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Old 4th August 2011, 11:31 AM   #1
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Default ONE MORE P3A QUESTION

here is another P3A related question

What can be done to achive an input imp. of about 100K without altering the safety status of the amplifier ?

Idea is that with 100k input the amp is going to be way too friendly for passive preamps and/or make a wonderfull pair with B1 buffer ....

kind regards sakis


ot remind the schematic
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 4th August 2011, 11:38 AM   #2
lineup is offline lineup  Sweden
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R2=100k
R5=100k
R4=4.7k
C3=22uF


would keep the dc-offset and gain, but make input impedance 100k

C1 could also be lower ~1uF or 2.2uF
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Old 4th August 2011, 11:53 AM   #3
AndrewT is offline AndrewT  Scotland
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I recommend similar. Just a few minor adjustments.
C1=1uF & R1=100k.
R5=100k, R4=4k7, C3>=33uF.
C2 & R3 adjust to taste.
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Old 4th August 2011, 01:14 PM   #4
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yeap this was my approach too ...

a) please comment on the idea
b) please comment regarding theissue of safety IE anyone things that this might effect stability of the amp?

thanks for your help

sakis
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Old 4th August 2011, 01:22 PM   #5
AndrewT is offline AndrewT  Scotland
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I don't recognise any safety issue by substituting any of those component values.

Line2 of post3 shows R1=100k. This must be changed to R2=100k. Sorry.

Just noticed you posted an ESP schematic.
Ask him first.
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Old 4th August 2011, 01:33 PM   #6
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The particular schematic is not protected .... its free in the internet ...

Thanks for your help Andrew .

kind regards sakis
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Old 4th August 2011, 01:41 PM   #7
AndrewT is offline AndrewT  Scotland
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Go and read ESP site.
One of the conditions of using the information he provides freely, is that you agree to not reproduce his drawings as you have done in your post.
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Old 4th August 2011, 01:57 PM   #8
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i apologize for that probably i have been reading it but dont remember

Moderators please remove my post or schematic .

Thanks Andrew for the notice .

Kind regards sakis
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Old 4th August 2011, 02:58 PM   #9
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Noise will be considerably worse, some hiss may come about.
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Old 4th August 2011, 05:29 PM   #10
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Default Bootstrap

Or, to keep the amplifier running as present, you could bootstrap the input as per Self's illustration. Chapter 4 in the current 5th edition of his Audio Power Amp. Design Handbook. I haven't tried it but this should keep internal noise at present level while matching to the higher impedance set by the resistive divider added at the input. ..... BTW, It's also part of the trimodal amplifier, if your edition is earlier.
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Last edited by Ian Finch; 4th August 2011 at 05:53 PM. Reason: add ref.
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