Testing and measuring bridged dual-mono amplifier
Bridging a dual-mono amplifier that was not originally designed for this task as a bit challenging, especially if it only has unbalanced RCA inputs. This is the case of my A250W power amplifier, however I was curious how it would work in the bridged mode.
A250W is a dual-mono amplifier, two completely separated power amplifiers with separate power supplies, all in one 4U x 19” case. The inputs are only unbalanced RCA. To make a bridged amplifier, it is necessary to tight output “Lo” speaker terminals of both channels together and the load is there connected between “Hi” speaker terminals of the left and right channels. This will interconnect power signal grounds of both channels. Now, we must not connect the signal grounds of the left and the right RCA input connectors together, as it would create a ground loop including the high current speaker ground returns and thus destroy the S/N of the amplifier, resulting in hum and buzz. At the same time, we must invert signal for one of the inputs to get the doubled voltage swing between the “Hi” output terminals of the left and right channel. This is the trick of the bridged amplifier, we get twice of output voltage of the single amplifier channel, thus four time the power (in theory). But, now any of the two channels sees half of the load impedance and must deliver the two times more current. So, the output transistors are much more stressed.
My test setup looks as follows:
L and R channels of the dual mono amplifier are completely separated. They are then connected by the “COM connect” wire between the OUT Lo terminals of both channels. The load is connected between OUT Hi terminals.
RCA input of the left channel is driven directly, input of the right channel is driven via the 1:1 link transformer in opposite polarity, which assures inverted polarity at the OUT Hi of the right channel. Load is thus not tied to signal ground and the measurement must be differential, as shown in the schematics.
Measurements
I have measured THD vs. output level for 4ohm, 6.8ohm and 8ohm resistive load. Then I have also measured a burst power into 4ohm.
THD vs. level at 1kHz
The amplifier keeps its low distortion even in the bridged mode. With 4 ohm load, it gave 526W at THD = 0.1% and 430W at THD = 0.008%. With 8 ohm load, it gave 460W at THD = 0.33% and 365W at THD = 0.002%.
Burst power
Burst power was measured by 7 periods of 100Hz sine, total duration 70ms, rectangular window (no smoothing of signal rise and decay). Burst power measured seems to be 770W/4ohm.
Conclusion
This amplifier rated at 2 x 250W/4ohm and 2 x 150W/8ohm gives 1 x 526W/4ohm and 1 x 460W/8ohm with a stepped level method of sine input. Burst power, 100Hz/70ms, is 770W/4ohm. The limiting factor here are the power supplies of the two channels, each based on a 300VA transformer. But the power in the bridged mode is higher than for the similarly rated class D amplifier.
Bridging a dual-mono amplifier that was not originally designed for this task as a bit challenging, especially if it only has unbalanced RCA inputs. This is the case of my A250W power amplifier, however I was curious how it would work in the bridged mode.
A250W is a dual-mono amplifier, two completely separated power amplifiers with separate power supplies, all in one 4U x 19” case. The inputs are only unbalanced RCA. To make a bridged amplifier, it is necessary to tight output “Lo” speaker terminals of both channels together and the load is there connected between “Hi” speaker terminals of the left and right channels. This will interconnect power signal grounds of both channels. Now, we must not connect the signal grounds of the left and the right RCA input connectors together, as it would create a ground loop including the high current speaker ground returns and thus destroy the S/N of the amplifier, resulting in hum and buzz. At the same time, we must invert signal for one of the inputs to get the doubled voltage swing between the “Hi” output terminals of the left and right channel. This is the trick of the bridged amplifier, we get twice of output voltage of the single amplifier channel, thus four time the power (in theory). But, now any of the two channels sees half of the load impedance and must deliver the two times more current. So, the output transistors are much more stressed.
My test setup looks as follows:
L and R channels of the dual mono amplifier are completely separated. They are then connected by the “COM connect” wire between the OUT Lo terminals of both channels. The load is connected between OUT Hi terminals.
RCA input of the left channel is driven directly, input of the right channel is driven via the 1:1 link transformer in opposite polarity, which assures inverted polarity at the OUT Hi of the right channel. Load is thus not tied to signal ground and the measurement must be differential, as shown in the schematics.
Measurements
I have measured THD vs. output level for 4ohm, 6.8ohm and 8ohm resistive load. Then I have also measured a burst power into 4ohm.
THD vs. level at 1kHz
The amplifier keeps its low distortion even in the bridged mode. With 4 ohm load, it gave 526W at THD = 0.1% and 430W at THD = 0.008%. With 8 ohm load, it gave 460W at THD = 0.33% and 365W at THD = 0.002%.
Burst power
Burst power was measured by 7 periods of 100Hz sine, total duration 70ms, rectangular window (no smoothing of signal rise and decay). Burst power measured seems to be 770W/4ohm.
Conclusion
This amplifier rated at 2 x 250W/4ohm and 2 x 150W/8ohm gives 1 x 526W/4ohm and 1 x 460W/8ohm with a stepped level method of sine input. Burst power, 100Hz/70ms, is 770W/4ohm. The limiting factor here are the power supplies of the two channels, each based on a 300VA transformer. But the power in the bridged mode is higher than for the similarly rated class D amplifier.
Last edited: