Hi guys

I have recently built an voltage controlled filter using the schematic from MFOS experimenter board. The circuit consists of a cut off frequency control and a resonance control. The resonance part of the circuit is working fine but I cannot get the cut off control to work at all. The cut off works through a PN4391 transistor set up as a variable resistor and I believe this is the component that isn't functioning as it should. I have checked the circuit over thoroughly and all connections seem to be correct, however when I go through the circuit with a continuity tester the Drain and the Source are connected, and they shouldn't be according to the schematic. So my question is, is this normal behavior for a PN4391 transistor? Unfortunately these transistors are discontinued and are being sold for ridiculous money on ebay. This is where I originally purchased 2 of them (generic brand), and I'm wondering could I have bought 2 dodgy ones as Ray from MFOS says they can vary in characteristics quite a bit.
I have now purchased 1 Fairchild PN4391 from ebay but I am waiting for it to arrive to test it.

schematic: (See schematic in post #3)

Any help with this would be greatly appreciated.

Thanks

 

Hello,

I can not find the PN4391 on the given page.
I only see 2 times the 2N3904 used as a switch.

Bertus

 

Hello,

I can not find the PN4391 on the given page.
I only see 2 times the 2N3904 used as a switch.

Bertus

Hi Bertus

It must have been the wrong page. This is the schematic:

 

Hello,

The page does not allow direct linking, but when you click your right mouse button, a popup will show.
Then you can choose : open frame in a new tab.
http://musicfromouterspace.com/analogsynth_new/EXPERIMENTERBOARD/page3.html

The main problem with jfets in their great spread in specifications.
That is why each filter needs a bias of its own.

Bertus

 

Hello,

The page does not allow direct linking, but when you click your right mouse button, a popup will show.
Then you can choose : open frame in a new tab.
http://musicfromouterspace.com/analogsynth_new/EXPERIMENTERBOARD/page3.html

The main problem with jfets in their great spread in specifications.
That is why each filter needs a bias of its own.

Bertus

I have built the circuit exactly as shown. I was just wondering why both circuits (vcf 1 & vcf 2) are acting the same with regards to source and drain being connected with a continuity tester, and seeing as the resonance part of the circuit is working I'm guessing it could only really be the transistor causing the problem. When i turn the pot for the cut off frequency nothing happens at all. As for biasing, I understand that is what the trim pot is for, but, with this part of the circuit not working properly, adjusting the trim pot also doesn't do anything. I was hoping someone might have a PN4391 laying around and could check if the source and drain are connected through a continuity meter, if that were the case then it suggests something else in the circuit is wrong but I have gone over and over it and cant see anything wrong.

 

Hello,

The following page has a way to test jfets:
http://stompville.co.uk/?p=112
The tests will show you the differences between the jfets.

Bertus

 

Hello,

The following page has a way to test jfets:
http://stompville.co.uk/?p=112
The tests will show you the differences between the jfets.

Bertus

Thanks Bertus, some of this is over my head tbh, but could these points be the reason I am experiencing problems with the source and drain being connected.

"“Depletion mode” because if there is no electrostatic field at the gate maximum current flows between drain and source."

"2. A lot of small-signal JFETS (of the types used for stompboxes and the like) are symmetrical. This means that the drain and source are interchangeable. This property is of practical use when designing a printed circuit board."

 

Hello,

Have a look at the following PDF where the jfet is used as a variable resistor:
http://educypedia.karadimov.info/library/Vishay-Silcnx_FET_VoltCntrlRES_AN105.pdf

This is one of the links of the EDUCYPEDIA page on fets and jfets:
http://educypedia.karadimov.info/electronics/composemijfet.htm

Bertus

 

Hello,

Have a look at the following PDF where the jfet is used as a variable resistor:
http://educypedia.karadimov.info/library/Vishay-Silcnx_FET_VoltCntrlRES_AN105.pdf

This is one of the links of the EDUCYPEDIA page on fets and jfets:
http://educypedia.karadimov.info/electronics/composemijfet.htm

Bertus

Ill check them out, thanks for the help Bertus

 

Unfortunately these transistors are discontinued

the important thing in those cases is to understand how those devices are used.

for example, if they are used as variable resistors, LED+LDR can do a much better job than a jfet will ever do, giving you far greater range of control voltage and range of resistance.

 

being sold for ridiculous money on ebay.

when you buy anything from ebay, you should assume them to be fake unless proven otherwise.

 

with regards to source and drain being connected with a continuity tester

If you mean you measure continuity between the source and drain, that's normal for a depletion-mode J-FET.
There are ON (conducting) with no gate-source voltage.

 

the important thing in those cases is to understand how those devices are used.

for example, if they are used as variable resistors, LED+LDR can do a much better job than a jfet will ever do, giving you far greater range of control voltage and range of resistance.

When you say LED+LDR do you mean gain or frequency controlled by light intensity?

 

If you mean you measure continuity between the source and drain, that's normal for a depletion-mode J-FET.
There are ON (conducting) with no gate-source voltage.

Yes thats what I mean. Thanks. So its called depletion mode. You learn something new everyday!

 

Hi guys

I have recently built an voltage controlled filter using the schematic from MFOS experimenter board. The circuit consists of a cut off frequency control and a resonance control. The resonance part of the circuit is working fine but I cannot get the cut off control to work at all. The cut off works through a PN4391 transistor set up as a variable resistor and I believe this is the component that isn't functioning as it should. I have checked the circuit over thoroughly and all connections seem to be correct, however when I go through the circuit with a continuity tester the Drain and the Source are connected, and they shouldn't be according to the schematic. So my question is, is this normal behavior for a PN4391 transistor? Unfortunately these transistors are discontinued and are being sold for ridiculous money on ebay. This is where I originally purchased 2 of them (generic brand), and I'm wondering could I have bought 2 dodgy ones as Ray from MFOS says they can vary in characteristics quite a bit.
I have now purchased 1 Fairchild PN4391 from ebay but I am waiting for it to arrive to test it.

schematic: (See schematic in post #3)

Any help with this would be greatly appreciated.

Thanks

It may not be meaningful but I am used to seeing the source of the JFET going toward ground.
(edited to add ...)
Never mind. It works either way.

 

When you say LED+LDR do you mean gain or frequency controlled by light intensity?

the original circuit used the jfet as a variable resistor.

a ldr is a variable resistor whose resistance is controlled by light intensity from an led whose current is in turn controlled by a user / signal.

 

Hello, gface83! It may be too late to be bumping this thread but I really need your help. I've run into the exact same problems that you did, with the cut-off frequency knobs being unresponsive. The reso works fine. Wanted to know if you found out what went wrong. I've been breadboarding the circuit exactly as its shown in the schematics but they all turn out the same way.
Thanks,
Awaiting anxiously!

 

Hello, gface83! It may be too late to be bumping this thread but I really need your help. I've run into the exact same problems that you did, with the cut-off frequency knobs being unresponsive. The reso works fine. Wanted to know if you found out what went wrong. I've been breadboarding the circuit exactly as its shown in the schematics but they all turn out the same way.
Thanks,
Awaiting anxiously!

Hi come_in_peace

This was a little while back now and it was a very annoying problem, but, ill try to help as much as possible. I didn't end up getting the circuit to work as needed, but, after a hell of a lot of research I stumbled upon a web page that stated an alternative for the Pn4391. I can't remember what it is now, but I will get back to you when I get up the studio and let you know. I ended up buying a few of these to try and unfortunately, I ended up with exactly the same results, so I really started to mess about with the trim pot after resoldering the original PN4391 back in. I had some sort of result, as it did resemble a cut off filter, but, with really distorted frequencies at certain frequencies. So it wasn't a smooth cut off as you would expect. TBH I left it as it was because I got fed up of spending so much time on such a simple circuit. So it's still a mystery really, but did have better result than the first time because it wouldn't do anything to begin with. Hope this helps a bit lol any other questions I'll be happy to answer!! Good luck my friend and let me know if you end up getting better results.