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Old 12th August 2010, 03:27 PM   #1
comasus is offline comasus  India
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Default 100-watt sub woofer amplifier

Hello

see the attachment

The output transistors getting hot very much Q7 is getting hotter than Q5
could you please tell me how can i reduce heat
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Old 12th August 2010, 04:33 PM   #2
AndrewT is offline AndrewT  Scotland
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check for oscillation.
If it is bad it will destroy your project.

I don't see an output Zobel.
I don't see an input RF filter.
that increases the risk of oscillation.

What is the output offset?
Have you tested the amplifier into an open circuit and into a 8r0 resistor?
Have you shorted the input to signal ground for these tests?

C1 & C2 don't look right for a sub-bass amplifier. They might just about work as a high pass filter for a bass amplifier.
I would prefer to see a separate pre-amp to tailor the signal to suit the speakers.
That would then leave the amplifier to pass all bass frequencies it receives on to the speaker.
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Last edited by AndrewT; 12th August 2010 at 04:40 PM.
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Old 12th August 2010, 06:09 PM   #3
comasus is offline comasus  India
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Dear sir

I am new to electronics could you please guide me
How can i change this to normal amplifier so that i can use this for keyboard

in the circuit diagram Q1 and Q2 are shown as bc108 but i have used mpsa42 and i am giving +/-45v supply

Last edited by comasus; 12th August 2010 at 06:22 PM.
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Old 13th August 2010, 08:23 AM   #4
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Quote:
Originally Posted by comasus View Post
Dear sir

I am new to electronics could you please guide me
How can i change this to normal amplifier so that i can use this for keyboard

in the circuit diagram Q1 and Q2 are shown as bc108 but i have used mpsa42 and i am giving +/-45v supply
Here is one that will work with your mpsa42's and will be 10 times better that the one you posted. Runs on 40-0-40vdc.
OS
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Old 13th August 2010, 09:26 AM   #5
comasus is offline comasus  India
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Thank you but i want amplifier using the transistor 2n3773
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Old 13th August 2010, 09:47 AM   #6
comasus is offline comasus  India
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Thanks but i want an amplifier using 2n3773
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Old 13th August 2010, 11:28 AM   #7
lineup is offline lineup  Sweden
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As AndrewT said, it is probably oscillation.
The lower output, Q6, Q7, is in fact a CFB, Sziklai.
In this Quasi output stage.

My guess is that Q6-Q7 are oscillating.
I would try first with a capacitor (can be ceramic) 100pF
between B, Base, and C, Collector pins of transistor Q6.
If this does not help I would try with 220pF or 470pF.
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Old 13th August 2010, 11:39 AM   #8
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you need to verify the idle current of the amplifier ...that is measuring on the R14- and R16 the voltage drop ( carefully there you are looking at millivolts range )

if then this high means that your amplifier idles in higher bias than it supposed to be

the reason for that is the poor vbe multiplier that exists on this circuit ....

to make this in plain greek there is a string of diodes D1 and D2 that are supposed to be in perfect physical contact with the heat sink of your amplifier carefully now cause you need good physicaL contact but absolutelly no electrical contact

While your amplifier operate temperature goes up ( that is why you need a good heatsink ) together with the temperature also idle or biass goes up all the above drive your amp to a think called thermal run away .... to avoid this the D1-D2 while attached on the main heatsink will sense and monitor temperature and reduce bias to avoide thermal run away

of course the two transitors need insulation from heatsink and most imortand proper heatsink .... proper means a quiet big size ...

check all these and get back to us also post pictures of your work might also help
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Old 13th August 2010, 12:28 PM   #9
comasus is offline comasus  India
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voltage drop at R14 is 23mV at R16 is 54mV
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Old 13th August 2010, 12:53 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
what have you got connected to the output?

Those different voltages across Re indicate massively different currents.
Where is that difference in current going to?
Back down the NFB connection? Check the resistor you have fitted.

Check Re resistances.
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