Class (A)B output stage: why 2 emitter resistors, why not 1? - Page 2 - diyAudio
 Class (A)B output stage: why 2 emitter resistors, why not 1?
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diyAudio Member

Join Date: Sep 2006
Location: Pleasant Hill, CA
Quote:
 Originally posted by deleveld Yes I agree that symmetrical clipping is a good idea!
Do you mean 0.06% of power loss in one rail must be compensated by equal 0.06% of power loss on another rail for some good idea?

What is the reason behind this idea?
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diyAudio Member

Join Date: Nov 2003
Location: Indiana
Quote:
 Originally posted by Wavebourn Do you mean 0.06% of power loss in one rail must be compensated by equal 0.06% of power loss on another rail for some good idea? What is the reason behind this idea?
To be a narrow mind.

 12th December 2006, 10:01 PM #13 diyAudio Member     Join Date: Sep 2005 Isn't odd harmonics one of the downsides to perfect symetry? No wonder so many things in nature including ourselfes are not perfectly symetrical.
 12th December 2006, 10:09 PM #14 diyAudio Member   Join Date: Apr 2002 Wavebourn, you make an interesting point. I think one should try to maximise output swing before clipping occurs. I agree thats not a very objective statement. If fact it sounds like an ethical statement and I am not sure why it seems natural to describe it in those terms. From an objective viewpoint you could consider that symmetrical clipping increases the probability that a random signal wont be clipped. Because you never know whether the signal peak will be +ve or -ve. An amp can only relaibly process signals smaller than its lowest clipping level. I think that with a single emitter resistor then the clipping difference could very well be on the order of a few volts. If the difference in clipping levels is on the order of fractions of a volt, then the point of diminished returns has been reached. But when the clipping difference can be a few volts I think that some attention should, if possible, be paid to fixing the problem. Doug
diyAudio Member

Join Date: Nov 2003
Location: Indiana
Quote:
 Originally posted by Nordic Isn't odd harmonics one of the downsides to perfect symetry? No wonder so many things in nature including ourselfes are not perfectly symetrical.
Isn't it odd, that harmonics are one of the downsides to perfect symmetry?

OR

Aren't odd harmonics on of the downsides to perfect symmetry?

Which statement is the one you wanted to make...

As far as we go, I agree, assymmetry is built into our genome.... is that why we prefer some forms of distortion?

 12th December 2006, 10:12 PM #16 diyAudio Member     Join Date: Nov 2003 Location: Indiana Doug, Would it be better opimized if you used a single emitter in a quasi-complementry OP stage?
diyAudio Member

Join Date: Sep 2006
Location: Pleasant Hill, CA
Quote:
 Originally posted by deleveld Wavebourn, you make an interesting point. I think that with a single emitter resistor then the clipping difference could very well be on the order of a few volts. Doug
In the order of a few volts? One tenth ohm against 4 ohm load?

Let's check... Suppose, output voltage is 10V, it means 2.5A current, and 0.25V drop on 0.1 Ohm resistor, it means 0.625 W of power.
Now, you add 0.25V more of voltage drop claiming that output power before clipping will be higher? No, it will be less. Unaudible less, but anyway less.

So, what is the reason?
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 13th December 2006, 04:25 AM #18 diyAudio Member     Join Date: Aug 2005 Location: K-town No emitter resistors in a BJT EF could make biasing a bit touchy. Hope you have a good stock of outputs. __________________ All the trouble I've ever been in started out as fun......
Account disabled at member's request

Join Date: Jan 2006
Quote:
 Originally posted by deleveld Did you only use emitter for top or for the bottom output transistor? I assume you had no particular problems with only one emitter resistor? thanks, Doug Eleveld

I put it in the top, but either way would have done. The resistor was 1 ohm, the output transistors were TIP31/TIP32 (that's what I had in my junk box). Driver transistor connections weren't an issue because there weren't any and the output devices were biased and temperature compensated with two 1N4001 diodes connected in series.
Pout was a massive 1.5 watts rms into 8 ohms.

Probably not what you were expecting!

diyAudio Member

Join Date: Sep 2006
Location: Pleasant Hill, CA
Quote:
 Originally posted by CBS240 No emitter resistors in a BJT EF could make biasing a bit touchy. Hope you have a good stock of outputs.
Why?
The same idle current flows through both transistors and their common resistor. The same biasing voltage is applied between their bases.

Where's the difference?
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