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Old 18th June 2008, 04:20 PM   #1
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Default voltage rating for Ci?

Hallo All,

What would the minimal voltage rating for a Ci capacitor in a LM3875 amp be?

I am thinking that the voltage seen on a Ci cap cannot be more than few volts maximum.

I have some panasonic FC 47uf 10 volt capacitor. Would these be ok?

I also have some nichicon 100uf 35 volt PW series. Would these be ok as well or are they too big?

If the above are not good to use, I am thinking of getting some panasonic bi-polar caps SU series in 47uf 10v, 25v or 35v rating. Would these be a better choice?

Thank You,

Robert
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Old 18th June 2008, 09:11 PM   #2
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I would go for the nichicon, it's always better to stay on the big side with this cap.

If you want to remove sub-bass do it on the input coupling cap.
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Old 18th June 2008, 10:25 PM   #3
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Thank you,

I do not have an input cap. I am an using audiosector kit. I am guessing that subsonic frequencies will still be there, but a Ci cap is better than DC offset.

I just wanted to know what to use in case I had DC offset. I will use this amp for my "B" studio monitors for recording. DC offset is is a no-no when recording.

Thank You again,

Robert
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Old 19th June 2008, 05:22 PM   #4
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Hello,

I figured that the 100uf cap with a 680ohm resistor would give a corner frequency of 2.3Hz.

It this low enough to avoid the electrolytic distortion?

Some one suggested on another thread, that using an electrolytic as Ci a corner frequency of 1 Hz would avoid the problems of the electrolytic in this application.

Also, How would I add an input cap? What size and where in the signal would it go?
Do I need an input cap?

Thank You,

Robert

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Old 20th June 2008, 12:09 AM   #5
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Input filters usually are 1uF + 47K. There is no problem with electrolytic distortion since it's easy to find a plastic one at that capacitance.

You have to play with three aspects here:

· Input impedance
· Type of capacitor you can use
· Resistor noise (and offset)

For the Ci i belive that a 220uF would do the job pretty well.
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Old 20th June 2008, 01:38 AM   #6
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Check this post for input cap: http://www.diyaudio.com/forums/showt...78#post1508778
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Old 20th June 2008, 11:48 AM   #7
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Hello,

Thank you to you guys.

I was asking about Ci, as far as the electrolytic questions were concerned.

So I did learn something here. I learned that Ci brings DC offset down to unity gain.

And the input coupling cap is what rids the inputs of any incoming DC offset.

I get it now, thanks again.

Therefore, if I was to theoretically use a Ci cap and a input coupling cap...I could use a 100uf for Ci in series with R3 (this is the correct place for it, ja?) and the input coupling cap goes somewhere in the input.

Does the input coupling cap go in series with the positive input lead (wire),before any volume pot?

This is where I am not so sure...
where exactly does the input coupling cap go? What size resistor goes with it? What is their order and placement in the circuit?

Does another resistor need to be used for this cap as well or does the 220 Ohm resistor that I soldered in the R1 position stay in place and then a Cap is added before it in series?

Or do I add another resistor of 47k with a 1uf cap in series in front of the R1 resistor? In what order do these components occur?

Thank You,

Robert

p.s. sorry for the ton of questions...I am compelled to know how to do this properly if needed in the future...
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Old 20th June 2008, 01:28 PM   #8
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If you decide to use input coupling cap, install it in place of R1. The volume pot goes before coupling cap, R1 is not needed.
No other changes need to be done to the circuit.
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Old 20th June 2008, 02:10 PM   #9
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You do not want to use a polar electrolytic there...given the choices available...
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Old 20th June 2008, 06:29 PM   #10
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OK, all is clear now!

Thank you again!

Robert
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