Trim pot bias on SparkleBoost

[Outlaws]

Okay, I am barrowing a DMM because I have to bias my trim pot on my sparklebooster I am building.

So what do I do?

[NaBo]

set DMM to 20V range, attach black probe to ground, attach red probe to drain of Jfet, adjust trim until you get a reading of ~4.5V... done!

[Outlaws]

set DMM to 20V range, attach black probe to ground, attach red probe to drain of Jfet, adjust trim until you get a reading of ~4.5V... done!

Thanks!  

[Outlaws]

So do I have to have the whole circuit put together (volume and gain pots) with the battery hooked up for this to work?

I populated the circuit board but I haven't hooked the rest up yet.  So maybe the battery is why it doesn't seem to work.

Otherwise, other than turning it two clicks to the right for 20v, what do I hookup to on this thing.  There is 3 reds and a black, so its a bit confusing for me at the moment.

(I am borrowing this so its not the end of the world if its not the right kind of dmm) :lol:

[NaBo]

nope, dont have to wire anything other than the battery.  As long as 9V+ has a path to ground, the circuit is "complete"... the circuit doesn't really care if it receives input or has controls, it's happy just sitting there being complete and wasting batteries ...  that's why bypass switching doesn't turn off power consumption, and most people use their input jack to switch power on and off.

for that particular dmm, two clicks right, plug red lead into the rightmost socket (the symbols above it tell you which tests to use this one for... voltage, resistance, diode continuity, etc), black to COM.  hook up the probes and bias away!

[Outlaws]

nope, dont have to wire anything other than the battery.  As long as 9V+ has a path to ground, the circuit is "complete"... the circuit doesn't really care if it receives input or has controls, it's happy just sitting there being complete and wasting batteries ...  that's why bypass switching doesn't turn off power consumption, and most people use their input jack to switch power on and off.

for that particular dmm, two clicks right, plug red lead into the rightmost socket (the symbols above it tell you which tests to use this one for... voltage, resistance, diode continuity, etc), black to COM.  hook up the probes and bias away!

Thanks I got it biased....I think.  I got it to 4.51v.  Is that close enough.  I just did it with my fat fingers.  I will get it closer if I need to.

I take it ~ is refering to give or take.

[NaBo]

yep... +/- 5% is generally within acceptable limits, so 4.51V should work perfectly.

[monkmiles]

So do I have to have the whole circuit put together (volume and gain pots) with the battery hooked up for this to work?

I populated the circuit board but I haven't hooked the rest up yet.  So maybe the battery is why it doesn't seem to work.

I have this same question. I'm about to start a Sparkle Boost and am a newbie. So do I need to hookup the battery in order to bias? I was of the understanding I didn't have to hook up any power and that the DMM would provide it?

[BoxOfSnoo]

So do I have to have the whole circuit put together (volume and gain pots) with the battery hooked up for this to work?

I populated the circuit board but I haven't hooked the rest up yet.  So maybe the battery is why it doesn't seem to work.

I have this same question. I'm about to start a Sparkle Boost and am a newbie. So do I need to hookup the battery in order to bias? I was of the understanding I didn't have to hook up any power and that the DMM would provide it?

The DMM will only power itself, a tiny amount for measurement purposes.  You need to use a battery for this.  The 9v input will be adjusted to somewhere in the neighborhood of 4.5v at the drain of the JFET by the circuit, you tweak the trimpot to get as close as you can.

[PRR]

> understanding I didn't have to hook up any power

No.

The point is to trim the battery and the trim-pot against your specific FET so that the FET sits at "about half battery voltage".

Without the battery nothing will happen.

If battery is 8V you aim for about 4V; if battery is 12V you aim around 6V.

No precision is needed. I'd take anything from (on 9V batt) 4V to 6V as a happy starting place.

[monkmiles]

Thanks guys, that makes sense.

So do you guys usually just quickly wire up the battery temporarily for this? Or do you do the biasing after the battery and/or power jack is wired to the pcb insdie the enclosure? And in that case why not just wire up the jacks and switch too?

[ayayay!]

I prefer to build it into the case as much as you humanly can first, and use the power supply to bias.  The PS is going to be consistently 9volts (actually more like 9.6v)

And as far as this little circuit, you can use your ears to bias it really.  4.5v w/ a JFET in this setup isn't necessarily going to give you your best result.  Start w/ 4.5v, then adjust to taste from.  Your final bias may be nowhere near 4.5v, but your ears will thank you. 

[MikeH]

Honestly, for the future, if you're just biasing the one JFET and it's a simple, single gain-stage circuit (like the sparkle boost is) you can just bias by ear.  Just adjust the trimmer until it sounds best to you, which will probably be the loudest setting.

In a circuit with multiple JFET stages this is not a good way to go as it will result in too much gain causing too much distortion and make the circuit sound like... hmm, what's the technical term?  Oh yeah... crap.

Oh, but it's a really great/important learning experience to learn to use a DMM- so in this case you should do that first.