Class AB MOSFET Amplifier Build

[hongrn]

Hi All,

After 2 months straight of tinkering, fixing and troubleshooting, I finally have something to show for. This is a class AB kit using a pair of MOSFETs, with a claimed output of 150W into 8R and 300W into 4R. This build in still in progress, but music is playing, so I decide to post this project.

1. Enclosure: the case is made out of 1/2" MDF laminated with black glossy formica. The top will be clear 1/4" acrylic with perforations for heat venting (still working on it). I lined the bottom of the case with a Home Depot aluminum sheet to provide a grounding plate for the equipment. Earth ground is connected to this sheet from the ground plug of the IEC receptacle. The front and rear panels are held by metal angles and screws, so can be removed if needed. The power button (Apex Jr.) is the same that Ron used in his project. And before Mr. Cottiers can photoshop my front panel, I will have a pair of VU meters installed soon. The rear panel houses the regular RCA and speaker terminals, the fan, fuse and the IEC receptable.

[hongrn]

Re: Class AB MOSFET Amplifier Build

2. Power Supply: Power is provided by a 800VA 40VAC-0-40VAC toroid, rectified to 56VDC using a 25A full wave bridge and 88,000uF/100V of capacitance. Four 1.8KOhm/5W resistors are used as bleeders. The PSU is also grounded to the bottom metal plate. Since this is a toroid from a Rotel amp bought used from the Swap Meet at diyaudio, with only one pair of secondary, I had to improvise by adding a small 12V/2A transformer to provide power for the LEDs and fan. I used a small 2A bridge rectifier to convert the AC power to DC, then a LM7812 to regulate the rectified power into 12VDC to power the fan and future LED arrays to illuminate the interior of the enclosure.

3. Power Soft Start: I decided to hand built this module as well from a schematic, so it was quite challenging to understand zeners, voltage dividers, and relays. After 2 weeks, I finally understood the concept. The final cost is less than $8 to build the soft start module, and it works beautifully. No speaker thumping when turning the power on or off.

[[email protected]]

Re: Class AB MOSFET Amplifier Build

Nice Hong. Very clever idea using the formica.

[hongrn]

Re: Class AB MOSFET Amplifier Build

4. The amplifier: This is a L7 amplifier kit using a pair of MOSFETs. Since this is very inexpensive ($40 for a stereo kit), I decided to take a chance and see if I can put together a cheap amp that sounds good. Well, mixed results. One channel kept blowing up, even after I replaced the damaged components. I ended up replacing the whole channel, and everything started working well. It took a long time to get things to work, since I had no instructions, no schematics, limited knowledge, and even less troubleshooting skills. Everything was from trial and error, with strict observance of the lethality of electrical power.

For now, the amp is playing at very moderate level so I can monitor the heat and determine whether the heatsink size is adequate. How does it sound? Very good, but we'll see how durable this amp is.

[[email protected]]

Re: Class AB MOSFET Amplifier Build

Is the fan blowing in or out?

[hongrn]

Re: Class AB MOSFET Amplifier Build

Blowing out to draw the hot air from the case, since this is an AB design, and efficiency is much lower than our IRS2092. I'll route 2 long strips on the top acrylic panel where the heatsinks are to aid ventilation.

Is the fan blowing in or out?

[jsr]

Re: Class AB MOSFET Amplifier Build

You've been busy man! Glad you got that soft start working. You're far more adventurous than I with electronics.

[tomzarbo]

Re: Class AB MOSFET Amplifier Build

Hong,

That's a good looking project. I like the finish idea with the Formica, and the power button is pretty cool looking too. I have a similar button I plan on using for my amp build upcoming. I've been pondering just how much to put on the front of an amp. I have a very cool spectrum analyzer that I had planned on implementing in my case, but then I see a case like yours, understated and classy with a few VU meters on the way, and I think maybe keeping it very simple is the way to go. Plus I think what if something breaks in the future and it can't be replaced easily, then I have holes/gaps in my amp front. I still am not sure what I'm doing, but your case is looking very cool.

Did you need an amp per se, or were you looking to expand your electronics knowledge and just see if you could do it? I kind of fall in the second category myself, but I had to send my amps/power supplies to a member of TT to get help with hooking it all up... it didn't help that some of it arrived from out of the country packed very poorly and damaged. :(

beautiful work, and it seems like you're enjoying the process, which is mucho important.
I hope my project turns out at least fractionally as good!

TomZ

[donradick]

Re: Class AB MOSFET Amplifier Build

Nice work Hong! Sheesh, you've been busy. Congrats on pushing yourself and getting the design working.
I like the removable front and back panels, and I def think you should put in the meters.

[donradick]

Re: Class AB MOSFET Amplifier Build

Nice work Hong! Sheesh, you've been busy. Congrats on pushing yourself and getting the design working.
I like the removable front and back panels, and I def think you should put in the meters.

[Paul K.]

Re: Class AB MOSFET Amplifier Build

Great job, Hong.
Paul

[lowpolyjoe]

Re: Class AB MOSFET Amplifier Build

Wow, that's awesome

[Sydney]

Re: Class AB MOSFET Amplifier Build

Nice
( Still love my old A/B amps )

[Beau]

Re: Class AB MOSFET Amplifier Build

Hong,

Looks great.

Beau

[hongrn]

Re: Class AB MOSFET Amplifier Build

Thanks everyone for your encouraging comments.

Tom, just like you, Don, Lowell and Johnny, I'm trying to see if I can beat Bob Cottiers... Not a chance. As you said, I just wanted to expand my electronics knowledge. Got to exercise my brain somehow, and troubleshooting this stuff really makes you think hard. As an example, when building the soft start, I burned a dozen resistors because I couldn't get the relay to close. I connected the module's ground to the case, but hadn't connected the PSU's ground yet, so they was no return for the Vcc+ feed that triggers the relay to close. A few aha moments later, weeks later, mystery solved. I think what's important is not to allow yourself to be frustrated and give up. I usually put everything away for a couple of days, then go over the circuits thoroughly again. And no, I don't need another amp, but I'd like to be able to build more and more complex amps, and DIY as many modules as I can. Thanks.

Hong