Class D Design Issues

[DrLex]

I've just built my first class D amp with two TPA3122d2 ICs in BTL mode. The layout is based on the evaluation module design from TI, but I squeezed two amps on the same board. It works well, but one of the channels has a high-pitched whine that gets louder when increasing the gain.

The whine continues when unplugging the power, while the amps run on the tiny bit of juice left in the decoupling caps, so the PSU is not the cause. After some more debugging I found that my first design isn't as good as I hoped and the whine is actually caused by RFI. If I disable the other channel, the whine is gone.

I made the mistake of placing the signal traces way too close to the inductors and output pins of the other channel, even though I knew it was a bad idea. I ignored my common sense due to laziness: by routing the signal traces this way on the cramped board, I needed less jumper wires. I had hoped that nearby ground traces would adequately shield the signal traces, but now I learned this is not the case. The TPA3122d2 switches at about 250kHz, and the coils (despite being shielded) and output pins transmit some of this ultrasonic signal to the nearby traces. This signal becomes the audible whine in the other amp due to intermodulation, because the amps don't switch at exactly the same frequency. The good news is that it's only really noticeable at high gain and I'm going to use the amp in an experimental mini-subwoofer anyway that's physically unable to reproduce trebles, so I don't need to try fitting a lowpass filter or worse, redo the board

So, even though you should already know this, it can't hurt to stress it again: keep signal traces as far away from the outputs of class D amps as possible!

[Electrone]

A possible solution might be to cut the problem small signal traces and rerun the signals through shielded wires.

[fooeywuffle]

I built this thing around 20 years ago. It still works. I just thought maybe somebody would be interested in an Old School design. FETs at the time I couldn't get to work right, so all is bipolar transistors and J-Fet Op Amps. Power supply does have 2 of those TO-3 BUZ things driven by a transformer. It is for Bass at cutoff of 500 Cy, but is actually good to 3 Kc. 60 watts a channel. It is unstable without a load. I use it to drive a pair of Eminence 18 inch woofers in my 57 Chevy. Also posted this in the photo gallery.

[basreflex]

I know that there is an article in the original dutch version of elektor: (elektuur) describing a classD audio amp with 2N2219 and 2N2905 output transistors. does anyone have a copy ? probably years 1968- 1971

[Electrone]

I seem to recall studying an article about constructing a bipolar transistor classD design. I think it featured in a 1966 edition of Electronics World. Probably not really HiFi, but interesting nonetheless. There seemed to be a stir of interest in classD around that time. Letters & Circuit Ideas In Wireless World/Electronics World.

[unclejed613]

i have a copy of the EEE magazine "Electronic Design Handbook" from 1965. a very simple class D amp (called a "bang-bang" amplifier in the article text) is on page 93. i thought i had scanned this image in when my scanner was working, but i have not been able to find the file. so i just did a copy of it in LTSpice. it's a free running PWM. the input requires a 0.7V bias to operate "linearly", and the output needs to be capacitively coupled. feedback can be coupled through an RC filter back to the input (so it's an inverting amp). the article claimed linearity with feedback to be within .0025 percent. maximum input swing is 100mV for a 20V output swing. correction, i modeled this in ltspice, input zero bias seems to work better. but a 100mV swing seems to "clip" the amp

[sweetperfume]

hey i want a simple design for class d amps......worked on some but dint get how these things work somebody help out