"The Wire AMP" Class A/AB Power Amplifier based on the LME49830 with Lateral Mosfets - Page 53 - diyAudio
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Old 15th December 2011, 02:48 PM   #521
opc is offline opc  Canada
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Hi Neel,

Sorry about that... the way I worded it is not quite right.

That is indeed class B, but it's obviously not "optimally biased" while remaining in class B. In other words, performance could be both better or worse than what's shown there while still adhering to the definition of class B. Worse if you reduce the bias further, and better if you increase it right to the edge of 180 degree conduction. Beyond that, and you're into class AB. With this particular amplifier performance will continue to improve well into class AB with the optimal point being about 350-400mA of bias current. This is way beyond the transition between class B and class AB, and you'd actually be operating in class A up to a little over a watt when the output current exceeds the bias current and stops both devices from conducting at 360 degrees.

Regards,
Owen
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Old 15th December 2011, 03:16 PM   #522
AndrewT is online now AndrewT  Scotland
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The phrase "optimally biased ClassAB" only applies to a complementary emitter follower BJT output stage.
It can loosely be applied to quasi BJT and common emitter styles of BJT output stages, but the results of "optimum" are not nearly as good. Look at D.Self's graphs.

mosFETs fall into a completely different biasing territory.
They don't have an optimal bias region. The crossover distortion gets less the more the bias is increased. So much so that the crossover distortion does not disappear until they are biased in ClassA.

I'll quote Borbely again.
The total output bias of a mosFET stage should be not be less than 500mA.

That's an ampere of ClassA output current. Equivalent to 4W of ClassA into an 8r0 resistor.
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Old 15th December 2011, 05:06 PM   #523
npdot is offline npdot  United States
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Owen & Andrew,

Thanks for the kind tutoring.

Neel
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Old 17th December 2011, 04:30 PM   #524
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Hi OPC..I've found this project really interesting.. could I ask for 2 pcb ?? Do You still have something ? Pls contact me.. thanks..
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Old 18th December 2011, 05:48 PM   #525
Hedser is offline Hedser  Netherlands
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Hi Owen, same for me if still available. Do you have 2 more pcb's?
Thanks, Hedser
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Old 21st December 2011, 02:16 PM   #526
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Owen, any thoughts on when kits might ship?
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Old 21st December 2011, 02:46 PM   #527
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Owen I dont want to go too much offtopic here but I was looking at some design points on this national chip and looking at the harmonic spectrum results in the datasheets and then a thought came up.
Have you thought of using TMC compensation on this design ??
Two pole compensation works, so TMC should also. I think it would further improve the design and TMC greatly improves crossover distortion which results in mainly odd order harmonics. When compairing the two schemes in a typical Self circuit youll find that TMC has several advantages over 2 pole.
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Old 27th December 2011, 05:49 AM   #528
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Quote:
Originally Posted by opc View Post

if you're alright with lower output power, I have a little treat in the works that should give about 16W output into 8 ohms and gives the same distortion and noise performance of the SE-SE headphone amplifier.

Cheers,
Owen
Any more info on this one Owen? 4 ohms, or bridging possible?

Also, have you received a PS from Roberto yet? Looking forward to seeing how it performs.....
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Old 27th December 2011, 05:57 AM   #529
qusp is offline qusp  Australia
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indeed, it would be interesting to try that on my tweeters as they shouldnt need more than that in a digital XO. also interested in the PSU experiments. Owen posted in the headamp thread that he expects to start accepting payments for the power amp within the next 2 weeks and shipping directly afterwards.
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Old 27th December 2011, 09:11 AM   #530
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