DS1 -> Rat mod

[Chistopher]

I did a quick little experiment this week. I converted my DS-1 into a fine little simulacrum of a Proco Rat. In their stock form they don't really sound like eachother at all, but topologically they are very similar.

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Because I believe knowledge should be free, I'll share my journey here. First I'll start out with how each circuit works.

A Boss DS-1 see the signal enter through a buffer, get a healthy amount of gain through a transistor, gets boosted again through an opamp, gets clipped by hard clipping silicon diodes, goes through a tone control, and then exits through the volume knob and then a buffer. A Rat enters straight into an opamp that gives the signal a TON of gain, gets hard clipped with similar diodes to a DS1, goes through a tone control, and then leaves through the volume control and then a buffer. The gain of both opamps is determined by the gain knob controlling positive feedback.

As you can see, from a big picture perspective, the biggest difference between a Rat and a DS-1 is that the Rat gets one big gain stage, and the DS-1 gets two medium gain stages. The second big difference is that the DS1's tone control cuts bass, mids, or treble, depending on its position, and a Rats filter knob functions the same as the tone control on a guitar

 

[Chistopher]

Now here's the big differences between a Rat and DS-1 in order of how the signal travels through the circuit:

1 - The gain staging. The DS1 transistor boost stage boosts all frequencies above 30 Hz by around 30 dB and then the opamp further boosts all frequencies above 70 Hz by another 30 dB. A Rat has one gain stage, but two filters. Frequencies above 60 Hz get a total of 45 dB of gain, frequencies above 1.5kHz get a total of 67 dB of gain.

2 - Pre-diode HPF: In both circuits, right before the signal gets clipped by the silicon clipping diodes, there is a bass cut. On the DS1 it cuts bass below 150 Hz, on a Rat it cuts bass below 30 Hz. This makes the Rat fuzzier at high gain.

3 - Tone controls: A DS1's tone control blends between a bass cut and a treble cut as your turn it from right to left. This causes a significant volume loss (12 dB I'm told, I've never bothered to check myself) and a pretty severe mid scoop. I'd say 70% of what makes a DS-1 sound like a DS-1 is the tone control. A Rat has a simple cap in series to ground and a potentiometer acting as a variable resistor. Turning the filter up also causes volume loss.

 

[Chistopher]

Here's the mods I made in order of the way I made them:
Tone Control:
R16: 1.5k
C12: 100 nF
R15: short
C11: short
R17: remove

This is actually a pretty unique way of wiring a tone control as far as I'm aware. The clean signal comes in through one side of the potentiometer, the treble cut signal comes in through the other, and the pot acts as a blend control. As soon as I made this change the DS-1 was a lot louder and no longer sounded like a DS-1. The tone control didn't make the pedal any quieter when turned down, and it sounds mostly like a Rat filter control to my ears, except by my tuning not as severe. I chose 100 nF by a quick calculation and my ear. If you want it darker, go higher in cap size. 150 nF would probably be near as makes no difference to a stock Rat, but the stock Rat goes a little too dark for me. Also this tone control doesn't have the big dead zone on the left side the a Rat does.

Rebias Q2

R9: 56R
R6: 56k

These values are the same values I always use for the transistor boost stage on a DS1. It lowers the gain just enough the transistor won't distort (that's what can make a DS-1 buzzy even at low gain settings) and coincidentally brings the corner frequency of the gain to the Rats value of 60 Hz. R9 is the big player at this stage. In the context of these mods, lower it and the pedal will get bassier and buzzier, raise it and the pedal will be lower gain and clean up better. I actually settled on 39R, mostly because it wasn't a big enough change to be worth changing back. It now sounds nothing like a DS1. Closer to an MXR Dist+ or a DOD250. I ended up flipping the ribbon cable to the tone control, so it is now wired backwards like a Rat. The tone control is linear on a DS1, so it didn't effect the taper.

Opamp
C8: 2.2uF
R13: 47R

These are identical values to a Rat. The signature sound of a Rat is caused by a massive spike in the 1.5kHz range and this is the part of the circuit that causes it. One interesting thing about my implementation is that the gain control of my pedal ONLY controls the frequencies above 1.5kHz. With a Rat the gain control controls the gain of all frequencies. You would think this might create an issue with the bass thinning out at higher gain levels, but it was actually a nonissue. I suspect it may be because the full spectrum was already so close to clipping that a mid boost changed the texture of the sound more than the EQ. One issue I came across. By making R13 so small, the gain knob didn't start playing a big role in the gain equation until the last 25% of its range. So I swapped the gain top to an antilog pot and now it works smoothly throughout the whole sweep. The pedals sounds almost identical to a Rat at higher gain levels, at lower gain levels it cleans up better than a Rat and does not suffer from thinning at low gain levels like a Rat.

Clipping bass filter
C9: 2.2uF

A lot more bass gets clipped now. It now clips frequencies above 30 Hz, as opposed to a DS1 clipping frequencies above 150 Hz. It sounds like a fuzz rather than a distortion. I made this change at the same time as I changed the gain structure, so I don't know how big a difference it makes, but I can assume it makes a big one.

 

[Chistopher]

Now here's a review

What it does better than a Rat:
Low gain sounds aren't thin
With a Rat the low gain sounds are bunched up between the first two notches, and everything beyond that is saturated. This one doesn't have that
The tone control doesn't cut volume
The tone control doesn't cut as much upper mids
The tone control has a better taper

Things a real Rat does better
The letters on my DS-1 don't glow in the dark
On bass guitar it functions as an overdrive rather than a fuzz. It's a good sound, but a different one. If you don't change R9 from stock, it will probably fix this. Heck, I may experiment with returning R9 to stock just to try it out. This would basically double the volume of the bass frequencies

At the end of the day this circuit is really just a Rat with easier to dial in controls. I've been a/b-ing it with my modded Rat2, and I probably couldn't tell them apart in a blind test. The 1.5 kHz midspike and getting rid of R15, R17, and C11 is the most important part of the sound, but there are a lot of other changes you can make to get various Rat sounds. Keeping R9 at 22R would be akin to the Ruetz mod. Changing the clipping diodes to LEDs, MOSFETs, or germaniums would give you a Turbo Rat, Fat Rat, or You Dirty Rat, respectively.

 

[GuitarStv]

Cool experimentation! Thanks for posting. Is there any different feel in the distortion between the rat and modded DS-1?

 

[Chistopher]

A dozen paragraphs and I forgot to mention feel :laugh2:

At high gain settings it compresses just as hard as a Rat, which isn't exactly hard to duplicate. The Rat and DS-1 both hard clip at 0.7v, meaning that the signal going to your amp is never going to get louder than 1.4v. It doesn't take a lot of gain to get a guitar signal up to 1.4v

However, compared to a DS-1 especially, it cleans up a lot better with the volume knob now, even at the maximum gain setting.

Much like a Rat, at the higher settings on the gain knob, even with vintage single coils, increasing the gain only really increases compression and sustain.

The big takeaway is that if you plugged into this and couldn't see the pedal and someone else fiddled with the knobs, it would be indistinguishable from a Rat. The biggest advantage however is that the controls are a lot less interactive then on a Rat and the sweeps are the controls are more consistent throughout their entire ranges.

 

[Aceman]

Hahaha - Orange Rat - AKA the Citrus Rat!

I find these convert this pedal to that pedal things intriguing. I once had a guy try to convert an SD-1 to a TS9...he failed on a technical issue, but still interesting

 

[Chistopher]

Yeah, if you know what makes a circuit tick it's not too hard to make something else do something roughly similar. Between a moderate understanding and just fiddling around with pedals, it's not too hard to get most any pedal to sound exactly how you want.

 

[NotDemanic]

Funny, as I recall, trying to minimize distortion was the goal.
Making the signal break up was easy. Anyone remember wiring the LED that told when the pedal was on, into the circuit?

 

[ErikH]

I find these convert this pedal to that pedal things intriguing. I once had a guy try to convert an SD-1 to a TS9...he failed on a technical issue, but still interesting

I did that exact thing some years ago. It wasn't hard, more tedious than anything. Comparing schematics was the fun part, going part by part thru the entire circuit. When I was done, it certainly sounded like an 808. Even A/B'd it against a buddy's reissue 808.

 

[GuitarStv]

Funny, as I recall, trying to minimize distortion was the goal.
Making the signal break up was easy. Anyone remember wiring the LED that told when the pedal was on, into the circuit?

Yeah, I did a few seeing eye mods for folks back when I was in university. Paid for beer.

 

[Chistopher]

I prefer IR leds. I bet if you did a seeing eye mod with those, you could generate some unexpected behavior from your TV set.

 

[GuitarStv]

I prefer IR leds. I bet if you did a seeing eye mod with those, you could generate some unexpected behavior from your TV set.

I miss the clicker for my big old GE CRT. If you fired that bastard at a humbucker you would actually hear a dull "pew pew" sound through your amp.

 

[Seashore]

Here's the mods I made in order of the way I made them:
Tone Control:
R16: 1.5k
C12: 100 nF
R15: short
C11: short
R17: remove

This is actually a pretty unique way of wiring a tone control as far as I'm aware. The clean signal comes in through one side of the potentiometer, the treble cut signal comes in through the other, and the pot acts as a blend control. As soon as I made this change the DS-1 was a lot louder and no longer sounded like a DS-1. The tone control didn't make the pedal any quieter when turned down, and it sounds mostly like a Rat filter control to my ears, except by my tuning not as severe. I chose 100 nF by a quick calculation and my ear. If you want it darker, go higher in cap size. 150 nF would probably be near as makes no difference to a stock Rat, but the stock Rat goes a little too dark for me. Also this tone control doesn't have the big dead zone on the left side the a Rat does.

Rebias Q2

R9: 56R
R6: 56k

These values are the same values I always use for the transistor boost stage on a DS1. It lowers the gain just enough the transistor won't distort (that's what can make a DS-1 buzzy even at low gain settings) and coincidentally brings the corner frequency of the gain to the Rats value of 60 Hz. R9 is the big player at this stage. In the context of these mods, lower it and the pedal will get bassier and buzzier, raise it and the pedal will be lower gain and clean up better. I actually settled on 39R, mostly because it wasn't a big enough change to be worth changing back. It now sounds nothing like a DS1. Closer to an MXR Dist+ or a DOD250. I ended up flipping the ribbon cable to the tone control, so it is now wired backwards like a Rat. The tone control is linear on a DS1, so it didn't effect the taper.

Opamp
C8: 2.2uF
R13: 47R

These are identical values to a Rat. The signature sound of a Rat is caused by a massive spike in the 1.5kHz range and this is the part of the circuit that causes it. One interesting thing about my implementation is that the gain control of my pedal ONLY controls the frequencies above 1.5kHz. With a Rat the gain control controls the gain of all frequencies. You would think this might create an issue with the bass thinning out at higher gain levels, but it was actually a nonissue. I suspect it may be because the full spectrum was already so close to clipping that a mid boost changed the texture of the sound more than the EQ. One issue I came across. By making R13 so small, the gain knob didn't start playing a big role in the gain equation until the last 25% of its range. So I swapped the gain top to an antilog pot and now it works smoothly throughout the whole sweep. The pedals sounds almost identical to a Rat at higher gain levels, at lower gain levels it cleans up better than a Rat and does not suffer from thinning at low gain levels like a Rat.

Clipping bass filter
C9: 2.2uF

A lot more bass gets clipped now. It now clips frequencies above 30 Hz, as opposed to a DS1 clipping frequencies above 150 Hz. It sounds like a fuzz rather than a distortion. I made this change at the same time as I changed the gain structure, so I don't know how big a difference it makes, but I can assume it makes a big one.

The 1.5kHz spike probably explains why I like a Rat sound on bass so much. 1.5k to 1.7k is where I always find a nice metallic bite that gives the bass good attack while sitting underneath the guitar's pick attack. Much higher or lower and it starts to get lost. Having said that, and keeping in mind that this might be a dumb question since I know next to nothing about pedal modding, is there a component value I could change to raise that spike up to between 2k-2.5k for guitar applications? Also, could the caps for the clipping bass filter be put on a switch to toggle between different values?

 

[Chistopher]

The 1.5kHz spike probably explains why I like a Rat sound on bass so much. 1.5k to 1.7k is where I always find a nice metallic bite that gives the bass good attack while sitting underneath the guitar's pick attack. Much higher or lower and it starts to get lost. Having said that, and keeping in mind that this might be a dumb question since I know next to nothing about pedal modding, is there a component value I could change to raise that spike up to between 2k-2.5k for guitar applications?

Yes, but it might not behave exactly as you probably think if you only changed those values. 1uf for the cap and 68R for the resistor is what I would recommend. This would make it so that all frequencies below 2.3 kHz distort a little bit. You would likely want to use the 39R value for R9 in this case.

Also, could the caps for the clipping bass filter be put on a switch to toggle between different values?

Yes, just take a 3P2T switch and wire the middle two lugs to where the cap was, wire the top lugs to one cap, and wire the bottom lugs to another. If you want to know where the cutoff frequency is, lookup HPF calculator and put 2.2k in for the resistor and experiment with cap values.

 

[Seashore]

Yes, but it might not behave exactly as you probably think if you only changed those values. 1uf for the cap and 68R for the resistor is what I would recommend. This would make it so that all frequencies below 2.3 kHz distort a little bit. You would likely want to use the 39R value for R9 in this case.



Yes, just take a 3P2T switch and wire the middle two lugs to where the cap was, wire the top lugs to one cap, and wire the bottom lugs to another. If you want to know where the cutoff frequency is, lookup HPF calculator and put 2.2k in for the resistor and experiment with cap values.

That's great info to have thanks so much.

 

[Artie]

Nice write-up. Good info. Now I wish I had my DS-1 back.