Battery is used in the examples above to simplify the theory of operation, in reality these are
programmable linear power supplies as shown in block diagram above. We have: speaker output, speaker
polarity switches (left), two equal resistors, two electronic switches and a battery. Batteries are a short
circuit for AC signals (audio signals). The resistors attenuate the battery voltage in order to generate an
analogue output signal.
A/D) Both switches to minimum, zero output voltage regardless of speaker polarity switch.
B/E) Battery voltage is divided in half by two equal resistors, speaker polarity switch determines output
signal polarity, attenuated output voltage.
C/F) Both resistors connect to plus, speaker polarity switch determines output polarity, maximum output
voltage.
In all 3 cases we have two resistors with value R in parallel and batteries are short circuit for AC. So
output impedance in all examples remains constant 0.5R regardless of switch position.
By changing the electronic switch settings we can have no voltage, half positive, full positive, half negative
and full negative output voltage.
In the practical converter we have one converter that is used in 4 different ways for generating the
balanced output signal (bridge matrix system).
This ensures that only the lowest bits have to change state around the zero crossing as the converter
starts at minimum and progresses up in small steps, not in the middle like with most multi-bit DACs.
The lower bits are switched with R2R, the upper bits are split up in sub bits (fractal system). This helps to
further lower output impedance to 2 Ohms, increase accuracy and minimise glitches.
The end result is an accurate and clean high power multi-bit bridge DA converter with low enough output
impedance to directly drive speakers This way all lossy analogue circuits and related unavoidable
degrading, typically required for conventional DACs and connected (pre)amplifier can be avoided.
