MXR Distortion Plus modded to BSIAB

[halen]

Can the MXR Distortion plus be modded to the BSIAB.

[frank_p]

Don't think so...

Dist+     is an opamp + diode design.
BSIAB   is a JFET design.

Do you understand schematics ?

[deadastronaut]

nope!...

[wavley]

Can the MXR Distortion plus be modded to the BSIAB.

You can, however, get some really nice sounds out of this box with minimal mods that give you a browner, more growly sound.  You can play with the input and output cap values and other such things, it's a pretty easy circuit to work with.  I ended up changing a few values in mine and adding one more germanium to one side of the clipping diodes and an LED to the other side for 4 diodes total, now it's less distortiony and more crunch.

[Fender3D]

Wouldn't be easier build a BSIAB from scratch?

[wavley]

Wouldn't be easier build a BSIAB from scratch?

Yes and no, it might not be if you're a beginner who already has a D+ and is just trying to find out how all the parts work together as a learning experience, or someone that already owns a D+ but doesn't like how it sounds it might be a good idea to mod a D+.

But if you're looking to build a new circuit then it is certainly a better idea to build the one you want instead of bending one to your will.

[halen]

Wavley what would need to be done to the D+ to make it more browner,Thanks for your help..........

[Mark Hammer]

I have no idea what "browner" means to you, but there are a number of things you can do to give it more muscle.

1) Replace the .047uf cap to ground on the path where the Gain pot is with .22 or even .47uf.  At highest gains, the standard D+ loses a lot of bass.  This will bring it back.  Since a lot of the signal lives in the bass end, bringing more bass in will produce more intense clipping without having to lift a finger.

2) There is not nearly enough treble trimming in the stock circuit, which makes for a shrill pedal that requires futzing around with either the guitar tone or amp tone.  Taming the top end will make something you can turn on and enjoy the sound without having to adjust anything.  Put a 22pf-47pf cap in parallel with the 1M feedback resistor of the op-amp (the one between the output and "-" pin.  22pf starts a rolloff at 7.2khz, and 47pf around 3.4khz.  Similarly, the .001uf cap to ground in parallel with the diodes can be upped to .0033uf or .0039uf, for a second rolloff at 4.8khz or 4khz.  Those two rolloffs combined will still provide plenty of bite, but remove a lot of hiss and fizz.

3) The germanium diodes distort a little more but silicon diodes will deliver double the output level, which will allow you to push your amp harder.  Swap those 1N60s or 1N34s for 1N914s or 1N4148s.  You'll still get clipping, but it won't sound as harsh, and yur amp will play along too.

My sense is that the D+ was originally designed to roll off bass at higher gains so that the expected hum from single-coil pickups of the mid-1970s would not be quite as loud.  It was also sort of intended to cover the terrain between booster and distortion, which is why it had that 500k C-taper pot.  If all you want is a distortion pedal, and hum is not a big issue for you, then you'll be fine with a 50k-100k linear gain pot, and with bringing all the bass back in.

All these changes are easily reversible, but I think you'll like them.

[WGTP]

You might try adding the tone control section of the BSIAB to the end of a Dist. +, but then would probably need to boost the output after that.   

[frank_p]

Maybe, take a look at other opamp and diode based dirt boxes and ask questions about what you find different.
A good start would be to compare it with the RAT.  Both are relatively similar and sound different.

Scroll down on this page.  You can compare both in the same window.

LINK:
http://www.gmarts.org/index.php?go=217

1) Replace the .047uf cap to ground on the path where the Gain pot is with .22 or even .47uf...

This is the one in the upper left corner of the dist+ drawing.  See on the Rat there is two of these arrangements in parallel.

2)...Put a 22pf-47pf cap in parallel with the 1M feedback resistor of the op-amp.... 

The opamp is the triangle symbol with + and - in it.  It's the only active element in the circuit, that is the only part that add power to the signal and amplifies the height of the wave.
The 1 M feedback resistor is just over the triangle.  In the Rat they had added the capacitor just over it.

Can the MXR Distortion plus be modded to the BSIAB.

You can, however, get some really nice sounds out of this box with minimal mods that give you a browner, more growly sound.  You can play with the input and output cap values and other such things, it's a pretty easy circuit to work with. 

The input and output cap are the ones that are just near, before and after the opamp.  One is tied on the + of the opamp.  The other is at the end.

Similarly, the .001uf cap to ground in parallel with the diodes can be upped to .0033uf or .0039uf...

Wooops

3) The germanium diodes distort a little more but silicon diodes will deliver double the output level, which will allow you to push your amp harder.  Swap those 1N60s or 1N34s for 1N914s or 1N4148s.  You'll still get clipping, but it won't sound as harsh, and yur amp will play along too.

The diodes are the two arrows with a bar at the end,  just before the final resistor (pot).  These are the ones that clips the signal just like they show on the first big blue figure.
When you change the diode type, the red bar gets closer or far from the center line.  The top of the bottom and top waves get chopped on the red line.

You might try adding the tone control section of the BSIAB to the end of a Dist. +, but then would probably need to boost the output after that.   

Note also the tone control at the end of the Rat.

Spot what you have different on the two figures and you already have a step of comprehension.