Sunday, November 2, 2008

The Mystery Box

A while back, I went to check out Tony's music shop in Taipei and noticed that they had this obviously home-made guitar pedal in the display case.

It was priced at NT$600nt (about US$20), and I was pretty much willing to pay that just for the parts I could use out of it.Anyway, I asked if I could try it out and they let me. It sounded superb.

There are two main amplifying components on it, a little 8-pin chip and a transistor.

The chip is marked:

And the transistor is marked:
C550B (This appears to be a BC550B)

There are three controls on it, labelled volume, tone and gain.
There are two toggle switches. The one on the side is a 3-way, with capacitors in 2 of the positions. It works as a tone filter I guess. At the back is a two-way switch, which appears to work as a kind of turbo overdrive.

Pot values are as follows:

TONE - B100K
GAIN - A50K (dual gang)

(A=audio taper, B=linear taper)

I traced the path of the dual-gang gain control. One gang connects between pins 1 and 2 of the OP275 and the other gang goes between pins 6 and 7 of the OP275. Shown below is an OP275. You can see that the gain control boosts BOTH amplifying stages at the same time. Serious boost there.

The circuit appears to be something between a Big Muff Pi and a Rat, and despite requests on 3 different sites for information, it appears to be a totally unique design. If you know otherwise, please do let me know.

Here's a couple of pics of the guts:

incidentally, they dropped the price to NT$500 while I was paying for it. Happy days.

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