read somewhere some time ago and saved...
*Clip out the C5 cap , no jumper required . ( this is to smooth out the clipping from factory , but if you add the the 1n34a germs, i feel this C5 cap is uneeded )
*Add a 1N34a Germainum diode to each of the clippers in D3 & D4 ( this will add a touch more gain , but a way warmer clipping , or more tube amp like tone etc )
*Also i would install a op amp socket & run a JRC4558D !( Classic Tube Screamer Tone , my personal fav Op amp ! )
For your Treble issue Increase the C7 cap to .47 uf . ( Note anytime you are adding Gain , you will have an added amount of noise etc . )
***** Boss SD-1 5-Star Ge Keeley Mod *****
Keeley Custom Shop designed this mod to enhance the tone and functionality of the SD-1.
Freely copy and distribute. We have the right to change our modifications at any time without notification. It is fun to watch what big name people use the mods.
* Board Number, New part, Old part, Reason
* C10 --> Metal Stack Film 1uF, remove electrolytic, increased fidelity
* C7--> Metal Stack Film 1uF, remove electrolytic, increased fidelity
* R6 --> 2.4k ohm metal film, 4.7k, increased distortion
* C3--> 0.1uf metal stack film, 0.047uf, fidelity and bass response
* C2--> 0.1uf metal stack film, 0.018 fidelity and bass response
* R2--> 620k ohm metal film, 470k ohm, increased input impedance, better bypass
* Add 47pF cap across diodes in clipping section to smooth out distortion, just like TS9
On some older units, short out D3 and the 470 ohm resistor in the power supply section to increase the voltage to the circuit. This increase the headroom and gain. Newer units have it shorted already.
Here is the tricky part of our mod, take a germanium (Ge) transistor and use the Base-Emitter junction (legs) of the device and use it in place of D6. If you use a PNP transistor, the Base will be the negative or cathode side, the Emitter will be the anode. Here is a diode drawing Cathode---|<----Anode. This increases the amount of distortion produced by the pedal, it also sound really nice. Try this! Don't leave it out! It's fun. Get your Ge transistors from Steve Daniels http://www.smallbearelec.com/
If you want to drill out the LED and install a 5mm big job, change R30 to a 1.2k-3.3k ohm depending on brightness and battery drain you want, replacing the stock 3.9k ohm.
The Boss SD-1
The Boss SD-1 Super Overdrive is one of the premier Overdrive pedals that's currently being manufactured. It's also probably one of the best overdrive pedals that's ever been manufactured by any company. (In my opinion, it's hands-down the best overdrive bang for the buck!) It's about half the price of an Ibanez TS-9, and it's also quite a bit more durable, especially the switch that controls the JFET electronic switching of the pedal. Here's the description of the SD1 from the Boss website:
"Provides warm, smooth overdrive tones that respond to your picking style the way a tube amp does, even at low volume levels. Exclusive BOSS asymmetric overdrive circuitry delivers a genuine overdrive effect for a classic guitar sound."
Believe it or not, the SD-1 actually lives up to the manufacturer's description, and then some! But like most mass-produced effects pedals, there's room for improvement. The manufacturers tend to use the parts that they can acquire the most of for the lowest cost, which usually means that they aren't the best sounding parts that could be used in the circuit, but they're good enough to make a lot of people happy...but we're not those people, are we? HERE is a schematic of the Boss SD1, unmodified from it's out-of-the-box condition. You'll also notice that the schematic has the component numbers that appear on the Boss circuit board...slick huh? The circuit is almost exactly the same as the TS9, with a few differences here and there that make it a little more "hairy" sounding. Also notice the difference in the clipping section of the pedal. The SD1 has asymmetrical clipping, which I believe produces a more natural sounding overdrive. The only reason Ibanez doesn't use asymmetrical clipping is because Boss/Roland holds the patent for it. The SD1 also doesn't have the 51pF capacitor across the clipping diodes, which would otherwise help to smooth out the distortion.
Now, let's take a look at the modifications that I performed on my pair of SD1s. These mods are based on Robert Keeley's "5-Star Germanium" mods, but I made a couple of changes and omissions. HERE is a schematic that has the changed components highlighted in red. And below is a table that lists all the changes made to the circuit.
Component Number Original Part New Part
C2 18 nF 100 nF Metal Film
C3 47 nF 100 nF Metal Film
C7 1 µF Electro. 1 µF NP* Metal Film
C8 47 nF 100 nF Metal Film
C10 1 µF Electro. 1 µF NP* Metal Film
D6 1N4148 Silicon 1N34A Germanium
LED 3mm Red LED 3mm White Super Bright LED
R2 470K, 5% Carbon Film 620K, 1% Metal Film
R6 4K7, 5% Carbon Film 2K2, 1% Metal Film
R30 3K9, 5% Carbon Film 1K, 1% Metal Film
U1 JRC4558DD Burr Brown OPA2134PA
* NP = Non Polar
These modifications add a great deal to the audio quality of the circuit. You can get a little creative when choosing a new OPAMP for your SD1. The normal choices are the original JRC4558DD, the Texas Instruments RC4558P, the Burr Brown OPA2604AP, the Burr Brown OPA2134PA, and even the Texas Instruments TLC2272ACP, which is a linear CMOS OPAMP. I chose the Burr Brown OPA2134PA because it's designed specifically for audio applications, and because it's super high quality and low noise. The Burr Browns aren't cheap though... the OPA2134 was about $2.50 for a single one and the OPA2604 is $3.50 for a single one. So here's what we're doing with the rest of the SD1's circuit:
Changing C2, C3, and C8 to 100 nF will increase bass response.
Changing C7 and C10 to 1 µF NP metal film will remove some harshness from the circuit.
Changing D6 from a 1N4148 Silicon diode to a 1N34A Germanium diode will help to further increase the distortion.
Changing R2 to 620K will increase the input impedance of the input buffer resulting in more clarity when the pedal is in effect more or when it's bypassed.
Changing R6 to a 2K2 will increase the amount of avalible overdrive.
Changing R30 from 3K9 to 1K will increase the current being supplied to the LED, therefore increasing the brightness of the new LED, but it will also put more of a hurt on the battery, if you use one.
Changing the original not-so-bright 3mm red LED won't do anything for the sound, but putting that 3mm water clear super bright white LED in there will certainly make the pedal look a little more "trick."
Some Final Points
If you own an older SD1, there will be a diode and resistor on the circuit board, D3 and R31 (not shown on the schematics), that are connected to the negative pin of the DC power jack. I would recommend that you remove these two components from the circuit board and replace them with jumpers. Doing this will increase the power being delivered to the circuit so that it will have even more "oomph." If you own a newer SD1, then these two components are already replaced with jumpers, so you won't have to worry about them.