Samuel Groner
Well-known member
Hi
It occured to me several years back that an anti-parallel series combination of two electrolytic capacitors (i.e. two caps in parallel but with opposite orientations regarding polarity) might reduce their low-frequency distortion compared to either a standard single capacitor with twice the value or a standard parallel combination (i.e. polarities orientated the same way).
Today I finally found time/interest to verify this. I used a 1k series resistor and two 22 uF caps to ground, fed from a +20 dBu source. The caps were used either in standard parallel combination or the new anti-parallel arrangement. Here's the result:
The anti-parallel combination cancels even-order distortion products; hence there is appreciably lower distortion, particularly towards higher frequencies. The rise above 150 Hz/50 Hz is due to noise, not actual distortion--the noise increase occurs because the low-pass filter configuration reduces test signal level at higher frequencies. Likely the cancellation shows some sensitivity with respect to capacitor matching; I've not yet investigated this, at least the two caps used for the measurements were just randomly selected devices.
To my best knowledge I'm the first one who writes about this, despite the striking simplicity of the arrangement. The use of series combinations has been suggested before (i.e. by Bateman). While this is probably even more effective in reducing distortion (essentially it lowers the AC swing seen by the capacitor) it also reduces the effectively available capacitance, which is inconvenient as usually the reason to use an electrolytic capacitor instead of a film dielectric is size and cost.
So I might suggest that such anti-parallel combinations should be used for cases where sonic transparency is the main goal. The cost increase is modest, and likely this will help much more than the let's-add-a-100 nF-film-cap for which I've not yet found any objective evidence for an improvement within the audio frequency range.
Samuel
It occured to me several years back that an anti-parallel series combination of two electrolytic capacitors (i.e. two caps in parallel but with opposite orientations regarding polarity) might reduce their low-frequency distortion compared to either a standard single capacitor with twice the value or a standard parallel combination (i.e. polarities orientated the same way).
Today I finally found time/interest to verify this. I used a 1k series resistor and two 22 uF caps to ground, fed from a +20 dBu source. The caps were used either in standard parallel combination or the new anti-parallel arrangement. Here's the result:
The anti-parallel combination cancels even-order distortion products; hence there is appreciably lower distortion, particularly towards higher frequencies. The rise above 150 Hz/50 Hz is due to noise, not actual distortion--the noise increase occurs because the low-pass filter configuration reduces test signal level at higher frequencies. Likely the cancellation shows some sensitivity with respect to capacitor matching; I've not yet investigated this, at least the two caps used for the measurements were just randomly selected devices.
To my best knowledge I'm the first one who writes about this, despite the striking simplicity of the arrangement. The use of series combinations has been suggested before (i.e. by Bateman). While this is probably even more effective in reducing distortion (essentially it lowers the AC swing seen by the capacitor) it also reduces the effectively available capacitance, which is inconvenient as usually the reason to use an electrolytic capacitor instead of a film dielectric is size and cost.
So I might suggest that such anti-parallel combinations should be used for cases where sonic transparency is the main goal. The cost increase is modest, and likely this will help much more than the let's-add-a-100 nF-film-cap for which I've not yet found any objective evidence for an improvement within the audio frequency range.
Samuel