Hello everyone, I'm trying to build my 3rd diy pedal but every time the final result is not working 100%. :icon_evil:
I followed this board: http://johnkvintageguitars.homestead.com/Effects/others/Sadowsky/SadowskyBassPreamp.png
Which is mostly based on this schematic: http://theernie.com/wp-content/uploads/2014/10/Screen-Shot-2014-10-16-at-11.05.05-AM.png
Modifications made to the board: removed the zener diodes ad placed the 47pf at the first gate (as the schematic) (optional 10k and 1nF not placed as the board layout - shorted opt.10k of course)
Alright;
I can't have almost any signal passing through the first fet, everything sounds heavily distorted and super low volume, I can hear something by cranking up the gain on my amp and still having troubles getting something out from it. I kinda solved that by placing a 47k between the gate and ground;
Now I'm stuck at the 2nd gate, I have signal passing through the 100nF but if I link it with the gate everyting goes mute (well.. super low volume with distortion like before)
I checked the 100nF, 1M resistor and the 300pF, they are fine, I just can't understand what's going on with this pedal, I already replaced the fets two times :icon_cry:
At stock I'm reading 3V on each drain, by increasing it to 5V (another 10k in parallel) the volume goes up to normal but still distorted and the signal struggles to get through.
All pots connected except the volume one, it's just 100% all the time.
Tell me if you need some other readings, I really can't understand why I'm struggling so much on this simple build!
Sounds like a biasing issue.
What JFETs are you using? IIRC, the original used 2N5457.
QuoteAt stock I'm reading 3V on each drain, by increasing it to 5V (another 10k in parallel) the volume goes up to normal but still distorted and the signal struggles to get through.
Probably 6V is about optimal although 5V shouldn't be bad.
The best way to tune the biasing on that circuit is to trim the 10k in the top left corner. You should be able tune both drains to reasonable voltages simultaneously.
If you are using J201's the bias might be harder to set. It might even be hard to get both to bias correctly unless you play with some drain resistors.
OK, I see the problem. You need to add 1k resistors in series with both 47uF caps.
Using 2n5457, sorry.
Let me show you what I get, I just noticed something odd (stock/no 47k or other changes):
1st FET:
D: 3.169V
G: 0.786V
S: 3.069V
2nd FET:
D: 3.109V
G: 3.076V
S: 3.079V
Just saw your reply, I'm going to try that
Quote from: Rob Strand on December 12, 2017, 05:19:54 PM
OK, I see the problem. You need to add 1k resistors in series with both 47uF caps.
no success
Quote1st FET:
D: 3.169V
G: 0.786V
S: 3.069V
2nd FET:
D: 3.109V
G: 3.076V
S: 3.079V
Hmmm those measurements look crap!
The layout matches the 2N5457 pin out so it's not a pin out issue - it doesn't hurt to check.
What voltage do you measure at the bias divider (100k+10k + 100n) in the top left of the schematic?
Quoteno success
While it's not the problem here, I do recommend leaving the 1k's in. The original is like that and it will prevent clipping, especially when the battery gets low.
[Edit: check for shorts and any track cuts you have missed.
The fact the two gates are at different voltages is maybe the key. Check around those.]
Quote from: Rob Strand on December 12, 2017, 06:03:11 PM
Hmmm those measurements look crap!
The layout matches the 2N5457 pin out so it's not a pin out issue - it doesn't hurt to check.
What voltage do you measure at the bias divider (100k+10k + 100n) in the top left of the schematic?
The pinout is correct.
Reading 0.85V right after the 100k. It's a 100uF btw.
Quote
While it's not the problem here, I do recommend leaving the 1k's in. The original is like that and it will prevent clipping, especially when the battery gets low.
[Edit: check for shorts and any track cuts you have missed.
The fact the two gates are at different voltages is maybe the key. Check around those.]
Isn't the second gate supposed to be higher? it's directly connected to the drain of the first one, measurement made with 100% on both pots
No shorts, I'm going mad at looking for them.
QuoteReading 0.85V right after the 100k.
It's "OK" but it's probably a little high *because* the gate voltage of the second JFET is high.
QuoteIt's a 100uF btw.
So it is.
QuoteIsn't the second gate supposed to be higher? it's directly connected to the drain of the first one, measurement made with 100% on both pots
The two stages are identical so they should bias identically.
Maybe the best place to look is around the date of the second JFET. That's the weirdest measurement at this point. It is possible DC from the drain of the first JFET is getting through to the gate. So maybe check around the 100nF on the gate of the second JFET.
QuoteNo shorts, I'm going mad at looking for them.
On the layout there the 5th track from the bottom has a cut, just under the 750ohm resistor.
The track would join the drain of the first JFET to the gate of the second.
[Edit: I simulated this fault and it shows similar voltages to yours.]
Quote from: Rob Strand on December 12, 2017, 06:28:36 PM
On the layout there the 5th track from the bottom has a cut, just under the 750ohm resistor.
The track would join the drain of the first JFET to the gate of the second.
[Edit: I simulated this fault and it shows similar voltages to yours.]
I'm doing it on a perfboard, I did all the tracks by hand, everything on that gate is correct :icon_frown:
I tried to desolder everything from the gate and I still read 3V from the pin itself. Is it ok? I'm extremely ignorant
Voltages from 1M and 100nF are correct
QuoteI tried to desolder everything from the gate and I still read 3V from the pin itself. Is it ok?
If the gate is floating then it's probably OK. 3V just means the JFET is fully on, like a short. When that happens the 10k and 5k fomr a divider giving 3V; 9*5k/(5k+10k) = 3V.
OK what about if you now connect the gate(s) to ground. That will tell us the JFET is "working".
QuoteI'm extremely ignorant
No. Debugging can be tricky. You have theory and pursue it. If it fails use that info and move onto the next theory. If it gets too hard divide the problem, ie the region of the circuit, down then debug the smaller piece. Eventually you will find the problem it just takes a bit of persistence.
Quote from: Rob Strand on December 12, 2017, 06:48:09 PM
If the gate is floating then it's probably OK. 3V just means the JFET is fully on, like a short. When that happens the 10k and 5k fomr a divider giving 3V; 9*5k/(5k+10k) = 3V.
OK what about if you now connect the gate(s) to ground. That will tell us the JFET is "working".
Drain 0.900V
Source 0.845V
edit: sorry, it's just the second jfet.
QuoteDrain 0.900V
Source 0.845V
Well that's a bit unexpected.
Now you've got me going Ahhh.
It's like the pinout is wrong. You only expect to see that if you gate pin is actually a drain or source.
Have you got the flat-side of the JFET pointing to the left, like on the layout?
Quote from: Rob Strand on December 12, 2017, 07:12:50 PM
Well that's a bit unexpected.
Now you've got me going Ahhh.
That's how I'm feeling every time it doesn't work :icon_lol:
Quote
It's like the pinout is wrong. You only expect to see that if you gate pin is actually a drain or source.
Have you got the flat-side of the JFET pointing to the left, like on the layout?
Flat side to left like the other one, just checked and it IS a 2n5457 (just to be 100000000000000000000% safe).
Well, it's 1.20AM and I think it's enough for today.
I'm sure it's going to haunt me in my dreams
Thanks anyways, Rob :icon_wink:
QuoteWell, it's 1.20AM and I think it's enough for today.
I'm sure it's going to haunt me in my dreams
Thanks anyways, Rob
No problem.
At this point, for my own sanity, I'd pull one of those JFETs out and check the pinouts with a multimeter. Using diode test, one pin to the other two (one at a time) will look like a diode. It will have 0.7V in one direction and nothing in the other. That pin is the gate.
Between other two pins, with the identified gate floating, you should see a few hundred ohms in both directions.
Ok then.
Diode checking:
(com > red)
1 > 2 : 0.062V
2 > 1 : 0.062V
2 > 3 : 0.738V
3 > 2 : 0V
3 > 1 : 0V
1 > 3 : 0.738V
Resistance:
1 > 2 : 37.5R
2 > 1 : 37.5R
2 > 3 : 6.45M
3 > 2 : /
3 > 1 : /
1 > 3 : 6.45M
Well, at least the pinout is correct
Oh God.
The second Jfet had a problem on a leg, by moving it around I got 1.2V to gate in the opposite direction, that's why the second gate was so high :icon_wink:
Now they are biased at the same way but still 3.1V on the drain and very weak signal coming out from the fets
QuoteWell, at least the pinout is correct
QuoteNow they are biased at the same way but still 3.1V on the drain and very weak signal coming out from the fets
OK great, looks like you have sorted out some of the crazy stuff.
If you can, short out the 10k bias resistor, the one near the 100k in the top left of the schematic. This is the same as setting both gate voltages to zero. Then measure the voltages on the JFETs. We need to do this test again because of the connection bug. What this does is give me an idea of the biasing and JFET.
At this point I can only think of shorts, missing cuts and maybe wrong resistor values. See how you go with the re-bias test and we'll take it from there.
[Edit: 1 > 2 : 37.5R
2 > 1 : 37.5R
These are actually quite a bit lower than I expected.
So *maybe* the JFET is low resistance and that is causing the issue.
The only solution then is to change the biasing.
]
I completely rebuilt the whole pedal following a similar board made by the same guy with the addition of a 330k from input to gnd.
Still 3.1V on drain and triple-checked everything, I read the reviews of this pedal and turns out I'm not the only one who's struggling on biasing these 2n5457 :icon_frown:
By shorting out that 10k I get around
G: 0V
S: 2.5V
D: 4.2V
G: 0V
S: 3.0V
D: 3.2V
still s#@t on 2nd fet, can't understand why..
EDIT:
Quote
[Edit: 1 > 2 : 37.5R
2 > 1 : 37.5R
These are actually quite a bit lower than I expected.
So *maybe* the JFET is low resistance and that is causing the issue.
The only solution then is to change the biasing.
]
Yeah, I think the only way to get a proper biasing is to rebuild that part from scratch.. but I don't even know where to start (dumb_doing_electronics.jpg)
Quote2n5457
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.
I did a simulation based on a JFET which is out of spec. I can created a situation where I see what you see.
Quote1 > 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.
]
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.
QuoteWell 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.
QuoteNow 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.
The new one insta-died ;D
Can you suggest me a good diy bass preamp (maybe an eq is better) ?
And make sure it doesn't involve 5457's
QuoteCan 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!
Quote
And make sure it doesn't involve 5457's
LOL. Yeah, not a good experience.
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
QuoteThank 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.
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.
QuoteSadly, 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.)
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.
QuoteDodgy 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.
QuoteWe 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.
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.
QuoteMaybe 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.
I built it according to original schematic, without 1k resistors in series with 47u capacitors. I will try that, too.
It wouldn't hurt to check the DC voltages on the JFETs. If they are off the JFET stages could clip prematurely.
Drain voltages:
J201 (first tranny) 6.3V
2N5457 (second one) 7.3V
QuoteDrain 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.)
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 (https://docdro.id/pxoBZag)
QuoteNo, 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.
I have used this schematic:
(http://theernie.com/wp-content/uploads/2014/10/Screen-Shot-2014-10-16-at-11.05.05-AM.png)
QuoteI have used this schematic:
It's more or less OK but it has some differences to what I thought.
The main issue is there should be a 1k in series with each 50uF cap. That will reduce the gain.
The original JFETs were 2N5457's.
Some other stuff:
- the optional parts were not present.
- the 47pF on the input wasn't present but there's nothing wrong with it.
(- the 300pF was present)
- I don't remember a 100uF on the divider. Seems crazy large. Maybe it should be 100nF.
- Minor stuff: Parts were 2n2 not 2n5 and 47uF not 50uF, 4k7 not 5k
- There was a cap across the power rail and a protection diode.
- There were two 12V zeners in "inverse series" at the output between the 5uF and the 1K then to ground; for protection
Like this? I hope I understood you well. :)
(https://i.postimg.cc/ZWXd1qK0/sadowsky.gif) (https://postimg.cc/ZWXd1qK0)
JFETs are doubled due to fact that I use both TTH and SMD parts. I also left trimmers on source resistors to simplify biasing.
QuoteLike this? I hope I understood you well. :)
This should represent the original (at least for pre 2000),
(https://i.postimg.cc/NKYPygPX/bass_preamp_fixes_v1.0.png) (https://postimg.cc/NKYPygPX)
QuoteJFETs are doubled due to fact that I use both TTH and SMD parts. I also left trimmers on source resistors to simplify biasing.
Sorry I crossed them off when I was looking at it.
From what I remember the drain voltage should be about 5V (this was *not* measured from an actual unit).
The series Schottky diode you had is fine.
hi,
I always see these warnings: "this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic"
well, if I was opening a new one, I'd definitely link this one in... first thing...
also, this topic will be relevant as long as people are interested in building a sadowsky bass preamp...
so I venture forth :)
...................................
what is the conclusion?
is it worth attempting to build one?
using J201? and doing other mods?
or maybe it was a period thing which worked with the former generations (and make) of the 2N5457?
is it that it has noise and distortion issues?
how could moodysounds' still work?
they use SMD in the first place, meaning different transistors, I imagine...
anyway, what is your advice to newcomers? :)
welcome to the forum!
never take that 120 day warning to serious, zombie threads aren't as scary as zombie humans
smd or through hole of the same type use the same silicon bit inside, only in a different plastic mold and with longer or shorter metal pins.
i don't think the J201 will sound the same, but it will probably work, as the biassing scheme is quite robust.
i can;t answer the other questions as i havent build this exact unit, still hope this helps.
cheers
thanks very much!
you know, I see the demo of the boutique "version", and sounds amazing, and I'll suspect they use different means somehow :)
now I know that those transistors aren't different...
if I could call a time-travel cab, I'd go back to when DIYers first built it, and would ask everyone to plz mention the transistor make,
and perhaps the hfe...
Build it, it sounds good!
the difference between different Jfets in such a circuit is a very marginal one.
also, Jfets don't have a Hfe/Beta, they have transconductance, but this circuit operates well within the margins of that gain limit that that parameter doesnt matter here
a thing to keep in mind is that a demo always sounds different than anything IRL.
i suggest you just try it out on a breadboard and then decide if you want to build a pedal out of this
cheers
There is a 100k + 10k divider that eventually feeds the gates.
If you change the 10k to around 22k (maybe upto 24k/27k) it will make the circuit with J201's behave *very* much like the original.