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Old 9th November 2010, 10:10 PM   #1
ChrisA is offline ChrisA  United States
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Default Need 1% tolerance caps

I want to build some audio filters as part of some home made test equipment. I need some caps with values matched at the 1% level.

Values are in the range .1 to .001 uF moderate voltages as I'm just working with line level audio using op amps and a +-15V supply.

Yes I know of a few work-arounds. I could buy 100s of the 10% type and sort them with a mater or use trimmers. But that is less practical them simply using 1% tolerance parts

I need a type with good temperature stability but I assume any 1% caps would have that.

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Old 9th November 2010, 10:13 PM   #2
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LCR do a selection of 1% polystyrene caps, although these wont suffer high temperatures very well.
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Old 9th November 2010, 10:41 PM   #3
jcx is offline jcx  United States
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simplest is goto DigiKey parts selection page, select film cap, then 1% tolerance, "apply filter"

I'd also check ceramic - NP0/COG may be good enough depending on your requirements

it is also possible to do much more with soundcards and clever test design and digital processing with reduced requirements on the analog circuit's exact matching or precise notch frequencies - the soundcard can calibrate out test circuit response ripple or operate at "odd" center frequencies, mathematically subtract out residuals, harmonics of the source..

especially powerful with today's cheap soundcards and processing power are indirect tests looking for IMD products with multiple test tones - resolution below your test circuit's active gain, filter stage's inherent linearity can be possible

ESI Juli@ is still a very good price performance card @ ~$130

Last edited by jcx; 9th November 2010 at 10:53 PM.
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Old 9th November 2010, 11:11 PM   #4
ChrisA is offline ChrisA  United States
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Yes I know about computers. I have a pretty decent studio grade audio interface that is spec'd at 0.003% THD which is far better than any of the valve power amps I'll be measuring.

But I want the ability to hear, over a speaker the distortion products with the signal removed and with zero digital effects. I'd like to see the residual waveform on an analog scope. I think you gain insight with analog test equipment. You get better numbers with digital signal processing.

Maybe I'm not looking hard enough but I'm not finding much at Digikey by putting "1%" into the parts filters. Is there a specific type of cap that might come in 1% tolerances?
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Old 9th November 2010, 11:37 PM   #5
jcx is offline jcx  United States
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select "film" under Capacitors in Product Index page - then filter by 1% tolerance

for ceramic you would also have to drag select C value range, dielectric and check "in stock" to get down to managable number of pages

Audio DiffMaker and a "studio grade" ADC could give you a preview without the hardware hacking (beyond a simple divider for the Amp output to ADC) for a null test

Last edited by jcx; 9th November 2010 at 11:42 PM.
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Old 10th November 2010, 12:35 AM   #6
amc184 is offline amc184  New Zealand
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I was recently in a similar situation when I built a phono preamp. I decided on Wima FKP2 capacitors (a stacked polypropylene film and foil type, nice for audio) which had a tolerance of 5%. I measured each one to get as close as possible match between channels, and I found that about three quarter were within a 1% tolerance. I had ordered twice what I needed and got a very good set out of that. The FKP2 series only goes up to 33nF, not the 100nF you specified, but there are other similar products that will cover the full range of values you want.

If you are fixed on capacitors with a tight tolerance, then polystyrene ones are the way to go. You'll probably pay about five times as much over the ones I specified above though, unless you go for NOS surplus ones (which may be out of tolerance due to aging anyway).

Be sure to steer clear of anything using a polyester dielectric, they have a high and non-linear temperature coefficient. Polycarbonate and polypropylene are okay in this respect, and polystyrene and teflon are the best.
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