What the hell is Class A3 ? - diyAudio
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Old Today, 07:38 AM   #1
kimbal is offline kimbal  Australia
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Default What the hell is Class A3 ?

Does any one know what Class A3 is ? Is it a modified "linear Class C" operation, or something crazy like that ?

I have been into electronics for decades and never heard of this Class, despite being an avid researcher on Audio design. I stumbled across it when looking at Electraprint output transformers as they make single ended Class A3 output transformers. The circuits Electraprint presents as A3, to me look like standard Class A1 or A2 designs - nothing special.

What advantages does it have over classical SE Class A designs ? How does it sound to the ear ? Does it have lower distortion, more power, better bandwidth; etc ? What drive requirements does A3 need ?

If my suspicions are right - this looks like one of those "new American inventions", where they change a part or two on on old well known design ( ie: re-invent the wheel ) and then claim they have invented something new; then suddenly rush out before anyone knows about it and get a patent put on it with the rights to sell it for a licence fee?
I say this because Electraprint won't allow you to commercially use their Class A3 tube designs without getting a licence from them. So my apologies for sounding sarcastic on these so called new-fangled inventions - as the electronics industry wreeks of such.

I also did a Google search and nothing came up that even discussed its operation.
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Old Today, 08:27 AM   #2
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Quote:
Originally Posted by kimbal View Post
I also did a Google search and nothing came up that even discussed its operation.
Because there is no "Class A3", it is just a marketing term dreamt up by E-P to sell their hi-end transformers. To use the A3 transformers, you need Sovtek 300B tubes that can withstand the higher B+, so "Class A3" operation seems to imply the following: higher plate voltage -> higher primary impedance -> lower idle current -> lower THD, but that's just speculation on my part.

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Old Today, 08:32 AM   #3
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"Class A3" is simply Class A2 where the Vgk never drops below 0V, and grid current flows through 360deg of the input cycle.

"What advantages does it have over classical SE Class A designs ? How does it sound to the ear ? Does it have lower distortion, more power, better bandwidth; etc ? What drive requirements does A3 need ?"

The main advantage is that it avoids the wild swing in grid impedance from a zero grid current condition to that sudden drop when grid current is drawn. This can reduce distortion since there is always some voltage dropped across the Zo of the grid driver. This should be of no consequence with a very Lo-Z grid driver, such as a source follower.

As for drive requirements, they're the same as Class A2: a very Lo-Z output, and adequate current sourcing capability, both easily obtained from a source follower.
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Old Today, 08:37 AM   #4
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Quote:
Originally Posted by Miles Prower View Post
"Class A3" is simply Class A2 where the Vgk never drops below 0V, and grid current flows through 360deg of the input cycle.
Is there a difference between Class A1 and Class A3 then?
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Old Today, 08:52 AM   #5
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Yeah, in Class A1 grid current never flows since Vgk never goes above 0V, and in Class A3, grid current always flows. They're kinda opposites.
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Old Today, 09:07 AM   #6
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Interesting, learned something new. Are there some readily available examples of Class A3 designs? Or just use an A2 drive circuit and make sure Vgk never drops below 0V.
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Old Today, 09:46 AM   #7
DF96 is offline DF96  England
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I had never heard of Class A3 before this thread, but it seems remarkably similar to screen grid drive.
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Old Today, 01:26 PM   #8
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I pulled the term A3 out of a warm dark place in my very first web page in 2003. At the time I was experimenting with the 811A and other tubes that worked with positive grid voltages.

The issue here is that the grid impedance changes abruptly during the transition from negative bias to positive bias causing the driver to distort unless it can source the required grid current. At the time I had not discovered mosfets or PowerDrive.

I had discovered that several tubes like the 811A and the 6HV5 can be operated on low enough plate voltage that the grid voltage remains in the positive grid region during the entire audio cycle. This eliminates the transition from high to low grid impedance. You can in fact drive the grid of a high Mu triode from the 16 ohm tap of the OPT in a conventional tube audio amp.

I got Jack to built the OPT's in my first Tubelab TSE amp. He seemed put off by the fact that I was using mosfets to drive the grid of a 45 or a 300B. Now he builds a special transformer to drive a DHT from a chip amp, and requires a license to build it for commercial use?????

A copy of my first web page used to be on my web site, but the link is broken now. I will be working on the site in the coming weeks, I can but it back. It may still be available on the way back machine but it was 11 years ago.
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Old Today, 01:26 PM   #9
Bigun is offline Bigun  Canada
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I learned something new !
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