Author Topic: another neg ground PNP fuzz experiment  (Read 246 times)

m7b52000

another neg ground PNP fuzz experiment
« on: April 09, 2021, 04:30:33 PM »
I've just built 1 of these on vero and it fuzzed mightily yet had an unbearable amount of hiss. I added a power filtering cap as prev described to no avail. At this stage I was using my Strymon wall wart, previously thought to be good. I was about to chuck it away but decided to try a 9v battery instead. It is now completely silent apart from the fuzzy goodness. I'm not quite sure what the lesson is here but I thought it was interesting...

Fancy Lime

Re: another neg ground PNP fuzz experiment
« Reply #1 on: April 09, 2021, 05:33:54 PM »
Some supplies don't like some circuits. I've had stuff on the breadboard and boxed up and even commercial pedals that are noisy with one ps but silent with others. And it's not always the same ps that makes trouble. In facts I have one that works perfectly with my age old unregulated dollar store ps but produces all kinds of hiss with better supplies, be they switching or not, regulated or not. Funnily enough, I have never had the infamous noise problems with any supply with PNP negative ground circuits, unlike most people it seems.

What you feed the whole thing into also makes a difference. I have an amp that does not have the signal ground connected to earth. If you plug anything into that, that is is powered by a switching supply, all hell breaks loose. My other amp is dead silent under the same conditions. If I use the noisy amp but connect it's DI out to the input of the other amp, I also get silent operation. If I use no amp but go directly into the USB interface hooked up to my laptop, I get noise only if the laptop is connected to it's power supply. Running off of battery works fine. So interaction between power supplies in a system without proper ground to earth referencing can also cause problems. Yippie!

Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

percyhornickel

Re: another neg ground PNP fuzz experiment
« Reply #2 on: April 09, 2021, 06:20:18 PM »
For me hiss noise has disapier almost complete after placing and soldering components in a pcb (rangemaster and fuzz face). Check you points and make sure your circuit is shield after putting all the stuff in a box.

Even check your potentiometers, try with new ones...    ...many times my pots are the blame of weird noises.


Saludos.

P.H.

R.G.

Re: another neg ground PNP fuzz experiment
« Reply #3 on: April 10, 2021, 10:27:28 AM »
Hiss can be the audible result of radio frequency oscillation inside a circuit that is low level enough to still let audio through. The hiss is actually the RF being "detected" into audio by the circuit, and the erratic nature of the RF noise being made into audio hiss. It can also be RF that's not persistent, but banged into existence as RF-ringing by noise on a power supply or ground, or by the RF impedance of the power supply itself.

1960's pedal designs often got away with crude layout and poor (by today's standards) circuit practices because the semiconductors of the day could barely muster RF responses. It was common for there to be special transistors available for radio use. Today, most semiconductors that are easily available have useful gain up into several hundred MHz to a GHz. At those frequencies, the self resonance of circuit traces can be a fine resonant tank circuit. Even if you have an oscilloscope, most non-specialized o-scopes only go up to 20MHz to 50MHz response and will not show any signals up in the UHF radio range or higher. So sometimes it's there and you can't see it.

With that as background mysterious hiss that starts when some logicaly-unrelated detail always makes me think "parasitic RF!" Switching power supplies generate switching noise; what distinguishes between them is how well they suppress this and eliminate the really high frequency pulses. But if your circuit has a built in tendency to sing, even tiny pulses can make for RF ringing.

Negative ground PNP circuits have always been tricky that way in my experience. I have usually been able to eventually cure them by a combination of good RF layout, decoupling, and burying mandrake roots at the crossroads at midnight. But some were impossible for me. As a result, my opinion is that it's far simpler to do a positive ground power supply for PNP circuits or convert the circuits to NPN that to deal with the often-unseeable RF issues. This is an engineering economy issue, of course. The objective may be to get the @#$@%# thing to work instead of understanding yet another layer of quantum physics.  :) 

But doing experiments for the sake of learning is also a good idea.  :icon_lol:
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

m7b52000

Re: another neg ground PNP fuzz experiment
« Reply #4 on: April 10, 2021, 10:52:28 PM »
Thanks!

fryingpan

Re: another neg ground PNP fuzz experiment
« Reply #5 on: April 11, 2021, 03:11:31 AM »
Try it with a conventional power supply (a non-switching one). The pedal enclosure *should* be enough to suppress RF, but what do I know.