how are you clocking the PWM rate? Also, on most Class D amplifiers, you see 2 inductors per speaker on the output filter, one on the “+” side and one on the “-” side.

What I would love to see here is an FPGA or something similar used to take a digital audio input (I2S for example), and convert it to PWM signals used to drive the MOSFET gate drivers.

Add a S/PDIF to I2S converter and/or an ADC with I2S output and you’d have a really sweet (and possibly much higher fidelity) class D amplifier

Use it to drive a sub with a low pass and the hum won’t be there. I’m not sure I would use class D for full range with plenty of class T digital full range chips out there.

Hint — Most mono amplifiers for 12v car audio applications are class D (or better) for subwoofer applications, while most full range amps are AB or T.

Sounds like 60 or 120Hz hum… perhaps line noise isn’t adequately isolated from the DC input to the board. Not terribly surprising with the step-down transformer to rectifier to capacitors that it looks like is being used, a linear regulator after that to drop the voltage just a bit would reduce a lot of the hum I think.

mIK – class T amps are just a product name for what are actually class D amplifiers. Seems that they use delta-sigma modulation and a digital feedback loop, but it’s still a class D.

I have 3 laptop bricks laying around, waiting to be used. I might use one for a quick bench supply.

@drone – I’ve used a bunch of OEM class-D boards by Sure Electronics sold on http://www.parts-express.com/. When I get some time ill post a 6×6 amp/matrix I build with them.

@mIK Your phrase “Class D or better” sounds like we’re classifying eggs or surface continuity…that Class A is the best class. Class A was just the first type, Class B the second, etc… And extending what Matt said, Class T is just Tripath’s brand of Class D amps.

@Drone Check these little beauties and their cohorts; they’re very easy to use, inexpensive and sound great:
http://focus.ti.com/docs/prod/folders/print/tpa3112d1.html
http://focus.ti.com/docs/prod/folders/print/tpa3106d1.html

@Matt
That hum comes form lack of negative feedback loop form speaker terminals to input signal terminals. This allows PSU hum to be modulated to PWM signal in output stage.
It just needs one more opamp per channel and some resistors and capacitors.

Hi everyone, thanks for your comments!

@Tim: the amp is of the self-oscillating type. On the breadboard, I measured the frequency at around 250kHz. With the decent PCB layout and much faster gate drivers in use now, I’d expect that frequency to be even higher.

@Matt: right you are… If I used a laptop power brick as I suggest in the writeup, the hum should be gone. I just used the transformer/rectifier/capacitor combo as I didn’t have a sacrificial power brick on hand.

@Chamon: I think it’s just a bad experimental power supply. The only ground connection is at the audio input. That’s not ideal, but the key is that there are no other direct ground connections.

@Drone: The L and C values shouldn’t depend on the supply voltage. They DO depend on the load (speaker) impedance, though. Right now, they are close to ideal for an 8 ohm speaker.

@space: There is negative feedback from the switching node of the output transistors. I didn’t take it from the speaker terminals due to the tendency of the RLC filter to cause instability when included in the closed-loop transfer function. You are right, though; with extra R’s and C’s I could make a compensation network and include the filter output in the feedback loop as well.

It’s true that Class-D amps are readily available these days. I just wanted to see how good I could make one from scratch. It worked well on the breadboard, hence the PCB…