Mu Doubler JFETs

[camsy83]

I've made a Mu Doubler as found on the Geofex site, using 2N5457 JFETs

Mu Doubler

These JFETs are listed in the article as being suitable for the circuit, but i'm struggling to get anything like the test values mentioned.


I've checked the Vgs of the JFETs using the JFET matcher found here

http://www.geofex.com/article_folder...h/fetmatch.htm

I'm using a TL072 for the opamp.


"Get JFETs with Vgs on the matcher of 3V or less. 1 to 2 volts is ideal"

the 2N5457s i've got are mostly reading around about the -0.4v mark on the JFET matcher. The highest Vgs of all the 2N5457s i've got is about -0.8v

I've used a few that have well matched Vgs - Q4 -0.347v, Q3 -0.346v, Q2 -0.360v, Q1 -0.409v; are these going to be any use?

I'm not noticing an octave up effect through the circuit, seems more like just a straight boost/mild distortion - but I have no idea how to bias the JFETs...

From the article - "You can adapt this to many JFETs by changing the value of R3 to get the source voltage to between 1.5 and 3V"

Reading voltage from the Q1 source to ground I can't seem to get close to this voltage, regardless of what resistors i choose.

From the article - "The signal canceling pair are biased by the 510 ohm resistor. You should see something lik 1.5V to 1.9V at the joined sources of Q3 and Q4"

Taking a reading between this point and ground i'm getting nothing like 1.5-1.9v, more like 0.5. Changing the resistor from 0.5K all the way up to 4.7K only pushes it up to about 0.7v. Am I still looking for 1.5-1.9 with these JFETs?

Which pair of JFETs are the most important to match closely? do I want to get my 0.347-0.346 pair in the Q2-3 slots or the Q3-4 slots for best results?

Are the JFETs i'm using unsuitable? Or do I just need to bias them properly?

Cheers,
Cam

[camsy83]

Ok, i've since studied the "JFET Doubler" circuit and explanation on the Geofex site, and think that i've answered one of my questions.

JFET Doubler

I think the reference to Q2 & Q3 needing to be matched on the Mu Doubler page may be a copy/paste carried over from this earlier design, and should read Q3 & Q4. (the earlier design had different numbers for the corresponding JFETs)

Also, since my previous post, i've been able to get the Q3/4 sources between 1.5 and 1.9 v, not much joy with Q1 though.

I don't know much about electronics, but i'm trying...

I think I understand the text on the Mu Doubler page, maybe someone can tell me if this is right -

1)Q1 is being used to split the input signal in two, with both outputs being equal in magnitude, but opposite in phase to send to Q3 and Q4.

2)The 2.2K resistors are 'biasing' Q1. The values of these resistors determine the level of the output signals from Q1's source and drain. Because the two resistors are equal, the source and drain levels are the same.

3)Q3 & Q4 are matched (mine both giving Vgs on the JFET matcher within 0.3% of each other). They must be closely matched so that they affect their inputs by a roughly equal amount. If each gets a signal of equal magnitude, exactly out of phase (as is supplied by Q1), and the outputs are added together, then all that remains is the distortion produced by the JFETs (which is what we're trying to get this pedal to produce).


Am I being too simplistic here? If I am, then please let me know what i'm missing (or point me somewhere I can find out!).

If i'm right (or close ), then is it also important to test the resistances of the 2.2K and 330K resistors to make sure they are also closely matched?

Cheers,
Cam

[guitician]

These types of distortions always sound like synths to me. It's too simplistic of and approach to harmonics. But then he explains, "to drive following stages", which may give it some interesting character.