Is a Power Filter Cap Necessary for Battery?

[Jdansti]

I'm going to install Tillman Preamps in a couple of my guitars, and to save room, I'm thinking about eliminating the 10uF cap across the power.  The power will come from a 9V battery, so would removing the cap have any negative impact?  If it could, I'll use a tantalum since they're smaller.


Copyright 1992, 2001, J. Donald Tillman.  All rights reserved.

[Mike Burgundy]

It'll work  - a battery has *no* ripple like a stabilised adapter does. However, batteries sometimes do make noise all on their own (rustle, hiss) but very, very low. Carbon batteries are loudest, but nobody uses them any more. Any supply noise is coupled directly to your signal line, so it might or might not be a problem, depending on the amount of noise. I'd leave it in to be sure (16V10uF are small enough), but if space is a huge (ahem) issue then just try it.

[Mike Burgundy]

Interesting:
http://tf.nist.gov/general/pdf/1133.pdf
317 regulated has -120 to -130dBV noise, batteries better than -180dBV. Shame they don't go into detail on the various batteries.

[chptunes]

I got good responses too, for the same question here:

http://www.diystompboxes.com/smfforum/index.php?topic=99905.msg877143#msg877143


-Corey

[R.G.]

Can you get by without a filter cap?  Most of the time.

Is it a good idea? No.

The filter cap increases the amount of the battery's discharge that can be used effectively by the circuit. You get more useful life out of each battery, and less exposure to the circuit doing funny things as the battery ages.

[amptramp]

Just put R1, R2 and Q1 in the guitar and the rest of the circuit in an outboard box.  With the output impedance of slightly under 6.8K, you can use as long a cord as you want.  Besides, we are all about outboard boxes on this site, but no stompswitch will be necessary.  You can make the battery filter as elaborate as you want.

P.S. If you go with tantalum caps for C2, some series resistance from the battery will be useful to avoid the black death short circuit failure mode.  47 to 100 ohms would be good.  C1 would be immune as it has all the series resistance it needs.

[Jdansti]

Interesting:
http://tf.nist.gov/general/pdf/1133.pdf
317 regulated has -120 to -130dBV noise, batteries better than -180dBV. Shame they don't go into detail on the various batteries.

Thanks!  A lot of stuff I never knew about batteries! 

[Jdansti]

I got good responses too, for the same question here:

http://www.diystompboxes.com/smfforum/index.php?topic=99905.msg877143#msg877143


-Corey

Thanks! I missed that one!

[Jdansti]

Thanks R.G. and Ron. Good reasons to keep the cap and probably add the resistor in series with C2.

[Jdansti]

I've got one of the preamps put together.  Turns out that I had a tiny 10uF cap for the power, but the 4.7u output cap was bigger in size!  I went with it anyway.

I decided to try just soldering everything together point to point and then potting it with hot glue. It turned out to be 1.75"x 0.5" x 0.5" and it tested fine. I think that it along with the battery will fit in one of the guitars.

I'm going to perf the next one. After doing the point to point and dealing with a couple of broken leads, I want to try it on a board.

Here's the blister pack mould I used.





Here's the finished product.  The mould is now part of the unit. It didn't like the heat from the hot glue and warped a bit. No biggie.







Thanks for everyone's help!