another jfet gain stage to try in a DIY preamp

Gus · Dec 23, 2018

Something different than another preamp gain stage.

Note the quasi constant current resistors in both stages.

It uses 1/2 of the available power supplies for the gain, - side is used for the quasi constant current.

You should bias up without any trimming needed.

R8 value sets the gain. EDIT Lower resistance gives more gain, max gain when shorted. Try a reverse log potentiometer 20k to 25k

A circuit like this can be found in a solid state Fender Harvard amp.

You can use a Darlington in the 2nd stage or a high hfe NPN or...
You can scale up the current in the 2nd stage.

CJ · Dec 23, 2018

Cool! looks small enuff to go on board the gitar,

Gus · Dec 23, 2018

I was thinking more in one of the small racks that people build modules for that have +- supplies. This also has a gain control that lets you set each stage gain, not leaving the gain stage on full and using a volume control after it.

You can use a number of JFETs 2N5457, 2SK30A, 2N3918 etc. and it should bias up. The source to ground voltage will change a little. Think Vgs at the drain current that the circuit stabilizes at. So as long as the drain current is not more than IDSS the source might move a few volts referenced to ground.
Lets make up some Vgs numbers
-16VDC(plus the Vgs at ID) via a 27k to ground. No Vgs drop added 16VDC/27k= .59mA
(16VDC +1Vgs)/27k=.63mA
(16VDC +3Vgs)/27k =.70mA
.70mA-.63mA=.07mA
.07mA x 12k= .84VDC change

You don't always need a active CC device if you have enough extra voltage you can get close enough with a resistor.

The gain stage is set up to work more in the + to ground supply range and the clipping should be interesting.

You can increase the output stage current, as shown it is about 5mA.

I first noticed this bias method years ago in the groove tube amp book with lots of schematics IIRC early 90's? used in a Fender Harvard schematic JFET preamp section. I am looking for the schematic on the web.

CJ · Dec 23, 2018

what is the overall gain of those two stages, and happy Holidays!

JohnRoberts · Dec 23, 2018

CJ said:
what is the overall gain of those two stages, and happy Holidays!

From a quick peek maybe close to unity gain inverting, or not...

JR

Gus · Dec 23, 2018

It sims inverted close to gain of one with R8 at 20K as John posted.

Make R8 a 20k or 25K potentiometer and you have a gain control.

VictorQ · Dec 24, 2018

What does quasi-constant current mean? R3 current is roughly constant (in the flat part of the logarithmic large signal TF), but those other ones seem pretty 'naked' to me?

Btw. was thinking maybe you can add a similar RC as the gain control across R1 and convert it to a distortion control (ie. control the percentage current swing of the FET without changing signal gain). Less useful to be sure, but the point of this circuit is color, no?

Gus · Dec 24, 2018

R2 and R4 go to -16VDC
So the change in current is less because the - supply is being used for setting the stages current R2 for the source, drain current and R4 for the emitter, collector current.
Because of the use of the -supply in this manner the currents in the stages stays a little more constant not as good as constant current devices but should be good enough for this circuit.

The nice thing is the jfet should not need selection. One can change the R1 and R2 values for the jfet used make sure there is enough drive for the follower and you don't add too much noise from the drain resistor value before the base.

The gain stage is in the +16VDC side. Input resistor value is limited by PCB leakage, think piezo. Useful output is limited by the + supply (output pair has one Vbe drop not two of a Darlington)

I am not a fan of the Hamptone design. People seem to like it and I have noticed people have racks with +- supplies. If I wanted clean I would use opamps.

I am surprised there are not more posts in this thread.

CJ two threads for more effect voltage range.
https://www.diystompboxes.com/smfforum/index.php?topic=103492.0
https://www.diystompboxes.com/smfforum/index.php?topic=93817.0

JohnRoberts · Dec 24, 2018

Gus said:
I am surprised there are not more posts in this thread.

For a discrete gain stage without gain? ;D

I've used several discrete JFETs over the years, but never to intentionally make distortion. (If I wanted more distortion I wouldn't have selected for matched Vgs in one of my old phono preamps).

JR

Gus · Jan 12, 2019

hint
Think
500
ha___
I thought there would be more posts

Gus · Feb 2, 2019

Nothing?

I post this nice gem of a circuit fragment and nothing?

people have 500 series and people seem to like the Haptone jfet circuit

Here is a better gain stage IMO

fazer · Feb 2, 2019

So I have an O16 ounces for grid input Tube. Had about 20Db gain. Put R8 variable 25k and I have a 40 dB mic pre with some dirt in it. The transformer Is plus 8 headroom so I need to pad as well? Haven’t tried the Hampton this is pretty simple I’m prolly missing coupling requirements

Gus · Feb 4, 2019

Found a link for the Fender Harvard in the technical section.

It has an interesting solid state preamp. Note the drain resistor value and the RP of 12AX7

https://el34world.com/charts/Schematics/files/Fender/Fender_harvard_reverb.pdf

clintrubber · Feb 5, 2019

This is a nice & remarkable coincidence, I was looking around for the thread to the 'Fender Harvard topology' (the discrete JFET version, as Gus has addressed before as well here & elsewhere), and by magic, there's this fresh thread!

Nice, should bve a trigger to finally test these.

clintrubber · Feb 5, 2019

Gus said:
Found a link for the Fender Harvard in the technical section.

It has an interesting solid state preamp. Note the drain resistor value and the RP of 12AX7

I recall we had a chat about this before, possibly elsewhere, yep, makes one curious how these two circuits (one in JFET style, the other tubed) compare in sound.

Gus · Jan 3, 2020

repost of the schematic

Gus · Jan 3, 2020

another one

PRR · Jan 3, 2020

> I post this nice gem of a circuit fragment and nothing?

Many folks here would not know a "gem" if it fell in their lap, and can't handle a "fragment".

But some old piece of utility junk from the 1940s, that's kewl, no matter how badly documented.

Winston OBoogie · Jan 4, 2020

PRR said:
Many folks here would not know a "gem" if it fell in their lap, and can't handle a "fragment".

Haha

Funny, but no doubt true.
Well, I appreciate the repost Gus, I missed it the 1st time round.
Cheers.

JohnRoberts · Jan 4, 2020

PRR said:
> I post this nice gem of a circuit fragment and nothing?

Many folks here would not know a "gem" if it fell in their lap, and can't handle a "fragment".

But some old piece of utility junk from the 1940s, that's kewl, no matter how badly documented.

Hey I'm old junk from the 40s....

JR

another jfet gain stage to try in a DIY preamp

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