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Old 24th February 2011, 04:25 PM   #1
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Default 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?

Last edited by yosha; 24th February 2011 at 04:37 PM.
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Old 24th February 2011, 05:19 PM   #2
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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.
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Old 24th February 2011, 05:42 PM   #3
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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
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Old 25th February 2011, 11:02 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by yosha View Post
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.
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Last edited by AndrewT; 25th February 2011 at 11:04 AM.
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Old 25th February 2011, 08:49 PM   #5
BrianVG is offline BrianVG  Belgium
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"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.
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Old 25th February 2011, 09:25 PM   #6
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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
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Old 26th February 2011, 09:17 AM   #7
BrianVG is offline BrianVG  Belgium
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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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