The following two screenshots are from the HK-3350 30-watt stereo receiver service manual. This a 1991 model.
The full SM with schematic is here:
https://elektrotanya.com/the_harman_kardon_hk3350.pdf/download.html
On page 34/36, the power-amp section, refer to caps c419/420 (L/R ch) near the input. 0.1uF/50v POLARIZED electrolytic .
That's a weird electro polarized value!
I'm a bit confused as to why:
A POLARIZED cap is used here?
An ELECTROLYTIC cap is used here?

If one can get away with non-polar here, then why not use a film? Maybe cost?
Or is there some ckt design issue that REQUIRES a POLARIZED cap here?

 

No circuit REQUIRES a polarised capacitor (did I mention this once before?)
It was used because it was cheap.

 

Yeah, ceramic or film at that LOW value (0.1 uF) is what I'm used to.

HOWEVER ... a polarized 'lytic at that LOW value is something I have never seen used.

If you look at the SM parts list, see link in OP, you'll note several 0.1uF mylars are ALREADY used extensively in the same receiver. And I even see a few MTL (metal ??) 0.1 uF on the power-amp board.

So, with c419/420 (L/R ch), I thought HK engineers were up to something clever that was TOPOLOGY-related. Like helping with feedback, etc. I mean how much more do ceramics or films cost for the all their benefits, especially in or near the signal path?