Help with SD-1/TS-9 tone control mods...

[MetalGod]

I'm messing around with the values of the resistor/cap combo (470ohm/.027uF) on the SD-1 and was wondering how to work out the effect each component has on the rolloff.

Anyone?

[wampcat1]

the resistor and cap and the tone control circuit is actually BOOSTING high frequencies - it isn't rolling them off like a LP filter used for most tone controls. This same type is also used by the boss cs-3 among many others.

bw

[MetalGod]

Thanks for the reply Brian,

So changing the resistor has what effect?

What about the cap?

I know I could solder parts in there and listen but sometimes it's nice to know what's going on (i.e. the theory)

[Celadine]

Someone had to do it:

http://www.geofex.com/Article_Folders/TStech/tsxfram.htm

read it, learn it, live it.

[MetalGod]

Someone had to do it:

http://www.geofex.com/Article_Folders/TStech/tsxfram.htm

read it, learn it, live it.

Thanks, but I've read that article and while it's pretty much the TS-9 bible it doesn't give me the info I want. 

[Mark Hammer]

I'm messing around with the values of the resistor/cap combo (470ohm/.027uF) on the SD-1 and was wondering how to work out the effect each component has on the rolloff.
Anyone?

The op-amp used for the tone control is set up as a noninverting op-amp.  That means that the gain of the stage is determined by the 10k resistor in its feedback loop, and whatever resistance there is to ground from its inverting pin (pin 2 if you are using an actual Boss SD-1).  That resistance to ground is made up of the 470R resistor, and whatever portion of the tone pot there is between the wiper and pin 2.  The upper and lower rolloff points are set by the .01 feedback cap (yielding treble cut starting around 1.6khz) and the .027uf cap to ground (low cut will vary).

Note that when the tone control is set to the bassiest side, the .027uf cap and 470R resistor are essentially placed in parallel with the .018uf cap that forms a lowpass filter after the clipping stage.  We'll ignore the 470R resistor for the moment and simply note that a lowpass filter incorporating 10k and (.018+.027uf) provides a treble rolloff starting around 353hz.  With the 470R resistor that will be a bit gentler.

When the tone pot is rotated to the treble side, the .027uf cap is connected directly to pin 2.  With the 10k feedback cap, that produces a gain of [10k+470R]/470R = 22.3 with a low-end rolloff at 1.5khz.  When the tone control is rotated to the bass side, the resistance to ground is 470R+20k, which gives a gain of [10k+20.47k]/[20.47k] = 1.5, with a low-end rolloff around 288hz.

So, you can see that a few things happen simultaneously as you turn the tone knob.  At one extreme the tone-section gain is reduced and the action of the RC lowpass filter just after the clipping stage is exaggerated.  At the other extreme, the lowpass filter is restored to its default state (more or less), and the gain of the tone section is increased, but with a noticeable reduction in lows and mids.

So, you can see that changing the value of any of the passive components in that equation will have an influence.  Make the .027uf cap smaller and you get less treble cut in the bass position (when it is in parallel with the .018uf cap) with more bass-cut in the treble position.  Make the 470R resistor smaller, and the amount of boost goes up in the treble position, except with even more bass-cut than before.  When the 470R resistor is increased to 680R, the amount of boost in the treble position is reduced, but a little bit more bass is allowed to pass.

[frokost]

Mark: Thank you! I was able to turn the tone control of my TS-808 to the right after reading this.... Now the knob is actually useful.