JFETS matched pairs used in voltage controlled oscillator

Thread Starter

spuddo

Joined May 2, 2013
72
Hi , i am building a copy of an old Moog synthesizer VCO.

The original JFETS used in this circuit (attached) are E402's , the ones in my vco are E420's.

These are obsolete , so i have found what are claimed to be substitutes , LS841s.

The following are tests of Idss and Vgs. . These seem to be the critical specs.

Idds(ma) Vgs(v)

E420(a) 2.1 1.5
2.1 1.5
E420(b) 2.8 2.0
2.8 2.0
E420(c) 2.2 1.5
2.2 1.5
What i see from this is each matched pair seems okay and even if there is some difference between pairs , that's okay.

The following are tests of the substitute fets

Idds(ma) Vgs(V)

LS841(a) 1.0 .72
1.0 .72
LS841(b) 1.0 .72
1.0 .72
LS841(c) 1.1 .75
1.1 .75

Are these subs good enough to do the job in this circuit?
 

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Thread Starter

spuddo

Joined May 2, 2013
72

Ian0

Joined Aug 7, 2020
5,169
It looks like that stage is just an opamp, the parameters may not be so critical. Have you tried something readily available such as On-Semi’s NSVJ5908?
Is R12 really 33.2Ω?
 

Thread Starter

spuddo

Joined May 2, 2013
72
It looks like that stage is just an opamp, the parameters may not be so critical. Have you tried something readily available such as On-Semi’s NSVJ5908?
Is R12 really 33.2Ω?
Hi , i need a TO-72 through hole can , so SMD is out.
R4 is 33.2k , R12 is 150k , but the schematic is full of errors.
Thanks for your reply.
 

Thread Starter

spuddo

Joined May 2, 2013
72
Can't you mount an SMD component Manhattan style?
This is a vintage piece of gear---70's single sided PCB.
Would like to keep it that way.
Thanks for your input.
Can you answer my Post#1 question.
Are these subs good enough to do the job?
Both groups of jfets are pair matched but vary in Vgs.
Is it the Vgs or matching that is more important.
I'm not an EE and don't know what the engineer would have required.
Any help appreciated>
Best regards
 

Thread Starter

spuddo

Joined May 2, 2013
72
Since you've already bought the LS841, why not try it and see?
That's an option , all the parts have not yet arrived.
I would still like to know how important the difference in these Vgs values effects the circuit operation.
the E402's around 1.5 to 2.0v , the LS841's around 0.72 to 0.75v.
Best regards.
 

Ian0

Joined Aug 7, 2020
5,169
That's an option , all the parts have not yet arrived.
I would still like to know how important the difference in these Vgs values effects the circuit operation.
the E402's around 1.5 to 2.0v , the LS841's around 0.72 to 0.75v.
Best regards.
They are in a long-tailed pair configuration, and the sources are driven by a constant current source. Vgs won't make any difference at all.
Gfs might, but they seem to be part of a feedback amplifier connected as an integrator, so I doubt it.
 

Thread Starter

spuddo

Joined May 2, 2013
72
They are in a long-tailed pair configuration, and the sources are driven by a constant current source. Vgs won't make any difference at all.
Gfs might, but they seem to be part of a feedback amplifier connected as an integrator, so I doubt it.
Thanks for your reply.
When all the parts arrive , i'll build the vco , and test all the suggested replacements for the original E402.
Best regards
 
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