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Old 24th August 2011, 10:44 AM   #1
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Post Pre amp oscillate

Hi,
I'm new here and need help from all off you, I just developed my new pre amplifier circuit and already build it, but it is generate the frequency even when I short the input to ground.
The output dc offset is 2.8 mv and the oscillate frequency is 102 hz.
I tried every way to solve, but it is not better.
I also enclosed the circuit here.

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Old 24th August 2011, 12:47 PM   #2
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please find attachment of my preamplifier ( with schematic )
Attached Images
File Type: jpg alps.jpg (13.6 KB, 209 views)
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Old 24th August 2011, 12:54 PM   #3
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I presume that you understand that the above is just a joke with a point ( though )


in a simple amplifier choise and matching of just one stage ...the LTP for example is very critical and use of unmatched parts might result to offset while closely matched parts will provide more punchy and tight bass with no offset .


in a circuit like the above which i cant understand the principal of operation but i can tell you for sure that looks like way too many areys of transitors are used and each and every one of them is an open door to paracistic oscilation...

That will be in simulation level cause if you try to do this in real life you will understand that a circuit like yours might also be very pcb demanting
slight mistake on the pcb and oscillation will be waiting arround the corner ....(unless you use the perfect transitors ...like the ones you have in your simulator )

just my opinion
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Last edited by east electronics; 24th August 2011 at 12:57 PM.
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Old 24th August 2011, 02:27 PM   #4
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Thanks to Sakis, It is the thing that I'm thinking about, I plan to replace the transistors to be 1N4148 that should be easier.
I enclose my more pic.Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.
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Old 24th August 2011, 02:48 PM   #5
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Sorry about large image size, anyway what are other opinions.
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Old 24th August 2011, 03:23 PM   #6
alexcp is offline alexcp  United States
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102 hz sounds more like ripple from the power supply, not oscillation. Are you sure your power supply works correctly?
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Old 25th August 2011, 01:37 AM   #7
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We can don't worry about power supply because of 3 stages voltage regulator working fine, my multimeter can measure at 0 hz ripple.
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Old 25th August 2011, 04:31 AM   #8
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Removing of transistor arrays still not working.
What I have tried are:
replace transistor arrays by zener diode. : not better.
Removed cascode transistor for both stages : not better.
I'm doubt now.
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Old 25th August 2011, 12:44 PM   #9
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Your schematic is too small for me to see the part numbering but you should improve your current sources to the LTPs. This 100 hz problem most probably is a powersupply or grounding problem. First incorporate supply decoupling like that used in Selfs blameless amplifiers to improve PSRR, also introduce a small resistor circa 47 to 68 ohms in series with the base of the current source transistor, if the current sources are oscilating this should put a stop to it.
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Old 25th August 2011, 09:16 PM   #10
jaycee is offline jaycee  United Kingdom
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1uF decoupling is most likely the problem - add some 100uF electrolytic decoupling with 100nF ceramics (yes ceramics!) and the problem will disappear
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