High Pass: Don't understand exactly

[hilbi]

I get the r c filter circuit in a TS

But i don't understand why the "keeley" mods report a more bass response because lowering the R From 4.7 to 2.2K and raising the C from 0.047 to 0.1uF results in keeping the same corner freq, so the bass roll off will be the same, but people report an increase in bass, i also experience this myself.

Can someone explain?

I used the website below as reference.

http://members01.chello.se/pastorn/index.htm

http://web.archive.org/web/20020824175704/www.guitarkit.com/distortion/GeneralHotrodding.html

http://www.musicianshotline.com/archive/monthly/pedal_power/0603.htm

What i want from my modified TS is less gain (cleaner) and more natural freq response, more transparant.

[MartyMart]

Your confusing two mods here, there is also a 51k resistor on the gain stage, which when lowered to about 20k, along with your posted change gives a "less and more" gain mod  
So the TS is "cleaner" on low settings and "dirtier" on higher settings of the gain pot.
Increasing the cap from .047 to .1 lowers the bass roll off on the TS from about 750hz to 350hz, so you will notice a "low end" increase, particularly on low "E" notes, I prefer to use a 0.068uf to split the difference as .1 seems too much to me.

Hope that helps,

Marty.

[hilbi]

yes i lowered the 51 k already and this indeed produced less gain on low settings, but what if i keep the 4k7, upgrade the cap to 0,1uF and lower the 51k to 20k?

Will i then have only less gain?
More bass?

I'nm figuring out the relation ship between these three components

[MartyMart]

yes i lowered the 51 k already and this indeed produced less gain on low settings, but what if i keep the 4k7, upgrade the cap to 0,1uF and lower the 51k to 20k?

Will i then have only less gain?
More bass?

I'nm figuring out the relation ship between these three components

Yup, more bass and just "less" on lower gain settings, see the above post as I just edited it !!
Marty.

[hilbi]

ok thanks, that's what i needed.

still not sure why with 2,2k and 0,1uF i do seem to hear more bass as well

[MartyMart]

ok thanks, that's what i needed.

still not sure why with 2,2k and 0,1uF i do seem to hear more bass as well

Hilbi, forget the 2k2 and the 20k mods, thats ONLY for gain changes, the .047 cap in your "Keeley" essay, is whats producing the "bass roll off" around the op-amp, a bit like increasing/decreasing the input cap on lots of circuits.
That cap helps give the "TS" its classic "mid hump" which many people like, I prefer a bit more bass, hence my 0.68uf suggestion.
Changing those two NP ( non polar ) caps from Keeleys essay is also worth doing, I liked the slight change in "clarity"

Cheers and enjoy,
Marty.

[cd]

Both the R and C on pin 2 (inverting input) set the -3dB rolloff point (bass) of the stage.  Formula is 1/2piRC (R in Mohms, C in uf)

Stock TS-9 = 4k7 and .047u for a rolloff of 720Hz

Modded TS-9 = 2k2 and .1u for a rolloff of 723Hz

i.e. double one, halve the other and the response will be the same.  Now why do you hear "more" bass (other than psychological trickery) - the 51k resistor sets the gain of the stage when the Drive control is on zero (all the way down).  Gain of a non-inverting stage is:

(feedback resistor (51k) + inverting input resistor (4k7))/inverting input resistor (4k7) = 12x gain

If you change the 51k to a 22k and leave everything else the same, you get:

(22k + 4k7)/4k7 = 5.7x gain

If you change the 51k to a 22k and change the 4k7 to a 2k2, you get:

(22k + 2k2)/2k2 = 11x gain - notice any similarity to above?

Provided you setup the stage for exactly the same amount of gain (say, 20x) and then change the rolloff point, you will hear more/less bass.  However if you change the rolloff point, you typically change the amount of gain, which can account for hearing differences.

Now where changing the 4k7 to a 2k2 makes a whole lot of differences is when the DRIVE control is maxed out (to 500k).  

w/4k7, total amount of gain: (500k + 4k7)/4k7 = 107x
w/2k2, total amoutn of gain: (500k + 2k2)/2k2 = 228x - A LOT more.

[hilbi]

Thanks guys, this has been really helpfull.

I think i want to go for the 4k7, 0,1uF 20K resitor to get less gain and less bass roll off. I will try this and see how this sounds.