FET Guitar Preamp

[Paul Marossy]

Anyone ever build that FET guitar preamp that's floating around on the net?
This one: http://www.till.com/articles/GuitarPreamp/

If so, how does it work? I'm thinking about putting it in one of guitars, but I'd like to get some input on it first, if possible.

Thanks.  

[petemoore]

I think that is one of the things I used to know, I have the schematic around here somewhere. I don't think I actually built that one though...
 I see no reason that one won't work, I didn't read again what it said about adjunting bias...ya know I like to set Q2 collector bias on fet...
 IIRC the author went to great lengths to get it down to exactly that and dexcribes why he likes it convincingly...I personally like Fet Transistor Boosters, the ones I've tried fire right up and do the Fet thing I like...J201 was louder than MPF102 which sounded great and boosted a plenty...no worries

[ErikMiller]

I built one a couple of years ago to go direct into my mixer. I like mine a lot. I think I used the NTE part instead of a real J201, which Tillman does not recommend, but it sounded fine.

Nice preamp; easy build.

I thought EVERYONE had built this. :-)

[brett]

I've used this and experimented with it a fair bit.  It sounds excellent, so long as you do one thing;
replace the 6.8k resistor with a 25k trimpot.

This is because there's substantial differences in the biasing needs of different JFETs.  With the trimpot it is quite obvious where the "sweet" spot is.  A few k either side of the sweet spot gives low gain and poor tone.

If you know the Vgs for 0.1mA in your FET (ie a 47k resistor in RG's FET tester), you can use a 4.7k resistor for Vgs around -3V, 6.8k for around -2V, 10k for -1.5V, 15k for -1V and 22k for -1V.  

Alternatively, use the following formula; R=33exp(0.84Vgs) where R is the appropriate resistance (kohms) and Vgs is the gate voltage for 0.1mA (a negative voltage).

Confused? Don't worry - just use the trimpot and get it spot on  :wink:

[Paul Marossy]

I must be one of the few FET guitar preamp virgins...  :lol:

I was already thinking about putting a trimpot on it, it's the obvious solution to getting it to bias properly. I will put a J201 in it since I have a few lying around.

From what I'm hearing, it sounds like it would be worthwhile to build. It's such a simple circuit, too.

[petemoore]

If you're out of those trimpots, and boneyheadded a little like me...you can do it the easy/hard way...sockeet where you would put the resistor, then try different reisistors in the socket !!! Use the DMM to see which way the voltage reading at Q2's collector goes, and get it bout where ya want it.
 The other 'other' way is...using the socket you put where the 6k8 resistor was to have gone, plug your pot in [use wires and a socket on the pot to make this easy]...while measuring with DMM the voltag between Q2's collector and ground...adjust the pot till you get 4.5v [whatever target voltage] at collector...then carefully do not adjust the pot, remove and measure the resistance of the pot...thats the R value you want in that position...[I socket tranny's and caps...this way I can get it setup without dropping the transistor four times and having the cap fall out].
 Then ya just build that exact value from resistors, or round to the neares available fixed value...and stick it in the socket...works great.
 After all that just put the pot in there if you got it...lol...
 I haven't blown Fet's that I know of...but if you we're to want to try different types this socket or the pot makes it easier.

[brett]

I forgot to mention...
because it's such a simple circuit, and hardly worth putting in a box of its own, I simply put one in each of the boxes that I use first in my chain (e.g. orange squeezer, dyna, wah).  It's especially good in the wah (which otherwise suffers from low input impedance).  

You could even do a simple surface-mount version like RG suggests for the Millenium2.

[javacody]

I built a version of it in my little gem project so I could add a tone stack. It sounds very nice. I used an MPF201, and with a trim pot (I used a 10k trim pot)  instead of the 6.8 resistor, it has plenty of gain.

[Ansil]

there is the one you put in a guiatr  cable.. i think i will do that before i go active again. but thats me personaly

[swangor]

The Preamp Paul talks about at: http://www.till.com/articles/GuitarPreamp/ seems pretty cool and very straight forward.

I also like the idea of changing the gate resistor instead of hand picking fet's as the site suggests.

One quick question:  I understand the idea of getting the FET buffer as close to the pickups as possible to avoid cable cap. and noise, but is ther really a dramatic difference if clarity, ect between an inside mounted pre am an one mounted with a small high quality jumper cable to the bufffer clipped onto the guitar strap.  I can understand there being a quality drop if the cord was long.  I don't mean to open a can of worms or lengthy discussion, but if that little difference between inside and outside made a difference than I think I like his preamp cable idea as a good compromise.

Thanks,
Rob S.

[aron]

I know Larry Carlton has a FET preamp in his Sho Bud volume pedal. I bet it's a simple thing like this one.

Of course wired by Dumble himself.

[Paul Marossy]

Ok, I guess I have to build a few of them. Seems like they could have all sorts of uses...  

[petemoore]

I run a Mosfet Booster er some Impedance altering efkt] just about the whole time anyway.
 Putting the booster right in the guitar makes sense.
 Smafkt I would prefer [probably] having the output vol of the boost as the first knob I can grab on the guitar as opposed to the pickups volume control attenuating before the booster.  Of course this would defeat the Rolloff of the FF efkt now that Im thinkin of these things.
 K: first guitar knob...guitar pickup attenuation [with small bypass cap for more highs as you roll off]
     Second guitar knob: booster output vol
     Third guitar knob [a third?]...bass roloff for the boosters output, if there's room left.
 Booster T.Bypass switch? nahh

[Paul Marossy]

I designed a PCB for this little circuit. I'll etch it in the next few days, and test it out. If it works ok, I'll put up a new post with a link to it. Maybe someone can make use of it.  

[Mark Hammer]

I've had a 2N5089-based one in my Epiphone since around 1983 or so, and keep meaning to switch to a FET-based one.  I had previously used a bifet op-amp and actually preferred the high-end on that one more.  Shows you how lazy I am I suppose.

The one piece of advice I'd offer is to try not to get the gain over 3 or 4 in the pre-amp itself.  A hotter output WILL overdrive amps nicely, make envelope filters do their thing efficiently, make most distortion pedals sound better (well, except for maybe Tube Screamers), and offers better all round S/N ratios but wreaks havoc with a bunch of other devices like chorusses, flangers and phasers (most of which will lack companding or anything to constrain excessive signal) if the gain is up too high.  Limiting the boost to just a bit will get you a nice balance.  Besides, the main thing you want from it is a buffered line anyways.

[Paul Marossy]

Good point, Mark. It seems that whole reason Don Tillman came up with that circuit is to compensate for treble loss in long guitar cables. Of course, now with "oxygen free copper" cables on the market, this FET preamp thing really isn't necessary.  :roll:

I'm building it anyway...

[javacody]

Well, buffered line, and nice warm tubey sounds from the JFET.  

[Jered]

If not being used simply as a buffer, but using it for slight OD or that warm tubey sound as mentioned previously, do this. With a trimmer for your drain Res. get close to half of the supply voltage going to drain. Then tune by ear, that is what your striving for, what sounds best to you. Fets vary greatly, you'd be surprised. Generaly with a nine volt supply your going to be in the 3.3vdc to 6.7 vdc ballpark. Measure the resistance on your trimmer and replace with a fixed resistor if possible. Trimmers are sometimes noisy in OD/boost circuits.
 OT note:     I've recently noticed that friends I've built pedals for that live near the ocean seem to be more succeptable to corrosion on the trimmers mentioned above. Anybody else notice this? Aron? Island life have any noticeable effect your aware of?
  Jered

[Paul Marossy]

Well, I finished the first one of these that I'm going to put in my guitars. I used perfboard for this one, I'll try out my PCB layout for the second one that I'm going to build.


http://home.att.net/~u2p0j0m4/EX370FM-Back.jpg">
http://home.att.net/~u2p0j0m4/FET-Brd-Angle.jpg">

This is mounted in one of my Ibanez guitars, in the control cavity. Sorry for the grainy pictures...  :oops:

[George Moore]

  Jared, we played the Beach Club on a hot humid summer day, the equipment surfaces melted and peeled away...strings sweat to break in 20 minutes, chrome peel from pole pieces within a few hours, the next day I peeled down to the heavy surface rust on the PA amp with a credit card, harvesting most of the case paint. Most of the metal surfaces were compromised also/including the bias pot in my Plexi.