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Old 22nd March 2006, 06:09 PM   #1
CJ900RR is offline CJ900RR  Sweden
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Default Input cap?

Edit: this questions is for the LM3886

Do you really need a input cap with a resistor to ground? I guess it acts like a low-pass filter, right or wrong?

I didnt use it in my first, exprimental, gc but i have added it to the one i use now, and i think that my first one sounded way better. Specially in middle and high frequensys. And it isnt present in the National datasheet.

And i have also added a 220pF cap (Cc in Nationals datasheet) between leg 9 and 10, as suggested in the datasheet (did that on my first amp to) and it made the amp more friendly to my ears.

But, if i still need a input cap with a resistor to ground, how do i calculate the requierd values for it? And regarding the cap between leg 9 and 10, does a bigger cap (470pF for example) means that it let thrue higher freq. and a smaller cap (100pF maybe?) means that it cuts earlier?

Best regards, Carl-johan...
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Old 22nd March 2006, 06:34 PM   #2
Deltat is offline Deltat  United States
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Actually that would be a high pass filter. You dont need it if your 100% confident your not going to be getting any DC on your input signal. The equation for the cut off frequency it 1/(2piRC)

So lets say you use a 10uF cap and 100kohm resistor you will have a cutoff frequency (or 3dB down point) of about 1.5Hz. This will block any DC and will not affect your passband range.
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Old 22nd March 2006, 06:35 PM   #3
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Default Re: Input cap?

Quote:
Originally posted by CJ900RR
Edit: this questions is for the LM3886

Do you really need a input cap with a resistor to ground? I guess it acts like a low-pass filter, right or wrong?
No but watch the dc offset
High pass

Quote:
Originally posted by CJ900RR

But, if i still need a input cap with a resistor to ground, how do i calculate the requierd values for it?
159155/(Hz * R)
159155/(Hz{cutoff frequency} * R{resistor to ground)= Cap value. What ever cutoff frequecy you want divide that by four

Quote:
Originally posted by CJ900RR

And regarding the cap between leg 9 and 10, does a bigger cap (470pF for example) means that it let thrue higher freq. and a smaller cap (100pF maybe?) means that it cuts earlier?
The larger the cap the earlier the cutoff
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Old 22nd March 2006, 07:11 PM   #4
CJ900RR is offline CJ900RR  Sweden
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Well there you see! Thank you both of you. My input cap now is 1uF and the resistor is 47k. To narrow for me. I will try a 10uF and a 100k tomorrow and see how it sounds.

Many thanks, Carl-Johan...
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Old 22nd March 2006, 07:41 PM   #5
Nordic is offline Nordic  South Africa
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Note, that the quality of this cap can make or break your amp.
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Old 23rd March 2006, 12:58 PM   #6
Deltat is offline Deltat  United States
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Quote:
Originally posted by CJ900RR
Well there you see! Thank you both of you. My input cap now is 1uF and the resistor is 47k. To narrow for me. I will try a 10uF and a 100k tomorrow and see how it sounds.

Many thanks, Carl-Johan...
A 1uF and 47k should be fine as the cutoff frequency is 3.386Hz which is still infrasonic.
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Old 23rd March 2006, 09:17 PM   #7
CJ900RR is offline CJ900RR  Sweden
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Quote:
Originally posted by Deltat


A 1uF and 47k should be fine as the cutoff frequency is 3.386Hz which is still infrasonic.
In fact it wasn't fine. The amp lost it's life with those values

But now whit the new values it's alive and kicking again The hig's and middle is way more seperated and it's a clear sound. Just the way i like it. Before, all the middle, high and sometimes lower frequens allmost floated together.

The only thing to adjust now is the bass, witch hopefully will be more sharp after some power-adjustements.
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Old 23rd March 2006, 10:43 PM   #8
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What is the value for the resistor and cap, you are now using?
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Old 24th March 2006, 05:16 AM   #9
Nordic is offline Nordic  South Africa
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The bass is in the cap, get a decent $10 input cap and you'll see.
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Old 24th March 2006, 12:46 PM   #10
Deltat is offline Deltat  United States
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Quote:
Originally posted by Nordic
The bass is in the cap, get a decent $10 input cap and you'll see.

Can you make a suggestion on where to look?
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