Building a pro co rat

[poppy man]

Im currently building this in my spare time. I know enough to transfer this to veroboard but dont know how it works basically, namely the distortion, filter and volume, also the transistor. I can only hazard a guess. Am i correct in these assumptons

The distortion pot and capacitor are a high pass filter . Only high frequencies are passed?
The 2 diodes grounded oppisite ways before the filter are what causes the cut in the signal and creates distortion?
As for volume im assuming its just the fact that its grounded that enables the volume to be adjusted?

Transistor i have no idea. something to do with amplification?

[crane]

what exactly do you want to know?
the transistor works as a buffer (or if you want to stick to the "something to do with the amplification" terms - it's a current amplifier).
There are plenty of layouts availible for this project. Building one does not require complete understanting of the circuit (debugging however is much easier if you have basic electronic knowledge)

[poppy man]

I just want to know how the signal is distorted, how the filtering part of the circuit works and how the volume part works

I will research more about the transistor being a current amplifier


Sorry if i was vague. My electronics is basic at best. I do appreciate the information you have gave me so far though

[crane]

Well I'll try to answer your questions one by one:
1)Distortion pot is not a part of the hi pass filter - it regulates gain of IC-1 circuit. IC-1 is used as a non inverting amplifier. Mathematics of this is well described here:
http://en.wikipedia.org/wiki/Operational_amplifier_applications#Non-inverting_amplifier
The only thing is that you have to substitute R1 and R2 with full AC impedance (eg in your schematic R1 = R4 + ZofC5 in paralell with R5 + ZofC6, while R2 = R3 in parallel with C3). C3 is actually a part of a low pass filter (it reduces gain for higher frequencies - if you're able to do complex math you can figure out IC1 gain equation and plot it frequency vs gain).
2)Yes - the two diodes are there to clip signal. The more gain from IC1, the bigger part of sinewave is cut away, the bigger distortion.
"Filter" pot is a simple first order RC low pass filter. Can't explain it better than this (imagine R7+R8 as R and C8 as C):
http://en.wikipedia.org/wiki/Low-pass_filter
It will be hard for you to google something about "current amplifier" as this particular circuit is usually called "drain follower" when done with a FET transistor (like Rat schematics), emitter follower when done with a bipolar transistor (like in many boss pedal schematics) and a buffer when done with an opamp (although emitter followers and drain followers are referred as buffers as well).
3)Volume control is just a voltage divider.
If you know Ohm's law it's not hard to figure it out. Imagine upper resistance of the pot as R1 and lower part of it as R2.
If you do some math with Ohm's law you will figure out that Uout=Uin*R2/(R1+R2)

Hope this helps a bit.

[electrosonic]

I find it annoying that the "filter" control is just a regular tone control wired backwards - it cuts more treble as you turn it up. It makes more sense to me to wire it the standard way.

the transistor works as a buffer (or if you want to stick to the "something to do with the amplification" terms - it's a current amplifier).

The transistor also separates the tone (filter) control from the output, so the tone control works in a predictable way. Without it, the loading  of the next device (pedal or amp) would alter the frequency response of the tone control.

Andrew.

[zombiwoof]

I find it annoying that the "filter" control is just a regular tone control wired backwards - it cuts more treble as you turn it up. It makes more sense to me to wire it the standard way.

the transistor works as a buffer (or if you want to stick to the "something to do with the amplification" terms - it's a current amplifier).

The transistor also separates the tone (filter) control from the output, so the tone control works in a predictable way. Without it, the loading  of the next device (pedal or amp) would alter the frequency response of the tone control.

Andrew.

Seeing that the "filter" pot is a 100k log pot, by wiring it backwards it would seem was a simple way to get a reverse audio taper from a regular audio pot.  Does this make sense?.  Maybe regular 100k log pots were much cheaper or easier to get than reverse audio (log) pots in that value, and that was the reasoning for the backward wired log pot.  Just a guess on my part.  If you wire the log pot the normal way, I think the tonal adjustments would be more gradual on the opposite end of the pot's rotation.

Al

[electrosonic]

I forgot about the lack of symmetry in this situation, reversing the pot will mess with the taper. I guess when the first RATs were made it was easier to source audio taper pots than reverse audio taper pots, so that is what they went with. I find it funny in some of their ads they talk about their "famous filter control" or some such, when it is obviously a compromise.

I am glad you mentioned that, I am putting together a couple of RAT clones and I noticed on the breadboard that the tone control (wired backwards from stock) did not have the right feel. I guess  a need to order a couple of 100k reverse audio pots.

Andrew.