Krank Distortus Maximus similar to Smash Drive + additional 2N5088 gain stage

[Carlos]

Hi everybody,

judging from the gut shots

the Krank Distortus Maximus could be similiar Aron's Smash Drive + tone stack, because you can see the LM386-3 in there
But what is the function of the 2N5088 transistor at the bottom left corner of the gut shot?
Is it an additional gain stage with a 2N5088?

Regards

Carlos

[raulgrell]

Could be a pre-distorion gain, post-distortion gain, pre-dist buffer, post-dist buffer... Could be alot of things... We'd need more info to give a definate answer...

[Carlos]

Look at the pedal from the top


If you look at the gut shot

first row of pots from left to right:
Treble 250KA
Mid 25KA
Bass 1MA

You can even see the C1 470pF ceramic, and .022uf caps for bass and mids (C4 & C6).
Bottom row: The 1MA pot is the master.
But what bugs me is the 50KA as gain pot. I haven't seen anything like it in the Smash Drive schematics or LM386 application notes.

I guess the additional 2N5088 is an additional gain stage  and the gain pot serves as a simple voltage divider at the input of the LM386-3.
I need to see pin1 and 8 of the LM386 more closely ...

Regards

Carlos

[Carlos]

If you look at the 3PDT closely, a PCB trace obviously goes to R10 (guess 470 k - 1 M to ground) via C3 to the base of the 2N5088. I will have a look now at Joe Davisson's 5088 designs and compare the two ...

Regards

Carlos

[ambulancevoice]

i can also see what appears to be a input pull down resistor

47M??? ha!

[doug deeper]

the 386d would be more crankeded. (k)

[Fender56]

Verify if the Drain of the 2N5088 is directly connected to the positive supply. If so, chances are good that it's a buffer/follower that can be used as an input buffer, to drive the tone stack or as an output buffer.

It looks like an easy board for reverse-engineering...

[Carlos]

Thanks! I have contacted the board member who took the photos to do some measurements for me.
The 2N5088, however, is NPN transistor, not a FET.
I bet it's a gain stage in front of the LM386, because the chip does not need output buffer, it can drive low output impedances, even speakers.

Carlos

[Fender56]

Oops! You're right! It's not a FET!!! I was thinking about the 2N5458...

Yeah, it got plenty of a output power to drive either the tonestack and the output. So it's maybe a gain-stage feeding the LM386 like you said, or an input buffer like most of the Boss pedals.

Is it possible that the gain of the LM386 is somewhat fixed and the Drive level is set via the gain-stage of the 2N5088? That can explain the "high" value of the Drive pot. Just wondering!!!

Keep us informed!

[Carlos]

Yes, I think the LM386 could have a fixed gain, if there is no resistor between pins 1 and 8, it's 26db or 20x. But then it wouldn't distort that heavily, would it?
We need a better picture of the PCB.

[ambulancevoice]

yeah, thats most likely whats going on
lm386 has fixed gain and the gain pot is before the lm386
pins 1 and 8 are probably connected together though
maybe

[Carlos]

You cannot really tell from this picture:

[anchovie]

With R3 being 430K and R6 being 43K, I've always suspected that this is a Sekova Power Booster with a 50K level pot running into a fixed-gain 386.

[Carlos]

Yes, I got your point, but isn't 43k too low as input impedance?
Then there should also be a huge difference, if you play with or without another pedal (=buffer) in front of the Distortus Maximus.
I can't really one side of R6 going to ground.

Regards

Carlos

[anchovie]

Input impedance will be set by R10, which as ambulancevoice pointed out does indeed look like 47M!

It's an awful PCB - it looks like it was drawn in MS Paint by someone with a broken mouse.

[mnordbye]

Whatever it is, it seems it would be cool to reverse engineer it. 

Magnus N

[Carlos]

If the gain pot is a voltage divider in front of the LM386, we should be able to mute the Distortus Maximus when the gain pot is turned down to zero, shouldn't we?
Can anybody verify that?

Regards
Carlos

[Fender56]

Not if there is an offset resistor between the Drive pot lug and ground. This resistor set the Min threshold, avoiding to mute the pedal.

However, if the pot lug is directly connected to ground, the pedal will be muted if the pot is turned down.

[Carlos]

Hi anchovie,

R6 is connected to pin 2 of the LM386 which should be grounded according to the application notes. It's grounded in the Smash Drive, too.
Regards

Carlos

[Carlos]

Hi anchovie,

I also believe now that the input gain stage is a Sekova Power Booster.
If you look at the 2N5088, the flat side of the tranny is orientated towards the 3PDT, so it's the emitter facing the gain pot. The 2N5088 an NPN transistor, therefore the emitter goes to ground. Here it's connected to a resistor. I can only see the first color rings: orange and white that is 3 and 9.
The emitter resistor in the  Sekova Power Booster is 390R

Carlos