Capacitor type for the bias circuit (polarized or bipolar)

Karl vd Berg

Super Member
Good evening to all,

I have a Rotel RB-981 and I'd like (just) to know if replacing the stock 10uF/50V polarized electrolytic cap with a bipolar 10uF/50V film cap (Muse, Wima, etc) would affect the behavior of the amp. I saw people replacing these polarized capacitors with bipolar film caps (including myself previously in another amp), but saw other people keeping a polarized one in the position because this is how engineers did for "some particular" models.

What do you think? Not a problem to replace the lytic with a film one, or keep the same type of original cap?

Here below 2 images illustrating the RB-981.

5cgigh.jpg


347gorp.jpg


Thank you.......
 
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The C612 capacitor in the schematic is actually the "Vbe multiplier bypass capacitor"; there is lots of information available regarding replacing/upgrading this capacitor if you search with this term. Personally, I typically replace with a small value film-type capacitor (e.g., 0.47µF-1.0µF) and call it a day.
 
That is correct, the absolute value of the Vbe capacitor is not critical; if you look at a number of schematics, you will see that the capacity rating for this capacitor ranges from 0.1µF to 47µF. Since its function is to insure good high frequency performance (by improving the voltage regulation across Vbe transistor at high frequencies), its beneficial to use a capacitor with a good (e.g., low ESR, ESL) high frequency performance.
 
The C612 capacitor in the schematic is actually the "Vbe multiplier bypass capacitor"; there is lots of information available regarding replacing/upgrading this capacitor if you search with this term. Personally, I typically replace with a small value film-type capacitor (e.g., 0.47µF-1.0µF) and call it a day.

I tried....

Apart from power up and down behavior of the amp if size is large, I do not know where the size selection for this capacitor comes from....
Basically I have no clue...I have seen a topic where a guy at DIYaudio forum tried from 10 nanofarad up to 470 microfarad hoping to hear or see the difference.
Another one tries to find impact on distortion after finding some manufacturer left it out at a later date....

Probably a small size film as you do is a good rule of thumb.
 
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