Mixer; Headphone AmpliEr; Frequency Response Shaping; 2-Way Crossover Lters - Kurzweil CUP2 Service Manual

Mixer

(page 2, zone 2)
The mixer stage, U5, combines audio from the Engine PCB entering at J507 with audio from the
Line In jacks through J509. Volume of both signals is controlled at the source. When on, Q6 and
Q10 will short-circuit the Amplier output to ground as the rst stage of audio muting (see
Muting on the following page).
Headphone amplier
(page 2, zone 3)
Q4 and Q5 boost the output current capability of the left channel mixer amplier in order to
drive two pairs of headphones in parallel. Q8 and Q9 do the same for the right channel. The
headphones are connected to J508. The same headphone signal drives the Line Out jacks
through J509.

Frequency response shaping

(page 2, zones A6 and C5)
A band-reject lter (U7 and the surrounding components) is used to counteract the main
cabinet resonance to make a atter overall response. Since the CUP2 woofers are mounted in
tuned, ported enclosures, this lter is not needed and the signals are shunted around it through
R115 and R116.
However there is some frequency shaping applied to the crossover lters and the power
ampliers themselves to further atten the response.
2-way crossover lters
(page 2, zones 7 and 8)
The audio signal coming back from the headphones through J508 drives the four crossover
lters, one for each speaker, through voltage dividers R38/R39 (left) and R40/R41 (right). The
voltage dividers adjust the overall volume of the speakers relative to the headphones. C73 and
C74 can gently reduce the high frequency response; but they are not needed in the CUP2.
The tweeter crossover (U8 and surrounding components) is a 3-pole high-pass lter that passes
frequencies above about 3KHz to the tweeter power ampliers.
The woofer crossover lter is in two sections. The rst section is a low-pass lter that blocks
frequencies higher than 3KHz from entering the woofers. This is followed by a high-pass lter
that blocks frequencies below the woofer's useful range thus reducing bass distortion. This
lter also slightly boosts frequencies near the bottom of the useful range to atten the bass
response.
Printed Circuit Boards
Audio PCB
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