Question about C31

Sorry for my newbness, this is my first synth build, and I still have a lot to learn.

I have a question about C31 though. The build instructions state its optional, and lowers the ESR… I have to say, I don’t really know what that means… do I want that? I don’t really know because I don’t know what a lower ESR will do here. Can anyone tell me?


ESR means “Equivalent Series Resistance”. The lower the ESR is the easier it is for the LT1054 to generate the negative 5V. As C31 is only about 54ct you should add it as long China does not suck up all available Tantal…

Resistors are electronic parts that dissipate energy. Capacitors are electronic parts that store, hold and restore energy, like a bucket. Except that they are not perfect buckets - there’s a always a bit of energy “leaking” when they are charged and discharged. So the real, actual capacitor you can buy is, from a modeling point of view, a perfect capacitor in series with a resistor (that represents its dissipative/leaky behavior). The ESR (equivalent serial resistance) is the resistance, in ohms, of a resistor that would leak the same amount of energy.

So now the bit about the LT1054. If you look at the 1054 datasheet, they recommend a low ESR cap (non-leaky) for the charge capacitor. With a high ESR cap, more energy will be leaked and the voltage at the output of 1054 will drop. Which capacitor should we use? There are different types of capacitors and they all have different capacities / tolerances to high voltages / ESR / prices. Tantalum (the common low ESR-type) caps are expensive and not available in the larger capacities. The best solution (as recommended in the Linear datasheet) is to use in parallel two caps of different types, a big electrolytic and a small tantalum ; and this has the effect of combining their performances (capacities and conductances adds up). Which is good!

But DIYers sometimes do not have those tantalum caps, and they can be a bit expensive and hard to find. Since the circuit works without the tantalum cap (the voltage drop is acceptable because the output voltage is regulated anyway), I mark it as optional. Of course if you have the tantalum cap in your kit, do not hesitate to solder it! But if you don’t have it, don’t feel stuck, move forward, the energy that won’t get lost in the cap will be lost later as heat in the 7905 anyway.

I actually figured this one out myself… its on the controller board, and part of the power circuit… I’m definitely adding the cap. Seeing the “optional” originally threw me, the note about ESR had me thinking it was part of the filter board… newb scared by acronyms

Thanks for explaining… it confirms what I was thinking, after looking more closely.