AMP CLASS A to B detection - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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
Reply
 
Thread Tools Search this Thread
Old 14th August 2005, 02:03 AM   #1
fab is online now fab  Canada
diyAudio Member
 
fab's Avatar
 
Join Date: Mar 2004
Location: Canada
Default AMP CLASS A to B detection

I have always worked and listened to class “AB” amps where the class A was until about 1W rms and then after the amp switched to class B for higher level. Class A heats too much to get a reasonable power for me. Just by curiosity I checked the other day with a voltmeter while listening to music what was the power that I needed 99% of the time. I have realized that I do not play my amp louder than about 10Wrms (and this was very loud) in my living room.

Thus, I intent to build a class A / B amp :
Class A until about 15 Wrms / 8 ohms
Class B up to about 50W rms / 8 ohms

The class B would allow me to use the amp for party or else but most of the time I would enjoy class A. Is class A so good compare to good AB?

I thought it would be fun to know when the amp begins to go in class B while I listen to music like with a LED indication.

So I thought that one way to detect the level of music that makes the amp to go into class B would be to measure the AC current in one of the output transistor and compare it with the DC bias current in the same transistor. If the AC current would exceed the DC bias current then a LED would indicate class B operation. Using the usual output transistor source or emitter resistor to measure DC and AC voltages across it could be easy. I have built a prototype and it seems to work (although my actual amp switches to class B at about 1 W. Of course this circuit is only useful with a reasonable class A power. The advantage of that technique - compared to just measuring the AC output voltage and then calculating the power – is that it considers the load impedance since it can vary and also if someone change the DC bias current then no adjustment is required for the Class B detection circuit.

Any comments appreciated?
  Reply With Quote
Old 14th August 2005, 09:15 AM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
a push pull ClassA amp will deliver almost 2 times Iq into the load before it leaves ClassA into ClassAB.
Say you set Iq = 1A then you have about 1.95Apk into any load and still working in ClassA. Into 8r about 32W, 6r about 24W, 4r about 16w.
A conventional ClassAB amp is often set to an Iq = 10mA to 50mA (like your's). If your heatsinks are large enough it is easy to reset the bias to increase the Iq to about 100mA to 500mA for increased ClassA drive ability. Once you approach the higher end of this Iq range, temperatures start to rise dangerously and you risk SOAR failures. KEEP it COOL.

As you rightly pointed out the ClassA limit is strictly current based and the ClassA output power drops as the load impedance drops.

Your idea to measure the Volts drop across a load monitoring resistor and comparing this to the filtered volts drop on the emitter resistors could be turned into a ClassAB detector.
It may even be possible to avoid measuring the output volts drop and instead measure the fall to almost zero current in either Re and latch on a led indicator. 2 comparators (one for each Re) should be able to be set up to do this.
__________________
regards Andrew T.
  Reply With Quote
Old 14th August 2005, 11:02 AM   #3
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
1A bias in a push-pull amplifier will allow 2A peaks before one device cuts off. That will be 1.414A RMS current, or about 16W ionto 8 ohms.

In a real amp with real losses and a real brute force unregulated supply it will take about ±40V to do 50W into 8 ohms.

1A bias will therefore be 80W heat to get rid of at idle, per channel.

This is about how much heat a 200W class AB amplifier has to get rid of at full power.

Nelson Pass has a patent covering how to reduce this to a sane level in a cacsode output stage. It is a very clever varient of a tiered power supply.

The easiest way to do this would be to run the outputs from a ±12V supply, biased at 1A, shifting to a cascode running from ±40V for peak signals above ±12V.

The power at idle would only be 24W instead of 80W, and the amplifier would run as a cascode above the 16W level.

Douglas Self has such a design, although his site looks like it may be down for now.
  Reply With Quote
Old 14th August 2005, 02:22 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Default OOPS !

DJK is right about rms power.
I quoted peak power by mistake because I forgot to divide by 2.
__________________
regards Andrew T.
  Reply With Quote
Old 14th August 2005, 03:00 PM   #5
Steven is offline Steven  Netherlands
diyAudio Member
 
Steven's Avatar
 
Join Date: Oct 2003
Location: The Netherlands
Default Re: AMP CLASS A to B detection

Quote:
Originally posted by fab

I thought it would be fun to know when the amp begins to go in class B while I listen to music like with a LED indication.
In my post http://www.diyaudio.com/forums/showt...339#post282339 I showed a non-switching class AB output stage with the intention that it will be used for the most part in class A. The post also mentions the possibility and way to add a class A to B indicator.
It was just a basic design idea and it still hasn't found a way into a tangible amplifier, but maybe this will change one day.

Steven
__________________
The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973
  Reply With Quote
Old 15th August 2005, 04:06 AM   #6
fab is online now fab  Canada
diyAudio Member
 
fab's Avatar
 
Join Date: Mar 2004
Location: Canada
Default class B detection

Thanks AndrewT, DJK and Steven for your valuable comments.

Your calculations seems to match with mine since the heatsink (SK93) that I have selected is able to maintain about 52 deg C at 90W. I also planned to use +/- 40V dc supply at 1A dc bias current as you suggested.

One thing I would also like is some appreciation if it really worths it in term of sound reproduction (from experience) to do 15W class A and 50W class B while with the same heatsink I could do 200W AB (1W class A). Suppose it is the same amp topology of course between the 2 options.

The cascode output stage idea seems interesting but has someone tried it to see if other problem could arise? Do you remember the patent number? Is it similar to class G?

AndrewT, is it possible that you draw the 2 comparators circuit that you mentioned?
  Reply With Quote
Old 15th August 2005, 07:27 AM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi Fab,
sorry, just a design idea for you or another to develop.
__________________
regards Andrew T.
  Reply With Quote
Old 15th August 2005, 07:50 AM   #8
Account Disabled
 
Join Date: Aug 2004
Location: Earth
Hi Fab,

You can achieve lower distortions from a WELL DESIGNED amplifier run in Class A because it doesn't have to deal with the x'over discontinuity. Having said that there are mfg's out there who design massive power wasteful Class A designs to produce small amounts of power with sympathetic distortions, for money, while hyping the need to achieve audio nirvana - see Luscious new amplifier thread.

Cheers,
Greg
  Reply With Quote
Old 15th August 2005, 09:57 AM   #9
diyAudio Member
 
Ultima Thule's Avatar
 
Join Date: Jan 2004
Location: Koskenkorva Land
Fab,

you are probably using Emitter/Source (hereon called E/S) followers as the outputstage!?


Here is one idea of several I see:
If you also have E/S resistors you can add a comparator on BOTH upper and lower resistors, and every time the voltage over the E/S resistors approaches zero you are entering the class-B working area for the output transistors.

And if you wish some persistancy to the reading of the LED light you can add a R/S logic circuit or 555 timer etc etc adding a small delay so the LED wont go off every time the outputstage current makes the zerocrossing(entering the class A region), Obs! it's not the voltage you want to know, thats beacause loudspeakers are not perfectly resistive.

Cheers Michael
__________________
"If transistors are blueberries and FETs are strawberries, then tubes must be.. pears"
Michael 29th January 2010
  Reply With Quote
Old 15th August 2005, 05:28 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi Ultima,
were you reading post 2 last para when you wrote that?
I'm glad you made a better job of explaining the method.
__________________
regards Andrew T.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
jack detection signal? Heimhenge Digital Line Level 10 8th February 2009 11:46 PM
Thesis idea: Speaker distortion detection LG Multi-Way 13 10th November 2008 03:19 AM
EAC Drive Detection Problem hajame Digital Source 7 23rd February 2008 06:40 PM
Dc-detection, is this correct? fixation Solid State 12 16th February 2006 07:08 PM
need help with DCF-77 time signal detection hugobross Everything Else 1 3rd May 2003 04:27 PM


New To Site? Need Help?

All times are GMT. The time now is 03:56 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2