#1
I have a couple simple pedals under my belt but would like to build this one and completely understand it and try to do my own mods and such.

I haven't started building but lets start talking about it... hopefully.

Here's the schematic I'm going to use

http://www.tonepad.com/getFile.asp?id=89

its from tonepad, and has tons of reliable builds.

so where to start. its a pretty simple circuit, I know most distortion works on driving a diode/op-amp(in this case)/transistor at a higher voltage to force it to clip correct? this one seems to be no different but thats spawning my first quesiton. the op amp at the heart of this circuit, which I assume is for the main distortion clipping has a cap and resistor in parallel in the feedback loop and a cap and resistor in series on the input. If memory serves correct, that is simply a band pass filter. so why do a band pass filter instead of say an inverting amplifier? or could I change that to like a Low pass filter and make a bass distortion pedal?
#2
Also, What do the Vb points connect to in the schematic? in the drawing above it they seem to just connect to each other, but why would anyone label them like that?

The pdf is attached, you should go to tonepad.com, because they have tons of schematic and such.
Attachments:
tonepad_rat-1.pdf
#3
the power section is comprised of a 47R resistor, 2 100k resistors, a 1M resistor (on the IC's non-inverting input), a 100uF cap, a 1uF cap, a 4002 diode, and a 10nF cap.

the 47R resistor is sort of over-voltage protection for an unregulated wall-wart. It is also the Drain load resistor for the JFET. the 100k resistors form a voltage divider that divides the voltage in half for the IC's bias voltage. the 1M resistor on the IC is for impedance purposes. the 1uF cap filters the bias voltage i guess. the 100uF cap is a power supply filter. the 4002 is polarity protection. in the event the power supply is the wrong polarity, this diode will short the supply rails (not the best way to do it but it works). the 10nF cap is an RF filter.

Vb is the bias voltage. the op-amp must be biased into linear operation to work properly. in some distortion circuits (not this one), op-amps (tubes, transistors) are biased closer to saturation. this makes them clip.

starting at the input and working to the right: the 1M resistor on the input is a pull-down resistor to eliminate switch popping.

the 22nF cap blocks DC and provides some simple EQing of the signal. larger values let in more bass.

the 1k resistor helps protect the IC from input over-voltage. the 1nF cap shunts very high treble freqs to ground and out of the signal. helps tame harshness.

the network across pins 2 and 6 f the IC is a negative-feedback loop. this is what controls the gain of the IC. the 100k pot controls how much output signal is fed back into the inverting input. whatever ac signal goes into the inverting input has the polarity flipped at the output. when you combine a signal and the inverted form of the same signal, it cancels out completely. the higher this pot value, the higher the gain of the op-amp (i've mostly only seen 500k and 1M instead of 100k).

the 100pF cap is a bypass cap. this is a very low value, so only high treble freqs are making it through to the inverting input.

the 560R and 47R resistors direct some of the fed-back signal to ground, keeping it away from the inverting input. the caps in series with these resistors are low-freq roll-offs. they keep the effect from being too bassy at lower gains. the 30pF cap on pins 1 and 8 is a freq compensation cap. the LM741 and similar ICs have this cap already built-in.

the 4u7 cap and 1k resistor n the output are similar to the ones on the non-inverting input.

D1 and D2 are what actually do the clipping. diodes clip the waveform because of their forward voltage drop. say the drop is .6v. any voltage lower than that will not pass through the diode. as a result, the large amplitude coming from the op-amp is clamped down to .6v. so diodes lower the output level and clip the waveform at the same time.

looks like the Filter pot works with the 1k5 resistor and 3n3 cap as a tone control. the rest of the circuit is a JFET output buffer and it's support components.

this is a bit odd as compared to most op-amp/diode distortions i've seen. the dual series RC networks on the inverting input really stick out to me.
#4
Quote by Invader Jim

this is a bit odd as compared to most op-amp/diode distortions i've seen. the dual series RC networks on the inverting input really stick out to me.


Yea, this does seem like a different circuit than say a fuzz face where you just have 2 transistors in series... but its supposed to be an exact clone.

First off I thought for the most part Diodes had a forward drop voltage of .7.

secondly, so in a basic summary your not clipping the op amp, your increasing its gain until it goes beyond that .6 or.7v so it clips the diodes and that gives you the distortion sound.

Well, I'm gonna build it when I get a chance, I have no parts here, but I might just order them. hopefully I can have some fun with it.

one more question, I have an enclosure from an old failed project but its got 4 holes drilled for pots... what would be a good thing to be able to adjust? the resistor between the op amp and the diodes? that seems like it could be a secondary gain control by making a larger voltage drop between them.
#5
What you could is put a switch in the last hole. The switch would take the diodes out of the circuit. Then you'd have the option for more of a clean boost.
#6
Quote by end_citizen
What you could is put a switch in the last hole. The switch would take the diodes out of the circuit. Then you'd have the option for more of a clean boost.


Thats a really good Idea, I think Ill do just that would I jsut need a switch to bypass them and thats all?
#7
Yeah. Just take the diodes out of the signal path. It won't be a pure clean boost because the Rat is very trebly, but it won't be super distorted like the rat can get, either.
#8
Quote by somekid413


First off I thought for the most part Diodes had a forward drop voltage of .7.

.



Forward voltage is a function of current, it's an exponential equation. For silicon diodes, it varies from .6v to .7v.

Those two RC networks are weird on the inverting input of the op-amp. It seems like they might be for filtering out two very specific frequency ranges (AC hum for the 560 and 4.7uF pair,).
Last edited by earthwormjim at Sep 23, 2010,
#9
the drop of rectifier diodes is around .7

silicon small-signal diodes range from .4 to .6-ish

small-signal germanium is from .25 to .35

schottky's are around .1

LED's vary depending on color. a green one has a drop of around 2.8v

these are all numbers I have actually seen. not arbitrary guesses.

Quote by somekid413
secondly, so in a basic summary your not clipping the op amp, your increasing its gain until it goes beyond that .6 or.7v so it clips the diodes and that gives you the distortion sound.

that's exactly right.
#10
Sorry, bout rehashing an old post, I finally got around to building the thing and is there any way to ad an led without a 3pdt switch? that seems like the simplest solution, but would also require me to order a 3pdt switch which I don't have on hand.
#12
If you really want to get into effects building, I would recommend 2 sites:
http://www.geofex.com/
http://www.diystompboxes.com/smfforum/