Silicon Chip – May 2019

70 Silicon chip Australia’s electronics magazine siliconchip.com.au

Fig.2: the circuit of the Bridge Adaptor, (also known as a BTL, or bridge-tied load adaptor) without the power supply

(shown in Figs.3&4). The incoming audio signal is AC-coupled to non-inverting input pin 3 of IC1a, which acts as a

buffer. The buffered signal is fed to CON2 and also IC1b, which inverts the signal and then feeds this inverted version

to CON3. By connecting CON2 and CON3 to two separate single-ended power amplifiers (or left and right channels in a

stereo amplifier), you can almost quadruple the power delivered to a single speaker.

via a 1kW resistor. This means both of its inputs (pins 5 &

6) have the same source impedance, as the two 2kW resis-

tors are effectively in parallel, given that both are driven

from op amp outputs, which have an effective impedance

close to 0W.

Power supply options

You can power the Bridge Adaptor from a 9-16V trans-

former, standalone or plugpack, or you can use a 12-40V

DC supply, a ±6-20V DC split supply, or an 18-32V centre-

tapped transformer.

That last option is most useful if you’re building this

project into an amplifier chassis. Later on, we’ll show you

how to wire up a switch so that an amplifier can be easily

configured as either stereo or bridged mono.

When the unit is powered from AC or a split rail DC

supply, signal ground is tied to power supply ground by a

0Wresistor (ie, wire link), as shown in Fig.3.

But if the unit is powered from a single DC supply (eg,

12V DC) then signal ground needs to be around 6V DC, so

that the AC signals have a symmetrical swing.

The power supply section is therefore reconfigured, as

shown in Fig.4, by omitting some components and replac-

ing others with wire links.

In this case, the 0W resistor is instead 10kW, and another

10kW resistor forms a voltage divider across the DC sup-

ply rail, to generate a half-supply rail for signal ground.

A 220μF capacitor between signal ground and power sup-

ply ground prevents any ripple or noise on the supply rail

from getting into the signal ground, and thus affecting the

audio signal.

It also presents a low AC impedance to the op amp

feedback divider, so that the unit’s frequency response is

not affected by the resistors used to generate the signal

ground rail.

Before construction

Before you start assembling the board, if you’re going to

be fitting it in a UB5 Jiffy box, place the board in the bot-

tom of the box and use a marker to place dots in each lo-

cation where a mounting hole is required.

We’ve provided four different PCB overlay diagrams,

to show what components you need for each supply con-

figuration.

Fig.3: this shows a “universal” power supply, suitable for a single ended DC input, a split DC input (ie, +V/0V/–V) or an
AC transformer with or without a centre tap. CON4 is used for single-ended DC or AC since it only has two contacts.
CON5 is used for split DC or an AC transformer with centre tap.