As I mentioned in another thread, I performed a set of upgrades to the F&D (Fenda) A520 Subwoofer Amplifier. The A520 sounds very good right out of the box - these upgrades are only necessary to improve the durability/reliability, and optionally to improve the sonics.
1) Change the secondary-side filter caps of the switching power supply, C5 to C8, to high-quality Japanese electrolytics. I replaced them with Rubycon ZLH 1000uF/25V, which are low-ESR and extremely durable. However, lots of choices are possible - the only constraint is that the diameter is 10 mm.
If you only want to improve the durability, that's all is required. The remaining upgrades are for sonics.
2) There are two ceramic input coupling caps, C15 and C16, which are redundant because there are additional coupling caps right at the chipamp inputs. Hence, these may be shorted out. I replaced them with 1.5 ohm Beyschlag thin-film mini-MELF resistors which are approximately the same value as the ESR of the capacitors which I removed, but a 0805 0 ohm resistor or even a plain wire jumper across each of C15 and C16 is fine.
3) (Optional) Change the -3 dB lower cutoff frequency of the HPF to the satellites. By default, this is around 400 Hz with the factory values of C13,C14=0.6 uF, C31,C37=0.1 uF and C27,C45=1 nF, all ceramics. I upgraded them to C13,C14=4.7uF/6.3V X5R ceramic; C31,C37=47nF/16V Panasonic ECHU PPS film and C27,C45=820pF/50V Panasonic ECHU PPS film. The net effect is to change the lower cutoff to around 130 Hz, which allows more of the lower-mids/mid-bass frequencies to go to the satellites.
4) (Optional) Also visible are changes to feedback resistors R31, R33, R27, R34 - all changed from 0805 SMD to 0204 thin-film mini-MELF. This improves the linearity of the feedback network.
5) (Optional) Change U1 and U2 from TDA2030 to LM1875.
The net effect of all the mods above is improved durability and transparency in audible sonics. No changes were made to the subwoofer signal path except the input coupling caps C15, C16 mentioned above. All changes are limited to the signal path and chipamps for the satellites only.
1) Change the secondary-side filter caps of the switching power supply, C5 to C8, to high-quality Japanese electrolytics. I replaced them with Rubycon ZLH 1000uF/25V, which are low-ESR and extremely durable. However, lots of choices are possible - the only constraint is that the diameter is 10 mm.
If you only want to improve the durability, that's all is required. The remaining upgrades are for sonics.
2) There are two ceramic input coupling caps, C15 and C16, which are redundant because there are additional coupling caps right at the chipamp inputs. Hence, these may be shorted out. I replaced them with 1.5 ohm Beyschlag thin-film mini-MELF resistors which are approximately the same value as the ESR of the capacitors which I removed, but a 0805 0 ohm resistor or even a plain wire jumper across each of C15 and C16 is fine.
3) (Optional) Change the -3 dB lower cutoff frequency of the HPF to the satellites. By default, this is around 400 Hz with the factory values of C13,C14=0.6 uF, C31,C37=0.1 uF and C27,C45=1 nF, all ceramics. I upgraded them to C13,C14=4.7uF/6.3V X5R ceramic; C31,C37=47nF/16V Panasonic ECHU PPS film and C27,C45=820pF/50V Panasonic ECHU PPS film. The net effect is to change the lower cutoff to around 130 Hz, which allows more of the lower-mids/mid-bass frequencies to go to the satellites.
4) (Optional) Also visible are changes to feedback resistors R31, R33, R27, R34 - all changed from 0805 SMD to 0204 thin-film mini-MELF. This improves the linearity of the feedback network.
5) (Optional) Change U1 and U2 from TDA2030 to LM1875.
The net effect of all the mods above is improved durability and transparency in audible sonics. No changes were made to the subwoofer signal path except the input coupling caps C15, C16 mentioned above. All changes are limited to the signal path and chipamps for the satellites only.
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