch voltage information and serial data used to
load the LED's on the front panel board.
CMRR for the two channels is trimmed by adjustment of VR3 and VR4. Apply the same signal
to both positive and negative balanced inputs of mic input A on each channel in turn and
adjust the preset for minimum output level to the console on that channel. Alternatively on
Issue 4 boards there are test points CH1P and CH1N (CH2P and CH2N) which are
balanced outputs which can be monitored.
Early modules had the headroom set in the gain control IC (U45) controlled by serial data
and clock from the processor. This gain block was driven by the headroom setting on
Encore. A headroom setting of 26dB required a gain of 0dB, a headroom setting of 20dB
required a gain of 6dB and so on. These ensured that a reduced external maximum
headroom would be amplified up to fill the available internal headroom on the ADC.
Issue 3 onwards has headroom set by a push on link on each channel. Link Headroom
settings are detailed on the board and can be selected from 26dB (0dB gain), 24dB (2dB
gain), 20dB (6dB gain), 18dB (8dB gain) and 15dB (11dB gain). The digitally controlled
gain block U45 is still in place and is used for fine gain setting on the mic input and line trim
for the line input.
After the gain block the headroom corrected signal is fed through a high pass filter. The
frequency of the filter is nominally set to 90Hz. If a different low frequency roll off is required
it is possible to remove R94, R96 and R93 to get a frequency of approximately 60Hz.
A trim of approximately +/-0.5dB is available to match an external peak signal of +26dBu
to 0dBFS in the ADC (26dB headroom setting). With line in selected apply a signal at 0dBu
and adjust VR2 (VR1 on the second channel) to adjust the level fed to the console. For
+24dB headroom for example a 0dBu signal should give -24dBFS.
The output from the filter is fed to a peak limiter comprised of VCA U19 and side chain IC
U21. VR8 (VR7) sets the threshold of the limiter. VR12 (VR11) sums a preset voltage into the
VCA control voltage to give a constant 9dB gain. This is counteracted by the console
reducing the gain into the limiter at the gain block U45. The threshold setting is then set to
limit the signal to the ADCs at approximately -1dBFS. VR6 (VR5) sets the distortion and
control feedthrough on the VCA to a minimum.
With 0dB input line signal applied and the limiter switched on, adjust VR12 (VR11) to give
-1dB of gain at the console. For +24dB headroom for example that would correspond to
-25dBFS. Now measure distortion either using a line out channel or using the balanced test
points CH1N and CH1P (CH2N and CH2P) and adjust VR6 (VR5) for minimum distortion .
Apply a signal to the inputs which is above the maximum headroom level e.g. +26dBu and
adjust VR8 (VR7) to give -1dBFS.
Overall this gives up to around 10dB of limiting on a signal that would have otherwise
passed into clip. This is true as long as the input signal (whether mic or line) is kept at least
10dB below the input clip level (+26dBu for line and +19dBu for mic). When working with
Digital Service Manual
Issue 3 Page 33