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Old 28th July 2005, 01:09 AM   #11
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Quote:
Originally posted by PSz.
Ok,
next crazy idea. How about using a tube amp output transformer INSTEAD of the standard output filter, and using the natural frequency roll off provided by the transformer for filtering. Has anyone tried it?

regards,
PSz.

I believe some of the very early didgital amplifiers, called class D, in the '60 used regular output transformers just as you suggested.

I guess that the modern approach is less costly.

Dave
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Old 28th July 2005, 01:49 AM   #12
PSz. is offline PSz.  United States
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So, if some of the early ones used transformers, then it should work! If anyone wants to donate some trafos I will tear into my demo board and give a full report!

As far as I know the rolloff of transformers is first or second order (I don't know much about 'em), so it seems that my other crazy idea might work too! That is unless the reason why early ClassD didn't sound good was due to the filtering! Then again, ClassT is spread spectrum so it might do better.

any thoughts?

PSz.
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Old 28th July 2005, 02:29 AM   #13
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Oh, I just had another one (crazy idea)! How about just using a cap across the output for a 6dB/octave rolloff and do away with the inductors. We would still have filtering in place, just lower order.
I was in the process of trying exactly that today, but after I took out the inductors and bridged the connections I managed to accidentally fry the chip when it was powered up before I could listen to how it sounded .
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Old 28th July 2005, 02:39 AM   #14
PSz. is offline PSz.  United States
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What value cap were you using?

PSz.
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Old 28th July 2005, 03:33 AM   #15
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Quote:
Originally posted by David Davenport



I believe some of the very early didgital amplifiers, called class D, in the '60 used regular output transformers just as you suggested.


Dave

A bit of trivia: Who invented the digital amplifier? Extra points for when.

In his article “Updating Pulse Modulation” in issue one/1987 of Audio Amateur magazine, Norman Crowhurst wrote “In 1966 I was issued a patent for a form of pulse code modulation that promised to revolutionize audio...”

That was part one of a three-part series. Part two, “Developing High Quality Pulse Width Modulation” was published in the two/1987 issue of AA and I can’t find where the third part was published. I only surmise a third part because Crowhurst referred to it at the end of the second part. These articles were an update to original articles “The Two State Amplifier” written in the July and August 1965 issues of Radio-Electronics magazine. I have only the July part of that series.

In the February 1966 issue of Electronics World magazine, Don Lancaster wrote an article “Amplification Using Switching Techniques.”

Any good technical library should have the 1965 issues of Radio-Electronics and the 1966 issues of Electronic World. You could get the 1987 AA articles directly from Audio Amateur Publications who now publish audioXpress. They also should be able to tell you what happened to the third installment of that series.

Dave
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Old 28th July 2005, 06:54 AM   #16
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Wow,
looks like I've got some reading to hunt down! Also, I was thinking more about the cap across the outputs... I think it ought to be set up like a zobel, so as not to appear as a dead short. Am I totally off with this?

PSz.
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Old 28th July 2005, 08:31 PM   #17
jkeny is offline jkeny  Ireland
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I know this isn't anything as radical as you guys but how about as a first step to experimentation platform:

Has anybody tried to bypass output circuit of t-amp by removing on-board inductors and connecting air-cored inductors to output diode and then using own output circuit with caps to ground and zobel network if necessary?

Signal after output inductors no longer contains high freq waveforms, so tight circuit with smd parts not necessary?

would allow experimenting with different quality & value caps as well as eliminating zobel.

John
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Old 29th July 2005, 11:20 PM   #18
Jorge is offline Jorge  Brazil
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A few comments:

A capacitor is like a short circuit at high frequencies, and inductor is an high impedance device at these frequencies.
So, removing the inductor will fry the chip...

Using a Zobel is about the same as connecting the speaker directly to the chip without any filter.

A transformer with a Zobel may do the trick - but where is the advantage? The inductor will still be there...

Crowhurst:

I remember his original article (yes, I was alive by then), and the AA one.
I understand he passed away before writing part 3.
But it is a very interesting reading - if I recall correctly, he was more interested in power efficiency than quality (the transistors of the time were quite slow).
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Old 30th July 2005, 06:31 AM   #19
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I got a different impression about hooking up with a first order (or no filter) from Tripath app note #4, bottom of page 3. They suggest that it is possible to hook up a speaker without any filter. AND that the quality of sound is indistinguishable from an amp with an ideal filter. This is what lead me to my crazy thoughts on the output filter...

Can you help me understand why the chip would blow up without the inductance? As opposed to a first order filter? Wouldn't a zobel that cut starting at say 50k hz. just cause the resister to get hot? And maybe the chip would actually require a heatsink...



regards,
PSz.
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Old 30th July 2005, 12:44 PM   #20
Jorge is offline Jorge  Brazil
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Quote:
Originally posted by PSz.

Can you help me understand why the chip would blow up without the inductance? As opposed to a first order filter? Wouldn't a zobel that cut starting at say 50k hz. just cause the resister to get hot? And maybe the chip would actually require a heatsink...



regards,
PSz.
Why does one needs a filter in class D amps?
Because one may blow the tweeter, radiate lots of electro magnetic interference, etc. But indeed, it will operate without the filter (the speaker itself will act like a filter).

A first order filter using only an capacitor can blow the chip, as I've posted. A capacitor with say 8 ohms impedance at 20kHz will have 0.8 ohms at 200kHz and 0.2 ohms at 800k - about the switching frequency, so almost like a short circuit to the chip.

Using a Zobel or no filter will have almost the same effect, since a Zobel is not a filter - it's an impedance compensation circuit (from a given frequency up the impedance is constant, the resistor value)..
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