Sadowsky bass preamp clone troubleshoot

[Rob Strand]

2n5457
still s#@t on 2nd fet, can't understand why.

It could be the 2N5457's are on the edge of the specs and the circuit can't handle it.

Alternatively,  the 2N5457's are dodgy and it's outside of your control.  A month or so ago there was another weird JFET case.  Whatever way we looked at it I didn't make sense. The JFET looked like it was behaving weirdly.

If you want to get what you have going:
- leave the 10k bias resistor shorted
- start to decrease the drain resistors until the drain voltage is about 5V (to 6V)
- re-measure the JFET voltages
- If the drain resistors required for correct biasing are too far off the original 10k we might need to tweak the source resistors.  Dropping the drain resistors might raise the source voltage too much - tweaking the drain resistors fixes it (but screws the bias again, a bit).

It can be done.

The other option is to buy some 2N5457s from a different vendor and see how things work out.

[Rob Strand]

I did a simulation based on a JFET which is out of spec.  I can created a situation where I see what you see.

1  >  2 : 37.5R
2  >  1 : 37.5R

These imply the "Yfs" JFET parameter is quite high.
But that alone isn't enough.
The drain voltage can't get much above 3V, so if we have a JFET with a high "VP" parameter, say above 3V
the 3V drain voltage isn't enough to turn the JFET off.  As a result the drain voltage gets stuck at 3V.

I will try a high "VP" JFET with a "Yfs" parameter that is large but still in spec and see what happens. ***


The other thing to note is your two JFETS aren't exactly the same.   You could swap their positions to prove that the problem follows the JFET.

[***  Here's what I got when VP is large and Yfs large but in spec according to the datasheet.
       (It is not normal to get both parameters high but anyway.)

       Bias resistor
       10k          Shorted
G   0.85V        0
S   2.9V          2.9V
D   3.1V          3.2V

As I mentioned the large VP (ie. > 3V) prevents the JFET from being able to be turned off.
That's the main issue.

I suspect your second JFET has a VP under 3V so when the 10k is shorted it is just enough to "save"
the situation.
]

[subnormalwater]

Well well well. By trying to increase the drain voltage I noticed that the first fet suddently died by showing 3V on the gate... what a great happening.

Now I run out of 5457s, if this one decides to die then bye bye pedal, you're going to live in my trashcan.

[Rob Strand]

Well well well. By trying to increase the drain voltage I noticed that the first fet suddently died by showing 3V on the gate... what a great happening.

What a pain.

Now I run out of 5457s, if this one decides to die then bye bye pedal, you're going to live in my trashcan.

LOL!

Honestly you can get that pedal to work.   I think you just got a crap lot of 2N5457's.

I've never noticed this before but that pedal doesn't handle the possible range of JFETs very well.

[subnormalwater]

The new one insta-died 

Can you suggest me a good diy bass preamp (maybe an eq is better) ?

And make sure it doesn't involve 5457's

[Rob Strand]

Can you suggest me a good diy bass preamp (maybe an eq is better) ?

There's too much of a preference factor.

There's a another forum (....org) which has a large number of threads.
Under topic "Electric Guitar Rewiring" there's:
- "MIJ Fender Jaguar Bass preamp";  This pretty much a transistor version of the Sadowsky.
- "Southbeach Bass Preamp (2-band)";  Standard bass + controls, from a unnamed popular preamp. 
- many other's.

Under topic "DIY Stompbox designs" there's:
- "Fodera, Sadowsky, aguilar, and other bass preamps"  a large thread with many preamps.
   In that thread. the Ken Smith one looks like it might be worth trying.

There's also a thread on the 2-band Music Man preamp.

The Sadowsky preamp is has a mid notch which you like or not.  It also boost + boost.  No cut.
The "Southbeach Bass Preamp (2-band)" is flat with boost/cut
The Ken Smith  preamp is flat with boost/cut but it roll-off the highs for a less "hifi"/"brittle" sound.
The Musicman has it's own sound.


Have a look!

And make sure it doesn't involve 5457's

LOL.  Yeah, not a good experience.

[subnormalwater]

Thank you very much Rob, I'm sad this turned out to be like that

I'm going to hear some other preamps and I'm quite sure we're going to meet again while debugging my next build LOL

[Rob Strand]

Thank you very much Rob, I'm sad this turned out to be like that

No problem.  That's life in electronics sometimes things are just a PITA.

[anotherjim]

Sadly, JFET quality issues now are making such designs a bit of a disaster area. They still have use, but it isn't smooth sailing unless you limit them to unity follower stages, where they do have sonic merit, albeit subtle most times.

For alternatives, it all depends whether you want on-board (some mentioned are primarily for putting inside active basses) or pedal/DI use, the sound you are after will decide the choice.
Old school Guitar type bass amp - Fender Bassman etc.
Old school specialist bass amp - Ampeg flip top etc.
Modern full range - Trace Elliot etc.
All are completely different animals, as are the speaker cabs employed which is a huge influence on the sound.

If I was in a hurry, I'd just go out and buy a Behringer BDI21. As long as you don't want an overdrive sound, those are absolutely fine for most bass tones. You can find schematic for DIY use (the Behringer really needs the plastic top interior spraying with conductive paint to screen it). Or a schematic for the Tech21 Bass DI of which the Behringer is a clone. No JFETS, all op amp design. Behringer use a TL074 for the drive/presence boost section and this sounds horrible if clipping. The original uses (IIRC) a TLC2274 CMOS op-amp (which is the entirety of its claim to "tube sound"). Whatever, the Tech21 design is know to sound fine overdriven.
Both contain a fixed cab simulator, which may be too much unless used with full range amplification.

[Rob Strand]

Sadly, JFET quality issues now are making such designs a bit of a disaster area. They still have use, but it isn't smooth sailing unless you limit them to unity follower stages, where they do have sonic merit, albeit subtle most times.

Do you know if the dodgy JFETs can be narrowed down to a particular manufacturer?

(I don't buy many parts these days as I have too much junk and I'm not build much anyway.)

[anotherjim]

Dodgy FET's can come in many guises, from quality failed parts to downright fakes that are not even JFETs. To avoid that, buy from proper distributors or audio specialists like Smallbear. Most of the 2N5457 or J201 parts we want are Fairchild ("F") branded and fakes often have this marking.
The big distributors probably won't even have any stock of TO92 JFETs, since this form isn't widely manufactured anymore (there is a company making them, but they turn out very, very expensive in small quantities).
We are now in the same situation with these JFET's that we are in with Germanium parts.

The way around this particular hell is to get the SMD SOT23 form, which are cheap and widely available. Due to wide variation in even the genuine parts, selection is still advisable, but at least you can get plenty to sort through. Although very small and fiddly to handle, the part only needs holding down on an SMD adapter pcb while you check how it biases.

[Rob Strand]

Dodgy FET's can come in many guises, from quality failed parts to downright fakes that are not even JFETs. To avoid that, buy from proper distributors or audio specialists like Smallbear.

Good advice.   

We are now in the same situation with these JFET's that we are in with Germanium parts.

Seems like it.  As they become scarce it creates a market for dodgyness. 

[rankot]

I've built this preamp too and also had distortion problem. Although I have a bunch various of JFETs, I wasn't lucky finding some that will work as I expected, so I removed source bypass cap (47u) on the second JFET. It was much better, but still distorted on accented bass notes, so I replaced first 2N5457 with J201 and it is fine right now. Maybe very little distortion on really heavy picking or slapping, not sure, but much much better than it was. Maybe I could remove first JFET's bypass cap, and return the second one? I'll try that too.

[Rob Strand]

Maybe I could remove first JFET's bypass cap, and return the second one? I'll try that too.

A simple way to decrease gain is to increase the 1k resistors in series with the source caps.

The second stage gain largely recovers the losses through the tone control.  The original value doesn't add much overall gain- so you shouldn't have to decrease the gain much here, if at all.

The first stage gain will cause clipping in both the first and second stages.  Maybe change the 1k source resistor to 2k2 or 3k3.

[rankot]

I built it according to original schematic, without 1k resistors in series with 47u capacitors. I will try that, too.

[Rob Strand]

It wouldn't hurt to check the DC voltages on the JFETs.  If they are off the JFET stages could clip prematurely.

[rankot]

Drain voltages:
J201 (first tranny) 6.3V
2N5457 (second one) 7.3V

[Rob Strand]

Drain voltages:
J201 (first tranny) 6.3V
2N5457 (second one) 7.3V

They aren't bad.   Probably better if they were lower.
I suspect the real issue is simply the first stage gain.

(BTW: If you play with the biasing it usually changes the gain as well.)

[rankot]

No, first stage definitely works fine - no distortion at all, but if I run second one with 47u source bypass cap, it gets heavily distorted. I also tried to add a resistor in series with the cap (470 ohm), but still distorted, so I left that cap out. It is probably possible to bias those stages, but I was too lazy to solder pots and do it (PCB is actually really small).

For those willing to build this pre, there is my PCB with 10k trim pots for biasing: https://docdro.id/pxoBZag

[Rob Strand]

No, first stage definitely works fine - no distortion at all, but if I run second one with 47u source bypass cap, it gets heavily distorted. I also tried to add a resistor in series with the cap (470 ohm), but still distorted, so I left that cap out. It is probably possible to bias those stages, but I was too lazy to solder pots and do it (PCB is actually really small).

I'd have to look at the fine details.  Maybe the gain of the J201 stage is lower than normal.

I'm not sure what schematic people are using for this but on the original the source circuit for each JFET is as follows:
- 4.7k resistors from Source to ground.   This sets the JFET DC bias.
- A 1k resistor in series with 47uF from the Source the ground.   This sets the gain.
  It also makes the gain more predictable and less dependent on the JFET characteristics.
  Increasing the 1K resistors reduces the gain without affecting the DC bias.

An additional feature of the circuit is the gate resistors don't connect to ground like in most circuits.  They connect to a 100k + 10k resistor divider to +9V.  If you trim either the 100k or 10K you can tweak the DC bias of *all* the JFETs at once.  This is good for batch tuning the DC bias or for tweaking the whole circuit when using different JFETs (in all locations).

The PCB doesn't look like it has the 1K resistors in series with 47uF caps.  This makes the gain higher and more susceptible to overload.  As mentioned above the gain is more predictable with those resistors.