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I have a pair of 20 years old electrostatic speakers that have an analog first order high-pass filter before the electrostatic panels, so there is a capacitor in the audio signal path. The original capacitors were electrolytic (I suppose the capacitors were about the same age as the speakers) with a capacitance of 100 uF which I replaced (by recommendation) with Mundorf MCap 250 MKP capacitors. After the switch I subjectively heard an improvement in the sound quality with a more refined upper end. I should note that I do use a calibration system with mic that should fix most big problems in the signal frequency response.

My question is if it just was an placebo effect or if there is actually some measurable quality in the capacitors that can cause the improvement in sound/signal quality? If so, what causes this and is it a consequence of the aging of the capacitors or have the type of capacitor a measurable effect on sound/signal quality (there are very expensive "high-end" capacitors sold by DIY speaker stores)? Does the same differences exist with different types of resistors that are in the signal path?

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    \$\begingroup\$ It could be real. As electrolytics get old they can show an increase in ESR. Did you save the old caps? You could measure the ESR. (Resistors won't make much difference... well old cabon composites have excess noise.) \$\endgroup\$ – George Herold Dec 9 '14 at 17:20
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Modern analyses of electrolytic capacitors definitely show an increase in ESR and a reduction of capacitance but these are modern capacitors and your last-century caps might be worse. See this document entitled "Experimental Studies of Ageing in Electrolytic Capacitors" and particular the graphs on page 4 and 5.

DC leakage currents may also increase with age and this could alter the bias points on the amplifier for your electrostatic speakers which, in turn, might cause higher frequencies to be attenuated more hence, your perception of better top-end response when the caps were swapped. See also this document entitled "Capacitors Age and Capacitors Have an End of Life".

If just used as a high-pass filter I expect that you might not have witnessed any aural change but given that bias points on amplifiers could change I think it's feasible.

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  • \$\begingroup\$ Thanks for the references about aging. Besides aging are there any other measurable differences between electrolytic and MKP (or similar) capacitors that could affect the quality of the audio signal (frequencies < 20 kHz, typically max 50 W)? I'm a firm believer in that if you can't measure it, the ear can't detect it (unlike some other audiophiles). \$\endgroup\$ – Jesper Nordenberg Dec 10 '14 at 11:20
  • \$\begingroup\$ I can't think of any that would be relevant in the type of circuit you have. \$\endgroup\$ – Andy aka Dec 10 '14 at 12:09
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20-years old electrolytics, especially if they are only rated to 85 degrees Celsius have basically exceeded their lifespan. The electrolyte dries out over time. You should really tell us more about the original caps, but assuming a typical 2000-hour cap at 85 degrees C, you can calculate their expected life at 25 degrees C (average room temp.) as 2^((85-25)/10)*2000 = 128,000 hours = 14.6 years. (For every 10 degrees under the limit temperature, the lifespan [given at the limit temp] doubles). I actually have a 1985 electrolytic relic, originally rated at 6800uF (physically it is thick as size D battery), which has only about 10% of its original capacitance left. So drying out is quite real.

As for whether the audiophile-graded caps you bought are really worth the dough, you should compare them with new, run-of-the-mill electrolytics, not with the old ones.

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I have replaced electrolytic capacitors in both my Sansui au-555 & sp-100 speakers (circa 1960's). The difference in sound quality in the speaker crossover capacitors was largest. 4x original electrolytics swapped for poly-propylenes. The sound was certainly slightly muffled and compressed in comparison to the new caps. The difference was more so in dynamic response than the freq. response. A much more open and lively sound. I measured this with my ears - I am trained in Audio production and live audio.

In regards to electrolytic vs Polyprop etc. Lifespan is certainly a well agreed difference. Sound quality I wont get into!

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