This 145W (8Ω) Class-AB stereo power amplifier with four Exicon lateral MOSFET output devices per channel (ECX10N20/ECX10P20) was built for midrange use in an active speaker system (around 130Hz to around 3kHz), but it would also be great for full-range stereo (with a mod. to the speaker protection module).
It’s not double-triple PHX ultra mumbo-jumbo certified, but doesn’t need to be as it isn’t for sale and it didn’t need to be “burned in” either! 😆
It has a soft starter, a safety ground loop breaker, speaker protection and muting and sounds as good as or better than any high-end audiophile brand!
These are ESP P101 (Rev. C) PCBs with the four Exicon MOSFETs underneath and good quality components (Panasonic electros, Wima film and Cornell Dubilier silver mica capacitors, metal film and Welwyn wirewound resistors etc.):
P101 is specified for up to 200W with 70V rails, but I reduced this to about 60V (using a 240V/45V transformer on 225V mains). The 625VA transformer looks ordinary enough, but is a little unusual:
It was custom-made with bifilar secondary windings:
They bifurcate to left and right “stereo” bridge rectifiers:
Those Category X capacitors across each diode of the bridges are to suppress switching noise. Thermal grease for cooling to the chassis floor.
It’s the next best thing to two separate transformers (which would not fit in the chassis) and is in effect the same as having two isolated 90V secondaries with centre taps tied and isolated from the chassis by the safely ground loop breaker:
Being “dual mono” each bridge rectifier feeds one of two CRCCc filters – again using butchered Altronics PCBs. The twelve electrolytic caps are 80V, 6800μF Panasonics. Instead of the chokes of the Mk II Class-A amp each supply uses in-line 0R1 resistors between the first two caps of each rail:
Since the voltage drop is minimal, the resistors are a more cost-effective than the chokes in forming an equivalent pi filter.
The drain resistors were increased to 6.8kΩ/2W to protect the power indicator LEDs from over-current with the 60V rails:
The Altronics PCBs certainly make building the power supplies simple, but you end up with a lot of left over under-rated capacitors.
The arrangement takes the essential features of the ESP-recommended hybrid power supply circuit, but being an AB amp with dynamic power demands, the over-the-top independent configuration (back to the secondary windings anyway) might provide some minor sonic improvement – perhaps in the stereo separation of the mid bass. Who knows. 😛
Not intended for continuous subwoofer use and having over-sized heat sinks, thermal cut-off switches were not needed. Other than that, the construction was the same – having the same soft starter, auxiliary transformer and speaker protection setups.
The speaker protection board is set this time to trip below 100Hz and I used slightly fatter speaker wires:
Testing and Calibration
DC offset of one channel was good at 15mV. The other had 70mV which was not great. Although of no real consequence, I managed to reduce it to 35mV by replacing all of the driver transistors. The bias was set to 50mA.
Been working well for 8 years now and sounding fine. Absolutely no audible hum, hiss or anything wrong. The only mod. has been to stick a Zener diode inline with each relay diode for quicker speaker release in case of a fault. 🙂