passive pickups need preamp?

Here is one very popular example of a minimal bare-bones DIY piezo buffer circuit you can build yourself, or by one pre-made from the author:

http://scotthelmke.com/Mint-box-buffer.html


Here is the $58 Schatten Mini Pre:

http://www.amazon.com/gp/product/B006AVA3TQ



Here is the $99 LR Baggs Gigpro:

http://www.amazon.com/gp/product/B001E95KEW


Here is the $29 Behringer ADI-21:

http://www.amazon.com/gp/product/B000KITQK2



And there are others, that cost more, as I mentioned in a previous post.

Not necessarily but it helps in my experience.

I only use passives. But I do use an off board pre-amp - I am not a fan of any of the prebuilt onboard powered pickups (the sort that run on button batteries) for a simple reason - an off board pre-amp of a decent quality (I use an LR Baggs) with a passive strip gives me

a) more tone shaping options
b) a far more natural tone to my ears.

Now - into an amplifier with a good acoustic stage - perhaps I can go straight without the pre-amp - it will work. But for recording / going into a desk - a pre-amp is essential for me.

My Baggs is super cool as together with a line out I can give an engineer an balanced XLR output from the uke too.

(this is the pre-amp I use - http://www.gotaukulele.com/2015/03/lr-baggs-venue-di-pre-amp-review.html )