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Hi !
A while ago, my son tried a breadboarded variant of our favourite rodent and really liked the sounds. I want to make him a proper pedal for his birthday and so purchased a RATT Deluxe kit.
I noticed however that a bunch of components values are fairly different from the “original”. The doc says the differences are minimal, but out of curiosity, I did the math based on the formulas in the Electrosmash analysis of the RAT, and I seem to get significantly different cutoff frequencies for the filters in the negative feedback path. I also have a concern with the coupling cap on the output of the op-amp.
The C5/R6 pair is roughly in the ballpark. The original cuts off at 1/(2pi*560*4u7) = 60.5 Hz roughly, the GPCB at (1/2pi*4k7*470n) = 72Hz. So we have a small difference in the lowest end cutoff, but not that important.
However it’s C6/R7 (or TR1) that baffles me. The original is 1/(2pi*47*2u2) = 1539 Hz. GPCB is rather far off. With the 100R resistor (or the trimpot set to max), we get 4800Hz. That”s more than 2 octaves higher. It would take a 330R resistor to approach the original valule (1461 Hz), or a 500R trimpot…
Finally, the coupling cap C7. Unless I’m mistaken (please enlighten me if I am !) it forms a high pass filter with R8 (and the path to ground via the diodes). With the original values, the cutoff is around 34Hz. With GPCB, I calculate 723Hz, which would cut off a lot of the mids & lows, wouldn’t it ? I’m not sure because I don’t know enough to fully understand the effect of the diodes in the above filter.
Am I missing something or will the RATT Deluxe with the GPCB values end up with a significantly different sound as a result of this ? I’ll try to recover the breadboard setup to try…
If I was to do the original circuit, I assume I would have to use tants for C5,C6 and C7 to fit the PCB, am I right ?
Cheers !
Ben.
You are seeing variations in component values between designs from different sources. I cannot offer reasons why these differences exist other than to say different authors may have felt that they preferred the resulting tones.
If your son liked the tone from the breadboard version, you may want to recover those values and try them with the GPCB board. Note that the diode switching arrangements here are quite different.
You can use MMLC capacitors for C5, C6 and C7. They will fit. We do not recommend using Tantalum caps unless necessary.
BTW, I noted that there is a low pass filter at (R9+FILTER)/C8 that has a low freq at 4089 Hz.
This was an original Tonmann design from Germany and unfortunately he is not around to answer details but this board has been in circulation for 11 years and many people swear by our values. One thing I can say at a glance is that some of the original values are obviously higher than need to be for a guitar pedal coupled with the fact that we do not like using electrolytics in the audio path which would explain most of it.
As Wilkie said you can put original values in and use MMLC capacitors which have a 5mm lead spread.
Here is a chart that shows the changes.
Here are some additional notes:
R5: This has been added to prevent the Gain pot from being turned completely off.
This is useful in that you can set the minimum gain (via the value of R5) when the Gain pot is at 0% rotation, this will give more usage out of the gain pot itself. There are two methods to adjust the value of R5 for minimum gain:
Method 1:Socket R5 and try different values –10kΩ is suggested as a starting point.
Method 2 (The second method is a bit more involved): Don’t install R5, wire lugs 2 & 3 of the Gain pot as usual. Wire lug 1 of the Gain pot to the right hand (from the layout diagram) pad of R5: Set the Gain pot to give the desired minimum gain. Remove the wire from the right hand pad of R5 and measure the resistance between lugs 1 & 2Choose the nearest standard value for R5 and solder it to the board.
R7 –TR1: This is a version of the Ruetz mod.
I don’t think this mod really warrants an external pot so I have used an on board trim pot. If you want to make this mod with an external pot, don’t install the trim pot and wire the external pot to the respective pads. It basically reduces the high frequencies a lot less than the bass and mid frequencies when the gain pot is turned down.The best results will be obtained when the R7 –TR1 value is between 0Ω (a wire jumper) and 100Ω, there are several methods to achieve this: Socket R7 and don’t install the trim pot. Try different values for R7 ranging from a jumper up to 100Ω. This is an unsatisfactory method as many people won’t have (enough) low-value resistors in their parts bin. Install the trim pot and don’t install R7, set the trim pot to taste. Using a 1kΩ trim pot (which is more obtainable) in conjunction with R7 at 120Ω or 100Ω will yield the desired results
Thanks for the replies !
So unless I screwed up my math, the easiest for me at this point is to use the GPCB values but with a 500R trim instead of 100R, which should allow me to reach around 300R and should provide the same frequency cutoff as the original for that part of the filter, while also allowing for the “GPCB” sound should my son decide he likes it. I might also put a socket for R7 so we can shorten the pot range with a parallel resistor if needed.
Just in case, I’ve also ordered online some MLC’s (sadly my local walk-in shop, JayCar, only has tants & electrolytics for 2.2uF and 4.7uF). I’ll recover the breadboard, test the whole lot, that way I can always go back to the originals if that’s what he wants.
Haha, my son is 🙂 (He plays bass)
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