Class D amp in N&V Q&A

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Joseph
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Class D amp in N&V Q&A

Post by Joseph »

Am I missing something or is something missing from that schematic diagram of a Class d amplifier? It appears that the output of the transformer can just drive the speaker in the positive direction.

I'm a big fan of class D topology and that is why it caught my eye, which is not too physically capable. Never could see too well.

I designed a few ampliverter circuits which use a SMPS to drive a speaker. I wonder if that is what the intention of the printed answer was.
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Re: Class D amp in N&V Q&A

Post by ringo47stars »

Maybe if you could out the page of the schematic and the month of the issue you might get a better response. I remember something about that but couldn't find it in the last three issues.
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Re: Class D amp in N&V Q&A

Post by CeaSaR »

Article is in June 2009 edition, under Q&A, pages 31-32.

I can't comment as this is the first schematic I have seen for a
class D amp. What I do remember is that they are called "Digital"
because they are either on or off, with the modulated pulse
smoothed out by the analog component of the speaker itself.
Does that mean that the cone only moves in one direction when
current is supplied, ie. the current doesn't actually alternate?

I think that would be a great article for a future edition of N&V.
Of course, I personally would welcome more audio articles/builds.

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dyarker
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Re: Class D amp in N&V Q&A

Post by dyarker »

Haven't seen June issue yet.

Is transformer primary center tapped? If so, driving one end of primary positive gives positive output, driving other end of primary positive gives negative output.

Another way to it is put the primary in an "H" bridge.

Either way allows bipolar output with a single supply.

A clue to which method could be the number of power transistors/FETs used. Center tapped tranformer can be driven by two power transistors. H-bridge requires four.

Looking forward to getting my copy of June N & V; in mean time that's my guess.

Cheers,
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Joseph
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Re: Class D amp in N&V Q&A

Post by Joseph »

Thanks CeaSaR for the fill-in. I have already misplaced my edition.
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Re: Class D amp in N&V Q&A

Post by dyarker »

Just got my June issue.

As I guessed, primary is center tapped (two windings in series) and driven by an FET on each end.

First look at D1 and D2 on secondary side and I thought maybe Joseph is correct. Then saw that low end of speaker is at half of supply voltage, and thought maybe circuit is okay, but it still didn't "feel" right. So reread the text and mentally pushed some electrons around. Now I agree with Joseph. I don't see any way for the high end of the speaker to be driven below half the supply voltage.

I'm pretty sure reworking secondary side of transformer without D1 and D2 would work, and reduce distortion at the same time. The modulator would have to be changed so that one primary winding handled pulses to shape positive half of audio wave, and the other winding the negative half of the audio wave.

In the last paragraph Russell said the power section was "blue sky". Sorry Russell, and I hope it's me that's wrong, but I think there are some unexpected clouds in this (one) time.
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Re: Class D amp in N&V Q&A

Post by CeaSaR »

Russell,

I've spent quite a while staring at that schematic and reading a lot on Class D
amps. Every one that I have found show a corrallation to a standard type of
linear amp where the speaker is either fed by the midpoint of a high/low driver
system or what amounts to an H-Bridge (BTL). I am having a hard time
understanding how your design works.

Is the bottom of the speaker (what is usually connected to ground, in this case
the positive of a 1000 uF polarized cap) supposed to be effectively isolated from
ground? Why is it at 1/2 VCC? Are the two 1000 uF caps there to act as some
sort of charge pump for the speaker? AND, both of your rectified outputs from the
"coupling" transformer feed the high side of the speaker through the low-pass filter
(scratches head in vain attempt to understand)..???

I guess I'm grasping at straws here, perhaps an explanation of the output stage
is in order for those of us who are lost by your design. The most important part to
clarify is after the MOSFET drivers, as they are in a standard configuration.
The rest is what has us baffled.

Hoping to hear from Russ Kincaid,

CeaSaR
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ringo47stars
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Re: Class D amp in N&V Q&A

Post by ringo47stars »

I didn't have the June issue at the time of my last post either but I went through the last year anyway and still couldn't find the schematic I was thinking of. The transformer in the circuit is the same type as the one in the battery charger circuit so it probably is applicable being the same type of function. Like a battery charges up in the car but needs a charger for in the house so the idea would be to create some modulation to alter the source voltage to be more like the source voltage in the car. So the transformer in the amp would probably have the same function. If it was a bigger transformer it would have a feed-back that would change the source voltage but the way it looks in the circuit, ic6 and related circuit is the feed-back.
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Re: Class D amp in N&V Q&A

Post by CeaSaR »

For those interested, I have submitted this thread to Russ through the Q&A email.
Hopefully he will answer either in print or here.

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Re: Class D amp in N&V Q&A

Post by russlk »

My apoligies for taking so long to get to this thread. I understand the confusion, I don't understant it myself! I don't know what I was thinking, I have only one half of the needed circuit. This circuit can only drive the positive alternation of the audio, as was correctly noted. I think one solution to this problem would be to seperate the positive and negative of the input audio and replicate the circuit from IC1 thru IC6, Q1, Q2, and T1. Diodes D3 & D4 would be reversed to provide the negative part of the signal. C5 and C6 are not needed and the speaker can be grounded.

There must be a more elegant solution because when one amp is working, the other is doing nothing. A single amp with switches at T1 output to change from positive to negative would be better.

Russ Kincaid
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Re: Class D amp in N&V Q&A

Post by ringo47stars »

I know that feed-back idea works with battery chargers but I think the technical term for it is some-what similar to memristors as stated in the nuts & volts mag June or July last year. If you wanted to figure out why one would be included in an amp circuit then the best way would be to add it on to an amp circuit that works to see if there is power dissipation or distortion. I was trying to think of a reason for there being one there but I can't remember trying that idea out. I believe you would call that a single amp with a feed-back circuit.
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Re: Class D amp in N&V Q&A

Post by CeaSaR »

Here's a good tutorial on how class D works:
http://sound.westhost.com/articles/pwm.htm
I know I'll be reading more on it.

Here's my search:
Yahoo! search

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Bob Scott
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Re: Class D amp in N&V Q&A

Post by Bob Scott »

If someone could shoot a graphic of the schematic my way, I could decrypt it. I'm quite good at reverse engineering and redesign.
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Re: Class D amp in N&V Q&A

Post by CeaSaR »

Shortly there, Bob! Look for a Yahoo! addy in your inbox.

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Re: Class D amp in N&V Q&A

Post by CeaSaR »

Bob,
On it's way.

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