Hi folks! I'm new here and don't understand JFETs

[David the Climber]

Hello everyone!
I was googling for information on JFETs and it brought me to this site. I hope you are a patient bunch because I am a slow learner and a little out of my depth...
Long story short, I am pottering with an old audio preamp. There is a resistor to ground in there, and that is bypassed with a capacitor which shunts some of the audio freq to ground to prevent the audible being swamped at that freq. I get that.
I thought it would be interesting to put in a couple of extra caps at different values paralleling that capacitor, each one with a resistor (100K ish) in series with it to ground. Then I could use a switches to bypass one or both of those resistors to alter the total capacitance, and therefore the frequency being sent groundwards. It seemed to work when I tried it with mechanical switches, noticable change to the audible, nothing exploded and I am still alive typing this with all my fingers and thumbs.
Now I would like to replace the mechanical switches and don't want to use relays because of the power consumption of the relay coils, I just don't have the mA to spare.
I think I remember reading somewhere that JFETs can do this... but that must have been a long time ago, I do not remember any detail. I think it was something like wire the resistor I want to switch in and out of the circuit across the drain & source of the JFET. Then when a bias voltage is applied to the gate, presto the JFET allows current to flow from Source-Drain, and the capacitor tail is to all intents and purposes "grounded" and my audible changes according to the cap value.
If there is anyone here that has some JFET knowledge that wouldn't mind letting me know whether I am heading in the right direction on this, I would be most grateful!!
Thank you for reading!!

 

[BobK]

First, we need a schematic. Not all audio preamps are identical.

There is a resistor to ground in there, and that is bypassed with a capacitor which shunts some of the audio freq to ground to prevent the audible being swamped at that freq. I get that.

If you are talking about the resistor from the emitter to ground, the capacitor is doing just the opposite of what you think. It is allowing audio frequencies bypass the resistor so that the full gain of the transistor is seen rather than reduced gain at DC. The capacitor is normally sized to allow all audio frequencies through. A larger capacitor will have no effect except to let inaudible signals through, and lowering it will make it act like a high-pass filter.

The typical N-channel JFET is on (or partially on) when there is 0 or positive voltage between the gate and source, you need a negative voltage with respect to the source to turn the JFET fully off.

Look at analog switches, they use a MOSFETs in a more friendly manner. A logic level signal can switch them on and off. The CD4016 is an example. Plus, you get 4 in a package.

Bob

 

[David the Climber]

Hey thank you for the quick reply!! I get how the filter works but I fuddled my words. I do that alot though!
I am sorry, I do not have a proper schematic to hand. But I do have paper and pencil! So, I have done a drawing of the bit of the preamp that I am talking about. There are not many parts. To the left is a drawing of the circuit as it was before I changed anything. In the middle I have shown the changes I made that seem to work but are done with mechanical switches. To the right is what my hazy memory thinks I read I could do with JFETs, for very little cost in terms of mA.
Thank you!!!

 

[BobK]

Wasn't expecting tubes! But the circuit is the same as that used with a transistor. The emitter or cathode resistor sets the gain to roughly it's ratio to the load resistor. The capacitor passes all frequencies above it's cutoff around the resistor giving you a higher gain, limited only by what the active device can do.

JFETs would work, but you would need a voltage of something like -2V to turn them off and +2V to turn them on. As I said before, an analog switch does the same thing using logic levels. 0V for off 3V for on.

What effect are you trying to get? If you think each capacitor will affect a different frequency range and they can be used independently, that is not going to happen.

The low frequency cutoff will be set by the sum of the capacitors that are switched in. With 22uF and 1.5K, as in the original design has a cutoff of 14.5 Hz, which essentially means it passes the entire audio spectrum. Taking all the capacitors out will give you flat response at lower gain. Leaving in smaller capacitors will cut essentially boost higher frequencies higher than the cutoff frequency.

Bob

 

[David the Climber]

Well what I'm trying to do is fill some of the many gaps in my knowledge, and hopefully have some fun along the way!
What I'm hoping to do here is be able to start with the .01uF, to hear how that sounds, then parallel in a 0.1uF for 0.11uF (tolerance permitting), hear how that sounds, then add in a 2.2uF, (for 2.31uF) etc and then that big 22uF.
I want to be able to hear it rather than just read waout what it will do. Also, because what is plugged into the preamp can be a bass or a guitar or a microphone or anything else on a 1/4" jack and because the preamp can be connected to different power stages... which may have different frequency responses... which may be connected to different speakers... it may be useful (and fun) to be able to adjust the frequency response of the preamp. Also, I'll have a very audible indication of whether my assembly and soldering skills are improving!
What I may do next if I can get this to work is see whetheer I can come up with some way of getting it to switch additional caps in or out depending on the output level because speakers respond differently with output level so that might sound groovy. Or it might not. Fun to find out though!
I thought I would try it with JFETs because I thought I'd read it somewhere, but maybe I heard someone talking about it... The only transistors I have here are in a small poly bag that someone has hand written J111 on (wasn't me, not my handwiting), but there are no markings on the transistors themselves. Google says that J111 is a JFET number but whether these transistors are J111s or not or are all different I don't know.
Analog switches are new on me. I'm pretty sure I don't have any, but if they're readily available I'm happy to learn them before I learn JFETs.
Thank you for your replies, I appreciate the information!! I hope one day I will have learned enough to be able to respond to a newbie with useful knowledge. It is very encouraging to know there are helpful souls on the internet!!
Thanks again!!

 

[Audioguru]

The old vacuum tube preamp has a 1.5k cathode resistor bypassed with 22uF. The capacitor simply reduces the gain -3dB (half the output power) at the cutoff frequency and reduces lower frequencies at -6dB (1/4 the output power) per octave. The 1.5k with a 22uF bypass calculates the cutoff frequency to be 0.16 divided by (1500 x 0.000022)= 4.8Hz so it passes all audible frequencies. If the capacitor is replaced with 1uF then very low guitar frequencies and a male voice will be cut. If the capacitor is replaced with 0.047uF then all low frequencies will be inaudible and mid frequencies and a female voice will be reduced. If the capacitor is replaced with 0.01uF like you show then low and mid frequencies will be inaudible and high frequencies will be reduced.
A capacitor that is about 0.1uF will make a sound like your stereo with its treble tone control turned all the way down. Muffled.

Your idea to use Jfets as switches needs a negative supply to turn off the Jfets, and will produce a POP sound each time a Jfet is turned on or off. The top end of each capacitor must connect to the cathode of the vacuum tube the same as the original capacitor. You do not add a resistor parallel with the Jfet.

 

[BobK]

The 1.5k with a 22uF bypass calculates the cutoff frequency to be 0.16 divided by (1500 x 0.000022)= 4.8Hz

My mistake, I had typed 500 instead of 1500 into the calculator. You are right, the cutoff is 4.8Hz.

Bob

 

[Audioguru]

I wonder what a 4.8Hz earthquake feels like? I have seen a subwoofer cone move at such a low frequency but I heard and felt nothing.

 

[BobK]

Well, if you want flat response down to 20 Hz, what corner frequency would you use? Clearly not 20 Hz, since it would be down 3 dB at 20 Hz.

Bob

 

[hevans1944]

I wonder what a 4.8Hz earthquake feels like?

Not sure what the "frequency" was, but I experienced a small earthquake on Okinawa sometime around 1958 or 1959 when Dad was stationed at Kadena AFB. I was outside when the ground under my feet began moving like it was alive. Very uncomfortable feeling.

The entire "event" lasted only a few seconds IIRC, but it was long enough to make me grateful that I was not inside a building when it occurred. I don't remember there being any particular sound associated with the earthquake event, although it is possible there were some inaudible infrasonic frequencies involved. It has been reported that exposure to high levels of infrasonic waves propagated through air (as compared to earth or water) causes a "feeling" of dread, perhaps a visceral response to internal organs being shaken. I think some "scary" movies (perhaps all of them) are deliberately scored with sound tracks containing low frequencies in an attempt to elicit the appropriate response from the audience.

The OP is clearly not taking a very "scientific" approach to his "experiments," having completely ignored, or not understood, @BobK's or @Audioguru's comments concerning how cathode bias works. So be it. Allow him to futz around and have fun, stumbling around in the dark while adding and subtracting components to make it "sound right." Sooner or later he will either tire of that game and learn electronics theory, or tell himself that his circuits are "gud enuf" and go on to something else.

If he chooses to learn some theory, maybe he could explore the venerable Baxandall bass/treble tone control boost and cut circuit. See image below.

Plenty of links for study can be found here. This one seems particularly apropos to vacuum tube amplifier circuits.

 

[BobK]

Okinawa sometime around 1958

OMG, I was in Okinawa in 1958! I don't remember an earthquake though, I was 6, and my dad was there as a CIA employee. We left in May of that year, so I might have missed the quake.

Bob

 

[hevans1944]

I might have missed the quake.

You must have missed it, because it's not something you EVER forget!

Although memorable for me, the quake I experienced was not large and it was short-lived. I don't recall that it was even the subject of discussion in the following days. Apparently earthquakes are a common occurrence in this part of the world, along with typhoons and monsoon rains. The natives have just learned to live with it, barring the occasional tsunami that wipes out a few villages or destroys nuclear reactors.

I did notice they don't normally build multistory buildings on Okinawa, an exception being Kubasaki Junior High School at which I attended the 9th Grade while Dad was stationed at Kadena. It was a two-story concrete building IIRC and a short military bus ride from Kadena to the school. More info can be found here.

I remember a, somewhat obese, female classmate who took the time to learn Japanese. I think she had a "crush" on me, maybe because I was the only student who would socialize with her. She practiced speaking Japanese on the Okinawan servers in the school lunch cafeteria, in order to obtain larger portions as we moved together down the line. This impressed me, as the only Japanese I learned were a few words of greeting and thank you. I now wish I had learned more about the people and the language during the short time we were stationed there, but it seems I don't have much of an "ear" for learning languages. That, and teenagers are not mature enough to realize what appears unimportant to them today can turn out to be quite useful tomorrow.

I was starting to enjoy our stay on Okinawa when it was cut short by Dad contracting tuberculosis in his kidneys. That ended his flight career, but the Air Force flew him, chop-chop, to Valley Forge Army Hospital near Phoenixville, PA, while the rest of our family departed on a former troop-carrier ship, the USS Breckenridge, for San Francisco with a detour to Yokohama, Japan to pick up passengers returning to the States. We were not allowed to leave the ship in Yokohama. So much for sight-seeing on the guv'mint's dime.

The voyage home took about a week, and after we arrived we boarded a train for Phoenixville via a change in trains in Chicago, which took another two or three days. This was my first (and only) trip on an ocean-going vessel, and only my second (and so far last) trip on a railroad train. You need deep pockets full of cash to indulge in ocean voyages and comfortable train rides (we had Pullman sleeper accommodations) today, but our "vacation" trip was fully paid from our tax dollars. That's not likely to ever happen again... unless my Powerball numbers come up this week and I remember to buy a ticket.

 

[BobK]

@hevans1944, you might have been at Kubasaki at the same time as my older brother. He was there in 56 and 57 for 9th and 10th grades. He played on the basketball team. Meanwhile I went to Kindergarten in 57 at, I think I remember Naha or Kui (spelling?) One of those was where we lived and the other where the grade school was.

I do now vaguely remember my parents talking about an earthquake in Okinawa, but I have no personal recollection of it.

On the other hand, I was in Palo Alto when the big quake that happened during a world series game hit. I was in my hotel room, and the furniture was moving and plaster falling from the ceiling. I got out as quick as I could, and remember seeing the water sloshing out of the swimming pool. It was frightening indeed.

Then, 10 years and 1 day later, I was at a Casino Hotel just outside do Death Valley, when a big one woke me in the middle of the night and I had to evacuate again.

Bob

 

[(*steve*)]

Not to self: do not stay in the same hotel as @BobK

 

[BobK]

Not to self: do not stay in the same hotel as @BobK

Yep, the earth shakes when I am in town!

Bob