DC coupled JFET source follower

[TejfolvonDanone]

Hi Everybody!

I was researching about valve amplifiers and found this article:
http://www.valvewizard.co.uk/dccf.html
The section above the scope traces made me think: in a valve if you bias the grid above the cathode voltage current starts to flow which loads down the previous stage just for the up going signal. I thought that a JFET could work just like that. So i came up with the following idea:
https://drive.google.com/file/d/0BzGRfrmHSjb4R01uSVVROVlubk0/view?usp=sharing

WARNING: This isn't normal operation of a JFET because the gate is biased ABOVE  the source with around 0.5V. This is not enough to really open up the gate-source diode so only small currents will flow. This is essential because we have to make sure the maximum gate-source current don't exceed the datasheet maximum (which is 10mA for the 2N5484).

The switch adds a bootstrap cap to the source resistor of the first JFET. This gives more gain as the R2 resistor appears as a current source.

Simulated outputs: Gain swept from 0% to 100% showed in the same graph.
Switch off
https://drive.google.com/open?id=0BzGRfrmHSjb4X1dlTUJ5MldnRXM
Switch on
https://drive.google.com/open?id=0BzGRfrmHSjb4bWkydHliNml4Rk0

I've put it on a breadboard and i've replaced R4 with a 10k trimpot. The bias point of the first JFET was higher but i could manage to bias the second JFET's gate higher than its source. Unfortunately I don't have the chance to preform any kind of test beside measuring DC voltages.

I share this because I hope someone is adventurous enough to test out it with an audio probe.

[Kipper4]

I'm no too up on transistors stuff. So I got to ask what is the expected sound?
Under what circumstances is it useful?
Is it all about the current?
Sorry for the dumb question.
I'd be willing to put it on the breadboard tomorrow.
Will it need a volume pot (load) before the amp? Edit I've just realised it has one.

[TejfolvonDanone]

First of all I'd like to thank you for your help Kipper!

I'm no too up on transistors stuff. So I got to ask what is the expected sound?

Because JFETs work almost like triodes and the effect is the same as in the DC coupled cathode follower I expect that it will sound really close to a high gain valve amp.

Under what circumstances is it useful?

If everything goes as i expect it will have a valve like overdrive sound. As the simulation suggests it will mostly generate second order harmonics with a little third. So basically another overdrive.

Will it need a volume pot (load) before the amp?

No, it is not necessary but it will have a lot of gain. I just had to put a resistor on the outside of the cap so i could create a simulation with zero DC on the output.

Is it all about the current?

Yes it is. The current of the first JFET is (more or less) proportional to the input voltage. If this current decreases the voltage drop on the drain resistor caused by it is decreasing which makes the gate of the second JFET rise. Because it is forward biased (but not with a full diode drop) more current (few uAs) starts to flow from it into the drain of the first stage. Therefore the current through the drain resistor is not the same as the drain current. Which means the voltage is not proportional to the input voltage it is compressed.
If the current increases the voltage on the drain decreases and the gate of the source follower will be less forward biased. Thus it will be proportional to the input.

I'll just copy the section from the article as it describes the valve DC coupled cathode follower (which is essentially the same as with the JFETs):
"So we now have a cathode follower that is permanently 'stealing' current from the previous stage; what is the result? If a down-going signal appears at the gain stage's anode the grid voltage of the follower is pushed down back into the area of "normal operation", and grid current stops flowing more or less immediately, for the duration of that cycle. But when the incoming signal is positive going the grid is pushed more positive, which induces even more grid current to flow into the cathode follower as it tries to maintain bias, which in turn 'drags down' the anode voltage of the gain stage which is trying to move positive! In other words, the up-going cycles are very heavily compressed, but the down going ones are not, which generates a lot of second harmonic distortion."

[Kipper4]

Ok I'm going in.
Subbing the 84 with an 85 It's all I gots.
Going for some crunch. Cover my back guys.......

[Kipper4]

Ok preliminary report.
It does have crunch. It reacts nicely to dynamics. The harder you pick the more crunchy it is.
I found no need for the 1M gain pot.

I liked the More gain cap (C2). Better than without for crunch sounds. However it has a feel of misbiased transistor about it at low volumes (its late Mrs Kipper is sleeping in the next room)
I do like the tone and feel of less dynamic playing without the more gain cap.

I can see how it might work well as part of a low gain preamp. Maybe for a brad Paisley kinda feel.
Good work. I'll look at it some more.
I'm not sure if C1 needs to be such a big value. hmmmm.
What says you?

[Kipper4]

wait i forgot the bypass 22uf cap. I'll be back.
Don't shoot......

[Kipper4]

Ok well with the bypass cap in. C3
I put the 1M gain pot back in the mean time
It's still nice and dynamic. I changed C1 for a 47nf better. Changed again for a 22nf even better.
Now I likes it more without C2

If you want a more distortion feel stick a 5mm red led on the source of T2 to gnd

[amptramp]

JFET pinchoff voltages can be all over the place, so JFET Vgs is not like a bipolar transistor where there is a well-characterized Vbe.  You would have to do a select-on-test if you intend to produce these in quantity, but for a single unit, you can shift the values around to accommodate the transistors you have.  Also the Idss can vary, so the design has to be able to accommodate that.  It is nice to see your design has produced very good results - just don't build large numbers of them and expect the same results from each one.

[Kipper4]

Thanks AmpTramp.
Which led me to up the T1 source R3 to 2k5 its a little better. not so unbiased. Still plenty of crunch with hard picking.
So Does one go aboutit by measuring each device or could you put a trimmer on the Drains or source and adjust to taste and sound.
Maybe I'm changing the wrong part already. Advise please.

[Kipper4]

[ElectricDruid]

That sounds really sweet. Nice'n'tubey.

Good work guys.

Tom

[TejfolvonDanone]

JFET pinchoff voltages can be all over the place, so JFET Vgs is not like a bipolar transistor where there is a well-characterized Vbe.  You would have to do a select-on-test if you intend to produce these in quantity, but for a single unit, you can shift the values around to accommodate the transistors you have.  Also the Idss can vary, so the design has to be able to accommodate that.  It is nice to see your design has produced very good results - just don't build large numbers of them and expect the same results from each one.

The good old JFET hassle. Although there are quite some JFET biasing technique to bias a single common source or source follower stage but i'm not sure which one can be used here. I'm also not sure whether or not i'll ever need to consider this.

But look what have you done?! You just put a little voice in my head which asks "But how can it be done?".   

[TejfolvonDanone]

Thanks for the quick demo! It sounds nice.
Could you please make a quick DC measurement around the JFETs?
It might be different as i intended because the '85s are higher Vp higher Idss types.
If not decrease R4 and increase R3 at the same time. My simulation gave me 3.3k for R3 and 330R for R4 with a considerable gain loss.

[Kipper4]

Will do brb.

[Kipper4]

T1 2k46 source R
D 3v16
S 2v06
G 0

T1 2k2 source R
D 2v6
S 1v95
G 0v04


T2
D 8v55
S 3v95
G 3v2

That make sense?
Edit. All volts taken without more gain cap.

[TejfolvonDanone]

That make sense?

It makes. Because of the substitution the whole thing doesn't bias as i'd like it to.
R3 as 10k and R4 as 680R could bias T2 as G:6.6V S:6.2V D:9V. If you could adjust those two resistors until the gate-source voltage is around 0.5V than the effect i'm looking for will occur.

[Kipper4]

With R3=8k5 and R4=680
i get
T1
D 6v2
S 2v5
G 0v08

T2
D 8v
S 5v3
G 6v

with R3 = 10k
T2
D 8v
S 5v8
G 6v5

[Gus]

Do a search for "Neumann U69 schematic"

[TejfolvonDanone]

With R3=8k5 and R4=680
i get
T1
D 6v2
S 2v5
G 0v08

T2
D 8v
S 5v3
G 6v

with R3 = 10k
T2
D 8v
S 5v8
G 6v5

That should be it.

[Kipper4]

I'll review it again with all the different resistor swaps after work tomorrow. I need a clear ear.
I've had a few happy accidents along the way that I liked too. I'll report back on that too.
Now I'm wondering if it's achievable with other fets. Eg 2N3819 2SK30a just cause I've got them in stock.