# ADHD jfet voltages



## DGWVI (Sep 18, 2019)

Have one of these worked up. The distortion and true bypass work, but the buffered bypass is faint and fizzy sounding. Anyone have voltages?


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## Chuck D. Bones (Sep 18, 2019)

I suspect you got one or two hot 2N5457s.  Their Vp can be anywhere between 0.5V and 6.0V.  Anything much higher than 3V won't work for Q6 in this circuit.   The gates (pin 3) of Q5 & Q6 are at 0VDC.  Measure the source lead (pin 2) of Q6.  if it's close to 4.5V then we know Q6 is too hot.  For comparison, measure the source lead on Q5.  I'm guessing it's over 5V.

I'm going to assume that Mr. PedalPCB replicated the OCD v2 faithfully and Mark Fuller wired Q5 upside-down on purpose.  JFETs will work either way, but the biasing and gain will be different. 

If Q6 turns out to be too hot, we have at least four choices:
1) Cherry-pick a 2N5457 with a Vp less than or equal to 3V.  This presumes you have a few spares.
2) Use a different part # JFET with a more favorable Vp.
3) Wire Q6 "backwards" like Q5 by swapping leads 1 & 2.  In theory, this should work, but I haven't tried it.
4) Don't use buffered bypass.

This will take some time to sort out.  If we're lucky, someone with a working ADHD / OCD v2 will chime in.


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## DGWVI (Sep 19, 2019)

Swapped out the 5457s for 2sk30s (didn't have any 5457 or 5458 left, and saving my j201s), and it works splendidly.


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## Chuck D. Bones (Sep 19, 2019)

Did you measure the voltages after the change?  Might be helpful for others having similar problems.


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## DGWVI (Sep 19, 2019)

Q5
D- 1.16
S- 7.65
G- 0.01

Q6
D- 7.76
S- 1.17
G- 0.01


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## Chuck D. Bones (Sep 19, 2019)

Q5's source and drain are effectively swapped.  JFETs can do that.  Drain current on both is around 110uA, pretty low, but if it sounds ok, then that's all that matters.  In this circuit, maximum headroom occurs with drain current at 225uA for both FETs.


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## DGWVI (Sep 19, 2019)

Chuck D. Bones said:


> Q5's source and drain are effectively swapped.  JFETs can do that.  Drain current on both is around 110uA, pretty low, but if it sounds ok, then that's all that matters.  In this circuit, maximum headroom occurs with drain current at 225uA for both FETs.



The buffered option doesn't sound appreciably different for the hardwire bypass, so I'm assuming it'll be fine


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## Bravin Neff (Sep 19, 2019)

I'm following this conversation intently, as I'm getting ready to build my own ADHD. What I'm missing is... my board doesn't have any of the transistors labelled as normal (Q1, Q2, etc.), but rather just by their type (2N7000, etc.). It's not hard to zero in on what you guys are talking about, except for stuff like this:



Chuck D. Bones said:


> I'm going to assume that Mr. PedalPCB replicated the OCD v2 faithfully and Mark Fuller *wired Q5 upside-down on purpose.*



and 



Chuck D. Bones said:


> 3) *Wire Q6 "backwards" like Q5* by swapping leads 1 & 2.



How does one tell what is backards and not, since graphically there is nothing to go by one way or the other?


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## Chuck D. Bones (Sep 19, 2019)

As long as your pickups aren't too hot or you don't have a boost in front of the ADHD, it should be good to go.


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## Chuck D. Bones (Sep 19, 2019)

Bravin Neff said:


> I'm following this conversation intently, as I'm getting ready to build my own ADHD. What I'm missing is... my board doesn't have any of the transistors labelled as normal (Q1, Q2, etc.), but rather just by their type (2N7000, etc.). It's not hard to zero in on what you guys are talking about, except for stuff like this:
> 
> 
> 
> ...




There is enough information, it just takes some effort to sort it all out.  The older build docs used to label the ref designators.  Made life real easy. Now, if we want to figure out which device is which, we have to do it the hard way.  It's kind of a ♪ Leg bone's connected to the hip bone... ♫ sort of thing.  Compare the schematic to the board (if you have a board) or the board artwork on the website (if you don't).  Look for traces connecting 2 or 3 parts together and then find those things on the schematic.  One of the JFETs has pin 2 connected to two 22nF caps, that has to be Q5.  One of the JFETs has pin 1 connected to a 100R resistor, that has to be Q6.

Now about upside-down JFETs...
PedalPCB uses JFET schematic symbols that are unambiguous.  In the case of the ADHD schematic, notice that on Q6, the gate arrow (pin 3) points right at pin 2.  That identifies pin 2 as the source.  Also, you can download the 2N5457 datasheet from the web and it says that pin 2 is the source.  It's the same way with Q5, except for whatever reason the pin numbers are hidden on the schematic.  The gate arrow points at the source and if you trace the board like I did, you can see that pin 2 of Q5 connects to C1 and C13.  
Now here's where is gets a little weird... 
JFETs don't really care if you swap source & drain, at least not at audio frequencies.  With N-channel JFETs, whichever pin is at the higher DC voltage, that's the drain.  So in the case of Q5, even though the board artwork and the schematic identify the lead that connects to C1 & C13 as pin 2, it behaves like the drain because it's at a higher DC voltage.

Clear as mud, right?


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## Bravin Neff (Sep 19, 2019)

Chuck D. Bones said:


> There is enough information...



I'm gonna have to read that a couple more times, LOL.


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## Paul.Ruby (Jan 19, 2020)

Building mine now and definitely must cherry-pick the jfets. I have 5 good and 5 bad out of 10 bought. I strongly suggest builders breadboard this and check DC bias levels using this circuit. Shoot for ~2V on the source and ~7V on the drain, but there is a wide range suitable, given guitar signals are sub 1VAC. Reversing the drain and source had no effect on any given device to be suitable or not. I would actually bet the "upside-down jfet" was a layout choice, not circuit function choice, of whoever did this originally. It's the kind of thing I would do when building a prototype or laying out a PCB by hand. FYI, I tested mine to be suitable up to 1Vpp at 1khz with the full circuit implementation. This simplified DC bias test is plenty good enough for cherry picking with confidence. My "good" jfets all landed at ~2.5V on the source, ~6.5V drain, which is ideal.  If you only have "bad" jfets, then the 10k resistors can be tweaked up or down BUT must both remain the same value. Use the breadboard method to pick resistor value instead. The 27k resistor in the full circuit to get unity gain does not need to be tweaked.


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## Paul.Ruby (Jan 20, 2020)

Also, the circuit could be made immune to jfet variation by replacing the two consecutive jfet stages with one source follower, like this. Can still be a buffered bypass. The only downside is it's not quite unity gain. Unity gain doesn't matter at all feeding to the OD circuit. If using a true-bypass instead of buffered, then no downside, simpler and immune to jfet variability. The original circuit is also limited by the zener diode to ~500mv negative swing, so I'm not installing that in my build and not shown here. This alternate circuit is also not limited by the zener if it is desired but I would still leave it out so that the circuit works fine up to 18V supply. (I imagine this might be rehashing old news, but I didn't see this discussed elsewhere.)


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