Polarity check

[neilbeith]

Hey,

So I thought I had all the components for a circuit, turned out I only had tantalum electrolytic capacitors when the circuit didnt specify electrolytic.

Am I right in thinking the function is the same as long as I get it the right way round?

I have marked on the picture below which way I think the +ve leg should point, but I'm new to this so they are probably wrong, can anyone check?




Cheers!

[mattthegamer463]

I think your C2 might be wrong, your C8 looks right, and I'm having a hard time determining which side of C9 might have DC current on it so I'm not sure about that one.  I would imagine that the + side should be facing the transistor base. 

Hopefully someone will double-check, but I think C2 and C9 are wrong, C8 is right.

[neilbeith]

Here is the nearest schematic I could find, I have highlighted the capacitors in question and put a plus sign where I think they should go, please note even though the schematic says C10 it matches C9 on my original diagram.

[mattthegamer463]

We can see that R12 is inserting a Vr bias voltage (V+/2) to the base where C10 is located, C10 should be facing this DC voltage.  C8 takes care of all DC coming from pin 7 of the 4558, but there is also DC coming up through the 100k linear pot R21 there, since it is connected to Vr. in that case, at best there will only ever be 4.5V on the op amp side of C10, but always will be 4.5V on the transistor side, so the + should still be facing the transistor.

I still believe that C2 and C9 (C10) polarity are wrong, and C8 is correct.

[PRR]

Tantalum will replace Aluminum electrolytics, may leak less, cost more, maybe smaller, and in Power situations will eventually explode POP! Not a problem.

When in doubt about cap polarity, assemble the circuit without caps, power-up, put your voltmeter where the cap will go, swap leads until the display reads positive (not negative). Now the red lead is on the + side. (There are a few cases where this won't be correct, notably oscillators.)

It is possible (but FAR more brain-pain) to figure it out.



Q1 Base sucks current down through R2; therefore Q1 Base is lower than 4.5V. Q1 Base-Emitter drops another 0.6V. Q1 Emitter is significantly NEGative of 4.5V. OTOH IC1a input pulls much less current through a smaller resistor and will be essentially 4.5V. The _right_ end of C2 is more-POSitive. Your interpretation is incorrect.

C10 has 4.5V on left and another transistor thing on the right. We don't have the 0.6V drop but we still have "some" voltage DROP in R12. Your interpretation is correct.

C8 left end follows IC1b at essentially 4.5V. C8 right end wanders through resistors to 4.5V and there is no other DC path, so it is at essentially 4.5V. Both ends of C8 are at the SAME voltage. In some similar cases there could be subsonic kicks of a part-volt; here we probably don't even have that.

So CAN we use a polarity-sensitive cap in a place with NO DC voltage across it? Or a small voltage of uncertain polarity? Yes. Electrolytics can stand up to 1V reverse polarity without problem. So while it "might be best" to select an NP/BP (Non/Bi-Polar) cap at C8, in fact production audio will generally use a plain electrolytic of arbitrary polarity.

[edvard]

I used non-polarized caps in the one I built for a friend.
Problem solved...