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Bipolar electrolytics vs back to back standard
Bipolar electrolytics vs back to back standard
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Old 18th April 2019, 02:38 PM   #1
davidsrsb is offline davidsrsb  Malaysia
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Default Bipolar electrolytics vs back to back standard

How do back to back polarised electrolytics compare with bipolars?
With back to back I can reform parts that have been sitting in a draw for years, with bipolars what happens as they sit idle?
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Old 18th April 2019, 02:41 PM   #2
DF96 is offline DF96  England
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I am not sure how you might reform a bipolar - use AC? Use DC both ways?
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Old 18th April 2019, 07:41 PM   #3
egellings is offline egellings  United States
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I used a pair of 10uF polar ones in series to get about 5uF. I tied the (-) ends together and this works fine. This replaced a 4.7uF nonpolar (also called bipolar) cap in a DC nulling servo used in a Yamaha C-70 preamp. The cap served as the feedback cap in the integrator circuit. Old cap leaked and the integrator would not zero out the DC offset. Offset is now okay, at about 0.1mVdc. A film cap would be best in this position, but I could not fit one in there because of space limitations. It's important that leakage current be similar in both caps so that DC voltage divides evenly between them when sizable DC is present.
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Old 18th April 2019, 07:48 PM   #4
MarcelvdG is online now MarcelvdG  Netherlands
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I guess you can reform a bipolar electrolytic (I mean the type with two formed foils) by using DC in both directions. Capacitor distortion is somewhat worse for back-to-back polarised electrolytics, see Cyril Bateman's famous series of articles in Electronics World.
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Old 18th April 2019, 08:13 PM   #5
scottjoplin is offline scottjoplin  Wales
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Is a bipolar signal coupling cap ever going to need reforming, I was wondering the same thing a while ago but couldn't find any info?
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Old 18th April 2019, 08:34 PM   #6
MarcelvdG is online now MarcelvdG  Netherlands
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I'm no electrolytic capacitor expert, but I can't think of any reason why it wouldn't. I mean, when the formed electrode in a normal electrolytic starts to leak excessively when there is no voltage across it for a very long time, why wouldn't the same happen with the two formed electrodes in a bipolar electrolytic when there is no voltage across them for a very long time?

As far as I know, an electrode that has not been formed can still handle about 1.5 V to 2 V, so maybe you can get away with it when the voltage across the capacitor always stays below 1.5 V in normal use.
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Old 18th April 2019, 08:39 PM   #7
Zung is offline Zung  Switzerland
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Do a search for Cyril Bateman Capacitors Sound: he's done extensive and definitive tests on this topic.
The short version, in his conclusion:
"Having measured a considerable number of aluminium electrolytics using test voltages from 0.1 volt to 3 volt, with and without
bias, a single Bi-polar type produced lower distortion than larger, more expensive, specialist polar capacitors..
Much better results were obtained by connecting two double capacitance value Bi-polar electrolytics in series. Using 1 volt or
smaller test voltages and no bias, distortions for a double Bi-polar and the metallised PET assembly were similar.
With increasing bias or with increasing test voltage, the metallised PET assembly produced less distortion than any electrolytic
I tested.
"
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Old 18th April 2019, 08:58 PM   #8
Elvee is offline Elvee  Belgium
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Each capacitor of a bipolar capacitor (I am talking about the two main capacitors, not the additional parasitic ones in series with those) behaves as an electrolytic diode junction when its voltage rating is exceeded.
The forward voltage is very low, like 1 ~2V, meaning that all the rest will be applied to the reverse-connected one, helping reforming it.

Not a very clean process, meaning a non-linear current, at least when the new voltage is first applied, which partly explains the non-linear behavior of these components.
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Old 18th April 2019, 09:00 PM   #9
indianajo is offline indianajo  United States
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Quote:
Originally Posted by MarcelvdG View Post
Capacitor distortion is somewhat worse for back-to-back polarised electrolytics, see Cyril Bateman's famous series of articles in Electronics World.
I tried two 5600 uf caps back to back on the speaker output of a .02% HD PA amp. At about 1/4 watt it sounded funny. Direct transistor outputs sound better, however dangerous that is to your speaker. Test track was top octave solo grand piano, Peter Nero Young & Warm & Wonderful.
Single supply amp with a polar electrolytic biased to 1/2 rail voltage at the output does not sound funny at the same wattage.
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Last edited by indianajo; 18th April 2019 at 09:03 PM.
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Old 19th April 2019, 12:01 AM   #10
davidsrsb is offline davidsrsb  Malaysia
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There is a absence of information on what happens to bipolars in use or in storage. I cannot see why bipolars should behave differently. I am going to try to measure this, with some old polars that have been in a drawer for 20 years. Connect back to back and apply a large AC signal to simulate what happens in a speaker crossover.
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