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Old 26th April 2008, 03:00 PM   #1
jaya000 is offline jaya000  India
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Default Amp is very hot

I got one subwoofer plate amplifier of a taiwanese company.
It is becoming very hot even at reasonable volume.
I am thinking of reducing the value of R225 connected to Q 211.
or any other better way to keep it cool?
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Old 26th April 2008, 03:34 PM   #2
jaya000 is offline jaya000  India
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Here is the circuit
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File Type: jpg amp 1.jpg (67.7 KB, 395 views)
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Old 26th April 2008, 03:44 PM   #3
AndrewT is offline AndrewT  Scotland
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reducing the value of R225 will increase the Vbe multiplier voltage.
This in turn will increase the quiescent current.
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Old 26th April 2008, 03:56 PM   #4
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Oscillation?
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Old 26th April 2008, 03:58 PM   #5
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I see no reason for this type of amp to run so hot
Did it came with a sufficient heatsink ?
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Old 26th April 2008, 04:04 PM   #6
jaya000 is offline jaya000  India
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Quote:
reducing the value of R225 will increase the Vbe multiplier voltage.
This means i must increase the value to reduce current.

Quote:
I see no reason for this type of amp to run so hot
Tried by increasing the heatsink.
It has tip 102/107 pair at the output. This is TO-220 , so very small metal on the device.
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Old 26th April 2008, 05:08 PM   #7
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Default this is not

a bad idea but but you have to increase it not reduce it !!!!!!!

it would be wiser to look first how much is the idle as we speak by checking voltage drop in one ( or all ) the emmiter resistors ....

if idle is moving with in reasonable limits then something else is wrong ..... of course a scope will tell you if you low too much then crossover distortion will apear .....

another question is that if the amp gets too warm even with no signal .....also q211 should be attached to the main heatsink ....to be able to compensate temprature and reduce idle ....if loose the amp will get warm


anyway the truth is that before you make any changes the correct thing will be to make sure that amp is working with in specs and then start to look if something is wrong ....

check tha basics first ...then you will see if there is something wrong witha the all thing ...

how much power this amp makes ????? it looks an ok circuit


let us know
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Old 27th April 2008, 11:47 AM   #8
jaya000 is offline jaya000  India
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The amp is rated for 200w@2ohms & 150W@ 4 ohms.
I will recheck everything tomorrow.
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Old 27th April 2008, 02:01 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by jaya000
The amp is rated for 200w@2ohms & 150W@ 4 ohms.
I will recheck everything tomorrow.
Hi J,
that 2ohm and 4ohm spec tells us a lot about how bad this amplifier is.
A good amp will drive the intended reactive load within it's temperature, power and current limits.
This stress driving a reactive load is quite severe.
An easy indicator to see whether the amp is capable of driving moderate, rather than severe, reactive loads is to see how well it drives a resistive load of half the intended impedance.

Your amp is specified as 150W into 4r0 and that equals 24.49Vac into 4r0. However this falls to just 20Vac when delivering 200W into 2ohms.
That voltage drop of -1.76dbV when delivering test current into half impedance tells us that this amp is not designed to even drive a moderate 4ohm reactive load. It might make a decent 6ohm capability amplifier but certainly not capable of driving a severe 4ohm load.
A good amp will deliver better than -1dbV in half load impedance and a very good amp better than -0.6db into half load impedance.
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Old 27th April 2008, 03:24 PM   #10
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Ohh come on guys, 200W/2r - 150W/4r from 3 pairs of TIP102/107?
Sry I wouldn't offend anyone but I must state that it's another bowl of rice from Taiwan!

IMHO it's oscillating, try lag compensation in the VAS stage (100pF cap between base and collector of Q204).
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