Another non-polarised 'BP' electrolytic use query.... - Page 1
Forum section: electronics / repair

author: peteb2
author_rank: Regular Contributor
timestamp: December 27, 2017, 10:34:32 am
content: I'm restoring an early Yamaha amp (A460) that uses an STK-3042II hybrid module that for basic principles sake is a sort of dual OpAmp and running in this circuit as a Pre-drive Stage.The input audio signal into the device passes through two 3.3u/50V polarized electrolytics wired in series. The incoming signal 'sees' the +ve end of the 1st cap, then it's -ve joins to the second cap +ve and its -ve connects into the STK "+" terminal along with a 10k resistor to star GND.At the junction of the two caps there is a 4k7ohm resistor that ties to the STK "-" terminal with a polarized electrolytic 100u/6.3V to star GND (+ to GND).Here's my question: can i get away with substituting replacement BP electrolytic devices into these locations or will 'BAD' things happen? I would hate for the STK to let go as i expect it to be fairly next to impossible to buy now.... ! I have some quality Panasonic BPs to hand and at this time of year being summer holidays in my part of the world to order in standard polarized devices there will be a lengthy wait.... My PEAK tester shows all caps removed are very sick... as in way off value (40uF for the 100 with crazy ESR of 0.46)....Thanks in advance...

author: stj
author_rank: Super Contributor
timestamp: December 27, 2017, 12:53:42 pm
content: it should work but they will be a lot bigger.i would try to use film caps myself.

author: peteb2
author_rank: Regular Contributor
timestamp: December 27, 2017, 10:43:50 pm
content: Interestingly the aluminum BP devices are physically smaller compared the original aluminum polarized if anything. The improvements in technology i guess... I suppose as long as the new BP devices block DC and pass the audio signals things will be ok. I expect that the amp will actually sound a whole lot better with the proper value / better ESR caps in place too.I've done a bit of back-engineering and made a rough sketch using the iPad pro to better explain the placement of the caps in question...

author: jaycee
author_rank: Regular Contributor
timestamp: December 27, 2017, 11:46:08 pm
content: The two 100u/50v capacitors on the supply rails are obviously supply decoupling capacitors, and thus shouldnt be BP types.Replacing the 100u/6.3v capacitors in the -ve feedback path with BP's is fine, and harmless. Personally I would use 16v rated types at least.Im really not sure whats going on with the arrangement between the + and - inputs of the STK! The effective DC blocking capacitance will be 1.5u which seems a bit low to me... I would not mess with this arrangement though as it might be important for the stability of the amplifier.

author: drussell
author_rank: Super Contributor
timestamp: December 29, 2017, 05:58:05 pm
content: Quote from: stj on December 27, 2017, 12:53:42 pmI agree! If you're replacing small-value electrolytics anywhere in the signal path, I would replace them with plastic film capacitors. Much, much more linear, lower distortion, etc... better all around.. and these days they really aren't very expensive.