Lateral MOSFET Power Amplifier

Abstract

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!

Amplifier Modules

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.):

Power Supply

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. 🙂

15 thoughts on “Lateral MOSFET Power Amplifier

  1. No brand. They came from a seller in China on the aliexpress web site. Easy enough to find, but I cannot recommend sellers who peddle in fakes, so if you find him be very wary of his amplifier modules which obviously have fake output transistors. They have the Motorola logo which is a dead give-away!

  2. Fantastic construction neatness. You’ve packed in all the stuff into one chassis so neatly that it makes me self-conscious of my own construction projects. In fact I find this so difficult that I can’t figure out how some DIYers pack preamp and poweramp into a single chassis and still keep signal cables away from high-current ones, keep small-signal PCBs away from the power transformer and bridge, etc. I know this one’s a pure power amp, but its neatness of cabling and layout is lovely.

  3. Hello:

    Where you buy the transistors?
    I try to buy in Profusion Plc but they haven’t.
    if you know where i can buy realy transistors, not fake i apreciate.
    Very good work, congratulations.

    Thanks, and sorry my english!
    Carlos

  4. Hello Carlos,

    I got them from Profusion. A very nice guy from the UK regularly on the ESP forum called “||81” had some Exicons from Profusion for sale a while back. He might still have them. You could go to the forum and ask.

  5. Thank’s a lot.
    Ok, I’m repairing a powered mixer “Hallen & Heat” from a friend, and i bought the bipolar transistor from Profusion and the mosfet they haven’t.
    To me is very dificult to buy electronic parts because local market has not, and if i bought from EBAY, aliexpress or other they sel me fake material.
    Is Profusion a good suplier?

    Thank’s for your help.

    Carlos

  6. That’s OK.

    Profusion seems fine.
    As far as I can tell:
    Output transistors from all Aliexpress sellers are fake.
    Output transistors from all China-based eBay sellers are fake.
    Output transistors from most other eBay sellers are fake.

    I think that there is a production shortage on the Exicons. If II81 does not have them any more I’d just wait for Profusion to have them in stock again.

  7. Hi,

    Excellent build quality, nice looking work. Couple of questions:

    1 – In addiion to the 10R resistor, did you also use a bridge on the safety/chassis ground?
    2 – To where did you tie the points on the P101 board marked P-GND? Did you run those to the center/star ground?

  8. Hello Gary and thanks for your comment.

    1 – Yes. You can just see the bridge rectifier below the 10R in photo 7 (the 10R is mounted to it). You can see its mounting hole in the chassis floor in photo 6 just below the chassis-isolated zero Volt line bolt.
    2 – For each channel, the P-GND goes to one of the Fast-on connectors marked GND at the output end of the respective PSU capacitor bank PCB. I departed fom the ESP PSU schematic here since my power supplies are “dual mono”. The zero Volt line bolt is at the input end of the PSU capacitor banks and the (10R + bridge + ceramic cap) safety loop breaker is between that bolt and the chassis/mains Earth. The other Fast-on at the output end of each capacitor bank (see photo 9) is for the speaker return. Most competing designs make the mistake of returning the speaker to the amplifier module. This amp has absolutely no audible hum.

    Cheers.

  9. Hi Ian,

    Nice project, I’ve looked over at Rod Elliott’s ESP site many times. I too am looking for a amp project, and either this MOSFET design or some where else.
    I then found this site http://buildaudioamps.com/ and he seems to be a straight-up guy.
    So please don’t think I’m spamming for his site, I was just wondering if you or anyone here has dealings with the guy?

  10. Thanks for the link. I really like the look of his Project 19 as a subwoofer amp. I don’t mind it if people post links to sites that people might benefit/learn from. And that site (which is new to me) seems to fit the bill. Cheers.

  11. Ian,
    Great work!
    I built the P101 several years ago, but I realize how much neater my project would have been with power supply boards like the ones you have.
    Are they custom work you submitted to Altronics, or do they stock these boards?

    Thanks

  12. Me? Custom work for Altronics? That’s funny. No they were the boards they used to have for their 20W Class A amp kits – now deleted. See my page on the 4 channel amp for similar boards from yeeBay.

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