TEST SHEET FOR PRO-CO RAT MODS

Layout

Taken from Schematic

I

A small series of experiments to test certain mods for the Proco Rat to voice the pedal more effectively for a 7 string bass in terms of frequency response and distortion tone.

In this investigation I will be testing two different types of mod to the stock circuit.

Firstly; mods to the differential gain network to alter the frequency response of the circuit in an attempt at voicing it more effectively for bass by changing resistor and capacitor values.

Secondly; mods to the clipping stage to alter the clipping characteristics and change the type of distortion in the signal by swapping diodes.

Differential Gain Network:R7 (560R) and C4 (4u7) controls the freq range receiving the default, overall gain.R8 (47R) and C5 (2u2) controls the freq range receiving additional gain to voice the circuit in the classic, harsh, Rat tone.

Clipping stage:D1 and D2 control the clipping of the signal. Different diodes will affect the signal differently giving its own characteristics with each diode pair, matching or non-matching.

I initially used a veroboard construction with only certain components socketed, however upon initial testing it turned out that I wanted to be able to alter the values of many other components than the ones initially socketed on the first test board. As a result, for the second round of testing I will be using a slightly different design. It will still be of veroboard construction however it will have all the components socketed to allow them to be easily replaced during the process of fine tuning this circuit. See below:-

1) Initial test board with only the input/output caps, 47R resistor from the differential gain network, the 30pF cap, and the diodes socketed: note that the 3.3nF cap in C9 is 4.7nF on this board as I had no 3.3nF at the time.

2) Fully socketed test board, pre population:

And after population – IC chip and off-board wiring not yet added:

PT 1: Differential Gain Network: GAIN AND FREQ RESPONSE MODS

1) Remove 47R from the differential gain stage to remove boost to mid-high freqs.

Hypothesis: Will reduce apparent output/gain level and dull the sound, both as a result of reduction in additional gain above 1.5kHz.

Results: Reduces additional gain of freqs above 1.5kHz. Flattens/warms the sound slightly as a result of the reduction in level of mid-high freqs giving the impression of more bass by reducing mid+treble.

Conclusions: Gives a warmer sound but not what I’m looking for in this case, if it were a regular bass with 4-5 strings tuned to concert pitch this mod would probably be enough. Given that this is a 7 string bass the frequency range it tends to occupy most of the time is much lower than a normal bass, as a result the circuit’s frequency response needs to be altered to distort more of the low frequencies present in the signal to balance the mid-high frequency content that occurs with the stock circuit.

According to my research before building this pedal there are three places in the circuit that you can change the frequency response:-The input/output caps: These set the overall frequency response of the circuit, controlling what ultimately makes it in and out of the jack sockets at either end. Apparently they already allow the entire useful audio range through already so increasing their value would be pointless.

-The differential gain stage which sets the boost to frequencies above a specific threshold depending on the value of the capacitors in that part of the circuit.

-After the filter control: C9 appears to have the function of dumping unwanted low frequencies to ground once the signal has passed the filter section of the circuit. At least that’s what I think it does. I was given some advice whilst researching this project and was advised to experiment with increases in the value of C9 (3.3nF) and note the effects.

2) Return the 47ohm resistor and replace the 3n3 cap at C9 with a 47nF cap and replace 100k ohm filter pot with 10k ohm filter pot to alter freq range and filter slope for tone/filter pot

Hypothesis: According to the advice I received whilst researching this project, I was led to believe that this will increase the bass response and balance out the extra high frequencies occurring as a result of the distortion. Increasing the value of C9 from 3.3nF to 47nF should give a notable increase in bass response and warmth.

Reinstalling the 47R will reconnect the part of the differential gain network that in conjunction with the 2u2 cap, controls the additional gain of freqs over 1.5kHz. This will result in added brightness/crunch in the effected signal due to the boost of these mid-high freqs.

Results: The new cap value of 47nF added a significant amount of low end to the signal, giving the distorted signal a much fatter, rounder sound than before. It is noticeably warmer due to the increased bass response and is almost fuzzy and saturated rather than the crunch of a straight up distortion.

Unfortunately I am unable to substitute the filter pot - as mentioned above - at the present time and am using the stock 100k log pot. I have wired the filter knob in reverse to use it as a tone knob as this makes more sense to my mind, however with it operating on a logarithmic scale, it causes masses of high end to be let in across a very small area of rotation just before the pot is maxed out. This makes it very difficult to achieve the desired tone setting as the slightest change can yield wildly different results.

Conclusions: Regardless of whether I change the value of the filter pot from 100K to 10K or not, it certainly could benefit from being changed to a linear scale to make it easier to achieve the required tone setting.

The value of C9 should be kept at 47nF to maintain the increased bass response resulting from the change in value from 3.3nF. I offered to try smaller values to slightly tailor the low end threshold but the customer was happy with the results, the sound was still not quite there so I must continue to tailor the responses of other parts of the circuit to achieve the desired effect.

3) Keeping the changes from test 2, change the 2u2 cap for a 3u3, (and then for a 4u7 simply for investigative purposes) to lower the cut-off point of the freq range receiving the additional gain from this branch of the differential gain network.

Hypothesis: The 4u7 cap sets the freq range receiving the initial “default” gain prior to any additional gain to the second freq range set by the 47R/2u2 branch network. The gain of the 4u7 branch is set by the value of the 560R resistor (R7). Since the 4u7 cap sets the entire freq range of its branch of the differential gain network, it is reasonable to assume that increasing the value of the 2u2 cap (that works with the 47R in the other branch of the differential gain network) to 4u7 would essentially result in additional gain across the entire freq response of the circuit as opposed to freqs above a certain threshold that would be provided by lower cap values.

Increasing the value of the 2u2 cap to 3u3 or even 4u7 will lower the cutoff point at the bottom of the freq response in this part of the circuit therefore adding additional lower freqs into the differential gain network than with a 2u2 cap; i.e. 1kHz and up with a 3u3 cap as opposed to 1.5kHz with a 2u2 cap.

Results: Swapping the 2u2 cap in C5 for a 3u3 cap added more low end to the frequency range boosted by the second branch of the differential gain network as expected, but it seems to be too much. There is now a lot of bass as a result of the increase in value of C5, and a lot of treble due to the distortion, as a result the mids now seem to have been swamped by the extra bass and pushed out of the equation somewhat.

Conclusions: There is definitely more bass in the signal now, but it is not having the required effect and is making the sound muddier rather than reinforcing it. The value of C5 may need to be changed back to 2.2uF and other options considered to tailor the sound. Perhaps changing the value of the resistor in R7 from 560R to 1K, thereby lowering the initial default gain of the freqs passing through the first branch of the differential gain network, could change the sound in the required way.

4) Alter value of R7 (560R) adjacent to 4u7 cap in the differential gain circuit for different overall gains across freq response band of the 560R/4u7 branch of the differential gain network.

Hypothesis: Gain and resistance have an inversely proportional relationship so the gain will increase as the resistor value decreases. I predict the signal will be louder and harsher with each decrease in resistance in the 560R/4u7 network, and smoother and quieter with each increase of R7. As the distortion is already quite harsh, even with certain increases in bass response being retained, I do not feel that more gain is the solution here and that slightly less gain could be the answer to rounding off our sound to a suitable effect. As a result, I will substitute the 560R in R7 for a 1K resistor with the intention of reducing the default gain of the differential gain network, and reducing some of the harsh top end in the distortion.

Results: The increase in resistance did decrease gain as expected, however the sound seemed to harshen rather than smooth out. This was possibly the result of the first branch of the differential gain network (now 1K/4.7uF) reducing the gain of the freqs, making the difference between the gain of the first and second branches more noticeable and giving the illusion of more high end.

Conclusions: This did not have the desired effect; in addition to the muddying of the signal by replacing the 2u2 cap with a 3u3 at C5, the reduced gain in the first branch of the differential gain circuit seemed to actually have the opposite effect to what I was looking for, making the sound rougher and harsher than I’d hoped. The value of C5 should be returned to 2u2 and results noted.

5) Keeping the 1K resistor in R7, return C5 from 3.3uF to 2.2uF to alter freq response and gain of differential gain circuit again.

Hypothesis: The new value of R7 (1K) will reduce the initial boost received by the freq range set by the 4.7uF cap in C4. This lower gain coupled with the gain from the other branch of the differential gain network across the newly reduced bass response of the replacement 2.2uF cap in C5 will hopefully create the required frequency balance to voice the circuit for this particular application.

Results: Much smoother sound overall now thanks to the reduced gain at the 1K resistor in R7.

Conclusion: Frequency response seems balanced enough to satisfy the customer’s ears at this stage. Continue to tailor the sound using the diodes in the clipping stage.

PT 2: Clipping Stage: DIODE/DISTORTION MODSIn previous tests I performed using the stock version of the circuit with a guitar set up, I made the following observations:

1N4148 diodes are stock value. All descriptions of alternative diodes are qualitative and in comparison to a stock Rat circuit which was A/B’d with the test circuit.

1N34A germaniums (pair) are noticeably quieter.1N914 (pair) are a little more fuzzy/warmer/tonal. A little more like a fuzz unit that a dist box.Green LEDs (pair) are markedly louder and a bit more distorted.Yellow LEDs (pair) are still louder than stock diodes but not as loud as green LEDs. Also gives a slightly smoother fuzz.Clear LED (single) sounded harsher and more clanky than stock, it was also louder than stock.Clear LED+1N914 was very nice warm and fuzzy, good tone and not too much difference in output level as the extra level from the LED countered some of the level reduction from the 1N914 silicon diode.

Winners out of the group would have to be Clear LED+1N914, and Green LEDS (pair). I would like to try these with RED LEDs (pair) but have none at present.

However, as an aesthetic choice the customer mentioned they would like to use two red LEDs as status indicators which would be incorporated in the paint job design on the enclosure. The idea was to have a rat painted on the enclosure with the LEDs being used for its eyes. Rather than have two red LEDs for status indicators plus two diodes, I decided to abandon the status indicator altogether and use a pair of red LEDs in place of the stock diodes. This meant that I could use the LEDs in the diode sockets as status indicators as well due to the fact they glow when current passes through them and will give the effect of the rat’s eyes pulsating as you play.

I was initially using a pair of red 3mm flat topped LEDs in the circuit that was being tested in part 1 of this investigation. In addition to these LEDs, altering the values of components within the circuit allowed me to tune its frequency response and gain to a reasonable degree, but I was aware that different colour LEDs had different tonal qualities and that this may also apply for LEDs of the same colour in different sizes so I swapped the 3mm red flat tops out for a pair of 5mm red round top LEDs.

There was a noticeable difference in the response from the 5mm round tops; more saturation, slightly louder, harsher clipping etc but it was a little too extreme and lost some definition in the notes and finger clack that the customer wanted to retain in the sound. As a result, we tried a combination of one 3mm flat top and one 5mm round top LED to see if the positive traits of each one would balance out the negative aspects of the other. This was a little better and made for a nice medium between the two but was still a little excessive so we reverted back to the original 3mm flat top red LEDs. Upon doing this, the sound cleared up and became better defined and the customer said this was the sound he wanted as it didn’t overpower the sound with harsh high end fizz, and retained the finger ‘clack’ with the note definition he was trying to achieve.

PT 3: Final Design of Modded Bass Rat Circuit

R7 now = 1K

LM308 now = OP07

3mm red LED

C9 now 47nF

100K Log Filter pot now 100K Lin R10 replaced with

25K Lin pot