Hello! 51pf CAP change in TS clone = more compression and bass?

[derek_32999]

I posted this on the BYOC forum also, but I heard somewhere that one thing you can do to decrease noise in the circuit is to increase the 51pf cap to 200 pf. I accidentally read the values wrong and put in a 1200pf cap.... Whoops!!!
So, I got a lot of added compression and bass/mids. Actually kinda cool. Sounded a little like my MI AUDIO blues pro did.

So, I put a switch to change on-off-on switch to change from 100pf to opf to 560pf. Nice range of compression. I have to play with it some more to see if something else works better, since this was all done at low volume testing. I am sure this has been done before. Just wanted to contribute a tad for all the stuff you guys teach me.

[ubersam]

The small 51pf capacitor across the diodes acts to soften the "corners" of the clipped waveform somewhat. This softens the distortion somewhat. The action of the 51pf is most noticeable when the drive control is maxxed out, so it softens the distortion most when the gain (and distortion) is highest.

The 51pf does not really "compress" per se. As per R.G.'s article on the TS, it just rounds out the clipped waveform, as in: it is not so much square anymore. I cannot remember offhand where I read this but, having a small cap in the neg. feedback loop also cuts the harsh highs out. So, the bigger the cap, the rounder the corners of the clipped waveform and the smoother the highs.

Having different values of this cap switchable is a cool mod. I did a three value option to a build I made for a friend. It had 120pF, 220pF, and 330pF.

[Mark Hammer]

When a cap is in the feedback loop of an op-amp, it rolls off high end in conjunction with the feedback resistance, following the formula F = 1 / [2*pi*R*C]  As C and/or R get larger, F gets smaller.    When the gain is adjusted via the feedback resistance, more treble is rolled off as gain goes up.  When the ground leg resistance is used to adjust gain, there is no change in the treble rolloff with gain increase.

In the TS-9, when gain is at maximum (pot = 500k, plus 51k series resistor) the treble rolloff is at 5.66khz (assuming all component values are precisely their stated values).  At min gain, the rolloff is above 60khz.  Increase that cap to, say, 100pf and the rolloff at max gain is at 2.9khz.

It does NOT increase mids and bass at all.  Rather, it makes them more noticeable due to the absence of treble.

[derek_32999]

It does NOT increase mids and bass at all.  Rather, it makes them more noticeable due to the absence of treble.

Thanks mark.  This makes sense as I got to try it out at louder volumes today.  Without a cap is a little too noisy for me, but the 560pf cap sounded NICE kinda like my MI AUDIO blues pro.  It didnt make my blackface fender fart out, so PERCIEVED increase in low end makes a lot more sense.  I do like the sound enough to keep the mod in there after trying it at high volume today.  Thanks again

Derek

[brett]

Hi
Here's a thread about a similar op-amp feedback pedal with 120pF in the feedback loop with a 470k resistor, and some discussion about the cap size.  This and a Rat are my most-used pedals.
http://www.diystompboxes.com/smfforum/index.php?topic=50984.0
cheers

[derek_32999]

Thanks brett.  Thats pretty nice!  I am gonna start looking for mods for my rat soon, so the hunt begins.    This DIY stuff is the most addicting hobby I have ever had.  I am already planning on reboxing my 808 clone in a bigger enclosure (currentyl 125B and has 3 on-off-on switches among the other normal parts...   That was a tough fit!!)  I am thinking of adding a bandaxall tone stack with a mids control as previously suggested by JHS.  Then after everything is switchable I will figure out what I like best and downsize it.   Man on Man have I got some reading to do.  Thanks for the great forum!

I will post pics when my christmas present gets here (hopefully a new canon digital cam) 

Derek