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Author Topic: Two-Stage JFET Preamp/Booster  (Read 14507 times)

rowdy_riemer

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Two-Stage JFET Preamp/Booster
« on: April 05, 2010, 12:49:23 AM »
Recently, I built a simple two-stage preamp circuit. Each stage is basically built using the basic criteria for the Fetzer Valve (see runoffgroove.com). If I were to use a more general biasing scheme (in other words, not using the Fetzer Value calculations for Rd and Rs), I imagine I could get all the gain I need with one stage. I chose the Fetzer scheme based on its supposed tube like qualities. There have been recent discussions on this site about possible better ways to emulate a valve/tube with a JFET, but I wanted to keep the circuit simple. I also have not yet compared this design to one using a general JFET biasing scheme. Because it is very simple and makes a good clean preamp or booster, I figured some on this site might find this design useful. The design is certainly not god's gift to guitar-related electronics and isn't all that original, but might be simple enough for a beginner to build while still providing good results.

If anyone uses this design and sticks with an 18 V supply, you may want to scale it down to 9 V if anything you put after this circuit cannot handle an output voltage approching 18 Vpp (The output of this will probably be a volt or two less).

For those who care, more info is available at http://www.riemer.us/two-stage-jfet-preamp-buffer/two-stage-jfet-preamp-buffer

Edit:  Oops, I should have used the word "booster" instead of "Buffer". I've got this fixed in the post title
« Last Edit: April 07, 2010, 03:44:45 PM by rowdy_riemer »

J M Fahey

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Re: Two-Stage JFET Preamp/Buffer
« Reply #1 on: April 05, 2010, 10:44:20 AM »
Nice classic design.
It should work very well.
Please place the missing values; although we know they depend on the particular Fet used, they give us a staring point.
Congratulations.

rowdy_riemer

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Re: Two-Stage JFET Preamp/Buffer
« Reply #2 on: April 05, 2010, 11:13:28 AM »
Thanks, JM

The first stage has an Rd of 6.9 k (actually 4.7 k + 2.2k in series) and an Rs of 517 (47 + 470 in series). The second stage has an Rd of 10k and an Rs of 680. I do not remember the measured Idss or Vth for either JFET. I'd rather not unsolder the JFETs to measure them to find out(Yep, I didn't use sockets). I'll try to remember to throw another one of these together in my protobox and measure and write down the Idss and Vth of the fets for a better example. Then maybe for comparison, I'll replace the Rd's and Rs's with ones using Findeton's biasing guidelines from this post: http://www.ssguitar.com/index.php?topic=1518.0

rowdy_riemer

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Re: Two-Stage JFET Preamp/Buffer
« Reply #3 on: April 06, 2010, 08:27:15 PM »
I put another one of these together in my protobox (cheap aluminum enclosure with a small solderless breadboard inside). This time, I measured the Vth and Idss of both JFETs. Both JFETS are 2n5457's. Q1 had a Vgs(off) (called Vp in the Fetzer valve article) of -1.59 V and an Idss of 3.4 mA. Based on these measurements,  the "Fetzer Valve Calculator" recommended a Rd of 3923 Ohms and an Rs of 388 Ohm. Using resistors I had on hand, I used a 3.3 kOhm resistor and a 680 Ohm resistor in series for Rd (3980 Ohm total) and a 390 Ohm resistor for Rs. Q2 had a Vgs(off) of -1.16 V and an Idss of 2.1 mA.

The recommendations for Q2 was 6720 Ohm for Rd and 458 Ohm for Rs. I used a 5.6 kOhm resistor and a 1 kOhm resistor in series for Rd (6.6 kOhm total) and a 470 ohm resistor for Rs.

The resulting preamp sounds very similar to the one mentioned above. I could have used trim pots for exact values for Rs and Rd, but I doubt it would be worth the trouble. For one thing, I do not think the measurements from my cheap multimeter are accurate enough to warrant precise values for Rd and Rs. Also, the calculations for Rd and Rs are based on a supply voltage of 18V. I'm using two 9V batteries for my supply voltage, so the voltage is rarely going to be right at 18 V.

For those who do not know how to measure Vgs(off) and Idss and would like to, checkout the Fetzer Valve article at runoffgroove.com. The test setup in that article works well. If I get around to it, I'll replace the Rd's and Rs's in my protobox with ones based on Findleton's biasing formula. If I hear a noticeable difference, I might record comparisons of the two.
 

phatt

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Re: Two-Stage JFET Preamp/Buffer
« Reply #4 on: April 07, 2010, 12:33:25 AM »
If all that fancy stuff about Fets really grabs you the have a looksee here;
    http://www.till.com/blog/archives/2004/12/interesting_lin.html

IMO, No matter how slick you make it look/sound most of this stuff tends to miss the point.

Distortion of active commponents is only one small part of a very big picture.

My attention would be aimed at tone shaping of each each and every stage from input to speaker.
exact replication of a troide will sound rather boring with a flat response.
Another thing worth noting;

Impedances dramatically alter when driving Valve amps into OD/comp/dist.

I'm not clever enough to give exact exacts but I can give a classic example of this effect.

If you ever find yourself in front of a really old model Marshall (not the crap sold now)
Then play at *Low volume (2/3)* and tweak the tone controls around. You know, turn knob play turn knob play, try every possibility.
*Pay very close attention to How much the tone control effects the sound.*

NOW crank the Volume to 7/8/9.
Go back and start turning the tone knobs again. 
If you've never done this,,You will be quite dumbfounded at how little effect the controls now have. (they are almost not working anymore)

THIS is part of the mojo magic of those old valve amps.
Better qualified folk might like to explain the details but from my understanding it's the loading effect of *High Impedance, High volatage* circuits.
From what I've read the *Internal impedances* dramatically fall when the whole circuit is under load. This changes the the *Tone shape* Hence also alters the colour of the distortion produced.
This phonomina happens to all the old valve amps, just very noticable in Marshalls.

Back to SS Emulation; IME You can achive quite stunning results just by a few simple tone tricks using basic SS circuits.
BTW, if it interests you my circuits produce the above mentioned quirk much like a Marshall rig.

Well lets just say I'm *more* than happy. :)
Phil.


rowdy_riemer

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Re: Two-Stage JFET Preamp/Buffer
« Reply #5 on: April 07, 2010, 01:45:00 AM »
If I remember right (and I might be full of crap), the impedance dropping you are talking about is when the plate becomes more positive than the cathode, causing a cathode-to-plate current and a major drop in input impedance. This obviously cannot be replicated with a fetzer valve stage. Probably not with a regular depletion-mode MOSFET stage either. Though they can operate in enhancement mode, I imagine their input impedance remains high. However, maybe with a diode between the gate and source, you could get the input impedance drop. But a triode does not have to be pushed to that point to be overdriven.

Whatever the case, the purpose of this circuit is to provide clean or mostly clean gain either as a booster pedal or a clean preamp. That's why there is no tone shaping. Of course one could place a separate tone control before or after the circuit. If someone is using hot pickups or a tone control with some gain before the circuit and happens to overdrive it, I want it to overdrive gracefully. It might be useful for "fattening" up the tone a bit, too. But distortion is not the main goal. Also, I want the circuit to be damn simple, and this circuit is.  When I use it for a preamp with my Crate GFX-212 (bypassing the regular preamp via the effects loop) it sounds great.

J M Fahey

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Re: Two-Stage JFET Preamp/Buffer
« Reply #6 on: April 07, 2010, 08:54:40 AM »
Hi Rowdy, I liked your contraption very much.
That's what I recommend everybody: make some kind of "usable" chassis with jacks, pots and switches bolted somewhere , not just hanging from loose wires, and take it to the battlefield (ahem! , practice room or even Pub stage), to be "combat tested".
Modify at will and commit it to PCB only when happy with it.
Lab is great but live sound is the real test.
I must confess being guilty of bringing terrible kludges on stage, although rather than plug-in protoboards I use the old nails-on-wood breadboards.
Your real world measurements show the only (big) problem FETs have; the terrible unpredictable spread in parameters they have, which makes them unusable in production amps.
There *are* tight controller (almost sadistic) biasing schemes that force them into a certain bias point automatically no matter what Fet you plug there, but they rob them all the "valve/tube-ness" they may have.
« Last Edit: April 07, 2010, 09:01:25 AM by J M Fahey »

rowdy_riemer

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Re: Two-Stage JFET Preamp/Buffer
« Reply #7 on: April 07, 2010, 09:21:11 AM »
Quote
Hi Rowdy, I liked your contraption very much.
That's what I recommend everybody: make some kind of "usable" chassis with jacks, pots and switches bolted somewhere , not just hanging from loose wires, and take it to the battlefield (ahem! , practice room or even Pub stage), to be "combat tested".
Modify at will and commit it to PCB only when happy with it.
Lab is great but live sound is the real test.
I must confess being guilty of bringing terrible kludges on stage, although rather than plug-in protoboards I use the old nails-on-wood breadboards.
Your real world measurements show the only (big) problem FETs have; the terrible unpredictable spread in parameters they have, which makes them unusable in production amps.
There *are* tight controller (almost sadistic) biasing schemes that force them into a certain bias point automatically no matter what Fet you plug there, but they rob them all the "valve/tube-ness" they may have.
C

Thx, JM. Using my protobox, I can also put the top on the enclosure to minimize electromagnetic noise from power lines, etc. which even enhances lab testing.  Except in this case, I scavenged my power jack for the more permanent build, so I had to plug the battery cable wires into the bread board. The noise still wasn't bad. I haven't had any live performance tests yet, unfortunately. I haven't been in a live performance since I played bass in a band in my high school days. Between work, school(lots of calculus homework lately), and family, I haven't found the time to get myself into a band. If I ever do, I'm sure I'll be bringing strange looking contraptions on stage with me.

After taking some classes related to automated manufacturing, I've wondered how hard it would be to get a pick-n-place robot to pick up a JFET, place it on a measurement rig, and automatically select the proper resistors for biasing, and reject JFETs that are too far out of specifications. If I ever have the time for such a project, I might try it. I might even have access to some pick-n-place robots at school. The professor in the Technology dept. is pretty cool, and I might can talk him into letting me play with one of theirs.


phatt

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Re: Two-Stage JFET Preamp/Booster
« Reply #8 on: April 08, 2010, 09:19:58 AM »
Hello rowdy,
               I spose It all depends greatly on your definition of *Clean*?

SRV, Mark Knopfler, Hank Marvin. To a heavy metal freak those 3 sounds would be considered clean,,, But if you care to notice there is still a lot of rattle, Certainly still far removed from SS HiFi clean.

Yes it's easy enough to get big fat distortion from most modern circuits but those in between sweet sounds are very tricky to nail. Frankly ultra clean guitar sounds quite dead.
Sorry if I'm getting off track.
Re the loading;
                     The Plate is always more positive than the Cathode.
I believe the issue has more to do with the control grid and plate.
The internal impedance of the Valve itself,, (Imp is just AC resistance and that changes).
I did read up on this long ago but sadly the wonderful books had to be returned, sadly my brain needed to learn more basic stuff at that time so was not able to fully absorb the complexity involved. At least I sucked up enough to realise that something weird happens under load.
Phil.

rowdy_riemer

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Re: Two-Stage JFET Preamp/Booster
« Reply #9 on: April 08, 2010, 09:45:59 AM »
Sorry, I actually meant grid. Actually, when I wrote that response, I almost called it a gate. Sometimes the words I write don't quite match the ideas in my head, like when I used the word buffer instead of booster in the original title for this post. I'd insert some sort of smiley face here if the toolbar icons were working. It's easy to see how grid-cathode current can make input impedance drop. That would certainly load down any tone controls in front of the tube.

rowdy_riemer

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Re: Two-Stage JFET Preamp/Booster
« Reply #10 on: March 15, 2011, 10:16:18 PM »
I added a tone control to this circuit for a treble booster. Here's the new schematic:



The tone control looks a little odd, but seems to work well. Here are bode plots (from simulation) with full bass cut and no bass cut:



With peak cut, this circuit has about 23.4 dB of gain at about 4.8 kHz and -4.5 dB of gain at about 32 Hz (according to simulation). When I put the real thing in front of my Valveking Royal 8, the bass cut makes a big difference in "de-muddifying" the sound. And with a mostly clean output voltage at around 16 Vpp, the Valveking has distortion suitable for Metal.

This is my first time really trying to roll my own tone control. Hopefully, my lack of experience isn't showing.  :) Either way, I like the results so far.

 

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