Voltage, polarized vs nonpolarized caps: general questions

[zjokka]

hi all,

I'm working on the tentative schematic of the rhodes preamp, started breadboarding it this morning. my questions are simple, but I'm afraid the answers aren't:

-When do you need a polarized cap (vs. a non polarized)? I don't have any polarized ones lower than 1uF.
-How do you know what power source a circuit really requires. Can you just run a 15V at 9V with some luck? Do you have to change the values of the components accordingly?
-How do I build a simple 15V power source from 2 9V batteries. Just tie positive leads together and put resistors to ground until I read 15 volts?


more on the dyno rhodes project here

thanks for reading this

[R.G.]

-When do you need a polarized cap (vs. a non polarized)? I don't have any polarized ones lower than 1uF.

Non-polarized is always preferred. You only use polarized when (a) you can't get enough capacitance in non-polarized and/or (b) you can't get a non polarized capacitor that's small enough for the available space and/or (c) non-polarized is too expensive in that value of capacitance.

Effectively, 1uF is the crossover point. Between 1uF and 2uF, you stop being able to get non polarized caps small and cheap enough. There is minor overlap in this range, with 1uF-2.2uF polar and non polar available. Non polar is available up to about 50uF but are very expensive and big. I don't know of any 100uF non-polar parts.

You can not use polarized caps unless there is a polarizing DC voltage. If there is no DC voltage to keep DC plus peak signal the correct polarity for the cap, it must be non-polarized. Otherwise you kill polarized caps.

-How do you know what power source a circuit really requires.

Sorry to be flippant, but you know about circuits and do the math. "Requires" can depend on everything, including what drives the circuit and what the circuit drives.

Can you just run a 15V at 9V with some luck? Do you have to change the values of the components accordingly?

It depends on how lucky you are. Some circuits will work fine, some will work at a reduced level of performance, and some won't work properly at all. In general, you have to know the circuits and do the math to be able to figure out what's needed. The other way is just to trust to luck and try it. It might work.

-How do I build a simple 15V power source from 2 9V batteries. Just tie positive leads together and put resistors to ground until I read 15 volts?

You put the two batteries in series, positive of one to the negative of the other, to get 18V. Then you regulate the 18V down to 15. However, 18V is much closer to 15V than 9 is to 15. The circuit is more likely to work from 18 than 9 if what it wanted was 15.

In the schematic you linked, the preamp will still do something on both 9V and 18V, as the JFETs are somewhat self compensating.

However, the variation in the JFETs themselves is large. Assume that you will have to tweak the 1K resistor in the source-ground of each JFET to get the drain up in the middle region of the power supply.