Onboard Tillman inspired JFET preamp

[axelrod]

Catalpa

 

[A9X]

This is a blast from the past. I built a few of these in ancient times, might try another for the few minutes it takes to build one. I know I have a lot of JFets in my store somewhere.

 

[40Hz]

I've built several of those J201 preamps and they sound better when the drain is half the supply voltage (4.5v) I substitute the 2K2 with a 5K trimpot

This!

I’ve built several Tillmans over the years. That was what I had in my first active bass (a JB) back in the days before the Earth was formed. I had read somewhere (Popular Electronics magazine?) about swapping a higher value trimpot for the fixed resistor. And the sound definitely benefitted from it.

 

[dwizum]

I use a 10k trimpot or just test and swap resistors when I want a really small solution. This design is so simple, cheap, and effective - and it sounds great!

 

[SteveCS]

I did one a few years back. Details here...
Onboard Buffer - Nice one Albert and Don

 

[bassically_eli]

I've built a few of the "Tillman" style preamps, into DIY pedals, cables, and into guitars. About a year ago I installed one in my Pbass. I've been playing it over a year and finally getting around to posting it here.

About the switch: I used a sub-mini toggle on the pickguard, mounted between the tone control and output jack, for active/passive switch, and it's powered by +9V via TRS jack, and TRS cable from my pedal board. Or I can use a standard (TS) cable, for passive only, in which case the switch acts as a mute or kill switch. The switch is silent, both electrically and mechanically. In the schematic below, you'll see the switch is only SPDT, merely selects the passive signal or the buffered signal. In other words the JFET gate is always connected, but keep in mind, that load impedance is on the order of 10M and very low capacitance too. Key point: not switching power to the JFET eliminates pops when changing the switch.

When active, the volume boost is moderate, I'd guess about 2 to 3 dB. One reason for this mod was to get a hotter signal for an envelope filter pedal that wasn't fully triggering. Since then I've ditched that pedal but I use this preamp, on and off, all the time. Note the volume control is before the JFET. At about 7 and above on the volume control, the boost transitions into mild compression. Being a FET, this is a nice sound, IMO, similar to a tube.

Note the drain and source resistor values need to be adjusted for the particular J201B. But once you get it dialed in, it's stable. I've been using a few of these style preamps in my rigs for several years and they're all still working great.

Here's the schematic. I had previously modified the tone control but that's obviously independent of the boost mod.

View attachment 3330643

Why use a TRS cable? Tillman's drawing shows phantom power on the tip. Is it because your power supply only provides phantom power on the ring channel?

 

[MikeTranch]

Agreed, with AC coupling that is possible, and i did it that way at first. But i have few active basses and guitars, and i power all of them with TRS cable instead of battery. With the exception of acoustic electrics with built in tuner/preamp, every active guitar I’ve come across uses TRS jack to complete the battery negative connection to ground, via standard TS plug shorting ring to sleeve. Therefore we can replace the battery with shorted battery snaps, drive +9V on the ring via TRS. The first pedal on my guitar and bass pedal boards supplies the +9V. I also mod’d my little Ampeg combo, replacing input jack with TRS so i can plug bass to amp directly. On the pedals, and amp, I have a mini toggle switch for the +9V source in case i want to use standard cable. Regarding acoustic electrics with tuners, they’re designed so the tuner can work even if no cable plugged in. So the 9V wiring is different and would require mods to the guitar to make this work, leaving battery operation unavailable. I did this on my Breedlove bass, works fine. Might seem like a lot of trouble nut i was electronics hobbyist before a musician, and in any case it’s all being working well for many years. My DIY buffers have evolved but all retain the feature the guitar still works in passive mode with a regular cable. The active / passive switch becomes a kill switch if no +9V available.

 

[Passinwind]

Why use a TRS cable? Tillman's drawing shows phantom power on the tip.

Link please?

 

[bassically_eli]

Link please?

Here's the link to the circuit I was thinking of, which is actually a different project (but very, very similar) than the one in the OP and linked to in post #2. I was confused, my mistake.

FET Preamp Cable

 

[Passinwind]

Here's the link to the circuit I was thinking of, which is actually a different project (but very, very similar) than the one in the OP and linked to in post #2. I was confused, my mistake.

FET Preamp Cable

Thanks, it's been several years since I last saw that. Whenever I see "phantom power" referred to I like to confirm or deny that it actually is.

 

[MikeTranch]

Here's the link to the circuit I was thinking of, which is actually a different project (but very, very similar) than the one in the OP and linked to in post #2. I was confused, my mistake.

FET Preamp Cable

yup, built one of those too. was a challenge to fit even such a small circuit inside the plug housing. As it turns out, I rarely use it, as I've added preamps to my passive guitars and basses. And update, I've switched to BS170 fets, they bias the same as jfets but are more robust. I did blow a couple JFETs over the years

 

[MikeTranch]

Thanks, it's been several years since I last saw that. Whenever I see "phantom power" referred to I like to confirm or deny that it actually is.

yeah "phantom power" is often associated with using existing conductors to carry DC. For that reason I coined the term "TRS+9" for my scheme (though I'm sure I'm not the first to do this!) I use a pTouch labeler on my pedals and amps that have this mod. And as mentioned, I use a mini toggle to turn the +9V on/off, and a LED to show its on (an not shorted by a regular TS plug!). I also have current limiting so no damage if shorted. Been doing this for years, no problems. Probably not worth the trouble to most folks, just to save on batteries. But when mod'ing a guitar to make active, NOT having to add a battery compartment is a pretty good reason. And as it turns out, powering an already active guitar this way is pretty straightforward.