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DIY Stompboxes => Building your own stompbox => Topic started by: eh la bas ma on April 07, 2021, 07:09:16 PM

Title: How to add a Bass knob on the Screamer ?
Post by: eh la bas ma on April 07, 2021, 07:09:16 PM
Hello,

 I found this mod on musikding's forum :

(https://i.postimg.cc/SYc4dnws/image.jpg) (https://postimg.cc/SYc4dnws)

1/ How can I wire a knob instead of a switch here ? Should I do the same wiring, keeping the 47n on the pcb, using one pot's outer terminal and the middle one, leaving one outer terminal free, or jumpered to the middle one ?

2/ From a previous experience, I am guessing I should use an A100k, wired as a variable resistor. Am I correct ?

3/ The highest value I have in my hands for unpolarized cap is 330n. I only have electrolytic cap for 1uf, could the electrolytic one be ok ? If it is, where should the + leg go ?

https://www.musikding.de/docs/musikding/screamer/screamerV2_schalt.pdf

Thank you for your help

Edit : I've found the answer for point 3/ : http://www.muzique.com/lab/fatt.htm The + side goes to the op-amp.

Title: Re: How to add a Bass knob on the Screamer ?
Post by: iainpunk on April 07, 2021, 07:47:35 PM
a simple mod i planned for a tube screamer.
(https://i.postimg.cc/8ffg2ShB/bass-tube-screamer.png) (https://postimg.cc/8ffg2ShB)

im also planning on replacing the capacitor and resistor to ground in the tone stack, with a gyrator band pass to create a strong mid scoop or boost eq.

cheers
Title: Re: How to add a Bass knob on the Screamer ?
Post by: eh la bas ma on April 07, 2021, 08:07:05 PM

im also planning on replacing the capacitor and resistor to ground in the tone stack, with a gyrator band pass to create a strong mid scoop or boost eq.

cheers

Cool, this will interest me as well, when the time comes.

If I read your schematic correctly, the positive 1uf 's leg is connected to the 4.7k Vr pad  (R7 on the Screamer), The negative leg goes to 1k resistor, this 1k goes to the A100k outer terminal, the middle terminal goes to the op-amp's side of the 47n ?
Title: Re: How to add a Bass knob on the Screamer ?
Post by: eh la bas ma on April 07, 2021, 09:12:24 PM
Thank you very much Iain !

I used the middle terminal to connect the 1uf and the 1k resistor, and the outer terminal goes to the 47n.

There is definitly more bass and this A100k pot is interacting with the Tone knob in a very interesting way.

Can I ask you what is the purpose of the extra 1k ?

Will you share your next gyrator band pass mod, someday, please ?

I have been looking for kits that offer this kind of possibilities.

I noticed Pearl OD-5 and the Super Tube Screamer (Ibanez ST-9) :

https://aionfx.com/app/files/docs/fractal_legacy_documentation.pdf
https://aionfx.com/app/files/docs/cirrus_legacy_documentation.pdf

May these schematics help your research.
Title: Re: How to add a Bass knob on the Screamer ?
Post by: iainpunk on April 07, 2021, 10:06:49 PM
hey, the 1k is to keep the gain in check, basically to keep stuff stable.

the mid mod is basically a daughter board with a single transistor 4 resistors and 2 capacitors. it has 3 wires coming off of the board, Vcc Gnd and signal.
the signal wire connects to the hole where the 220r adn 220nf used to be in the tone stack, and the other wires go to the vcc and ground of the original board. this one isn't confirmed to work. i also have some diode changes in mind, but i want an actual unit to test on before i can confirm anything.
this mid mod is basically the same i did with the treble control on my Behringer Super Fuzz (hyper fuzz clone), i call it the ''swedish chainsaw mod'' on that pedal since it can get extremely brutal
https://www.diystompboxes.com/smfforum/index.php?topic=123374.msg1211856#msg1211856

cheers, Iain
Title: Re: How to add a Bass knob on the Screamer ?
Post by: ernest21 on April 07, 2021, 10:31:31 PM
On the off chance that I read your schematic accurately, the positive 1uf 's leg is associated with the 4.7k Vr cushion (R7 on the Screamer), The negative leg goes to 1k resistor, this 1k goes to the A100k external terminal, the center terminal goes to the operation amp's side of the 47n ?

[googoo spam link removed]
Title: Re: How to add a Bass knob on the Screamer ?
Post by: Mixa on April 08, 2021, 05:33:49 AM
You can use the Jan Ray bass knob, the pot blends between the two capacitors
Title: Re: How to add a Bass knob on the Screamer ?
Post by: fryingpan on April 08, 2021, 10:41:23 AM
One way to add bass to a TS is to put a larger cap on the ground path in the feedback of IC1a (increasing the resistor will shift the lowcut frequency, but it will also lower gain). Another is to add a low shelf to the second opamp stage.
Title: Re: How to add a Bass knob on the Screamer ?
Post by: iainpunk on April 08, 2021, 02:33:56 PM
On the off chance that I read your schematic accurately, the positive 1uf 's leg is associated with the 4.7k Vr cushion (R7 on the Screamer), The negative leg goes to 1k resistor, this 1k goes to the A100k external terminal, the center terminal goes to the operation amp's side of the 47n ?

[googoo spam link removed]
welcome to the forum,
my schematic has it saying 1uf NP which stands for non-polarised. as long as there isn't a huge AC swing input, its probably alright, but you might want to replace the Vr with ground if you want to use a polarized capacitor, putting the (-) leg to ground. i think non-polarised and going to Vr is better and more stable, less chance to get problems, oscillations or unintentional fireworks (capacitor goes bang!)
one of the pot's lugs is connected to the 1k and cap, the other two are shorted together and wired to the (-) pin of the op amp.

One way to add bass to a TS is to put a larger cap on the ground path in the feedback of IC1a (increasing the resistor will shift the lowcut frequency, but it will also lower gain). Another is to add a low shelf to the second opamp stage.
the two bass addition options might have the same strict frequency response on paper, but they sound quite different when distortion is added. the one has pre-clipping  bass and the other post-clipping, which makes big difference in characters, pre-clipping ads more fuzz to the character, making it less quackey while post clipping just adds more bass while retaining the quackey character, both are usable, but for different purposes.

cheers
Title: Re: How to add a Bass knob on the Screamer ?
Post by: POTL on April 08, 2021, 05:08:30 PM
you have 2 ways how to do it (actually much more, but these are the most obvious). 1) Classic Bass Control - see Timmy diagram. This method is simple, but this is not quite a classic bass adjustment, the more bass, the more fuzzy the sound, since this is a pre-distortion setting. 2) Active bass, after the amplification stage, see Way Huge green rhino diagram, you can just add a control next to the tone control, you can add it with a separate amplifier. This is a classic bass control and will work as you expect.