Switching Positive to Negative ground (Octavia)

Vendt · Post by **Vendt** » 05 Jun 2011, 10:25

Hi there,

I did a search but I could not find a reassuring answer.
I think I figured out the positive/negative ground thing.

Let's say I want to build an octavia:
http://fuzzcentral.ssguitar.com/schematics.php
http://www.tonepad.com/project.asp?id=62

And I want to be able to switch transistors NPN and PNP versions.
To make this switch I have to:
Change the voltage supply (in my case switch the DC jack so + becomes - and visa versa)
Change all components affected (Diodes and Elco's)

Do I have to change the Germanium Diodes (1N34A)? On the fuzzcentral schematics they do not switch. Why is that?

Thank You

ansil · Post by **ansil** » 22 Jun 2011, 02:48

so you want to replace all npns with pnps and pnps with npns? is this what i see that you are asking.

Vendt · Post by **Vendt** » 22 Jun 2011, 20:02

Yep...I know it sounds a bit stupid...but I'm learning here...

It's just that I don't understand why the diodes after the transformer don't change...And As I'm writing this I'm starting to understand I think...The diodes are there to stop current flow back to the transformer....and therefore should not be reversed..
Is that correct?

Sorry for the silly questions. I'm trying to learn

Thanx

mysticwhiskey · Post by **mysticwhiskey** » 23 Jun 2011, 00:46

Vendt wrote:Yep...I know it sounds a bit stupid...but I'm learning here...

It's just that I don't understand why the diodes after the transformer don't change...And As I'm writing this I'm starting to understand I think...The diodes are there to stop current flow back to the transformer....and therefore should not be reversed..
Is that correct?

Sorry for the silly questions. I'm trying to learn

Thanx

The transformer + diodes form a full-wave rectifier, effectively doubling the input frequency (hence the octave effect). For example, using a sine wave as input:

: FullWaveRectifier.png (1.77 KiB) Viewed 1635 times

The diodes being germanium probably means they smooth out the rectification transition point (where the signal goes from negative to positive), but that's just speculation on my part.

Vendt · Post by **Vendt** » 23 Jun 2011, 14:55

Yes I get it thanx!

If I would reverse the diodes I would get no signal at all because the signal (being all positive now) can't pass the diodes.

mysticwhiskey · Post by **mysticwhiskey** » 24 Jun 2011, 05:55

Nope, you'd just get an inverted (negative) output if you reversed the two diodes:

: NegativeFullWaveRectfier.png (1.8 KiB) Viewed 1620 times

Vendt · Post by **Vendt** » 24 Jun 2011, 15:22

lol...I am learning.

So I f there were no diodes there would be both a negative and positive signal that would counterbalance with result no signal...?

For arguments sake...If there was only one (lets say top) connection from the transformer to the output I would end up with only the positive part of the original sinewave. Switching the Diode would give me the negative.
And that reversed for the bottom part.

One output of the transformer "lifts" and the other "lowers" the signal.
It doesn't matter if the ground is 0V or +9V

As the sine wave is now doubled in frequency you get the octave up.

I'm still struggling with what the dotted line is..I mean is that 4.5V (ideal conditions)?

I'm sorry I can't quickly draw a picture explaining what I mean. Hope I'm clear enough.

mysticwhiskey · Post by **mysticwhiskey** » 25 Jun 2011, 00:12

Vendt wrote:lol...I am learning.

So I f there were no diodes there would be both a negative and positive signal that would counterbalance with result no signal...?

No, you'd still get a signal out.

For arguments sake...If there was only one (lets say top) connection from the transformer to the output I would end up with only the positive part of the original sinewave. Switching the Diode would give me the negative.

Yes, assuming you mean the top diode is removed completely so there is no electrical connection between the top of the transformer's winding and the output resistor, then you end up with a half-wave rectifier:

: HalfWaveRectifier.png (1.62 KiB) Viewed 1611 times

And yes, reversing the diode gives you only the negative part of the signal.

I'm still struggling with what the dotted line is..I mean is that 4.5V (ideal conditions)?

It's just the AC ground reference - in the Octavia there's a capacitor immediately before the transformer that removes the DC component from the signal, so in this case the dotted line is ground (0V).

Vendt · Post by **Vendt** » 25 Jun 2011, 08:00

Thank you so very much... for answering such beginners questions in such a very helpfull way.

Vendt · Post by **Vendt** » 22 Sep 2012, 21:43

Lol reading this topic back after a year....

I just didn't get it!
I do now....just letting you all know I learned so much here!