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Old 29th November 2006, 12:04 PM   #1
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Question Matching capacitors

Hi all,

I need to match some capacitors for a RIAA network on a phono stage and since I don't have a way to measure them I am wondering if it can be done with a 555 IC and a frequency meter (my multimeter has a freq. meter up to 200 KHz).

With a 555 timer in astable oscillator, if I choose the resistors (1% or better) for a target frequency I think I could easily match the capacitors and get a very good match. What do you think?

Thanks im advance.

Luis
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Old 29th November 2006, 12:38 PM   #2
SY is offline SY  United States
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The easy thing is that as long as you get the channel-to-channel match down to a fine point, the actual deviation from nominal is less important. So relative matching is more important than absolute matching, IMO.

If you've got a two channel scope, you can use a 1kHz square wave and match caps by putting them in an RC lowpass and measuring rise-time.
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Old 29th November 2006, 01:24 PM   #3
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Hi SY,

I understand that in the case of a RIAA network the matching between channels is more important than the absolute values of the caps.

I don't have a 'scope, that is why I was thinking of using a 555 and my multimeter as a frequency meter.

Luis
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Old 29th November 2006, 11:55 PM   #4
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I think you may have a good idea for a cap tester using the 555 with precision resistors and a counter. At least you can cull out caps which deviate far outside the center value, and you can match any two caps to each other pretty closely. The 555 spec sheet indicates that accuracy of the frequency of an astable oscillator will be about 2%, plus another few points over temperature. If you happened to have a known precision (1%) capacitor, you could "calibrate" the jig. For a few pennies, or Eurocents, you can easily give it a try. Let us know how well it works.
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Old 30th November 2006, 12:22 AM   #5
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Hi Brian,

Thank you for your kind reply. I'll order the capacitors in a few days and will post my results.

Luis
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Old 30th November 2006, 10:49 PM   #6
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One gets a Japanese RCL meter for the cost of a moderately expensive multimeter (at least here in the RSA). If electronics is your hobby, it might well pay you to get such an instrument. I find it very handy and use it often. I am amazed at the amount some caps can be out, often barely (or not) making 10%. Also, the same make and value might all be out but all almost equal (same batch?)

Do consider buying such an instrument.

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