I'm using a 10k pot to bias the JFET's in my Orange Squeezer clone. I made a few in the past for friends and I really like the last one I built for my friend. So I decided to build one for myself.
The one I built for my friend sounds better than mine. The funny thing is I'm using the exact same parts to the "T" and it's setup the same way. I did some reading up on these things and what I thought was a threshold pot (the 10k) is really a biasing pot.
So the question I have for you, is there an optimal bias setting in volts etc. or maybe a trick via ears that you can hear it pull into optimum operation?
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Personally I'd just set it in the middle and turn left and right a bit to try and find "the spot".
Quote from: Merlin81Personally I'd just set it in the middle and turn left and right a bit to try and find "the spot".
I see what you’re saying. I set it to the same "spot" as on my friends and it's not doing it for me. I'm thinking that there must be a stock bias voltage for the "spot". I know it sounds simple to do by ear, but those little trimpots jump up or down at the slightest touch of a screwdriver. I guess I could take his apart and measure it. It just seems like there must be a stock bias setting to get the best tone out of it.
R.G. speaks of this in some notes on the GGG site but does not give a reading. I wonder if there is some old Dan Armstrong build notes floating around in someone’s archives? Hint, Hint...
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I dunno, but i see 4.5volts getting thrown around alot when it comes to biasing, so you might try that?
Merlin - The 4.5 volt thing is a different circumstance, and applies more to biasing a single transistor than *balancing* two JFETS. Vanessa - In what way do your previous builds sound *better* than your current one? Are you actually A/B ing the the units, or relying on memory? The *trick via ears* is to sweep the trimpot and find the spot where gain reduction starts to kick in. JFETS are pretty notorious for inconsistency from part to part. You might just try sorting thru a stack of them and seeing if you get better results from another pair.
Kerry M
Quote from: bwanasonicMerlin - The 4.5 volt thing is a different circumstance, and applies more to biasing a single transistor than *balancing* two JFETS. Vanessa - In what way do your previous builds sound *better* than your current one? Are you actually A/B ing the the units, or relying on memory? The *trick via ears* is to sweep the trimpot and find the spot where gain reduction starts to kick in. JFETS are pretty notorious for inconsistency from part to part. You might just try sorting thru a stack of them and seeing if you get better results from another pair.
Kerry M
Yes I'm A/B ing the units. They are close. I guess I should just count my chips and give up. But I'm sure that in the original notes on the OS there was a spec that showed the bias of those units. It would save the time on counting through a stack of JFET's to find a magic pair.
I wonder if those guys over at Run off Groove know some info on this?
They do some great stuff on JFET's, I should ask them too.
I'm no expert, but i think it's normal for them to sound different with the trimpots set to the same spot. Because of the variation in parts (especially FETs), each unit's "sweet spot" will be at a different trimpot setting. You should be able to make them sound the same by setting the trimpots to different spots.
But i too had wondered if there is some optimal voltage at some point, that the sweet spot of the trimpot corresponds to.
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emre
Check the voltage in your friend's 'Squeezer.
Quote from: Merlin81Check the voltage in your friend's 'Squeezer.
I thought about that. I would have to tear it apart (bummer). It just seems that it could be biased via a DMM. I guess I'll have to tear his apart. Then see if mine can be dialed in via DMM off his settings.
You might just want to try another pair of JFETs and see if it makes a difference. There is a lot of info on JFET matching in the archives and at RG's site ( www.geofex.com ) . Could you describe in subjective terms the difference you are hearing? Is it a difference in the amount of compression, or is it a difference in tone? The JFETs would most likely only affect the former.
Kerry M
Quote from: bwanasonicYou might just want to try another pair of JFETs and see if it makes a difference. There is a lot of info on JFET matching in the archives and at RG's site ( www.geofex.com ) . Could you describe in subjective terms the difference you are hearing? Is it a difference in the amount of compression, or is it a difference in tone? The JFETs would most likely only affect the former.
Kerry M
It's the compression, it's hard to describe. The one I built for my friend seems a bit *smoother*. Mine has a little more of that modulation that R.G. describes. And from what I have found on the topic this is a bias issue with the fets. The ones I'm using are Fairchild 2N5457's. I guess matching them first might benefit this circuit? Mark Hammer has stated that it should not matter. I would think it would not either having a bias pot.
I'd be curious if you find much difference with another pair of JFETs. I also suppose it's possible that just part tolerance variation with other sections of the OS could do it. I'd have to reread all the saved stuff I have from this forum on the OS to remember the anatomy of what does what though. If you get a chance, try a *fancy* opamp ( Burr-Brown OPA2604) in your OS. I like the slightly *hi-fi* transparency this adds. Or I at least I convinced myself thats what I heard between opamp swaps :lol:
Kerry M
Here's an "ears" trick.
Put the OS after some pedal that can give you a clean boost. Like a 7-band EQ set flat, or a booster, the cleaner the better. Use the boost to hit the OS input hard. This will make the compression easier to hear. Adjust the trim pot.
This may not be the best way to judge sound quality, but you will hear the compression effect kick in.