how to correctly bias and Orange Squeezer?

[Sir_Ian]

Ok folks....I built myself an orange squeezer and everything "works." However, I just can't seem to get it to bias right. I don't know....partly I'm confused about where and how to bias it.

I've heard some people say..."do it by ear." Others say, "turn the trimpot all the way up, then turn it back to the point where it shuts off, and then back again a lil bit." And I've also read somebody say "there is only a 60 ohm range where it biases correctly" and if thats the case....I think I might be screwed.

So turning it all the way counter clockwise shuts it all off. At about halfway it kicks in. And from there on out it increases the gain. I tried measuring voltages as I had my sister smack some strings just to see if things were working. And they were.

I guess what I'm saying is...I'M CONFUSED!!!!!

I hear some people say its a small range to bias it, and others say its not. I don't hear any noticeable compression at all, but that's probably because I've been playing with that stupid trimpot on and off for 4 hours. In the other time I've been searching here and all over the internet. And nowhere do I find any good instructions on how to bias it. And since so many people here have built this, I'm wandering if there is just some concept I'm missing out on. (I mean....I know its supposed to be a subtle effect, but dang....It kinda just seems like I'm getting a volume boost out of the thing.)

So...How did all of you guys bias your Orange Squeezers and find that "sweet spot."

(side note...I added in a sustain control.....I changed the 100k to a 50k with a 100k pot in series. I don't hear much difference in the sustain, but I'm thinking thats because I only have a 100k pot in there. Later...I'll try a 500k and I'm sure that'll be more noticeable. I just thought I'd mention this in case it had any bearing on the biasing.)

[Mark Hammer]

The FET acts like a resistance to ground, such that it attenuates the incoming signal being fed to the op-amp.  Although you can, technically, dicker with the gain of that op-amp to provide greater and lesser amounts of attenuation/squish, in theory the gain is set optimally.  So, what you should listen for is a discernible constancy in level when you hit the "sweet spot" on the trimpot.  How will you know if you've hit it?  Plug in and strum softly, then harder, then harder still.  If you have a nice twangy bridge pickup, pick near the bridge and dig in a little harder each time.  Each 3-4 stage "test" really only takes 5 seconds or so to do, so you should be able to nail something suitable in a matter of a couple minutes at most, assuming the trimpot is of acceptable quality.

Keep in mind that FETs and guitar signals vary quite a bit, so the ideal trimpot setting is probably something that will not be at the same spot in every case.

[Mick Bailey]

With my OS the adjustment is knife-edge and there is one very narrow sweet-spot where it gives good sounding, tight compression. I built mine with a sustain control originally, but found this to be pretty useless as a panel mounted control and made it a trimpot instead to 'set and forget'. Once you've hit the spot it's a nice sounding compressor.

From what you describe the control behaves like my own build. I've played about with settings quite a bit and it is sometimes difficult to get the trim just right. Seems to be within a degree or so of rotation.

[freak scene]

maybe a multiple turn trimmer.

[R.G.]

As in all things with JFETs, the results you get including how tricky it is to set the bias and the exact bias point all depend on the exact JFET you have in there. Two of the same type number will not necessarily act the same.

There's no question what the biasing does: it sets the voltage reference for the JFET which acts like a variable resistor. Each JFET variable resistor is likely to need a slightly different value here unless they are hand selected. The trimming lets you set it for whichever JFET you put in for the variable resistor. But each JFET variable resistor will also have its own range of control voltage over which it varies from an open circuit to a low resistance. The bias does not change the width of that range.

[George Giblet]

"
April 2008

http://www.diystompboxes.com/smfforum/index.php?topic=67365.msg536659#msg536659


Firstly there is no "correct" value for the Q1 source voltage.  The reason a pot is
used is to *change* that voltage to suit the *particular* JFETs you have in your hands.
Voltages in the 1 to 2V range seem about right for the JFET part you have chosen,
but could be outside this range.  The bottom line is don't set this voltage to
something that someone else has  used - their JFETS aren't yours!

The voltages you measured look pretty good - so I'm having doubts something is wrong.

The best way to test the compression is working is to play through the unit then adjust the 10k pot.
You should find a region where the volume is loud and the trimpot has no effect, then you will find
a point where the pot has an effect, the beyond that point the volume gets lower and lower.
if you can get this behaviour it's highly likely the unit is working.
The "correct" point is around where the onset of volume reduction occurs.

The os is a quite a subtle compressor.
"