help with this Vibrato Circuit

[nordine]

also..

a pair of questions about your schematic:

-are the transistors critical for th eeffect? or can i use any general purpose PNP?? ( i've read somewhere that i can place a NPN instead of a PNP if i switch all polarities... that means i could use the 3904's right?)

-why the 4.7uF capacitor appears as non-polar?? is it that way?

-where are both pots??

-now about the original effect ( and yours)... why is there a 100k resistor at the very beggining? ..i know you said it is there to create a voltage divider.. now, am i suppossed to hear the original audio signal when the circuit is not voltage feeded?

thanks again
cya!!

[nordine]

about the caps...

what are their specific polarities? (if there are any)

..by the way.. i assembled your version of the circuit.. and the only thing i get is a pulsating noise through the audio signal ( this when i switch the 100k res for a 220ohm res) ...damn, i want it to work...

[tschrama]

Hi, don't worrie, I think you might have overlooked something. It's a simple circuit and it will work.... gonna read you posts now and get back later... cya,

Thijs

[tschrama]

i'm feeding it w/ 12Volts... would that be the problem? thats the max voltage i can feed it with, however..... is there a simple mod that could allow me feed it w/9V?? ( and still be using the 3904's)

12V or 9V shouldn't matter.. 2n3904 should work at both values... I use 9V so I can use a single battery.. easy and noiseless....

-are the transistors critical for th eeffect? or can i use any general purpose PNP?? ( i've read somewhere that i can place a NPN instead of a PNP if i switch all polarities... that means i could use the 3904's right?)

The transistors aren't critical... I use BC560b , Hfe was about 400.. you can use PNP or NPN, just reverse polarity... PNP uses a -9V supply, NPN uses a +9V supply... the second transistor can be either NPN or PNP.. doesn/t matter...

-why the 4.7uF capacitor appears as non-polar?? is it that way?

It's a polar electrolitic... -side goes to the other cap.. +side goes to the first...transistor..

-where are both pots??

I didn't use pots.. just used resistors with proper values instead..

-now about the original effect ( and yours)... why is there a 100k resistor at the very beggining? ..i know you said it is there to create a voltage divider.. now, am i suppossed to hear the original audio signal when the circuit is not voltage feeded?

The 100K resistor is there to weaken the signal. It is a essential part, do not remove it. When the second transistor is driven 'ON' by the oscilator (1st transistor) the second transistor has a very low resistance, even in absence of a voltage supply.. So the signal get's attenuated
39K / 100K+39K or 3.6 times.. about 11dB
When the second transistor is not driven but is 'OFF' , the signal get through the 100K, but doesn't get attenuated through the 39K... so it cycles through attenated/not attenuated.. just the effect we are looking for..

When the whole circuit is left without a voltage suypply, you should here the original signal, since the second transistor cann't be driven 'ON'.. it might be a weaker signal because of the 100K that's allways there.. that's why I used a high impedance buffer...

..by the way.. i assembled your version of the circuit.. and the only thing i get is a pulsating noise through the audio signal ( this when i switch the 100k res for a 220ohm res) ...damn, i want it to work...

A pulsating noise is good that means the oscilator is working.. it is pulsating at a proper frequency (about 8Hz)? The 100K is essential.. do not modify it..


goodluck..

[tschrama]

what are their specific polarities? (if there are any)

I just realised... in my version (using PNPs and a -9V supply) the only polar cap is the 4.7uF.. the -side should go to the transistor, the +side to the other cap...

In the original version (using NPNs and a +9V supply) this cap should be flipped around...

[nordine]

it's alive!!!

man, first of all, i want to THANK you for being so patient to explain me every detail of this thing..... for giving me strength, and even for having make an alternate version of it

well, bet you couldn't guess were was located the problem...

it was external

it was the defficient audio probe i set up for testing purposes.... the out impedance was insufficient, so i could not hear anything, even when the circuit was running perfectly

fixed that and NOW i can hear the audio tremulating and stuff

after some tweaks ( like reversing that polar cap, or altering the value of the 39k filter stage), the tremolo can variate from a very subtle oscillation to a very deep alteration of the original source, to a chopper-like effect, which is cool as hell!!

thanks a lot, i really appreciate your kind efforts
i've learnt a lot and now i want to do more effects

thanks for everything, cya!!

[tschrama]

no problem, I'm glad I could help.. You're right about the modulation depth: best to controll it by using a 47K ..100K? potmeter instead of the 39K resistor... Mine works pretty good also.. allthough it picks up a lot of hum... have fun.. come back with some other guitarFX if you want...


Cheers,
Thijs