siliconchip.com.au Australia’s electronics magazine May 2019 69
Features & specifications
- Up to four times the power into a single speaker, using a
stereo amplifier or two mono amplifiers - Low noise and distortion
- Powered from 9-16V AC, 12-40V DC, 18-32V centre-tapped
transformer or ±6-20V DC (split rails) - Low current draw – around 10mA
- Fits inside a compact, low-cost UB5 Jiffy box
- Can also be mounted inside an amplifier chassis
- A switch can be used to change amp mode between stereo
and bridged mono - Uses low-cost, commonly available parts
- Suitable project for beginners
Fig.1: the Bridge Adaptor is connected to two power amplifiers (separate mono amps, or two channels in a stereo or
multi-channel amp) to drive a single loudspeaker. This doubles the maximum voltage across the speaker, and increases
the output power to up to four times the original. Only the active terminals of the amplifiers are connected to the
loudspeaker while the ground terminals are not connected. Note that this will not work with an amplifier that’s already
internally bridged, ie, where both the positive and negative outputs are actively driven!two channels within a single stereo amplifier), the outputs
swing in opposite directions.
That means the voltage between the outputs is double
that of a single output. This arrangement is shown in Fig.1.
Since the power into a load can be calculated as V^2 ÷R, if
you double the voltage but keep the impedance constant,
you quadruple the power.
Of course, this is assuming your amplifier is capable of
delivering that much power. But if you use an 8W speaker,
since most amplifiers will happily drive a 4W load, it should
be capable of it.
You do have to be careful if using a 6W or 4W speaker
since many amplifiers will not be very happy with a 3W or
2 effective load.
Performance
We ran our prototype through a number of tests using our
Audio Precision System Two. We haven’t reproduced any
of the resulting graphs here since the results can be sum-
marised in just a couple of paragraphs. We used a 15V AC
plugpack as the power source during these tests.
The distortion and noise levels are very low. The signal-
to-noise ratio is 114dB with respect to 2V RMS (a common
signal level from a CD/DVD/Blu-ray player), with a meas-
urement bandwidth of 20Hz-22kHz. The frequency response
is ruler-flat, being only 0.2dB down at 20Hz and less than
0.1dB down at 20kHz.
THD+N is 0.0005% from the non-inverting output and
0.0006% from the inverting output over the 20Hz-20kHz
range with an 80kHz bandwidth.
Measuring with a 20kHz bandwidth, these figures reduce
to 0.0003% and 0.0005% respectively. The distortion across
the two outputs (ie, what you would actually hear) meas-
ures the same as the inverting output.
Updated version
We published a Bridge Adaptor in the July 2008 issue. This
one is considerably smaller and will fit into a UB5 Jiffy box
for convenience. But you also have the option of building it
into an amplifier chassis if that’s what you want.
This design also has much more flexible power supply op-
tions. It will run off AC, DC or split rails. It also uses parts
that are easier to get, and cheaper, than our last design.
The circuit diagram is shown in Fig.2. The input audio sig-
nal is fed in via RCA socket CON1, which has a 100kresistor
to bias the input signal to 0V (if it’s floating). That signal then
passes through two “back-to-back” electrolytic capacitors.
We’ve used this arrangement, rather than a single non-
polarised (NP) electrolytic capacitor because the size and
cost of NP capacitors can vary dramatically. By using two
small, low-cost regular electrolytics, we get the same effect
with a low price and small footprint.
The signal is DC-biased to signal ground (more on that
later) with a 10kW resistor, and RF signals are filtered out
by a 100pF ceramic capacitor. The signal is then applied to
the non-inverting input, pin 3, of low-noise op amp IC1a.
IC1a acts as a buffer, feeding both non-inverted RCA out-
put connector CON2, and the inverting stage, which is based
around op amp IC1b. The signal to CON2 passes through
another pair of 22μF DC-blocking capacitors and is re-biased
to 0V DC via a 100kW resistor.
The 100W series resistor protects IC1a against an output
short circuit and also isolates any cable capacitance to pre-
vent oscillation.
IC1b is configured as a classic inverter with a gain of -1,
set by the ratio of the two 2kW resistors. These values were
chosen to keep noise to a minimum without unduly load-
ing the output of IC1b.
After all, it has to drive whatever is connected to invert-
ing RCA output connector CON3 as well. The signal is cou-
pled from IC1b to CON3 in the same manner as described
for CON2 above.
IC1b’s non-inverting input, pin 5, is tied to signal ground