Don Tillman's discrete FET guitar preamp

[wnorcott]

I have built Don Tillman's superb and easy to build FET guitar preamp. I would like to share a PCB design and layout I created for it.

Don Tillman's FET preamp is designed to always be in the signal path, and not a "sound effect" pedal per se. It is a beautiful sounding preamp one of the best I have heard. The only thing that makes it stomp box like is that it is portable: you plug your guitar into it first, and then into a regular guitar amp.

The Tillman FET preamp uses a single J201 field effect transistor. The J201 JFET is a common part that should be familiar to many of you. What is different here is Tillman is not trying to drive it to saturation, he is keeping it clean and linear. The circuit has a very high input impedance of 3 Megohm which means it does not suck the tone out of your guitar and in that respect it also acts as a buffer. The tone quality reminds me of the very best of high end tube amplifiers, flat response and no noticeable tone coloration but still has that je ne sais quois that we know it when we hear it. It is just amazing that such a simple, spare design sounds so good. It only has seven parts and one of them is optional.

First the schematic this is from Don's site.



These can be built real small but my days of building a ship in a bottle are over so I scaled it so it is easy to build not clumsy. It will fit in the smallest project box that can hold 2 guitar jacks and a 9V battery.

Here is my parts layout design for it. As seen from the component side.




Here is my PCB mask to etch your own printed circuit boards, the image may display different sizes in your browser. When you print the finished size should be about 1.2" tall and 1.5" wide. The easiest way to print might be open the image in Paint and print on plain paper first, scaling it until the scale is correct. The size of the PCB is not so critical.



The preamp is intended to be always in line and always on. There are no controls it is a fixed 3dB gain amp, and operates on a single 9 volt battery. I wired mine with a simple SPST toggle switch to power it on. IF you forget to turn it off I would not sweat it: A 9V battery will last about 300 hours.

I built it with 1% tolerance metal film resistors. There are very few components in this build so use good quality. There is only 1 capacitor in the signal path, C1 is a DC blocking capacitor on the output. So it is not cricitcal, but I used good quality electrolytic. Use shielded cable on the input.

No project page would be complete without a demo so here it is. I have a YouTube demo of it very clean guitar sound:



Stereo link for the above since it is a guitar duet one in left speaker the other in the right:
http://www.youtube.com/watch?v=RDeJXaxguqU&fmt=18

It would be interesting to use the Tillman preamp as building block for a DIY amplifier head, I would say right after the input jacks and before the tone controls. You would probably need and additional clean gain stage to drive a power amp since the Tillman is fixed 3dB gain. Just a thought.

And last but not least, here is the link to the original project page by the master himself, Don Tillman:

http://www.till.com/articles/GuitarPreamp/index.html

I HOPE YOU LIKE IT!


Bill

[pgambon]

I've tried this curcuit and it sounds great! 3db is a good boost. Really lets your amp tone come through.

[udimet720]

I amde one of these for an acoustic that had no built in pickup/preamp. Very easy to build. I've wanted to do the preamp cable version, but I don't really have any need for it.

It worked great with the RadioShack piezo I got for a pickup.

Also, you can include a volume control by making R4 a 50K pot. This is handy in some situations....

[mmaatt]

Hi,

I'm interested in making this and found this http://www.muzique.com/schem/multi.htm looks quite good, has any one tried these PCB's.

Finally how do you identify the pins on the J201 and which one goes where ie Ground, Source & Drain.

Many thanks

Matt

[wnorcott]

Originally Posted by mmaatt

Hi,

I'm interested in making this and found this http://www.muzique.com/schem/multi.htm looks quite good, has any one tried these PCB's.

Finally how do you identify the pins on the J201 and which one goes where ie Ground, Source & Drain.

Many thanks

Matt

Anything made by Jack Orman is good: Jack is the Godfather of pedal design. Orman is the most experienced guy out there by a factor of ten as far as I am concerned.

To identify the pin out on J201 you can refer to the chart that I added earlier for that purpose:

Transistor Pin-outs Chart Transistor Pin-outs Chart

Bill

[mmaatt]

I'm waitning for my board to arrive from Muzique and was thinking of putting a switch then an LED (check light) after the 9v+ will this affect anything?

Many thanks

Matt

[wnorcott]

Well as you can see I put a toggle switch on mine which is fine. A LED will draw a lot of current. I am not a big fan of LED's. On the Tillman an LED will use up more power than the Tillman preamp does: without the LED a fresh battery will last up to 300 hours.

You should not put a LED in series with the +9V the best way to do it is you have to connect the LED beween the power rails i.e. between +9V and ground, after soldering a suitable resistor to one leg of the LED. Another thing to keep in mind is the current draw of an LED can cause a popping sound when you switch it on

Bill

[mmaatt]

Hi,

I tried to make one these using a Jack Orman mulit purpose PCB and followed the componts/values as below.

PCB layout here: http://www.muzique.com/news/tillman-fet-preamp/

R4 - 3.0M
R5 - 6.8k
R6 - 2.2k
R7 - 51k
R9 - Jumper
C1 - Jumper
C2 - 4.7uF
C4 - 10uF
Q1 - Jfet transistor (2N5457, J201, MPF-102 or similar)

It just buzzes, I wire a switch between battery and P+ rail, but it doesn't seem to make any difference to signal. what should i check first.

Many thanks

Matt

[wnorcott]

Did you check the pinout on the transistor and make sure it matches the proper orientation on the circuit board?

[mmaatt]

Got it working, the problem was with one of the jack sockets

I had to change R5 from 6.8k to 2.2K to get the J201 to bias a 5.7v, with 6.8k it was just over 4v measured between drain and gate.

One thing I have noticed, which I'm not sure is right is, if I play through the preamp, the volume is lower than if I play straight into my amp?? I thought this would be the other way round?

Apart from that the tone is great.

Many thanks

Matt

[wnorcott]

No it should be louder but only 3 dB louder, when you play through the Tillman. But the volume does not matter much, as your point out the Tillman is about its beautiful tone.

[mmaatt]

Thanks wnorcott,

I've realised what I've done, by trying to get the bias right I've reduced the output inpedance, the drain resistor should be 6.8k for this reason.

I'll reduce the source resistor which will increase the drain voltage (I think!!).

Thanks again

Matt

[Jack Knife]

Would this circuit work to warm up the sound of a SS amp? I've got a nifty little Yale Reverb that's slightly sterile sounding played clean.

mmaatt, did you get the JFET that was recommended in the article or did you use another brand?

[mmaatt]

I used a J201 - N-Channel JFETs - Vishay Siliconix.

I've read that the tolerances of these things are quite big, I ordered three an they were all similar, I had to use 4.4k resistor on the source (R2) and 5.6k on the drain (R3) to get just over 5v between the drain and ground. I dont know if this due to the varying tolerances of J201's.

R3 dictates the output inpedance so bias R2, I had R3 at 2.2k and this lowered the output so much that guitar straight amp was louder.
Hope this helps


Cheers
Matt

[11 Gauge]

Originally Posted by mmaatt

I used a J201 - N-Channel JFETs - Vishay Siliconix.

I've read that the tolerances of these things are quite big, I ordered three an they were all similar, I had to use 4.4k resistor on the source (R2) and 5.6k on the drain (R3) to get just over 5v between the drain and ground. I dont know if this due to the varying tolerances of J201's.

R3 dictates the output inpedance so bias R2, I had R3 at 2.2k and this lowered the output so much that guitar straight amp was louder.
Hope this helps


Cheers
Matt

JFET's are notorious for having gain levels that are all over the map. This makes trimpots worth their weight in gold. As a plus, you might even be able to get right up to around unity with a 2N5458 or similar unit.

[Jack Knife]

How about warming up a SS amp? Does it work good for that?

[wnorcott]

It makes nearly everything you play sound better on nearly every amp. I recorded it the demo (above) on a 1973 Peavey theClassic

It makes the tone clearer but if you pick sloppy it makes the mistakes clearer too so it is a double-edged sword.

Bill

[zorgzorg2]

Hi guys,

I looked at this preamp for a while now, but what I would like is that it would accept phantom power (48v)... I know the preamp cable of the same author does this, but what about this preamp ? Is it possible to adapt the schematics to get 48v instead of 9v ?
Sorry if this is a stupid question, but my electronic level is not great...

M.

[wnorcott]

You can drop 49V down to 9V with a simple voltage divider I would use 1% metal film to get as accurate as possible since this is a divider.

Vin = 48V
R1 = 1K
R2 = 240
Vout = 9.29V



http://www.raltron.com/cust/tools/voltage_divider.asp

That is pretty close to what a 9 volt battery actually puts out.

If it were me I would us a 9V battery instead of expose my expensive phantom power supply to the smoke test but the numbers work out.

Bill

[zorgzorg2]

Thanks for the answer !

To clarify a little, my thoughts were to have phantom power as an extra option, so that the battery doesn’t get empty when recording on a mixing console...

If I end up with something else than smoke, I’ll keep you updated ;-)

M.

[mmaatt]

I just bought 10x J201's, as the first 3 I had took some messing about about with values of R2 & R3 to bias properly.

I've just checked all 10 J201's with R2 @ 2k2 and R3 @ 6k8 the drain voltage was between 4.1v and 4.3v. Changing R2 to 3k9 got my highest J201 (4.3v) to 5.19v, which I think I'm going to live with. Reducing R3 also affects the output, so you lose the 3db gain (not this this is important).

I was wondering what, if any effect these two have on the tone of the thing (my test rig was on protoblock, without in/out jacks).

Cheers

Matt

[wnorcott]

Changing R2 should not harm the tone since the purpose of R2 is just to bias the J201. Before we can worry about tone the transistor has to conduct. But it will increase the sensitivity of the amplifier and hence the volume: even without having to touch R3, when properly biased the FET starts amplifying at a lower input signal voltage, making it more sensitive to your picking and sound more 'responsive'. I wish I knew why Jack renumbered all the components on his redrawn schematic and starting with R4 etc I did a double take when I saw that.

If you are getting the preamp to amplify then you are fine. Changing R3 that amount should not harm the tone. It is still medium impedance output.

I would suggest at this point focus on your ear so time to hook up the guitar jacks and play through it. The ear is the best judge after all when you are done it has to sound good. When I design mods I start with theory but past a certain point it is the guitar in one hand and the soldering iron in the other.

Bill

[zorgzorg2]

It’s me again. I was almost buying the components today but then I startet doubting : would the preamp be sufficient to plug my tele into a line input of a mixing console ? I know the impedance is corrected, but what about the gain ? Is 3dB enough, or will it still sound faint and noisy ?

[11 Gauge]

Originally Posted by zorgzorg2

Is 3dB enough, or will it still sound faint and noisy ?

If you've used mass produced pedals (i.e. Boss, Ibanez, etc.) then you've experienced unity bypass buffering. Actually, it's a bit of a fallacy, because it's actually close to unity. Chain enough buffered bypass pedals together and you'll get signal attenuation.

Unless there's some sort of radical EQ alteration, 3dB should be plenty.

Lots of pedal geeks like to tack a jFET onto the beginning and/or end of their circuits to finesse the buffering and input and output impedances.

[wnorcott]

By the way the transistor pins in this layout are D-S-G reading from top to bottom in the layout.

The pinout of the J201 is 1-D 2-S 3-G

So for a J201 you need to orient it with the flat side of the transistor facing the LEFT which is opposite direction it is in this picture. Sorry about that.

Originally Posted by wnorcott

Here is my parts layout design for it. As seen from the component side.

[scitizen17]

I have developed a PC board design that is 30mm x 17mm and includes a 10K trimmer in place of R2. This should help with matching specific J201 devices to the circuit. It includes the required 9v-GND, SIG IN-GND, & SIG OUT-GND solder points. It also includes a location for C3 (I'm using a 4.7uF) which is in parallel with R2 for additional gain if you require it. If anyone is interested I will post the board design.

[teletin]

Originally Posted by scitizen17

If anyone is interested I will post the board design.

Please do, I would appreciate it.

[vikki]

Don't know if anyones interested but here's a variation on the J201 preamp.....
http://www.guitar-repairs.co.uk/jfet_guitar_preamp.htm
Not built it though...there's an extra buffer transistor in there....some clips down the page too...
Vikki

[FenderLover]

These simple boosters are essential in my book - and too simple to screw up if you build it yourself. You could spend another $100 to have a Keeley or Fulltone name on it, but we all know better.
Keep in mind that these single stage designs invert the signal. That may come into play if you are recording or mixing your signal. If you use just the buffer version, you get nearly 1:1 gain and no inversion. Even with no gain, a buffer goes a long way to helping you signal.
If you use a slightly larger box, like a Hammond 1590DD, you can easily fit a buffer, a booster, and a looper in one box for for an essential Swiss army guitar setup.

[toolguy]

Sorry if this was asked before and I missed it.

Building this tonight and I had a question on the inputs/outputs.
Do the ground leads from the jacks get connected to the ground circuit of the preamp or connected to each other (lug to lug).
Searched google but can't find any good pics of a built unit.
Thanks
Jd

[RomanS]

All ground connections (plugs, circuit, power plug, etc.) have to be connected to each other - doesn't matter which point you use to connect them to, for practical reasons, the ground lug on one of the jacks is a good place for soldering all the ground wires to...

[jkstigma]

hello my friends!!!
i just build this box and its sounds great

is there any problem if i use this preamp with a 9v adapter and not with a battery?

[droscigno]

I was thinking that my iPhone might make a handy practice amp since I always have it with me anyway. I looked for an interface cable, but the one that is commercially available has many reviews that say it does not amplify the signal up to mic levels, so it does not work so well. Is the 3DB gain of the Don Tillman circuit the right amount to bring an electric guitar output up to the level expected at the iPhone mic input?

Edit: From further reading it is likely that the real issue with the commercial cable is an impedance mismatch, not the output level of the guitar pickups. From reading Don Tillman's design considerations, the input impedance of his circuit is 3 Mohm, and the output impedance is 6 Kohm. One replacement headset for the iPhone lists its mic as having 2.2 Kohm impedance. Does this sound close enough to people who know what they are doing :) (I don't!)

Thanks,
Dan

[limbe]

Originally Posted by jkstigma

hello my friends!!!
i just build this box and its sounds great

is there any problem if i use this preamp with a 9v adapter and not with a battery?

If the adapter works with stompboxes it will work just fine
Avoid adapters made for calculators or office machines.
Usually they aren´t filtered enough.The preamp will hum.
Limbe