Fairfield Barbershop - HIGH Gain.

[SeaWitch]

Hey all.

So, I've been building my way through bass overdrives, and so far, the Fairfield Circuitry Barbershop is winning.

I swapped the 2n2222a for a MPSA18, and the growl is closer to what I'm looking for.  So, I said to myself...how about an even higher gain MPSA13?  I put the MPSA13 in there, which is maybe double (or more) the HFE of the MPSA18, and there was really no discernible difference.

Are there more value changes I need to make when using ultra high gain transistors?  What component is limiting the high gain POWER of the MPSA13?

If you have any suggestions or insight, let me know!  Thanks!

[induction]

The 2n2222 in this circuit is just an output buffer. It's never going to go above unity no matter what caps or resistors you use or what transistor you replace it with unless you change the topology.

If you want more distortion, consider reducing the 33k between the two jfets. If you're using IvIark's layout, it's the one in the middle of the board, connected to Drive 3. That's what Guillaume suggests.

He also recommends tweaking the 8k2 and 9k1 until you get 6V on the drains of the jfets.

[SeaWitch]

Hey thanks for the info!

So, here is my plan:

Replace the 33k resistor with a B50K (distortion) pot.
Replace the B10K (sag) pot with a 10k (sag) trimmer.

Since the (sag) controls the amount of voltage to the J201s, I was thinking I'd just adjust the sag trimmer for 6V on the drain of the jfets and call it a day.  The sag pot really didn't do enough for me to want to keep it external...

Am I approaching this correctly?

Thanks again!

Here is the layout for quick reference:

[induction]

Seems reasonable. A couple of things to check:

1. It's possible the sag trimmer won't get you to 6V on both jfets at the same value. In that case, you might have to adjust one of the drain resistors. (If it was me, I'd adjust both drain resistors manually and leave the trimmer out. Trimmers are expensive, take up space, and can be noisy.)

2. 50k might be ok, or it might be unusable at either or both extremes, leaving you with a tiny bit in the middle that gives good tones. Linear taper might be ok, but you won't really know until you try it. For that matter, the whole approach might lead to sounds you like, or it might not. I'd test it before I built it. In any case, don't forget to wire the Distortion pot to decrease resistance as you increase the setting. (I.e. variable resistor between lugs 2 and 3).

Bottom line: Your approach seems reasonable, but you won't know for sure until you hear it. This is why breadboard everything before I build it. This is especially true for mods and redesigns. In the end, go with what sounds good. It's much easier to figure that out on a breadboard instead of installing and removing things on a vero.

Let us know how it goes.

[SeaWitch]

OK.  So I decided to follow the directions and properly bias the (2) JFETs to 6V.  It took a couple 22k resistors to get Q1 to 5.98V and Q2 to 6.10V.

Now the Sag pot works a lot better (big surprise, right?)

But, I felt like I wanted even more distortion, so I used a socket for the 33k position and put a range of resistors in there.  I tried 5k1, 15k, 33k and 68k.  I can't say that the distortion was increased or decreased with the various resistors, they just seemed to make the Drive pot work more erratic.  To me, the 15k seemed to offer the best "range", but really the 5k1 really didn't offer more distortion and the 68k really didn't offer less distortion...

Anyway, this is my update...any more suggestions/comments are appreciated!

[induction]

 When the drive pot is all the way up the gain seems pretty much maxed out on this design.

Short of redesigning the topology, here are three ideas:
1. Increase the source capacitors (say, 22u). I'm guessing this won't help, and if it does, the distortion will get farty, but I could be wrong.
2. Put a booster in front of it. Something full range like a MicroAmp would be my suggestion for bass, but you could try any booster.
3. Add clipping diodes just before the output buffer. (Edit: This will require a coupling cap and re-biasing of the output buffer. You could more easily put them between the jfet stages, after the coupling cap. I couldn't say which of these would sound better, or whether the output volume would be drastically affected.)