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Old 11th June 2014, 08:42 PM   #1
davym is offline davym  Scotland
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Default DC blocking for LM3886 chip amp

Hi all, I have a Brian Bell (Chip Amp) LM3886 kit which I'm building into a new enclosure for our lad. When I last used it I didn't bother fitting DC blocking caps because I knew my sources did not output any potentially damaging DC but I can't be sure that will always be the case in the future. There is no provision on the PCB's for a blocking cap and lack of space inside the enclosure means it would be difficult to fit them in.

What if I fit a couple of axial 1uF caps inside the separate passive preamp I'm assembling for use with this amp? I'm thinking about putting them between the volume pot output terminals and the signal output sockets. Is there any reason why this would not be a good idea?
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Last edited by davym; 11th June 2014 at 10:32 PM. Reason: Brain fade
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Old 11th June 2014, 09:32 PM   #2
sesebe is offline sesebe  Romania
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Here you have some variants:
Film Capacitors Product List | Capacitors | Newark element14 US
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Old 11th June 2014, 10:31 PM   #3
davym is offline davym  Scotland
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Cheers sesebe, it's really the value of the caps I need to consider that I'm questioning, I thought 1uF axial types would do the job if I got a decent quality cap because it will be directly in the signal path. I might send an email to Brian Bell to see what he thinks.
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Old 12th June 2014, 10:21 AM   #4
AndrewT is offline AndrewT  Scotland
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I have many 1uF 50V and 63V MKT capacitors. They are 0.2" pin pitch and are 7mm x 5mm x 3mm. They don't need much space. They are smaller than the RCA input socket that fits on the inside of the Case.
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Old 12th June 2014, 02:53 PM   #5
davym is offline davym  Scotland
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Fair point Andrew, I have a couple of quite large axial 1uF capacitors I was thinking about fitting inside the passive preamp but I could always buy in some smaller ones. I suppose either solution would work as both units will always be used together.

Found these on ebay - http://www.ebay.co.uk/itm/2x-DC-Film...item2a24c067fc

I wonder what effect they will have on sound quality, probably not much.
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Last edited by davym; 12th June 2014 at 03:01 PM.
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Old 12th June 2014, 04:51 PM   #6
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A properly employed capacitor in a properly designed circuit will have for all practical purposes no audible effect on performance. The distortion introduced is so small that its consideration is academic.

Capacitor choice becomes much more critical in filter and tone control circuits where the capacitor has significant impedance inside the audible range (or within a decade).

There's nothing wrong with using nice film capacitors for coupling. They do have a different distortion profile than electrolytics.
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Old 12th June 2014, 04:52 PM   #7
sesebe is offline sesebe  Romania
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Seem to be OK.
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Old 12th June 2014, 05:08 PM   #8
davym is offline davym  Scotland
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Thanks fellas. The amp PCB's have a position for an optional feedback capacitor listed as CI = 47uF 50V Panasonic FC.

The kit instructions state that a lower DC offset can be obtained if this cap is fitted but possibly at the expense of sound quality. Hmmm
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Last edited by davym; 12th June 2014 at 05:16 PM.
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Old 12th June 2014, 05:46 PM   #9
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Quote:
Originally Posted by davym View Post
Thanks fellas. The amp PCB's have a position for an optional feedback capacitor listed as CI = 47uF 50V Panasonic FC.

The kit instructions state that a lower DC offset can be obtained if this cap is fitted but possibly at the expense of sound quality. Hmmm
Not if it's done right.

Calculate a pole that's at least lower than the input low frequency pole. Do not use a larger capacitor than necessary. You can bypass this electrolytic with a nice film capacitor in the neighborhood of 1 uF if you like.

Anything that happens below 2 Hz is sonically unobtrusive. 3 dB down at 2 Hz = 0.1 dB down at 20 Hz and phase shift in the audio range will be negligible too.

This is what I mean by "properly designed." If you understand and quantify your parameters, you can manipulate them to your needs too.
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Old 12th June 2014, 06:54 PM   #10
davym is offline davym  Scotland
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Thanks Eddie, I didn't even think about bypassing the electrolytic but there is space on the underside of the PCB where I can fit a film cap. Need to do some calculations now...
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