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Old 24th October 2007, 11:38 AM   #1
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Unhappy Speaker output with offset..

Hi All,

this is my 1st time i build transistor amplifier...

when i measure the output of the speaker, i found it have offset -1Volt. may i know which component is defect or can i turning the VR. because when i try to turning the VR from 1k to 3xxOhm, the offset voltage is near to zero. but the Q2 is getting hot compare with Q1... :-s

VR1 = 1K
Q1 = Tip42
Q2 = Tip41
IC = MC1458

C7 = 22pF

Thanks all
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Old 25th October 2007, 12:54 AM   #2
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attachment below is my resistor value...

hope can anyone help me...

thanks u all very much..
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Old 25th October 2007, 02:17 AM   #3
Leolabs is offline Leolabs  Malaysia
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Ning Hao!

The output of the OPAMP should go to the center point of the bias diodes,not the upper half of the VR.
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Old 25th October 2007, 08:15 AM   #4
AndrewT is online now AndrewT  Scotland
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will his CFP output work without emitter resistors?
There are no collector resistors on the drivers. They will switch off with the outputs, instead of retaining some bias current.
Will the circuit work? or work well?
regards Andrew T.
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Old 25th October 2007, 11:31 AM   #5
Donk is offline Donk  Europe
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This circuit is from an old Texas application note form the 70's.
I clearly remember that emmiter resistors were needed (0.22...0.47 Ohm will be fine) and a Zobel (10 Ohm + 100nF poly) on the output is needed to.

See this page for a working "Texas"-amp:

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Old 25th October 2007, 11:31 AM   #6
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just now i try connect 1Khz sinewave to the input, but i can't get any output...

i think this circuit is not a complete circuit.. :-s

what value of resistor should i add in to the collector? can anyone help me? .. thanks AndrewT

tonight i will cut off the track that from OPAMP connect to VR and see the result. Thanks Leolabs
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Old 25th October 2007, 11:34 AM   #7
djk is offline djk
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You are best to drive the top side and use a pull-down to the negative 15V rail.

Click the image to open in full size.
Candidates for the Darwin Award should not read this author.
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Old 26th October 2007, 03:05 PM   #8
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I try connect the output opamp direct to the center point, the o/p offset get more higher.. :-s (-2V++) and also cannot function.

later i will try put 100 ohm resistor to Q3 and Q4. take R3 and R7 out.

my q3 and q4 is using C1815 abd A1015. hope the 100 ohm resistor not destroy it..
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