FET-2B Bass Preamp, my first "design"

[Ripthorn]

I decided that I wanted a bass preamp so that I wasn't just running my bass directly into my soundcard, but don't have the means for tube stuff right now.  So I decided to adapt the Alembic F-2B preamp using JFETs.  The schematic is shown below and has power supply filtering, trimpots for drain biasing and I changed a couple values to what I had on hand.  I breadboarded this up and it gives a very nice range of tones from funk to rock and I am sure there is more (I didn't play with it for too long, I want to get it on some pcb).  There was some pretty major hum, but I think that has more to do with the breadboard than anything else.  If I find otherwise with pcb, I will report back.

It is nice becuase there is a minimal component count and the tonestack really shapes the sound well.  The bright switch makes a noticeable, though not huge, change, but I like it.  The value of the bright cap could be changed to suit your tastes.  I put together a layout, but it is ugly because it was my first go at that as well.  PM me if you want the eagle files, but other than that, let me know what you think and what I did wrong.  Just as a note, I have never played the Alembic pre, so I don't have any real comparison, but this sounds MUCH better than direct in.

[bassmasta17]

got sound?
im interested. This might be my next build...

[Ripthorn]

Unfortunately my sound card is out of commission (sending it back to get fixed, hopefully it won't be too long).  The best way I can describe the sound is that it adds a lot of clarity and can fatten up the bottom end really nicely.  It makes my piezo on my bass really sound great.  As soon as I get my soundcard working, I will post clips.

[Gus]

For your piezo bass try 2.2meg to 10meg for the first gate to ground resistor. 

[Ripthorn]

I think I'll have to try that, I may need to put in a switch for piezos versus mags.

[jaysg]

You might want to rescale the tone stack for lower noise.  Resistor values are divided by 10, caps multiplied by 10.

[Ripthorn]

I did notice a bit of noise, but I thought that might have to do with no shielding, but I will probably have a go at it (except that I don't think I have the correct pots at the moment, guess it's time for yet another order...).  Thanks for the suggestion.

[Güero]

Hi Ripthorn,

I made a layout for your FET Bass Preamp.
Maybe the layout would be smaller, but whatever.
I am a bass player too.
Please, if everything was fine and verified, I request your authorization for post this layout
in our brazilian forum named HandMades ( www.handmades.com.br ) a lots of info and layouts there. Registration needed.

Here is the link for your layout:

http://www.mandamais.com.br/download/?codigo=zpaa41200932542

Cheers!

[Ripthorn]

Hi Guero, I have no problem with you posting on the other site, but I was unable to open the layout to be able to verify it.  I will try again a little later to see if it works then.  Thanks for your interest.

[Eric H]

For your piezo bass try 2.2meg to 10meg for the first gate to ground resistor. 

Note that this is only useful if the piezo pickup is not already pre-amped --either on-board or with a floor -box.
Gus, I'm finding raw piezo pickups to be rare these days.
How about making that large-value resistor switchable?

-Eric

[Güero]

For your piezo bass try 2.2meg to 10meg for the first gate to ground resistor. 

Note that this is only useful if the piezo pickup is not already pre-amped --either on-board or with a floor -box.
Gus, I'm finding raw piezo pickups to be rare these days.
How about making that large-value resistor switchable?

-Eric

Eric,

This resistor is switchable in my layout and the tone stack was rescaled as you can see :

Layout:




Ready to transfer ( The PCB is in actual size in PDF ):






Schematic:

[bassmasta17]

Me not having the skills or equipment to make my own PCB's, does any one have a perfboard layout of this?

[Gus]

What is the output resistance of the first triode of the tube pre this is based off before the tone stack?

What is the output resistance of this circuit first FET?

Do you want to scale the tone stack?

Think about the output R vs the tone stack
Duncans tone cal has a correction for the source and input resistance

It would be better to leave the 10meg always in circuit and switch another resistor in parallel for a lower value because you always have a DC path to ground and can use a SPST

Eric  I was thinking raw piezo as you noted.   Good point about piezo guitars and basses having on board amps.

[Ripthorn]

I have an onboard piezo buffer, so the circuit without the 2.2M/10M resistor sounds really good as is.  I still need to see what scaling the tonestack does, but haven't placed an order for the parts yet.  I also haven't calculated any of the input/output impedances yet, school just started back up and I will have to see if I can find time. 

@bassmasta, I actually did this on perfboard instead of pcb (I had a perfectly sized little perf piece and no patience).  I could draw one up maybe later today and post it.

[Ben N]

Nice job. I imagine that besides the bass players, the Gilmour-heads will want to have a go at this as well. It could also be the basis for an ROG-style analog Blackface sim, by adding PI/power amp and cab sim sections.

Just out of curiosity: Since it sounds like the eq is the biggest part of what you are getting out of this circuit, had you considered using an ROG Tonemender instead? Or the similar (but more complex) Albert Kreuzer fet-based bass preamp? Another interesting approach using mu-amp stages is here.

[Ripthorn]

I have considered both of them, but I liked the simplicity and also it was nice to use the j201's I had on hand instead of getting a chip to make the tonemender.  Basically I built it because I had everything on hand (I had pulled the jacks and pots from an old practice amp just the day before and they were the perfect values).  I am not an actual bassist, but I wanted to have some toys for my bass for when I record, but I think I might try a bass cab sim and maybe make a bass amp sim by combining this circuit with the cab sim.  Not sure yet, I will have to look into it.  I think more than anything, this was an experiment to see if I could do it, turned out to be simpler than I thought.

[CraigR]

Guero

Unless I am reading it wrong, it looks like you have the treble cap in your revised schem as 25n, that is to say scaled up by a factor of 100, versus 10.  ie. 250pF treble cap should become 2n5.  An easy mistake to make.

Craig

[Ben N]

Actually, I think a cab sim makes less sense for bass than for guitar--when I mentioned that I was alluding to the fact that the F2B is really a guitar preamp, being borrowed from the Fender SF/BF guitar amps. If one wanted to get the "full" Fender BF experience (or as much as one could hope for by subbing FETs for tubes), once could try to sim a power amp, and then perhaps a 2x12 cab sim. But for bass, I'd bet that what you did is better. Kreuzer's design is really the same thing, just gussied up some.

[Güero]

Guero

Unless I am reading it wrong, it looks like you have the treble cap in your revised schem as 25n, that is to say scaled up by a factor of 100, versus 10.  ie. 250pF treble cap should become 2n5.  An easy mistake to make.

Craig

Oh, you're right....
Sorry for that, my mistake!

Thanks

[bassmasta17]

@bassmasta, I actually did this on perfboard instead of pcb (I had a perfectly sized little perf piece and no patience).  I could draw one up maybe later today and post it.

Thanks that would be sweet.