Pure Class A Push Pull

megasat16 · 24th March 2011, 07:11 PM

I know there are many Class A amps using Push Pull Output transistors (BJT or MOSFET).

I am asking coz I am increasing seeing manufacturers use the term "Pure Class A" for the Class A amps with Push Pull output stage.

Traditionally and for purists, "Pure Class A" always apply to SE type and PP always is just called Class A (neither pure or otherwise in front of Class A). I have a few real (means not using clever biasing scheme such as Sliding Bias, etc.) Class A Push-Pull amps that were made in the late 70s and 80s.

The question is can a Push Pull always remain CLASS A (never leave Class A) till it reaches max output and never transaction into Class B but to clip?

And while maintaining the Class A, can the push-pull arrangement produces more current as the load impedance decreases?

Please discuss!

GoranB · 24th March 2011, 07:38 PM

I think that with enough bias current an Push-Pull amplifier will stay in class A without clipping.

megasat16 · 24th March 2011, 07:51 PM

Quote:

Originally Posted by GoranB

I think that with enough bias current an Push-Pull amplifier will stay in class A without clipping.

I have no doubt about it and agreed.

The problem is with the word "Pure" when Class A can become a Class AB with the high input signal.

lineup · 24th March 2011, 07:57 PM

I think you can call all true Class A for 'pure'

Regardless if it is SE or PP

megasat16 · 24th March 2011, 08:01 PM

Quote:

Originally Posted by lineup

I think you can call all true Class A for 'pure'

Regardless if it is SE or PP

That I also agree. It's True Class can be Pure Class A.

But when the True Class A leaves from Class A into Class B under some conditions, then, it's no longer True Class A.

So, how do we limit to the True Class A?

GoranB · 24th March 2011, 08:03 PM

You can overdive any A class push-pull amplifier using a high level input signal. But under normal conditions, with normal level input signal and good PS there shouldnt be any clipping.
About word "Pure" (A class)...I think its used for better selling of audio devices.

DF96 · 24th March 2011, 08:18 PM

"Pure Class A" is a marketing term, not a technical term. An amp is either class A or not. Peak clipping does not violate Class A, but cutoff does. Class A can be either SE or P-P.

This is what I understand.

megasat16 · 24th March 2011, 08:19 PM

Yes! But in Pure Class A PP design, we don't want to go into Class B or leave Class A at all.

Otherwise, it would be like what is already stated. "Pure" is used for marketing the audio devices and nothing more. I guess in Pure Class A PP design, you can limit the over driven signal with the NFB and auto adjusting the overall gain or using rails voltage to clip after the max Class A watts are reached so the output devices doesn't go into Class B when the input signal is higher than the max bias Class A bias current.

When input over driven the PP amp out of Class A, the push-pull Class A is no longer a Class A but a Class AB. And it certainly doesn't fit the "Pure" or "Class A" bill under that condition.

So, in order to stay Pure (or True) Class A, the gain needs to be auto limited with FB network, and rails voltage and power supply design.

GoranB · 24th March 2011, 08:25 PM

Maybe im wrong but i dont think that an class A amplifier can leave class A and work in class B or AB. It can clip, but it will stay in class A, no matter of distorted signal. You also dont need NFB to prevent it of clipping (under the normal conditions).

DF96 · 24th March 2011, 08:46 PM

NFB does not limit a signal. If the output clips NFB will push the signal even harder against the stops - the opposite of what might be wanted.

Over-driving does not push A into AB. Excessive (i.e. too low resistance) loading might do it, but that is not the amps fault.

In Class A design, the aim is not to go into cutoff. There is not some stronger aim of really really really not wanting to go into cutoff, which is "Pure Class " or "True Class A". It is either Class A or not, under specified conditions.

24th March 2011, 07:11 PM	#1
megasat16 diyAudio Member Join Date: Mar 2009	Pure Class A Push Pull I know there are many Class A amps using Push Pull Output transistors (BJT or MOSFET). I am asking coz I am increasing seeing manufacturers use the term "Pure Class A" for the Class A amps with Push Pull output stage. Traditionally and for purists, "Pure Class A" always apply to SE type and PP always is just called Class A (neither pure or otherwise in front of Class A). I have a few real (means not using clever biasing scheme such as Sliding Bias, etc.) Class A Push-Pull amps that were made in the late 70s and 80s. The question is can a Push Pull always remain CLASS A (never leave Class A) till it reaches max output and never transaction into Class B but to clip? And while maintaining the Class A, can the push-pull arrangement produces more current as the load impedance decreases? Please discuss!

24th March 2011, 07:57 PM	#4
lineup diyAudio Member Join Date: Dec 2005 Location: the north	I think you can call all true Class A for 'pure' Regardless if it is SE or PP __________________ lineup

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
2A3s in push pull; class A	Sir Trefor	Tubes / Valves	39	30th January 2011 10:03 PM
A question about Push-pull class-a	mutter	Pass Labs	8	17th July 2010 07:04 PM
Class B Push-Pull	rsumperl	Tubes / Valves	31	16th July 2010 06:18 PM
Class A push-pull	Nikolas Ojala	Pass Labs	5	24th August 2009 12:58 PM
EL34 Push Pull in Class A	arnoldc	Tubes / Valves	29	17th May 2006 11:52 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

24th March 2011, 07:38 PM	#2
GoranB diyAudio Member Join Date: Mar 2010 Location: Poland	I think that with enough bias current an Push-Pull amplifier will stay in class A without clipping.

24th March 2011, 08:03 PM	#6
GoranB diyAudio Member Join Date: Mar 2010 Location: Poland	You can overdive any A class push-pull amplifier using a high level input signal. But under normal conditions, with normal level input signal and good PS there shouldnt be any clipping. About word "Pure" (A class)...I think its used for better selling of audio devices.

24th March 2011, 08:18 PM	#7
DF96 diyAudio Member Join Date: May 2007	"Pure Class A" is a marketing term, not a technical term. An amp is either class A or not. Peak clipping does not violate Class A, but cutoff does. Class A can be either SE or P-P. This is what I understand.

24th March 2011, 08:19 PM	#8
megasat16 diyAudio Member Join Date: Mar 2009	Yes! But in Pure Class A PP design, we don't want to go into Class B or leave Class A at all. Otherwise, it would be like what is already stated. "Pure" is used for marketing the audio devices and nothing more. I guess in Pure Class A PP design, you can limit the over driven signal with the NFB and auto adjusting the overall gain or using rails voltage to clip after the max Class A watts are reached so the output devices doesn't go into Class B when the input signal is higher than the max bias Class A bias current. When input over driven the PP amp out of Class A, the push-pull Class A is no longer a Class A but a Class AB. And it certainly doesn't fit the "Pure" or "Class A" bill under that condition. So, in order to stay Pure (or True) Class A, the gain needs to be auto limited with FB network, and rails voltage and power supply design.

24th March 2011, 08:25 PM	#9
GoranB diyAudio Member Join Date: Mar 2010 Location: Poland	Maybe im wrong but i dont think that an class A amplifier can leave class A and work in class B or AB. It can clip, but it will stay in class A, no matter of distorted signal. You also dont need NFB to prevent it of clipping (under the normal conditions).

24th March 2011, 08:46 PM	#10
DF96 diyAudio Member Join Date: May 2007	NFB does not limit a signal. If the output clips NFB will push the signal even harder against the stops - the opposite of what might be wanted. Over-driving does not push A into AB. Excessive (i.e. too low resistance) loading might do it, but that is not the amps fault. In Class A design, the aim is not to go into cutoff. There is not some stronger aim of really really really not wanting to go into cutoff, which is "Pure Class " or "True Class A". It is either Class A or not, under specified conditions.