New to the forum and already asking some questions

[Johnny G]

Hi
as most people are gonna figure out im new here and im interested in building guitar FX. i have a fair bit of experience with electronics through school and my own research on the net and id like to think i knowwhat im talking about to a certain extent

sp far ive only built one working effect, the green ringer off general guitar gadgets and its ended up working quite nicelly. im having alot less luck with a big muff based distortion box but i think i know why.

anyway, enough with the introduction, i actually have a couple of questions that i want to ask.

the first is about simple single bi polar transistor booster circuits that use capacitors in between the collector and base  of the transistor. i realisethat this is because the signal on the collector is 180 degrees out of phase with the input and so the capacitor is there for negative feed back of the high frequencies. what i dont know is how you would go about working out the cut off point for the filter. any help withthis would be much apreciated.

second quesion is about octave effects such as the green ringer. from what i can see the general way of creating an octave effect is to get the original signal and an inverted version of it and then feed each of them through a diode to efectivlly rectify the original signal. what id like to know os whether there are any more ways of creating a pronounced octave up effect and isthere any way of doing it fairlly cleanlly, eg without distortion


thatsabout all i really want to ask for the moment, cheers for any help you guys feel like giving

[petemoore]

Hey, Johnny G. !!!
 Congrats with the Green Ringer!!!
 I just adjust by ear and caps from Base to Collector to fine tune them.
 I suppose a formula could be worked out for instances of this...
 A Small pot in series would allow some tuning...I just use different values, till the result is close to what I want, and call it a day, perhaps not the best' way, certainly not the most scientific', but it works for me, I think that's the method most tweekerz use.
 I'm refraining from mucking about in octave speak, perhaps someone else types about it who is more familiar.

[Chris R]

RG has a CMOS octave called mos doubler on http://www.geofex.com  i havn't built it myself yet.. but it's on my short list ;p

Also you can look at TIM E's page for a few octave circuit snippets. (Octup!, Octup Blender)

http://www.geocities.com/tpe123/folkurban/fuzz/snippets.html

Chris

[MarkB]

The only way to get a clean pitch-shift of any kind is using DSP..  
There are only a few places in the guitar chain where digital is generally better - and thats #1 (reverb and delay are #2 and #3)
"-)

[bioroids]

I think the cut off frequency can be calculated with the capacitive reactance formula, using the input impedance at the base as the resistance and (obviously) the value of the cap. How you calculate the input impedance on bipolar stages is another matter...

Luck

Miguel

[Ansil]

johhny g are you from the projectguitarforum?

[Johnny G]

thanks for all the help guys. the idea of putting a resistor in series or using the input impedance to work out the cut off point both make sense to me so cheers for that

ill have a look at some more octave effects to see how its done in a few more of them. im thinking of trying to make something that gives a more subtle octave overtone to use with clean guitar instead of sillah overdriven wailing (not that im notall for that of course )

and yes ansil man its me lol. good to see some familiar faces here. i figured i should probablly stop clogging up the PG forum with all my effects questions lol

[Transmogrifox]

The cap from collector to base has an effect called "Miller Multiplication", or the "Miller Effect".  The purpose of this is usually for the purpose of rolling off radio frequency signals, so the poles are far above the audio range.  However, this can be used to roll off the highs if you like.

Basically, the equivalent model is to break the connection between the collector an base.  Model it as one capacitor to ground from the base of equivalent capcitance as the value shown.  Then add a capacitor from the collector to ground, and multiply its value by a factor dependent on the gain.  I can't remember it exactly off the top of my head, but if I remember correctly, the factor is approximately the voltage gain from base to emitter for large gains (like a gain of 50 or 100 or more).  It's a little less for smaller gains.

I hope that helps

[Johnny G]

wow, thanks man. i'll go have a quick flit round google for some stuff on it