History Of Series; Improvements And Explanations; Sense Of R174 And R175; Sense Of - Spectral Audio ProTone Service Manual

3 History of Series

There where 3 different series made:
Serie # 1 : 960000 .. 960020 with golden front panel
Serie # 2 : 960021 .. 960161 with red front panel, C66 and C67 added
Serie # 3 : 960162 .. 960470 with red front panel, now with switch and trafo for 115V / 230V; R173,
R174, C63-C65 inbuilt; R174 and R175 (NTCs) added (by hand); GND Connection to case
This manual describe the last serie.
This manual contains no the hole schematics. Spectral Audio may send it to you for important
reasons, please write to info@spectralaudio.ch.

4 Improvements and explanations

4.1 Sense of R174 and R175

These two resistors helps to increase the lower limit of the cutoff frequency. Otherwise a ploop-
noise may occur when reaching 0 Hz cutoff frequency with a high resonance. You may reach the
0Hz using the LFO.
They are in serie connected from U20 pin 16 to GND.
4.2 Sense of C63 – C65
With time, the pots may become noisy. You may hear this effect especially at the Cutoff pot. Using
the C63 – C65 solve this problem.
They are connected from the middle pin (+) of the pot to GND (-). Resonance has negative voltage,
so middle pin is (-) and GND is (+):
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