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Old 25th February 2012, 05:15 AM   #1
AlexQS is offline AlexQS  United States
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Question Coupling Cap at input?

I have one silly newbie question. “Do I need a coupling capacitor at my LM3875 input?”

I measured the pre-amp output of my receiver by turning the dial on my multi-meter to DC-Voltage section, at the setting 200m It read -05.6 on left, and 01.7 on right.

Does that mean I need a coupling capacitor, or is there an acceptable limit that it should be under before I would need the cap?

By the way, at the time of measurement, I was listening to music, about ¼ to 1/3 volume level I think. Did I do it right, or do I need to measure a sine wave or test tone or something, or am I doing it completely wrong?

Thanks,

AlexQS

p.s. I have searched before posting. I can find where to put the cap, and what value I would use. I just don't know if I need it or not. I think I need to find out if preamp is direct coupleing. Could I find out by measureing if there is DC Voltage on output?
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Old 25th February 2012, 05:32 AM   #2
GloBug is offline GloBug  Canada
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If your getting DC offset from your preamp then you should use the coupling cap.
The amp will amplify whatever DC it gets, making it worse at the speaker terminals.

If you measured say under 70mv at the speakers, no music, preamp connected, then you might be OK.

It is common practice to have these caps on All amplifiers.

You can only delete them if your positive you wont have a DC offset problem and won't be using it on other devices.

Check again at the speaker terminals, you likely need the cap.
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Old 25th February 2012, 05:42 AM   #3
AlexQS is offline AlexQS  United States
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Thanks GloBug. That makes perfect sense.

I thought I could maybe I could measure preamp before connecting to the amp.

So excited chips and pcb's will be here anyday, ...but it sounds like this is something to be measured on speaker out at the amp. Do I build the 3875 first without it the coupling cap, try it and then go back and add the cap if needed?
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Old 25th February 2012, 05:44 AM   #4
AlexQS is offline AlexQS  United States
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Quote:
Originally Posted by GloBug View Post
It is common practice to have these caps on All amplifiers.

You can only delete them if your positive you wont have a DC offset problem and won't be using it on other devices.

Check again at the speaker terminals, you likely need the cap.
My last question I see you already answered,.... sorry
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Old 25th February 2012, 07:48 PM   #5
AlexQS is offline AlexQS  United States
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Okay,… so maybe I don’t know the value I need.

The recommendation I am following is for “…if you need coupling capacitor to protect the amp from DC that may be produced by a source component, a small electrolytic cap can be installed here: 4.7uF or bigger”

So I found these capacitors at partsexpress.com , but they are also rated in voltage.

Any suggestions as to which one I should use? Feel free to suggest another parts-vendor if that would be better.

Other info you may need, each channel will have it’s own 330VA 25v + 25v torodial transformer. As far as capacitance, I will start with 2 x 10uF on each power-supply section + the 2 x main filter capacitors on each amp pcb. No snubber to begin with. I may tinker and add more capacitance after a while of forming my listening impressions, but am not planning to make lots of changes -as I have another project to finish first.

I realize that this may be a question about personal-taste, but I think it’s great that experienced people sharing their opinions helps others to learn and form their own opinions too. I hope someone will try to guide me as a I’m a new guy, and you probably have more experience than I do. J
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Old 26th February 2012, 12:22 AM   #6
PRR is offline PRR  United States
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Use the cap.

For most chip-amps, 0.47uFd will be ample for the high impedance of the chip bias resistor; and avoids troubles with electrolytics.

Use Film (poly etc), not ceramic in such large values.

Nominally you need at-least 0.006V rating. More pessimistically you should allow for some disaster, like a source's 15V rail shorting to output, so 15V-25V. You will probably find you can't buy less than 50V in a film cap.
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