adding PNP modulation transistor to PT2399 causing lots of HISS

[loss1234]

I am trying to add a PNP setup (sort of like in the ECHO BASE) to control delay time modulation

however it is adding hiss.

I am using a pretty basic datasheet style pt2399 circuit which, when simply controlled by a pot only sounds noisy at LONG delay times.
But once i hook up the PNP it gets really noisy, really quick.

I was wondering if the transistor TYPE matters much? (i was simply using a 2n3906)

are there any other projects BESIDES the echo base that use this simple PNP style modulation control?

anyone else had this same problem or find a fix?>

thanks so much

[frequencycentral]

It doesn't answer your question, but I've found an LED/LDR combo in parallel with a 100K delay time pot works beautifully with PT2399, as in the Clari(not): http://www.diystompboxes.com/smfforum/index.php?topic=74294.0

[slacker]

I don't think the transistor should add any hiss, it's just working the same as a pot or any other resistance you attach to pin 6 of PT2399. Have you put a resistor in parallel with the transistor like on the Echo Base schematic? Without that the resistance will get much too high for the PT2399 which will cause all sort of weirdness.
The type of transistor shouldn't matter, people have used different ones the Echo Base and I as far as I know they haven't had any problems.

[loss1234]

yes i added the resistor

the only thing i can figure is that maybe its because my wires are too long?

i soldered up the delay on stripboard

it has an input wire and an output wire going onto a breadboard with jacks

the power comes off the breadboard and feeds wires to the stripboard

the LFO is on the breadboard, as is the transistor and resistor, pot,etc and these all feed a wire BACK to the delay pin


is it possible this is adding problems that would go away if i soldered it up?


thanks so much!

(too bad i cant just find a single supply npn expo pair!)

[anchovie]

Is the hiss constant or only on the repeats?

I had a Rebote delay on veroboard hooked up to a buffer on the breadboard last night and there was a constant hiss when I first powered it up, but the hiss went away after I poked a few of the breadboard components. There must have been something not sitting tightly.

A little bit of noise on the repeats at longer delay times is normal with the PT2399, but I've always found that if there's any silent time in the sample being repeated then the hiss effectively gets gated.

[loss1234]

no the dirtyness seems to come as soon as the delay gets higher than slapback

it is only there when i try to add the modulation

if i just hook up a regular pot and the 1k resistor to grnd, i get a pretty clean delay (with some noise on LONG delays)

i am stumped

but this is what you get when you start trying to add new  features to a circuit


i might try swapping out all the resistors and transistor to see if i had a wrong or bad part

thanks!

[George Giblet]

Try adding a 47nF to 100nF cap from transistor base to ground.

Does it add noise when the modulation is on minimum?