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Old 10th January 2006, 11:05 PM   #1
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Default Help designing Input and Vas stage's

Hi everyone,
my previous thread which i think was going completely off topic so i decided to make a new thread.
I have two PCB's i salvaged from an old amplifier which i would liek to Reuse as part of my first DIY solid state amp.
Iv'e been reading up on amplifier design ont he internet and in my electronics books but I need a little help designign the input and Vas stages for the amplifier.
I will post what i have come up with so far for the Input stage, not really sure how well it will work or what values the components should be or what transistors to use so any suggestions for components or improvements to the design would be great. Also what would be best to use as the CCS? i have been lookign and i have found sao many diffrent designs for CCS's, just wondering what is simplest to build and what you'd all recomend.
The schematic for the pcb's are atached.
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Old 10th January 2006, 11:06 PM   #2
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and the schematic that i have came up with so far for the input stage.
Many thanks,
Owen
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Old 10th January 2006, 11:20 PM   #3
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Quote:
Originally posted by DoomPixie
and the schematic that i have came up with so far for the input stage.
Many thanks,
Owen
looks alright
if you split up resistor R1 in two resistors

see the this image, version to the far right
http://www.dself.dsl.pipex.com/ampins/dipa/dpafig8.gif

=============

we are many that have read this investigation of distortions in power amplifiers
and learned a thing or two
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

The Blameless Amplifier has been viewed by thousands
and is not a bad start, from which you can make your own version
of a good audio amplifier
http://www.dself.dsl.pipex.com/ampins/dipa/dpafig33.gif

there is a lot of stuff at Douglas Self Amplifier Institute
http://www.dself.dsl.pipex.com/ampins/ampins.htm
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Old 10th January 2006, 11:26 PM   #4
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Thanks for the links lineup, i will go and read them now.
Have modified the schematic and replaced resistor R1 with two seperate resistors. Hope this is what you meant.
many thanks,
Owen
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Old 10th January 2006, 11:32 PM   #5
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yes, that is correct
in that current mirror, each transistor has its own Emitter resistor
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Old 10th January 2006, 11:43 PM   #6
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I have added the CCS circuit that they used in that distortion in power amplifiers link. that is a very usefull link, makes interesting reading.

Thanks,
Owen

edit to point out my deliberate mistake.. Q5 is the wrong way around.. Will fix next time i change the schematics. (this note is more to remind me than anything) lol
Owen
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Old 11th January 2006, 01:15 PM   #7
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Just a little update, i have added the values of all the components on the output boards. only thing im not sure abotu is the capacitors, i used two diffrent online calculators and one said 6uF and the other said 3uF neither of which seem right judgeing by the size of them.. they are the size and shape of resistors, have a pink body and the colour bands are blue, black, orange, black, brown. Anyone have any idea what the correct value is?
Thanks,
owen
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Old 11th January 2006, 04:59 PM   #8
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Quote:
Originally posted by DoomPixie
I have added the CCS circuit that they used in that distortion in power amplifiers link. that is a very usefull link, makes interesting reading.

Thanks,
Owen

edit to point out my deliberate mistake.. Q5 is the wrong way around.. Will fix next time i change the schematics. (this note is more to remind me than anything) lol
Owen

Hi Owen,

You might be better off to get rid of the voltage divider bias for the diff. for the sake of PSRR and use one resistor from ground for the +in side, and use a DC blocking cap for the -in side. The DC bias for the -in side comes from the output via the -feedback resistor. Looks about right otherwise.
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Old 11th January 2006, 05:29 PM   #9
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Ok, i have added a Vas stage and corrected a couple of errors i found.

CBS240: i made the changes you sugested as i understood them but im not 100% sure if thats what you meant?

anyone have any sugestions for transistors to use? i will make a list of what i have later but if they are no good i can always order more..

many thanks,
Owen
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Old 11th January 2006, 06:39 PM   #10
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ok, i just checked what transistors i have here.
i have:
2N3906
2N3055
2N3700
BC107
BSX20
BD140

any of these any use in this amplifier? i dont mind buying diffrent transistors if they are needed or would be better.
looks liek i have at least 10 of each, most i have more off though. Was sure i had a load of BC108 somewhere aswell but i couldnt find them. Have a "transistor lucky bag" on the way from maplins aswell that i threw in to my last order to make the total up so i could get free postage.

many thanks,
Owen
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