Monday, January 16, 2012

Dipping a toe into the pedal world

All right folks, I've finally decided to take the plunge and try my hand at building a few effect pedals.  After building so many guitar amps, pedal building seems pretty much like instant gratification.  Once I got my initial supply order from Small Bear Electronics, it only took about ten minutes to get my first effect up and running on a solderless breadboard.

Anyone that is interested in guitar effects owes a debt of gratitude to R.G. Keen, maintainer of the well-known Geofex website.  The first circuit I looked up was what seems to be everyone's first pedal (it only has one transistor)... the Dallas Rangemaster treble booster.  The basic Rangemaster sounded great on the breadboard, so I figured it would also be a good entry into PCB fabrication.   Did I mention that I also picked up a copy of RGs book on PCB layout for musical effects?  It's true - despite the mountains of point-to-point I've done in my amp projects, I never made a PCB.  So I'm going to try my hand at making a Rangemaster for a first PCB using the toner transfer method.

However, me being me, I always get ahead of myself and get slightly overambitious.   It turns out that the stock Rangemaster has a couple of quirks.  First, it uses a slightly unusual germanium PNP transistor, and therefore requires a negative -9V power supply (i.e. the +ve terminal of the 9V battery is considered ground, which is the reverse of the usual convention).   This is not a big deal when you're using a battery, but it certainly is if you are using a standard pedalboard power supply.  It also has no LED indicator to show when it is active.  I'd like to incorporate some updates to the Rangemaster to address these shortcomings.

Thankfully, RG has come to the rescue again. On the Geofex site, he details a method for converting standard +9VDC supplies into either bipolar +/-9VDC, or (if you use just one output) -9VDC.  This is done using the Maxim1044 Switched-capacitor voltage converter IC.   Similarly, he has spent some time developing bypass methods that allow you to incorporate an LED, while retaining the DPDT stomp switch of the original design. Since I have the bits on-hand, I'm going to try his "Millennium 1" bypass.  All that really needed to be done on my part was to combine all of the bits (and to throw in a couple of pull down resistors on the inputs and outputs to boot).

Anyway, nothing is built yet.  But here's a first draught (unconfirmed!!) of a PCB that (I think) should build in the charge pump and RG's Millennium I bypass, all while remaining true to the original PNP/germanium Rangemaster:


Dimensions of the board are 1.25" H x 2.5" W.  Please note: these are UNCONFIRMED DRAUGHTS and likely contain errors.  I'll post final validated boards in later follow up post with the completed and verified build.

Update 1:  If I get rid of the on-board LED option, and squeeze things a bit, I can get it down to a 1.25" x 2.25" form factor, which I think makes it practical for a Hammond 1590N or standard B size case...thoughts?

Update 2:  Here's a more complete schematic of what I'm contemplating.  Note that this schematic includes an additional capacitor, C6, which decouples the -9VDC output from the charge pump to the the charge pump ground.  I haven't quite decided whether I need both C6 and C4 (which decouples the -9VDC to the effect circuit ground), or just one or the other.  I'm worried about noise from the charge pump.  Comments appreciated.

Update 3: Here's another version that makes room for C6, giving the option to decouple the -9VDC supply through the charge pump ground instead of just through the circuit ground.

Update 4:  Typos pointed out by Davent on the DIY stompboxes forum: The 1nF output cap (top right of layout) is actually supposed to be 10nF.  R3 on the schemo is supposed to be 6.8K, not 68K.  Please note also that the values show for R5 and R1 should be selected to bias the transistor appropriately, per the guidance in RG Keen's treatise on the Rangemaster. Finally, R8 on the schemo is shown without value, and this should be adjusted so that the LED is fully off when it should be, but sufficiently bright when on.  The default value is, I believe, 4.7K.

Update 5:  Some very helpful comments from Paul R. on DIY stompboxes has lead to a reorganization that should greatly improve the ground scheme, and enhance the supply filtering of the -9V rail  There's an added dropping resistor here now between C6 and C4 that provides the series resistance in  an RC lowpass filter with C4 that should greatly reduce switching hash from the regulator.  Also, the grounds have been reworked so that all power grounds are collected together and kept separate from the audio ground.  Paul pointed out that it's a bad idea to include the LED for the bypass in the audio ground, so that and the bypass JFET are moved to the power ground. This could be further improved by powering the LED from the raw Vin+, but I'll file that hint away in the "for future reference" file for now.  This is the board I'm gonna try this weekend:

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