Biasing help

[Mrcephlapod]

Hi I searched for this and didn't really find what I was looking for I'm very new to electronics but not electricity and its been awhile since I've even messed with dc... I built a bazz fuss and then got confident and I built the Devi ever hyperion I have the correct transistors on the way but I wanted to play with the circuit and I found some good and bad sound but learned that transistors need to be biased what is that and how do I do it or is that better left to a pro for now?

Also if there's a good page you could link me to or a thread I missed that would probably be sufficient thanks...

[Digital Larry]

I think that you will find that "conventional wisdom" about biasing transistors has little place in the world of guitar fuzz.  Almost all technical information about using transistors to amplify audio concerns itself with minimizing distortion.    The other thing about transistor biasing is it tends towards getting a repeatable result even though the characteristics of the transistors themselves might vary over a wide range.  This is pretty much the antithesis of almost all fuzz designs I have seen.  What I've seen of Devi Ever's designs goes even further into the wilderness.  And that's not a bad thing.

That said, transistor fuzz does have its own set of special requirements for bias, or lack thereof, but its best to follow some well known circuits and let your ears be the judge.  I would suggest that you make medium-sized changes to the values of resistors and capacitors of a given circuit, one at a time and see what the sonic effects are.  Generally speaking you can increase the size of resistors without fear.  Decreasing the size of resistors may eventually cause the circuit to blow up because too much current was drawn.

[Mrcephlapod]

Cool that kinda what I was hoping to hear now hopefully it will sound right when I get the right parts in!

[PRR]

> transistors need to be biased what is that

Audio swings both ways. (AC)

Transistors only swing one way. (DC)

We get around that by "biasing" the transistor to about half of the maximum signal. Now it can swing both ways from the bias point.

"Half-signal" is a complicated point; and as Larry says, good fuzz is often "bad bias".

The fuzz circuits you have been plagiarizing are *already* "biased" (or pleasingly misbiased). Make small (2:1) changes in resistors to try other slants.

[bluebunny]

... transistors need to be biased what is that ...

I'm going to have a go at doing one of Paul's famous metaphors...



Little Johnny here needs to "bias" (hold) his rope somewhere above the ground (well-named!) so that he can skip successfully (US: "jump rope").  If he "biases" it too low, it will smack against the ground (and he'll find it hard to skip).  If he's in a room with a low ceiling and he "biases" too high, then it will smack against the ceiling (ditto).  He could even hold it on the ground (no "bias" at all), but then it will do little more than flick up off the floor from time to time.

Where he's holding the rope now is around 4.5V (assuming the little fella is in a 9V-high room).  All is good.

[Digital Larry]

Where he's holding the rope now is around 4.5V (assuming the little fella is in a 9V-high room).  All is good.

Nice one!

[bluebunny]

I'm just waiting for Paul to come back with an even better one involving his driveway and a barbecue.   

[bluebunny]

BTW, the slapping of Little Johnny's rope against the ceiling or floor fits nicely with the "flappy, farty" description often quoted when there's "a biasing problem".   

[R O Tiree]

I'm just waiting for Paul to come back with an even better one involving his driveway and a barbecue.

... or a piece of 2x4 and a bandsaw...

[duck_arse]

.... car jack ....

[LucifersTrip]

I built the Devi ever hyperion I have the correct transistors on the way but I wanted to play with the circuit and I found some good and bad sound but learned that transistors need to be biased what is that and how do I do it

the fun of DIY...sub pots for the 4 resistors, either one at a time or in combos and see if you can get a sound you like better that the original.
start with the 10K's.

[Gus]

[/quote]

the fun of DIY...sub pots for the 4 resistors, either one at a time or in combos and see if you can get a sound you like better that the original.
start with the 10K's.

[/quote]

IMO You need to adjust both the collector to + resistor and the collector to base resistor.  If you change the C to + the output resistance changes, if you change the C to B resistor the gain changes
There is a picture of a sim with a circuit using an emitter resistor in the following link.  The C to B resistor value is changed with different transistors
http://www.diystompboxes.com/smfforum/index.php?topic=103683.0

[Mrcephlapod]

Thanks for the info this has helped me greatly and I will try your suggestions as well I'm still waiting for my transistors so no update but I'm more confident I have it set up right on my proto board now so thanks again