Sunday, December 23, 2012

Forking to a New Blog

In order to keep this blog focused on the technology of musical sound, like instrument and musical electronics building, I will be starting a new blog shortly. It will still cover some music-related topics, especially the "independent" music business and related tools.

The new blog will be found here: Oak Blood Three

Sunday, July 3, 2011

Grounding and Shielding a Danelectro '63 Baritone

Supposedly the electronics in the original Danelectro guitars were very heavily shielded. When I bought this reissue '63 Dano baritone, I was more than a little disappointed that aside from the "lipstick" pickup covers, the instrument was practically unshielded. The back of the pickguard had a skimpy little strip of aluminum tape between the controls.
It wasn't a great surprise though, as this is a very noisy instrument.

The compartment cover was just a plastic disc held in place with an ungrounded metal strap. Of course grounding was also poor, with ground loops, potentiometer cans used as ground points, etc.

Although there wasn't any internal shielding (not even conductive paint— I checked with my ohmmeter) the pickup mounts seemed to be designed to work with shielding, since they had grounded copper spring mounts.

There's some decent advice on improving the shielding and grounding in a Stratocaster available at Guitar Nuts here: Quieting the Beast, Shielding a Strat™. I adapted some of the implicit goals of that article to the Dano '63. The most significant differences are related to the relative simplicity of the Dano: Two pickups, single volume & tone, single-pole selector switch with pickups in series. The way a Dano selector works is by shorting out the unused pickup when not in the center "series" position.

I used Copper foil tape with conductive adhesive to shield the electronics compartment, pickguard and back cover. Since the peculiarities of Danelectro body construction leave lots of inaccessible voids, I had to make little foil dams near the controls compartment.

I didn't end up creating a signal ground terminal as described in the article, since there were so few signal grounds to tie together: One from the jack & volume, one from the tone capacitor, one from the pickup selector. I just soldered them together and added some heat-shrink to insulate them from the shields.

I also omitted the large ring terminal under a potentiometer to connect shielding, as the pickup selector switch had a nice solder lug on its shield. I did use ring terminals to attach the bridge wire and pickup shields to the foil shielding.

I ended up replacing the tone capacitor, since the original was soldered onto the back of the potentiometer and had its leads cut too short to b
e usable in any other configuration. The new tone capacitor as well as the new 0.33μF coupling between ground ans shield are affixed with double-face foam tape.


Wednesday, March 23, 2011

Discrete SS Mini Amp

Do you have a desire to build a fractional-watt miniature solid-state guitar amp? Apparently many people do, since there are several schematics out in the wild on the DIY discussion boards. For some reason, they tend to be built around little "chipamps" like the LM386 and its successors. They work off a 9V battery and are pretty easy to build. Make magazine even featured one version that was assembled into a cracker box.

I'd bet (though I haven't done any "reverse-engineering" to verify) the mini-amps from the big name makers are not much more—they probably just have an op-amp distortion and tone control circuit ahead of the chipamp.

Seems like it would be better to do it with discrete transistors so that some of the magick-mojo properties of tubes could be included, e.g. low parts count, no negative feedback.

There are some good JFET-based overdrive/distortion stompboxes out there, so that part should be doable. JFETs still seem like a good fit for the power amp, since the low voltage and power demands are well within their typical rating, their transfer curves are pentode-like, and (unlike MOSFETS) the gate draws current when driven positive, just like a tube grid (not the same characteristics as a tube grid, but some current does flow.)