POWERSYM - very simple bridged
amplifier
I have decided to build a new class AB amplifier, that would be
simple enough but would have parameters good enough and some
power for a common home listening conditions with average
speakers, 85 – 90 dB/W/m sensitive. I did not want to make a
complicated SOTA design, and also wanted to utilize my nice power
supply with a 300VA double secondaries toroidal transformer,
which has quite a low output voltage. With this transformer I get
2 x 25Vdc supply voltage, with separated windings for left and
right channel rectifiers and filters, which eliminates loop
issues.
The amplifier circuit is based on a 50 years old Sinclair Z-30
topology, however considerably modified to better output devices
(MJL21193/94) and with highly improved PSR by new input
differential stage CCS design. The goal was to get more than 100W/4ohm,
so the bridged concept was used as the only possibility with only
2 x 25V power supplies. Bridged amplifier has some special
properties like cancellation of even harmonics distortion,
cancellation of CMV supply ripple components and no pollution of
signal common (ground) by speaker currents. It also has 4 times
higher output power than the same single amplifier operated with
the same power supply voltage. As of cons, output devices have to
handle 2x higher output current and output impedance is 2x higher
compared to a simple amplifier. Complete
schematics as follows:
Fig.1. Schematics of one channel of the POWERSYM amplifier
The schematics is complete, only supply rails bypass capacitors are not shown. R10//C11 (load) are for simulation purposes only.
The amplifier must be driven from a preamplifier with a balanced output, to get full power.
Basic specifications
Measurement of distortion
Fig.2. THD and THD+N vs. output power, 1kHz, 4ohm load
Fig.3. THD vs. output power at 60Hz, 1kHz and 6kHz, 4ohm load
Response to 10kHz square wave
Fig.4. Response to 10kHz square wave
Inside the amplifier
Pavel Macura 2019
last edited 10/2021