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Mr. Black - BB-74x clipping question.  [traced]

Posted: 01 Apr 2019, 18:51
by The Rotagilla
I came across the schematic for the Mr. Black BB-74x and have two questions.

1) A couple of people stated it's a TS variant but I'm not really seeing that. Am I missing something?

2) What do the 100r resistors in series with the clipping diodes do, both from an electronically and sonically.


Re: Mr. Black BB-74x clipping question.

Posted: 01 Apr 2019, 20:45
by karul

Re: Mr. Black BB-74x clipping question.

Posted: 01 Apr 2019, 21:03
by ppluis0
Hi Rotagilla,

Looks as YATS to me... :roll:

But, this design employs also something borrowed from the Proco RAT, and this is the couple of R-C networks at the inverting input of the first op amp.

These four components act as a combined low pass filter: the pair comprising C3 and R6 has a cut-off freq of 34 Hz and the other pair (C4 and R5) cuts at 1600 Hz

Also, the combination of C2 and the "Volume" pot (an elegant way to not to say "Drive"...) has a cut-off of 3100 Hz when the gain is at max setting.

Interesting is the use of same value and curve of pots (50KB) in the entire design...

Regarding the couple of 100 ohms resistors in series with the clipping diodes, I'm not 100% sure, but perhaps this round in some manner the clipping action of this diodes :hmmm:

Perhaps Mr. Jack Deville can bring us more information about this design. I hope.


Re: Mr. Black BB-74x clipping question.

Posted: 03 Apr 2019, 10:29
by Manfred
Here the curve trace of a 1N4148 Diode without and with a 100 Ohms resistor in seriell to a 100 Ohms resistor:
1n4148 in seriell 100 Ohms resistor.jpg
I ran a simulation of the Mr.Black circuit with an input of 50 Millivolts @ 1000 Hz.
and found no difference of the shape with an without 100 Ohms resistor.
Maybe there will be a difference with an higher input level which causes a higher current into the clipping circuit.
I am busy in the moment, I will run a further simulation concerning this matter.
1n4148  100 Ohms resistor compare.jpg

Re: Mr. Black BB-74x clipping question.

Posted: 03 Apr 2019, 23:48
by Manfred
I ran simulation of the MrBlack circuit using a higher input signal level
but there is no difference too.
I guess anything went wrong, so I ran the simualtion of the clipping circuit only.
I fed in 5 Milliamps@1000Hz using a signal current source and read the voltage over the clipping components.
There is a diffrence now the signal with R3,R4 = 100 Ohms becames more rounded.

Re: Mr. Black BB-74x clipping question.

Posted: 07 Apr 2019, 06:55
by ppluis0
Nice work, Manfred !!

Look as the addition of these small resistors softens the clipping action of normal silicon diodes.

Will be interesting if any member of the forum has the chance to apply a couple of resistors in an existing overdrive (as Tube Screamer or similar layout) and tell us if this produce noticeable sound changes.


Re: Mr. Black BB-74x clipping question.

Posted: 07 Apr 2019, 07:45
by Manfred
Thanks Jose,

but I should add this:
The simulation of the clipping circuit does not correspond to a real situation,
its purpose is to clarify how what effect the altered curve has.

I very busy in the moment when I get more time to myself again, I will build the real circuit for taking some measurements.
Then I will see whether and to what the simulation matches the real circuit.


Re: Mr. Black BB-74x clipping question.

Posted: 08 Apr 2019, 21:35
by Jack Deville

Hey doods!

Suppose it was only a matter of time with this one? People really seemed to like this little OD. Admittedly, it does sound great. :D

The above published is not perfect, but its close enough for punk-rock, worth noting: there is a long story behind the development and release of this dirt box which I am legally bound from disclosing, and whether yall choose to believe me or not: it is not a clone descendant of anything I have seen, but I can add one tidbit that yall may enjoy:

Component and value selection are critical and deliberate.

As always: have fun and please give cred where cred is due.

Re: Mr. Black BB-74x clipping question.

Posted: 09 Apr 2019, 08:11
by Dirk_Hendrik
I really fail to see why this circuit would be referred to as a Yats, other than that it has antiparallel diodes in the opamps feedback loop. The whole circuit bears far more resemblance to a RAT than a TS.