SP-1
Surround Control
Amplifier
Far left is the input. The volume setting is managed by a sixfold stepped attenuator with 24 positions. In various SA-CD's we detected differences in the levels of the six channels. And there's also the difference in loudspeakers, placement of the loudspeakers and room acoustics. That's why we added 6 individual mono potentiometers in order to level those effects out.
Every input has a capacitor preventing DC to enter the circuit. It's the only capacitor in the signal path.
All SA-CD's offer six channels with equal specifications. Therefore six identical amplifier circuits are used. We used operational amplifiers with specific audio qualities, a wide bandwidth and a high slew rate (20 V/µsec). The bandwidth is limited to 200 kHz by way of C3. SA recordings (may) contain frequencies up to over 100 kHz. In order to avoid phase shifts in the audible band until 20 kHz the bandwidth has to be tenfold that frequency. But on the contrary it makes sense to limit the bandwidth and avoid slewing of the connected power amplifier. There may occur "overshoot" or oscillations for instance caused by the connecting cables. This is suppressed by C2.
On the right side you can see the way the power supply is configured. This is done individually for each channel. Transistors T1 and T2 together with the zenerdiode Z1 act as a kind of "super Zener diode" functioning in parallel to the load (the op amp). The entire circuit now behaves like a "shunt regulator" (and a very fast one!).
The circuit is separated from the mains supply by series resistor R16. In this way a good isolation from disturbances from the mains is acquired. The total suppression measures 130 dB. The negative voltage is regulated in the same way. Only two small electrolytic capacitors were needed taking care for the effects of low frequencies at high volumes.
The outputs are "loaded"with a current mirror. In this way the internal chip temperature is forced to a higher level suppressing "thermal distortions". This could also be seen as a way to let the circuit function in "Class-A".
Thermal distortion occurs when an input signal (a varying voltage) causes a varying current in a transistor. Hence the temperature will vary and so will the internal capacities (causing phase shifting in the signal). A higher operating temperature in the entire circuit will make temperature variations, caused by the signal, relatively small.
At the bottom left there's a three position switch. In the first position all 5 channels are summed, amplified and filtered giving the low frequencies from all channels together at the SUB output. This position is useful if small loudspeakers are connected.
The second position offers the sub-channel directly at SUB out.
The third position is used if the sixth channel is used for heigth information (for instance with some Telarc discs). Then also the other channels are summed and available at SUB out. The heigth output is an extra output. It should, via an extra small power amp, be connected with a small loudspeaker positioned at some 2 - 2,5 meter above the center loudspeaker.
Not in the drawing is the input switch. That switch offers a choice out of four sources, two 6 channel sources and two stereo sources, such as tuner, CD, tape, aux etc. These all are switched by relais.
The main power supply is relatively simple. All rectifiers are connected with the transformer via series resistors. Those suppress spikes originating from the silicon semiconductors in the rectifiers. The 30 Volt input is stabilized down to + and -24 Volt. The shunt regulators bring this further down to + and -17 Volt.
All components are placed on just one printed circuit board. Only two switches are connected discretely with wires, the input switch and the SUB-out switch.