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Old 6th August 2008, 12:15 AM   #1
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Default Class A, push pull amp using transistor

hi,

i am wondering about pure class A Push pull amp. i know as its require less power supply and output is high efficiency, anyone build it? where to find some design?

i have searched can't find push pull class A circuits.
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Old 6th August 2008, 12:31 AM   #2
Zen Mod is offline Zen Mod  Serbia
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Default Re: Class A, push pull amp using transistor

Quote:
Originally posted by space2000
hi,

i am wondering about pure class A Push pull amp. i know as its require less power supply and output is high efficiency, anyone build it? where to find some design?

i have searched can't find push pull class A circuits.

A75

Krell KSA50 and KSA100
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Old 6th August 2008, 02:17 AM   #3
fab is offline fab  Canada
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Default Re: Class A, push pull amp using transistor

Quote:
Originally posted by space2000
hi,

i am wondering about pure class A Push pull amp. i know as its require less power supply and output is high efficiency, anyone build it? where to find some design?

i have searched can't find push pull class A circuits.
You can almost take any class AB design topology and increase bias current to the class A power level you want. If you want maximum power in class A you will still need bigh heatsinks even though push-pull class A requires half the current as single ended class A.

If you want power amp examples of maximum class A in push pull,. check for the "F4" and "F5" power amps on the Pass forum.

On my side, I use a compromise, a 70W Amp with about 5 to 20 W in class A (8ohms) since I usually listen to no louder than that. The Amp operates in class AB for the remaining watts until 70W when you want your neighbords to join your party...

Good luck
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Old 6th August 2008, 11:10 AM   #4
mikee55 is offline mikee55  United Kingdom
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Default Sorry to butt in

I've read various info on this. Apparently increasing bias raises distortion and listening fatigue, and how do you know if you're in Class A anyway? An Led indicator type circuit would be nice.
http://www.tnt-audio.com/clinica/bias_e.html

TNT Audio discusses doing it. Or am I barking up the wrong tree???

Mike
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Old 6th August 2008, 03:13 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

Doubling the bias current in a class aB amplifier is pointless
and will cause more distortion than the optimum setting.

The tntaudio article is very poor and ignorantly generic.

There will be an optimum setting for bias which depends on
the topology of the output stage, any other values are wrong.

Cranking it up "to be more class A" is very mostly wrong.

/sreten.
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Old 6th August 2008, 03:21 PM   #6
sreten is offline sreten  United Kingdom
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Default Re: Class A, push pull amp using transistor

Quote:
Originally posted by space2000
hi,
.... i know as its require less power supply and output is high efficiency ....
Huh ? run that past me again ? /sreten.
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Old 6th August 2008, 03:55 PM   #7
Eva is offline Eva  Spain
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Quote:
Originally posted by sreten
Hi,

Cranking it up "to be more class A" is very mostly wrong.

/sreten.
This just demonstrates the power of placebo effect. Something with a big "Class A" sticker on it always sounds better. Even if it's a gainclone
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Old 6th August 2008, 04:29 PM   #8
mikee55 is offline mikee55  United Kingdom
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Default De Ja Vous

I think I asked this myself, yonks ago. I did it to an amp I had. I put extended heatsinks on it. When I reversed it, it sounded no different.

Its right though. It says CLASS A. and I still got a look.

Mike
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Old 6th August 2008, 04:45 PM   #9
AndrewT is online now AndrewT  Scotland
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Hi,
but, if you increase the ClassAB bias to give an effective half power while still in ClassA that transfers to ClassB for the final 50% of max power, then an amp that has 20dB of overhead will have 17dB of ClassA overhead and an additional 3dB of ClassB overhead.

When you listen to this amp you will as likely as not remain in ClassA for 99.9999% of the time and only use the last 3dB of max power during the very few occasions that have extreme transients.
It is quite likely that the extreme transient that needs this last bit of power is so loud and short lived that the extra distortion due to transferring to ClassB from the High Bias ClassA will go unnoticed.

Effectively the high bias has hidden the non optimum ClassAB bias by ensuring that most signals never require the amp to go to ClassB.
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Old 6th August 2008, 05:02 PM   #10
Mooly is online now Mooly  United Kingdom
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It's swings and roundabouts. Upping the quiescent current ( if the heatsinks and thermal management of the amp can cope long term ) will increase the ripple on the rails. Many amps have poor supply rejection for various reasons and compromised wiring layouts. You may find the increase actually worsens the noise and distortion rather than improve it at lower levels. And it's at low levels where it matters most IMO.
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