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SSassen 4th June 2005 01:49 AM

My NON-discrete SODFA class-D amp

After about a week of getting reacquainted with analog signal analysis and running through a whole set of batteries on my HP calculator I've come up with a NON-discrete class-D amp that should work well for my application. That application being a top-of-the-line subwoofer.

I'm about to start construction on the subwoofer, which will feature one of the new Adire Tumult 15" woofers ( in a closed box of about 35-liters. Of course with the Vas volume of the woofer, 266-liters, the F3 is around 100Hz and thus the drop-off is -12db/oct.

To compensate for that I use a simple filter that is the reverse of that curve, +12db/oct and hence will flatten the curve again. This principle is known as ELF or Bagend and is used by manufacturers such as Velodyne.

Of course the amplifier power needed is substantial, due to the +12dB/oct boost. For 110dB at 100Hz amp power is 50-watt, then at 50Hz > 200-watt and at 25Hz > 800-watt and so on. Obviously for efficiency and power handling reasons I want a class-D amp to power the sub.

I've been reading up on class-D amp design in the past week(s) and designed the following schematic by borrowing ideas from around the web, here at, in application notes and at the German website. What do you think, am I on the right track here? With a power supply of +/-80-volts and a 2-ohm impedance I should be hitting my mark of about 800-watt RMS right? And please let me know if there's any glaring errors in the schematic.

Class-D sub amp schematic

Looking forward to the (many) replies.

Best regards,

Sander Sassen

classd4sure 4th June 2005 04:56 AM

Hi Sander,

Hysteretic self oscillator with pre filter feedback... yeah I think it fits the application and is a cool circuit/project.

I wonder if the integrating cap could drift or saturate, maybe a 1Meg in parallel with it or so, and the precision of it could possibly make a difference, as well as low input offset.

600 watts could be tough to get working reliably, guess you'll be looking at protection circuitry?


JWFokker 4th June 2005 08:40 AM

Bagend as in "Bag End", home of Bilbo, and later Frodo Baggins?

ssanmor 4th June 2005 08:55 AM

The way you get the invert version of the PWM and at the same time level shift with T2 is very elegant!
First I thought it was a mistake, but now that I have analyzed it, I understand how it works.

Best regards,

IVX 4th June 2005 09:03 AM

A) input buffer error.
B) hard to matching HIN-LIN falling edges. (maybe baker?)

SSassen 4th June 2005 10:30 AM

Chris, Ivan et al,

Thanks for the feedback, made a few changes to the schematic to incorporate some of the suggesties as well as a fix for some errors, the schematic in the fp is the latest rev.:

- a few polarized caps were placed in the wrong orientation
- input buffer had +/- inputs switched
- two zener diodes instead of one as BZX maxes out at 47-volt
- two zener diodes also better distribute the load

However by my calculations it will be a challenge to get to 800-watts RMS simply because 800-watts at 2-ohms needs 40V_eff and 20A_eff.

- 40V_eff * sqr(2) = 57V_s.
- max V_eff output is about 60% of VCC/VDD : VCC/VDD = +/-94V
- including losses across the mosfet's this will be +/-100V

Also I'm a little worried about power supply pumping due to the half bridge, maybe I'm better off doing a full bridge? I'd be happy to hear your suggestions about that as well.

Ivan, you mention it will be hard to control the falling edges of Hin-Lin, are you referring to the pre-driver circuitry or after the IR2110? And what do you suggest instead?

Best regards,

Sander Sassen

SSassen 4th June 2005 11:19 AM

Alright, I took Ivan's comments to hart and went looking for a comparator with both a normal and inverted logic/ttl compatible output so I can better interface with the mosfet driver. The LT1016 seems made for the job and I have updated the schematic to include it. For every revision I will now increase the file number of the schematic so the highest file number wil automatically be the latest schematic.




Thanks for all of your input, one more question though, can I interface the LT1016 directly with the IR2110, omitting the two transistors and four resistors? Or would it then go out with a bang?

Best regards,

Sander Sassen

classd4sure 4th June 2005 05:38 PM

Try it?

IR2110 has mos inputs do you know? Should consume little current unless switching.... maybe a small resistor would do

SSassen 4th June 2005 06:08 PM


Yeah, I already ordered parts, but I'd like for the circuit to be as simple as possible. I just don't feel comfortable to omit T1, T2 and the four transistors yet, as both parts of the circuit operate on a different voltage level. The GND of the IR2110 is tied to Vdd which will be at -50...80V whereas the LT1016 is at +/- 5-volts derived from Vcc and Vdd.

If I calculate the Vdiff between IR2110 GND and -5-volt at the LT1016 the difference in voltage is -50- -5 = 45 volts and that could very well cook the LT1016 hence I put the transistors there as a precaution. But maybe I'm just being too cautious, what do you think?

Oh, and you mentioned protection, you're indeed correct, it'll need both DC protection and a softstart and possibly overcurrent protection. That's why I picked the IR2110, as it features a enable pin (Sd) so you can disable the MOSFETS when needed which allows for a simple softstart scheme with just a few parts. However any suggestions there are also appreciated.

Best regards,

Sander Sassen

classd4sure 4th June 2005 07:42 PM


1. Try increasing C19 drastically, 10 uF or more if you can.

Don't think you can omitt the transistors as you say there needs buffer after the op amp.

So a baker clamp like Ivan was sugesting on the buffers might be worth a shot, and certainly transistor selection could make a difference but those seem like a decent type to try it with.

Can you simulate this? Did you have any plans to?


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