Reverse engineering of a 2.1 computer speaker system

Thread Starter

LordOfThunder

Joined Jun 27, 2018
80
Hello!
I bought this computer speaker system a couple of years ago (out of warranty) and it died on me shortly afterward. The brand and model of the system are not relevant for the sake of the question.

Now, the sanest thing to do would be to say "shit happens" and go on with my life. However, I took this as a chance to reverse engineer the PCB and fix it by myself. I hope to learn something new in the process because this is the first time for me to reverse engineer anything (I am still a student).

I am at a good point in the process of laying down the schematics but there is a part of the circuit that I do not understand at all.
https://drive.google.com/file/d/15M6652Yy4cdyIDQcpn84TPJSrajZP-OP/view?usp=sharing

As you can see after the high-pass filter there is a series of capacitors and resistors in seemingly random order. I am talking about R4, C4, R7, R8, C5, CE2, R24, C15, R27. I could not determine the capacitor values without desoldering them. Anyway, I just want to understand the qualitative behavior of the circuit. My question is:
  • Do you know what kind of purpose those components (R4, C4, R7, R8, C5, CE2, R24, C15, R27) have?

BTW I have emailed the company asking for the schematics but, understandably, they refused my request.
Thank you
 
Last edited:

Delta Prime

Joined Nov 15, 2019
1,187
Hello there
:)
I hope this helps.
This filter design is a two-pole filter comprised of two single-pole filters followed by a non-inverting

Sallen-Key_HPF10d1a69de0e742efb91266c7d308bc37.gif
amplifier.The critical frequency of this filter
second order sailing high pass filter(8)6aab53309cf64d0680ef39915a88a49d.gif

If the two resistors are equal and the two capacitors are equal, or if

CodeCogsEqn (9)1d96ca46798641c187550e3343ffcba8.gif
And
CodeCogsEqn (10)b25f516ffa134704a3550052fd2f4382.gif
Then the critical frequency becomes
CodeCogsEqn (11)cb462a8ef81d42dc9996539d2d89cf91.gif
The gain of the amplifier is
CodeCogsEqn (12)ab74ad1ce7534ba0bfd9c7539f9ffe88.gif
The amplitude and phase as function of frequency are given by
CodeCogsEqn (13)9669866cf10945bca03285870cfe9ad9 (1).gifAnd
CodeCogsEqn (14)5a42a960df8d42148bf45543b252d678.gif
A Bode plot of a second-order Sallen-Key HPF

38b837291ab2947d58c291bec3a22d1c6.png
 

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Thread Starter

LordOfThunder

Joined Jun 27, 2018
80
Thank you very much. It is a very good reference. However, I would like to know the purpose of the components that come AFTER the Sallen-Key high-pass filter.
 

Thread Starter

LordOfThunder

Joined Jun 27, 2018
80
I know. I mean the purpose of the R4, C4, R7, R8, C5, CE2, R24, C15, R27 components after the output of the opamp. please look at the pdf attached to the first post.
 
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