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yosha 24th February 2011 04:25 PM

Need help
 
hey , need a professional software to simulate an AB amplifier
i have a problem with crossover distortion and i dont have an oscilloscope

why do they add current mirror in the front end of the class a amplifier?

why when i add a pot and transistor to the bias section the driver transistor will heat?

firechief 24th February 2011 05:19 PM

For the simulation software I would suggest you check out the fourm called "software tools". Your other questions are kind of scattered. Meaning too little information on two wide of topics, but I will try. What makes you believe that your have crossover distortion if you dont have a scope? What is the brand and model of the amp? You must give information if you want others to help.
Bob

yosha 24th February 2011 05:42 PM

im workin on a class ab amp , i found the schematic on 300/500W Subwoofer Power Amplifier
my supply : +-56v 5A
load : 8 ohms
output stage:2sa1943 - 2sc5200

so the shcematic does not include a bias regulation..so i added it by a potentiometer and a transistor to the both of the base of the driver transistors..so when i switch the pot the drivers will heats and when i reduce the amount they will cool..so when they heat they are sonducting more current and then the crossover distortion will be reduced right?that's what i read thank u

AndrewT 25th February 2011 11:02 AM

Quote:

Originally Posted by yosha (Post 2481601)
my supply : +-56v 5A
load : 8 ohms

that supply can feed a 10r resistor with current.
The maximum AC output power using a sinewave will be ~50^2 / 10 / 2 ~ 125W into 10r.

A speaker can draw upto three times as much current as the equivalent resistor.

5Apk will restrict you to using a 20ohm to 30ohm speaker.
The maximum power into a 30ohm speaker is ~42W into 30ohm.

If you want to use an 8ohm speaker then you must find a way to supply at least 15Apk (preferably >=20Apk) through the amplifier to the speaker. This will allow a maximum power of ~ 150W into 8r0 and be capable of driving an 8ohm speaker.

BrianVG 25th February 2011 08:49 PM

"Because this amp operates in "pure" Class-B (something of a contradiction of terms, I think), the high frequency distortion will be relatively high, and is probably unsuited to high power hi-fi. At the low frequency end of the spectrum, there is lots of negative feedback, and distortion is actually rather good, at about 0.04% up to 1kHz. My initial tests and reports from others indicate that there are no audible artefacts at high frequencies, but the recommendation remains."

I got this from the page you gave us...
So why did you add a potentiometer?
And the current mirror will make the amplifier more linear, improves "PowerSupplyRejectionRatio" and it wil make the slew rate better. Someone correct me if I'm wrong.

yosha 25th February 2011 09:25 PM

brian : the pot i added it to control the bias current..and at low signal level i can hear a few distortion so i change the value of the pot and the distortion will disapear but the driver transistors will get more heat like they are conducting all time...so how can i convert this amp to class AB...and how can i calculate the current mirror and how do i know how many transistors will i add..or simply do u have a gd schematic to work on? thnx

BrianVG 26th February 2011 09:17 AM

This amp is designed as a subwoofer amplifier. When you change the value, you will change the quiescent current. That's the current that will flow throug your transistors, all the time. If you want to make a powerfull hifi amp, this is not the right choice.
Anyway, I think you've already made it a Class-AB by adding the pot. It's absolutely normal that your transistors heat up.


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