quick question about input cap (electrolytic polarity)

[chromesphere]

Hi all,
I built a bazz fuss recently (for a video), and in it i used a 2.2uf electro on the input, cause their cheaper.  I wired it so that the negative goes to the base of the mpsa13 and the plus goes to the input.  Someone commented and said, this is the wrong way around.  I had a bit of a look into it, and i also think that it could be the wrong way around...
Except...it still works...
I would have thought that the wrong polarity would block the input signal completely...so yeah, the question is, why does it still work?
(cheers )
Paul

[Kesh]

Yeah it's wrong. A pedal's input is nominally at zero volts and a npn transistor's base at least half a volt above zero.

Electrolytics however can cope with small amounts of reversed polarity, but you may shorten its life.

[chromesphere]

I find it bizarre that it even worked!  Thanks for the info Kesh i will change it.
Paul

[PRR]

If in doubt:

Build it without the cap. Use your voltmeter, watch Red and Black; this says which-way is + or -.

[chromesphere]

If in doubt:

Build it without the cap. Use your voltmeter, watch Red and Black; this says which-way is + or -.

Thats a great tip Paul thanks!
Paul

[Kesh]

btw, 2u is doable cheaply with film caps, just parallel two 1u metallised minatures.

(does anyone know why size and price shoots up suddenly between 1u and 2.2u for poly film?)

[ubersam]

I would have thought that the wrong polarity would block the input signal completely...so yeah, the question is, why does it still work?

I think it's because your input signal isn't DC, that's why it isn't getting blocked.

[chromesphere]

I would have thought that the wrong polarity would block the input signal completely...so yeah, the question is, why does it still work?

I think it's because your input signal isn't DC, that's why it isn't getting blocked.

So, for an electro input cap, it makes no difference which way around you have it in the circuit?

Edit: no the proper way is Kesh's answer.  The base of the transistor is higher then input signal and your meant to hook these up with lower voltage on negative.

[PRR]

> I would have thought that the wrong polarity would block the input signal completely

No. It will work until it blows-up.

In a typical input situation, the blow-up won't be BOOM, it will just leak-off all the bias on the first transistor which will quit doing what you want.

As Kesh says, some very-low voltage may not "blow-up", or not for months or hours.

There are thousands of commercial units with 0.1V reverse voltage working for many years.

But in general, you want a polarized cap going the right way, because you can't really know what will happen if it is in wrong.