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Old 18th November 2012, 08:39 PM   #11
Bigun is offline Bigun  Canada
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yes, sorry, sounds like I was pouring cold water on it. what would be nice is some idea of whether it sounds good ? subjective of course, but we are emotional animals
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Old 18th November 2012, 08:53 PM   #12
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No need for apologies. You were right: for a long time there are already very low distortions amps available.

Cheers,
E.
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Old 19th November 2012, 03:21 AM   #13
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I like how he graphed open loop stability over many levels of static offset.
Always wondered how best to proof stability through the crossing handoff.
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Old 19th November 2012, 03:41 AM   #14
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Quote:
Originally Posted by Edmond Stuart View Post
No need for apologies. You were right: for a long time there are already very low distortions amps available.

Cheers,
E.
The article claims low distortion before closing the loop. Is it attractive?
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Old 19th November 2012, 05:15 PM   #15
Struth is offline Struth  Canada
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Hi Guys

Anything that reduces let alone eliminates crossover distortion is worth investigation.

However, a traditional output stage with a simple bias regulator that monitors output current is a simpler solution, requiring just a two-BJT regulator. Idle current is rock solid and fairly easy to set by selecting Re for the outputs. The downside with this simple approach is that Vq must be one Vbe drop - or two - where class-i and Doug Self's class-A bias servo (three BJTs plus two current sources) both allow a Vq that is quite low, accommodating low Re values.

If you follow the links to the article and read the three parts available, you'll see that device matching is important in class-i.

Have fun
Kevin O'Connor
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Old 19th November 2012, 05:32 PM   #16
ilimzn is offline ilimzn  Croatia
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Just for fun look up the Musical Fidelity A1, namely the output stage... so this has been done before, quite a long time ago, actually. There is also a better way of supplying the bias voltage, by injecting a small surrent into the feedback node. Then the input side of the differentials can be simply connected together (A1 connects only for AC using one coupling cap per side but this can be simplified to a straight-through DC connection).
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Old 19th November 2012, 08:14 PM   #17
Struth is offline Struth  Canada
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Hi Guys

The A1 was clever but not in a good way. Overall THD was not that good.

Have fun
Kevin O'Connor
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Old 19th November 2012, 09:21 PM   #18
ilimzn is offline ilimzn  Croatia
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Hi Guys

The A1 was clever but not in a good way. Overall THD was not that good.

Have fun
Kevin O'Connor
Yes, for a number of reasons, one of which was deliberately low diff stage gain. But, if you look at it, it's exactly the same approach as here, except the bias offset voltage is added on the feedback end.
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Old 19th November 2012, 10:48 PM   #19
Struth is offline Struth  Canada
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Hi Guys

Where the class-i circuit is strictly a unity-gain output stage, the A1 was a complete amplifier. Diff gain and out device gain both worked towards linearity and bias control. Something is always compromised that way.

Have fun
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Old 20th November 2012, 12:16 AM   #20
wahab is offline wahab  Algeria
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Originally Posted by ilimzn View Post
Yes, for a number of reasons, one of which was deliberately low diff stage gain.
They did squeeze as much gain as possible from the input stage
by keeping it undegenerated but the following stage has only
a symbolic gain to add to the chain , hence about 54dB OLG
at most , although with a 100Khz bandwith.
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