Look Over Here! A Distra... Um Distortion Pedal (rev3)

The BFA (Big F***ing Amplifier) Overdrive Pedal

A BYO (build your own) distortion pedal with an audio isolation transformer to simulate the output stage of a Marshall or a Vox tube amp. It drives a clipping circuit, the primary cause of distortion in any amp, but smoothes that clipping with the transformer, what a valve amp does, particularly something like a Marshall. I call this circuit the BFA, short for "Big Fucking Amplifier." *Note: The TM022 transformer now substituted for a Bourne LM-NP-1001-B1L PCB mount. (Link = data sheet) The higher inductance makes for a more rectilinear distortion shape, still with the nice rounded corners on the signal and the bandwidth is closer to 10kHz.

BFA circuit schematic.

The design uses a low voltage, rail-to-rail dual op-amp, the LM358. Op-amp "a" is wired as an inverting gain stage, to push guitar signal way above the clipping voltage of a pair of shchottky diodes, wired opposite hand, to create the necessary saturation point. The pre-gain is set to not quite clip with the knob all the way counter-clockwise, and clip like a monster, all the way clockwise. Then we feed the clipped wave through the iso transformer, in the case of my prototype, a 42TM022RC*, with a 1.5k primary impedance. We drop the voltage a little first, though, to keep the loaded output of the secondary at roughly a little above normal guitar levels.

The secondary of the iso tranny is loaded by a 600Ω resistor, then buffered by the second op-amp. The secondary is wired opposite phase to counter the inversion of the gain stage. From there, we output the signal via op-amp "b", wired as a buffer, to the level pot and a capacitor. (The 50kΩ resistor represents the input impedance of a guitar amp.) The 4.7kΩ resistors and 10uF capacitors divide the supply to run a centre ground. This means a special input socket is required that has separate switching for power, rather than the usual grounding of the supply negative via the ring and ground of a TRS socket. Separate switching is better, anyway, incase you plug in a TRS lead and wonder why the pedal won't turn.

There's a gerber file for the PCB (zipped) here, and the image of it is here -->

The board is 52mm wide and 54mm high.The pots mount on the underside so that the parts can be seen with the battery cover off the housing. I recommend NextPCB for getting the board made. I got 5 prototype boards made for US$2 and the China Post Airmail option gets the boards to you in a month and a half at the latest for a total of US$12 or so. If you prefer PCBWay, they're good, too.

As I mention above, I've replaced the TM022 with a proper wideband audio transformer. You will definitely get better results for wideband audio with the Bourne LM-NP-1001-B1L. But also, hunt around a bit, see what you can find. There are many available wideband trannies, but needs to be between 2 and 4 Henries. Also, please note that the large capacitors and resistor labelled, "©2023", "Filthynoises" and "The BFA", respectively, are just to trick Fritzing into letting me add my own "branding" to the board. They're not required for the circuit and you really should leave them out as they'd do nothing but waste space.

Feel free to build this circuit for yourself or friends, but plese don't profit off it. Permission to use this design is by Creative Commons Share Alike, Non-commercial. Please abide by this. The web links in the parts list are to Pedal Parts Australia, if you're not in Australia, try Pedal Parts (USA) or Pedal Parts EU, depending on where you live. There are a few parts that Pedal Parts don't carry, I've linked to Element-14 for them.

And now, here's the parts list.

Bill of Materials: opAmp-NoFreqComp.fzz

Assembly List
Label	Part Type		Properties		Web Link
Primary/Secondary (Transformer)	lm-np-1001-b1l		https://au.element14.com/bourns/lm-np-1001-b1l/transformer-line-matching-1-1/dp/1929741
LVL	Rotary Pot 100kΩ	(Vimex 9mm) Log		https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=12_110&products_id=2164
DIST	Rotary Pot 1MΩ		(Vimex 9mm) Linear	https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=12_110&products_id=2254
LM358	LM 358 Dual Op-Amp	package DIP8		https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=78_92&products_id=2000
D1	Schottky Diode		1N5818 (or similar)	https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=81_95&products_id=2190
D2	Schottky Diode		1N5818	"     "							"
LED1	LED 			5mm Red			https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=17_101&products_id=2073
C1	Electrolytic Capacitor	1µF, 50V		https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=82_87&products_id=1770
C2	Electrolytic Capacitor	10µF, 50V		https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=82_87&products_id=1774
C3	Electrolytic Capacitor	10µF, 50V		https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=82_87&products_id=1774
C4	Electrolytic Capacitor	1µF, 50V		https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=82_87&products_id=1770
R1	100kΩ Resistor		100kΩ 1/4W	-
R2	1.2MΩ Resistor		1.2MΩ  "	 |
R3	1kΩ Resistor		1kΩ    "	 |
R4	3.3kΩ Resistor		3.3kΩ  "	 |
R5	1.5kΩ Resistor		1.5kΩ  "	 |-	https://www.pedalpartsaustralia.com/index.php?main_page=index&cPath=89
R6	680Ω Resistor		680Ω   "	 |
R7	1kΩ Resistor		1kΩ    "	 |
R8	4.7kΩ Resistor		4.7kΩ  "	 |
R9	4.7kΩ Resistor		4.7kΩ  "	 |
R10	470Ω Resistor		470Ω   "	-

Mechanical Parts

Case	125B Diecast Enclosure				https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=3&products_id=2415
Switch	Compact 3PDT footswitch				https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=1_13&products_id=1463
Input socket with power switch				https://au.element14.com/lumberg/klbpss-3/socket-6-35mm-jack-chassis/dp/1217020
Output socket						https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=4_9&products_id=1155
2.1mm panel mount Barrel Jack (DC socket)		https://www.pedalpartsaustralia.com/index.php?main_page=product_info&cPath=4_6&products_id=1146
9v battery clip						https://au.element14.com/mcm/29-130/9v-battery-snap-connector/dp/2783529?st=9v%20battery%20snap
Knobs for your pots as per your preference.

Construction Notes

Any 119mmx66mmx40mm diecast box will do.
The hole spacing between the 2 potentiometers is 26mm, the LED hole should be exactly in the middle between these. It doesn't change with the pushbutton, it just indicates there's power applied to the circuit, the switching for which should be handled by the input socket switch.
Whatever knobs you desire will work, provided they have 7mm centres for the shafts of the 9mm pots.
Your 2.1mm barrel jack should switch the battery out of circuit when using a plugpack power source.
The footswitch needs to be at least DP2T, input tip to Com1, NO1 to board input ("GTR") and NC1 to NC2 (creates the bypass), N02 to board output and Com2 to output tip. If you want the LED on when overdriving, you'll need a 3PDT switch with LED cathode wired to ground via the third bank.
The 9v battery negative should not be connected to ground, it has it's own pin, the one below "+9v". Ring and sleeve in should be connected "GTR" ground. If using a metal case and input jack with a metal mounting thread, no ground is needed for the output. If your switching input jack doesn't directly connect its ground to the diecast box, then output ground should be wired to the output jack ground pin. either method should ground the case, but don't double-ground the board to a metal case, as this may cause earth loop hum.

Transformer Notes

I'm settling for a Bourne LM-NP-1001-B1L audio transformer for my prototype, but it only has a 10K bandwidth. In my sims, if you go below 2H inductance, you'll get way too many even harmonics in the distortion and a really spikey waveform. The 1001 is 2.8H and this figure shows rounded corner square waves in the sim, and no measurable harmonics in the really basic spectroscope my sim software generates. This is what we want for that smooth tube amp clipping overdrive sound. The Vigortronix VTX-101-1602 appears to calcualte out to 12H and has a great response in the low end, for those who want a bass overdrive with an octave box in the line as well, but you'll lose distortion from 12th fret on your bass' G string and above. Maybe even noticeably down lower than that. I'm still only modelling at this stage. The Bourne (2.8H) should be crunchy for an octave more. Select on the sound you're trying to create. Another Vigortronix option is the VTX-101-1602, at a calculated 5H inductance, giving closer to the Bourne's distortion shape, but it would require mounting off the board as it's got larger pin spacing. I'm doing a rough calc of impedance at centre frequency using L = 2πf/X. Your measured mileage may well vary by a country mile.

There are heaps of audio transformers out there, if you want wider bandwidth, but not all list the inductance figure in the spec sheet. In a compact, circuit board mount transformer, you're probably not getting much more than 3 or 4 Henries and keeping 10kHz or more bandwidth, but that's going to vary from item to item. A search for "audio transformer" on element-14 gets these, and they are many! Select a transformer between 2 and 4 henries, that fits the 2.5mm x 12.7mm pin spacing of the Bourne. You may want higher bandwidth than 10kHz. You will pay more than a few dollars for the nearer to "perfect" transformer. If you find one with a different impedance (the Bourne is 600Ω for both windings) then change your resistor values on primary and secondary and the dropping resistor above the primary shunt such that 40mV RMS in (a hot guitar) results in a little more than 40mV RMS out at the top of the level pot across the output. (50mV should do it.) If the inductance is not listed in Henries, don't bother, it'll likely not be suitable, either for bandwidth reasons, inductance or both.


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