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Old 23rd October 2005, 11:14 PM   #1
TroelsM is offline TroelsM  Denmark
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Default The cheap simple ClassD

Hi!
Having build an amp similar to the "amp30" at this page: http://listen.to/AUDIOEXPERIMENT I got curious and started looking for at way to use the very simple approach with higher power, only N-channel Fets and without ANY parts that are not common.

The result is shown in this post and there a few sim's are in the next post's.

It is only a model for simulation-purpose and therefore there is no bypass-caps and there is no input filter. The goal is to build this amp with normal leaded components and see if works. The best results be with SMD, but I want to keep it simple and see if a good PCB-design cant save som off the "damage" done by the leaded components.

The Fet's are to be IRFP264 as these seems to be very common and I have a lot of them. I dont have a PSpice model for them. Anyone?
R9, R10, L1, R14, L3 and R17 simulate the PCB. - Or something like it..
U1A and U1F drive the BD's and at the moment of switching they have to deliver a high peak-current to the BD bases. Therefore The inverters should probably be made up of 3-4 inverters in parralel.
Deadtime is taken care of by the RCD-networks: D1, R5, C2 and D5, R19, C5.

All comments are velcome. My Questions (for now...) are:
1) What do you think of the driver-curcuit? I want to use discrete componets to make the the amp cheap and easy to build without any "special" parts.

2) I would like to add feedback after the filter but I hav'nt found a way around the phase-shift-problems?

3) If this amp is to be build as a bridge then only one modulator should be used, but then feedback is only taken from one part of the bridge. I have used this approach before and it works, but some sort of feedback from both sides of the brigde would be better.

4) The LM311 does a level-shift from +/-12V to -48/-60V.. is this ok?

5) Q2, R2 and R8 does the levelshift to the high-side-driver. I'm told that this part off the amp is critical and any better solutions are welcome.

6) If the output is ringing there should be added snubbers. What about diodes across each of the IRFP264's? I have some RHRG30120 (30A, 1.2kV, "Hyperfast"), but BYW29 might work?

That all for now. Sims and comments will be posted soon.

TroelsM
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Old 24th October 2005, 01:38 AM   #2
TroelsM is offline TroelsM  Denmark
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Default I Can't edit my own post-- strange...

First: a better Png-version of the Schematic:

TroelsM

Note: I remember something about getting a better PSRR if there were a resistor from each power-rail to the feedback-point...?
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Old 24th October 2005, 01:45 AM   #3
TroelsM is offline TroelsM  Denmark
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Default Voltage at the gate og the highside Fet

Seems to be fast and clean, but of course that's gonna get alot worse in the real world..

TroelsM
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Old 24th October 2005, 02:12 AM   #4
TroelsM is offline TroelsM  Denmark
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Default Simm of the amp

Here's the first set of sim-result's:

The amp dos'nt osc at startup but it starts after ˝ a period when the signal goes negative. I don't think this is a problem.

At high pos. voltage at the output the freq drops very much. Hav'nt figured out why. It's very late 4.08 in the morning). I´m not gonna work on that now.

TroelsM
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Old 24th October 2005, 02:30 AM   #5
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Hi,

Excellent effort.

My only comments at this time are, remove or lower R22 and R17 if you really want the current to flow to the BD's.

There's a few projects around which answer alot of your questions, dig up the "reference design" thread it's too full of good info to ignore.

You're drivers will hold a small amp together, should be fine for the experiments you're doing, that doesnt' mean that they won't need all kinds of tweaking though, parallel as many inverters as required to get the job done.

3. correct.

4. I highly doubt it, check the data sheet look into AC coupling or level shifting properly. Was smart to ask though, simulators aren't all telling.

5. It is. Opto or magnetic coupling are options, it'll depend if your current solution is OK or not.

6. Diodes across the fets aren't mean to cure ringing, they're meant to prevent the body diode from becomming saturated which will then cause some wicked shoot through/dissipation/failure. You really need the right diode for the job, schottky's.

Keep at it, this is great stuff.

Chris
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Old 24th October 2005, 02:36 AM   #6
TroelsM is offline TroelsM  Denmark
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Hi. Spanks for the fast answer.

It's getting too late here, but one thing: Can you suggest any improvements to the drivers? If its not gonna ruin the whole keep-it-simple-and-cheap-thing, then any ideas would be welcome.

TroelsM
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Old 24th October 2005, 02:47 AM   #7
tlf9999 is offline tlf9999  United States
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is this really that simple? it looks pretty complicated to me.

if you want to stay all discrete, you may want to look into TA-N88, a sony digital amp from years ago - it actually looks quite similar to your design.

a few questions - I am learning Class D myself as well:

a) why not use a gate driver, like ir2011 or the lm chip I mentioned in another thread? what is the advantage of having all discrete gate drivers?
b) why use the comparator for level shifting? wouldn't a simple transistor do that just as well?
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Old 24th October 2005, 04:13 AM   #8
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Quote:
Originally posted by TroelsM
Hi. Spanks for the fast answer.

It's getting too late here, but one thing: Can you suggest any improvements to the drivers? If its not gonna ruin the whole keep-it-simple-and-cheap-thing, then any ideas would be welcome.

TroelsM

Nah I'd say start with that and see how it works. Simplest is best to start with and that seems fairly simple.

When you do it with discretes like that you're going to learn alot so keep at it. When you want better there's already circuits on here that will teach you even more, and meet all your criteria. Lucky you

Regards,
Chris
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Old 24th October 2005, 06:32 AM   #9
JohnW is offline JohnW  Hong Kong
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Default Sony TA-N88 Class D amplifier

I've heard alot about the Sony TA-N88 Class D amplifier - does anybody have the circuit of this beast?

Thanks in advance,

John
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Old 24th October 2005, 09:29 AM   #10
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I don't own the TA-88N schematic I only know its block diagram (after which I developed my first class-d amps). And I might find out the patent number again.

I also once used BD135/6 as drivers that were themselves driven by 4049 CMOS drivers.
This topology was actually quite fast but I started to experiment with the fist IR2110 as soon as it came out.
Specialised driver ICs are indeed simpler to use but the ubiquituous BD transistors are more easily obtained of course.

Regards

Charles
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