Gonna "bust a cap" in my LM3875.. What size cap? - diyAudio
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Old 7th May 2014, 03:09 PM   #1
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Default Gonna "bust a cap" in my LM3875.. What size cap?

Hey guys..
I am working with a Audiosector LM3875 KIT and recently added 2 x Panasonic 10,000uf caps to the PSU and I quite like the ummph in the bass end.

So, I was thinking "lets add some BIGGER caps".. is that a sane idea? If so, what is the max anyone has ever added? What difference did it make?
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Old 7th May 2014, 03:19 PM   #2
AndrewT is offline AndrewT  Scotland
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I use an input filter of ~90ms for my Power Amps.
That suits a +-20mF of smoothing feeding an 8ohms amplifier channel.
If I were using 4ohms speaker I would have to increase the smoothing to +-40mF per channel.

If you high pass filter your power amplifier at a lot less than 90ms, then you can manage with a lot less capacitance.
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Old 7th May 2014, 03:23 PM   #3
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Hi Andrew, What is an input filter? I am sorry but it is a little beyond my knowledge
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Old 7th May 2014, 07:16 PM   #4
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Hi Andrew, Just seen this thread where I think you tried to provide a similar answer to above dumb question..

Buffer/Gain stage suggestions for LM3886
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Old 8th May 2014, 09:27 AM   #5
AndrewT is offline AndrewT  Scotland
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Does your further reading reveal what an input filter is?
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Old 8th May 2014, 09:30 AM   #6
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A little, it's all very confusing tbh. I am not a hardware guy I guess.
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Old 8th May 2014, 10:04 AM   #7
AndrewT is offline AndrewT  Scotland
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There are usually two input filters for any audio gear.
A Low Pass filter, it attenuates the very high frequencies. This is used to reduce the effects of VHF interference from motors and lighting control and relays switching and wireless telephones and mobile phones and wireless alarm systems and WiFi and etc......
This a resistor in series with the signal followed by a capacitor to Signal Return. It is seen as an RC and is a single pole when just two components are used.

A High Pass filter, it attenuates very low frequencies. This is used to reduce the effects of VLF interference and blocks DC from faulty equipment, or from poorly designed equipment (with inadequate instruction manual).
This is a capacitor in series with the signal followed by a resistor to Signal Return. Again this is a single pole filter when just two components are used.

These two input filters should always be used on every piece of audio equipment until you understand why they are there and in what circumstances you can design out their need at the inputs.
Together they cost just 4 cheap components.

Finally, you should, in my opinion, not be modifying audio equipment designs by guesswork.
In the very early stages you can COPY existing good designs and as your skills accumulate you can apply science to what effects your proposed modifications may have on performance.

I never recommend guesswork modifications. It can lead to gross errors in performance.
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Last edited by AndrewT; 8th May 2014 at 10:09 AM.
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Old 8th May 2014, 10:40 AM   #8
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Quote:
Originally Posted by dakku View Post
If so, what is the max anyone has ever added? What difference did it make?
The max I've added to a chipamp has been around 200,000uF per channel. Definitely it improved the bass 'snappiness' over 100,000uF. I plan to go higher with my latest build, at least 400,000uF. Note this is being fed from a SMPSU, not a trafo plus bridge rectifier. If you went the latter route I'd recommend CLCLC filtering before the final wallop of capacitance.
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Old 8th May 2014, 01:39 PM   #9
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Quote:
Originally Posted by AndrewT View Post
There are usually two input filters for any audio gear....
Hi Andrew,
Thank you again for taking the time to reply. It is much clearer now I assumed it was only the speaker crossover that had hi and low frequency filters.

The CAP I was reffering to was following on from your suggestion regarding adding Smoothing caps to the PSU. So, I had added 2 x 10,000uf caps as per this pic posted by 6L6:

Click the image to open in full size.

So, my question is, should I up the caps to bigger ones? Would it make a noticeable difference?
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Old 8th May 2014, 01:40 PM   #10
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Quote:
Originally Posted by abraxalito View Post
I plan to go higher with my latest build, at least 400,000uF




400,000uf. What would that look like?
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