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Old 2nd September 2003, 03:23 AM   #1
eehobby is offline eehobby  Canada
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Default Need 33uF coupling capacitor

I am contemplating building a pre-amp with a digital volume control. The circuit is shown here on page 5:
http://pdfserv.maxim-ic.com/an/en/app161.pdf

The capacitors are 33uF. The supply for the ciruit is just 5V. The only 33uF capacitors I can find at digikey are electrolytic or tantalum. The author of the application note doesn't indicate a polarized capacitor, so I'm stumped.

Q1. Is there such a beast as a 33uF non-polarized capacitor?

Q2. Where can I buy them?

Note: I have built many digital projects as an electronics hobbiest, but this is my first forray into diy audio projects.
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Old 2nd September 2003, 03:57 AM   #2
sam9 is offline sam9  United States
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A- There non-polar electrolytics, Panasonics has a small section in the Digikey catalog, and there are Black Gates;

B- If physical space will accomodate them look on page 63 of the latest PartsExpress catalog. These ate intended for speaker XO's but I dopn't se why you can use one such as the Solen 38mm x 53mm 2 US$9.25 each.

C- Maybe some other poster will correct me, but I can't see the utility of a 33uF cap in that application. I would think a 10uF metal film or even a 4.7uF would do. A cap in parallel to the input or output might raise cautions regarding oscillations, but in series... If it were me I would just use a Panasonic 10uF metal film from the Digikey catalog and not watch the results carefully. If then everthing seems OK, I would just shrug and not worry about it.

BTW- I've read that if electrolytics are of a large enough value, they have such a low ESR in the audio range that they efectively don't "exist" at audio frequencies. However, in this context the values seem to be 220uF or larger. 33uF certainly seems a little small for this argument.
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Old 3rd September 2003, 02:53 AM   #3
eehobby is offline eehobby  Canada
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Quote:
Originally posted by sam9
C- Maybe some other poster will correct me, but I can't see the utility of a 33uF cap in that application.
Sam9,
On the input, the capacitor and the wheatstone bridge act as a high pass filter, so reducing the value of C will raise the cutoff frequency. In this application, that won't bother me at all because all I am doing is controlling the volume of speakers on the deck.

On the output, I dont think it will matter what the value is (in fact the author of the App note hints that the capacitor can be removed if the next stage is capacitively coupled). I think the author choose the same value to keep the part-count down. So, I'll use the same value as on the input.

In summary, I'm going to take your advice and use a 4.7.

Thanks,
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