RAT project

[rangermaster]

Hi there.

I want to build my self a RAT. I found this PCB: https://www.musikding.de/The-Rat-pcb-Distortion

Is this the same schematic as the original RAT ?

Thanks.

[iainpunk]

hey, yes that's a RAT clone

cheers

[antonis]

Is this the same schematic as the original RAT ?

Strictly speakig, No..!!
(neither from schematic nor from BOM (parts list) point of view..)

But yes, you can proceed building that kit which is close enough to "original"(*) Pro co RAT..

(*) there are more than a decade "original" versions designed by Pro Co..

[rangermaster]

Thnx for the replies.

Is a LM308M the same as the LM308 ? I can't find any info about these...

[antonis]

It's just the same..

Musikding kit declares LM308 and OP07 fully interchangable for particular schematic/layout but, IMHO, in case of using OP07, frequency compensation cap (C4, 33pF) should be left out..

[rangermaster]

I am using a LM308M then if it's the same as LM308. Isn't the 33pF in the original 30pF ?

I'm also using a 2N5458 instead of the 2N5457 that musikding uses in there schematic.

[rangermaster]

Ok i compared tye original schematic that i found on electrosmash with the one from musikding.

Almost everything is the same except a few components.. see photos in attachments. Is this a big deal or does this not affect sound ?

[idy]

Hard to tell which components you are concerned about. There are sites that have lists of the (slightly) different values for every year/model of rat... they don't make big difference. Well, turbo rat is a bit different because LEDs...Check out here towards bottom of page:

https://www.coda-effects.com/2016/01/proco-rat-white-face-1985.html

30pf vs 33pf is no big deal. The tolerance on the parts means 10% difference is within margin of error or close. This part just shaves off the highest frequencies. Since the curve is gradual, beginning a "whole step" or so higher is trivial.

The FET is just a buffer and so adds little character of its own. 5458 will probably work. They are a little different, but if it works at all here it will be fine.

[rangermaster]

Well there is an extra 10k resistor before the output.. why?
And the 1K and 4,7uF are reversed in the musikding schematic.. why is this and does it affect sound ?

The power section is also different..

[duck_arse]

the extra 10k at the output is in parallel with the 100k volume pot. you have two choices - fit a 10k pot and no extra resistor instead, or just not fit the extra resistor. or, third option, fit the 100k pot and the 10k resistor.

series parts order makes no difference. the signal goes in one leg, and can go nowhere but out the other leg, so what happens inbetween is a mystery to us. cap first/resistor second [4u7/1k] or other way, makes no difference.

one important point you have missed relates to the difference in the supply section. electrosmash shows ground referenced gate for the out fet, whereas musicding takes the gate to a positive bias. this results in more clean swing for the fet - it is a point many rat builds [search] have problems with. the divider resistors and cap values will have little influence over the total sound.

also note that controls named "tone" and "filter" generally do the same thing, but in opposite rotational directions.

[rangermaster]

Ok thanks for the info!

So is it possible to adjust the IC +9v and ground issue you mentioned ? Just like the original rat ?

[idy]

By "ground issue" maybe you mean the biasing of the FET. The pcb you are talking about has done that...they have chosen the "better" biasing arrangement for the FET already. That is the only issue that is likely to "bite" a beginning rat tamer.

Otherwise you should be fine with the PCB and parts listed. The chart I referenced is if you get more advanced and want to "collect them all." Although rat lovers tend to be satisfied with one (correct me if I'm wrong)...Big muff builders tend to be more into the many possible variations.

[antonis]

I think Gord (GGBB) is more suitable person for enlighten RAT variations..

[rangermaster]

1. ok so it doesn't matter if i'm using the 10k resistor before the output or not.. i just don't understand why it's there  what's the use of it..

2. if i understand correctly.... the IC LM308 is wired better/wiser then a typical rat ? And what did you mean exactly by this results in a more clean swing for the fet ?

3. i understand now that reversing the 4.7uF and the 1k doesn't change the sound.. either way the end has the same value and results.

sorry maybe stupid questions but i'm kinda new to all this and trying to learn.

[idy]

2) Not sure what you meant by "IC +9 and ground issue." The other fella mentioned:
"one important point you have missed relates to the difference in the supply section. electrosmash shows ground referenced gate for the out fet,...."

...and that was what I was talking about. The transistor, the FET, has its gate "biased" by a resistor to ground in some versions of the Rat, and in others by a pair of resistors, one to +9 and one to ground, that are called a "voltage divider." Here the resistors are equal (2.2M), so the voltage is 1/2. This arrangement is better, your musikding schematic includes it.

The simple explanation of bias and "swing;"
Your signal may be traveling along a wire swinging up and down from 0v, say from +1 to -1, "2 volts peak to peak." When it goes to minus, a transistor will turn off, causing nasty noise. If you bias the wire by adding a DC voltage to it, say 4.5v, your signal swings between 3.5 and 5.5. Same size signal, but now the transistor stays on the whole time, and what comes out is still sweet music. Opamps also require bias, in the big Rat comparison Antonis posted above it is R4 connecting the input to the voltage divider...

You can read more about biasing buffer stages here: http://www.muzique.com/lab/buffers.htm


What I can't explain is why it would sometimes be OK to have a FET biased to ground only, like in some RATs.

[PRR]

....What I can't explain is why it would sometimes be OK to have a FET biased to ground only, like in some RATs.

What's the worst that can happen? Distortion?

[ashcat_lt]

I mean, the most it can swing in the "wrong" direction going into that buffer is about 0.6V anyway, and don't we want this point to be more negative than the junction of the transistor and R9 anyway in order to keep the body diode reversed biased?

[GGBB]

Well there is an extra 10k resistor before the output.. why?
And the 1K and 4,7uF are reversed in the musikding schematic.. why is this and does it affect sound ?

The power section is also different..

The "extra" 10k (musikding R10) comes from the "RAT-bypass" status LED circuit (similar to the millennium bypass) introduced in the original RAT2 (no longer used in true-bypass RAT2s). It is meant to be paired with 10µ for the output cap C10 instead of 1µ so that the low-pass filter formed by the cap and the volume pot in parallel with the resistor doesn't change from the original setup with just the cap and the 100k volume pot. LEAVE IT OUT (unless you are building a RAT2 clone complete with RAT-bypass status LED). See R15, C13 and notes in the Multi-RAT schematic below.



The electrosmash schematic is wrong - but the 1k-4.7µ order is unimportant.

The power section ... The change to the biasing of the output FET (musikding R11, R12, C14, R9) is as already mentioned designed to bias the gate at ~3V. It is similar to the biasing used in the Turbo RAT (Multi-RAT R16, R10) which was a necessary modification of the original biasing because the Turbo RAT op-amp output voltage swing was greater than the RAT because of the higher forward voltage of the Turbo RAT's LED clipping diodes. The musicding setup is potentially less noisy than the Turbo RAT and is helpful most of the time even in RATs where a common problem is having the FET go into cutoff. KEEP THIS BIASING. I use something similar in my clones.

Note also that musikding's R3 is not original - but is helpful.

[idy]

Thanks. Yes, that signal is pretty small after the clipping, and no, distortion at that point wouldn't mess up a pretty hi-fi sound. And the gate should be more negative than the source... making sense.

[idy]

So the extra resistor was there to help the LED switching find a good path to ground if the volume was all the way up? And then you need a bigger output cap to keep the same response... OK.