Sadowsky JFET Preamp Noisy

[lars-musik]

Hello folks,

a bass playing friend of mine asked me whether I could provide some preamps for onboard operation he'd like to build into his basses. As usually I couldn't say no. Now I got a little test rig in an old and mis-drilled enclosure here sporting a Music Man Stingray preamp, Bajaman's "Low Noise, Low Current Preamp" and a Sadowsky preamp. The latter has (a) a LOT more output on tap than the other two and (b) is a LOT noisier, independent of input gain.

Here's the schematic. Would you spot a place where I could modify anything to tame the noise? It's just some white noise, multipliable by the treble pot.

Thanks a lot in advance!

[lars-musik]

My first workaround is an additional volume pot at the output.  I guess that already does it.

[antonis]

In case of noise is comming from oscillation, place 100pF Miller caps Between Drain-Gate of Q1 & Q2..
In case of power supply oriented, place a 100nF ceramic cap between U1 & U4..
(it should also be a good idea to place a LPF right after U1..)

P.S.
Is R1/R2 values ratio correct or it's just a typo..??

[lars-musik]

Hi Antonis,

thanks for your answer. The thing is noisy even battery powered, so it's definitely not the PSU. I'll try those miller caps (although it's become quite hard since JFETS are SMD).

The super asymetrical bias voltage seems to be a thing in those circuits. I think it's been around 5 years (what? already again?) since I last trief myself on RG Keen's Jfet papers. It's time for another try...

Here's another schematic with those values
https://www.diyaudio.com/community/threads/bass-guitar-fet-preamp.357731/

and here a layout, obviously built mutiple times

https://tagboardeffects.blogspot.com/2013/04/sadowsky-preamp.html

[Steben]

These electrolytic caps are huge.
Probably a bass thing.

[Rob Strand]

The Sadowsky preamp should have 1k resistors in series with the source bypass caps.  That reduces the gain and will reduce the noise as well.

The large unbypassed source resistors ensure you can use fixed drain resistor values (10k on original) instead of a trimpot and still get good biasing.  (As per original design.)

If the JFETs aren't real 2N5457's it's possible the bias and gain are a little off and you could tweak the gain using the 1k resistors in series with the source bypass caps.   However, tweaking the bias point with the drain resistor may or may not work, I would have to check, since the gate bias voltage VB has been set-up for 2N5457's.

[lars-musik]

Thanks Rob, I will try the series resistors for those caps.

We had a test session yesterday with the three preamps and my friend liked the Bajaman preamp most. So the Sadowsky will probably be shelfed for the time being.

[Eddododo]

Are you breadboarding it currently? I have had issues with breadboards while doing elaborate FET preamps, and I've found that taking it apart, and putting it back together can help, and if not, then changing the layout a bit. I don't know if its the tray capacitance from breadboards, or what, but its a common thing I've seen. You may also increase the 300pF to ground a bit, its not like this preamp struggles with a lack of brightness..


but, maybe you're better off with the bajaman if he likes it.. the Sadowsky pre has been around for AGES and it's a little bit dated of a sound that a lot of bassists may find a little too cheesy marcus miller-esque in its emphasis of the highs.. I've never been a big fan of the circuit.

[Rob Strand]

We had a test session yesterday with the three preamps and my friend liked the Bajaman preamp most. So the Sadowsky will probably be shelfed for the time being.

Technically the Sadowsky is a boost only type tone control.  It's more flat with the controls set near zero and the gain is kind of set-up for unity there.

If you set the controls to 12 O'Clock it's sort of like a bit of Bass and Treble boost which is equivalent to a mid notch + level boost.   That's the Sadowsky sound people are used to.

[lars-musik]

Are you breadboarding it currently?

No, quite the opposite. I am not much of a breadboarder but rather prefer the anguish of home-etched 0805.

Rob, I'll just box the Sadowsky and give it to my bassist friend for further experiments - a boost/EQ can come in handy.

One thing I hadn't considered (but the bassist excitedly pointed out) is, that the Bajaman Preamp uses an existing volume pot at the input, so we tested it with a 250K-A which works fine and allows for the some pot to operate in active and in passive mode. So in the end, I decided on a DPDT for as passive/active switch, that also takes a stereo tone pot (one layer for active, and one for passive operation) into the circuit.


Here's our test rig

[antonis]

Toggle switches levers are dangerously close to stompswitches..
(unless you have a No 6  shoe size..)

[lars-musik]

Toggle switches levers are dangerously close to stompswitches..
(unless you have a No 6  shoe size..)

Toggle switches levers

That's true. But this here was tested on the lap and the final product will be built into a bass. So no danger here.

[Rob Strand]

Here's our test rig

Jigs like that are good to compare circuits.

You can also tweak the frequencies of those circuit by changing the caps.  For example if you like the treble control frequencies of one circuit you can (say) decrease the cap on another circuit so the treble control suits your taste - same goes for the bass.  For the three preamps you have chosen they are different enough that each one will still have their own character even after the tweak.

For the Sadowsky, you can using larger resistor in series with the caps.  If you judge the normal/good sound with the bass and treble slightly advanced then unity gain will correspond to 2k2 or 2k7 instead of 1k.