Baja low noise low current drain bass guitar preamp 121218

bajaman · Post by **bajaman** » 12 Dec 2018, 07:21

Hi folks
Merry Christmas to all

Here is a very simple but extremely nice sounding discrete bass guitar preamp for you all to build. Current draw is around 80uA

I recently wound a Music Man style alnico bass pickup - (low inductance but series connected - 3500 turns of 41 awg polynylon per coil

) and i needed a preamp for it. Everyone seems to be going crazy for the Aguilar / Sadowsky two band bass preamps and i have seen a schematic for the Sadowsky on the web, but thought i could do a little better

The Sadowsky schematic shows two 2N5457 low noise jfets but runs at a considerable current drain. I did not want to be constantly changing battery so I set to work designing a similar preamp but running at about 10% of the battery current draw. I was successful, and installed it in the bass, but it had too much gain and i found i could only set the bass volume to about 10% rotation, so i went back to the drawing board and thought i could do better - the circuit below. is the result and i am very pleased with it, and it has a very detailed sound to it - in my opinion far nicer than all those active op amp versions. In this two stage preamp, the jfet provides the high input impedance and sonic detail and the pnp transistor supplies the current gain and low output impedance to drive the tonestack and the proceeding effects or amplifier. The gain of each stage is set by the ratio of the 100k feedback resistor and the undecoupled 18k resistor - a voltage gain of 6.55 or +8dB for each stage. Higher (or lower) gain can be achieved by lowering (or raising) the 18k resistor value, BUT the decoupled 18k resistor will need to be increased (or decreased) by the same amount to maintain the jfet bias point. For example: changing to 10k gives +10dB, but the decoupled 18k then needs to be increased to 26k to maintain the same jfet bias

Have fun, and if you build it let us know your thoughts, suggestions etc.
Cheers
bajaman

the following graphs show: both controls at minimum, both controls at 50% resistance, both controls at full, treble at full rotation and bass at zero, treble at zero and bass at full rotation.

bajaman · Post by **bajaman** » 12 Dec 2018, 11:03

For even lower current drain - 46uA

Comparable to the original Music Man Stingray bass preamp current draw of 44uA with the LM4250 adjustable op amp, but with much better specs

Here is my 2SK117(Y) version

cheers
bajaman

Jarno · Post by **Jarno** » 14 Dec 2018, 12:48

Nice work! Keen on giving this a try.
I think I'll do a double sided layout and have some boards made (because I do not self etch boards these days), but I did make a single sided layout, needs only a single wire strap.
Let's see if I can upload it here.

Unverified, of course. Let me know if anyone spots something amiss.

And, Merry Christmas!

bajaman · Post by **bajaman** » 14 Dec 2018, 20:33

Jarno wrote:Nice work! Keen on giving this a try.
I think I'll do a double sided layout and have some boards made (because I do not self etch boards these days), but I did make a single sided layout, needs only a single wire strap.
Let's see if I can upload it here.

Unverified, of course. Let me know if anyone spots something amiss.

And, Merry Christmas!

Looks good Jarno - I built mine on veroboard with resistors standing vertically - high density small board to fit inside the small bass guitar cavity just under the treble and bass controls similar to the original MM Stingray preamp board.
cheers
bajaman

Jarno · Post by **Jarno** » 14 Dec 2018, 22:58

Thanks!
Also did a SMT version this afternoon (well, all the regular resistor values, like 1k, 10k, 100k, 1M are SMT, left the rest TH as I still have a very big pile of parts on stock, same for the transistors), which is slightly smaller. That one is double sided though, so not much fun to self etch.

Rallinger · Post by **Rallinger** » 20 Dec 2018, 10:12

bajaman wrote: Looks good Jarno - I built mine on veroboard with resistors standing vertically - high density small board to fit inside the small bass guitar cavity just under the treble and bass controls similar to the original MM Stingray preamp board.
cheers
bajaman

I recently built a Sadowski Clone with a Layout I found on guitar fx Layouts but I had several Problems and wasn't too impressed. I would Like to try out this one now. Since you built your Version on vero - May I ask you to provide your stripeboard Layout? Thank you very much.

bajaman · Post by **bajaman** » 20 Dec 2018, 10:52

Rallinger wrote:
bajaman wrote: Looks good Jarno - I built mine on veroboard with resistors standing vertically - high density small board to fit inside the small bass guitar cavity just under the treble and bass controls similar to the original MM Stingray preamp board.
cheers
bajaman
I recently built a Sadowski Clone with a Layout I found on guitar fx Layouts but I had several Problems and wasn't too impressed. I would Like to try out this one now. Since you built your Version on vero - May I ask you to provide your stripeboard Layout? Thank you very much.

sorry - no layout - i just built it as i went - pretty simple - two identical sections with the tone control components mounted directly on the control pots

mistermikev · Post by **mistermikev** » 30 Dec 2018, 20:36

I decided to do a layout for the 2sk117 version, excited to try it out in a future bass build. This layout is setup to mount on a push/pulll pot w a board mount pot for the treble. Posting here in case anyone cares to try my humble layout.

Thank you so much Baja for all of the wonderful things you share!

thetragichero · Post by **thetragichero** » 04 Jan 2019, 14:40

I REALLY like this because it uses standard guitar pot values, which would make testing/installing it way easier. I've got an itch (and the ICs in hand) to try the sabre ii preamp first but I'll probably throw one of these on perf to try anyway (maybe my 85 rb850 gets drilled for another pot to accommodate it... it already has the push-push pots for coil split (which I find useless on bass... maybe add a parallel 'musicman' option) ... as you can see my neurons are firing!

thetragichero · Post by **thetragichero** » 08 Jan 2019, 22:30

2 resistors and 2 capacitors soldered directly to the pots because it's gonna be a tight fit into here:

thank you for this, man!

thetragichero · Post by **thetragichero** » 09 Jan 2019, 23:37

okay so it sounds like a sick fuzz face (lower output, gated, and fuzzy)
I used 2n5087 instead of bc557 because that's the kind of pnp transistors I have
I deduced from internet research that pinouts were rotated 180 degrees from each other (this is quite possibly where I went wrong?)

reading schematic from left to right:
q1 (2sk117)
d 9.09v
s 3.05v
g 4.56v

q2 (2n5087)
e 9.6v
b 9.05v
c 9.56v

q3 (2sk117)
d 9.04v
s 3.28v
g 5.42v

q4 (2n5087)
e 9.6v
b 9.02v
c 9.56v

so did I over think the pnps?
only other difference was I used a way over spec 2.2uF (measured at 2.8uF) output cap because the only 4.7uF caps I have are electros (rc filter calculator put hp filter corner frequency at 0.2hz instead of 0hz, so I figured this would be okay)

thanks in advance for your help!

thetragichero · Post by **thetragichero** » 10 Jan 2019, 04:31

sorting through some scavenged parts, I also have two 2sa1015gr I got out of a tv set. besides that it's npn silicon and a few pnp germs

bajaman · Post by **bajaman** » 10 Jan 2019, 04:35

thetragichero wrote:okay so it sounds like a sick fuzz face (lower output, gated, and fuzzy)
I used 2n5087 instead of bc557 because that's the kind of pnp transistors I have
I deduced from internet research that pinouts were rotated 180 degrees from each other (this is quite possibly where I went wrong?)

reading schematic from left to right:
q1 (2sk117)
d 9.09v
s 3.05v
g 4.56v

q2 (2n5087)
e 9.6v
b 9.05v
c 9.56v

q3 (2sk117)
d 9.04v
s 3.28v
g 5.42v

q4 (2n5087)
e 9.6v
b 9.02v
c 9.56v

so did I over think the pnps?
only other difference was I used a way over spec 2.2uF (measured at 2.8uF) output cap because the only 4.7uF caps I have are electros (rc filter calculator put hp filter corner frequency at 0.2hz instead of 0hz, so I figured this would be okay)

thanks in advance for your help!

You did use 2sk117 (Y) types ?
Ideally you should see around 5v on the 2N5087 collectors

thetragichero · Post by **thetragichero** » 10 Jan 2019, 04:56

looks like they're BL type

edit: they might be counterfeit? laser etched bought off eBay.... now that I think about out, when I was breadboarding the orange otr120 od circuit it sounded the same regardless of orientation (I have a habit of wiring things backwards so I tried a bunch of things to get it working)

I have the small bear jfet assortment if one of those would work without changing an of the other values (since the 2n5457 version has a couple changes, I see)

edit 2: oooh 22k parallel with 100k makes 18k.... if I have to use 2n5457, it'll be ugly but doable

bajaman · Post by **bajaman** » 10 Jan 2019, 05:12

thetragichero wrote:looks like they're BL type

edit: they might be counterfeit? laser etched bought off eBay.... now that I think about out, when I was breadboarding the orange otr120 od circuit it sounded the same regardless of orientation (I have a habit of wiring things backwards so I tried a bunch of things to get it working)

I have the small bear jfet assortment if one of those would work without changing an of the other values (since the 2n5457 version has a couple changes, I see)

edit 2: oooh 22k parallel with 100k makes 18k.... if I have to use 2n5457, it'll be ugly but doable

Don't forget the 1M resistors go to ground in the 2N5457 version

thetragichero · Post by **thetragichero** » 10 Jan 2019, 15:29

i can paint by numbers but i'm curious - what specs are we looking for in the jfets? a look at the data sheets makes me think 2n5484 is similar (BF244A seems less so, BF245A about the same as BF244A, J201 and MPF102 seem far off) Idss min and Vgs(off) seem what my father would call "close enough for punk rock," but my knowledge of the underlying circuit design theory is lacking at best
maybe this is beyond the scope of this thread

bajaman · Post by **bajaman** » 10 Jan 2019, 20:52

thetragichero wrote:i can paint by numbers but i'm curious - what specs are we looking for in the jfets? a look at the data sheets makes me think 2n5484 is similar (BF244A seems less so, BF245A about the same as BF244A, J201 and MPF102 seem far off) Idss min and Vgs(off) seem what my father would call "close enough for punk rock," but my knowledge of the underlying circuit design theory is lacking at best
maybe this is beyond the scope of this thread

Unless you have 2SK117(Y) not (Bl) not (G), use the 2N5457 and it's schematic - you can't go wrong ( 2n5087 should work fine too)
cheers
bajaman

marcao_cfh · Post by **marcao_cfh** » 11 Jan 2019, 02:23

Jarno, your layout looks great! Just a few suggestions to make it more DIY friendly:
- bigger spacing between tracks and ground plane;
- wider tracks;
- bigger jumper pad.
The way you made is fine for professional pcbs/photosensive transfer, but for toner transfer it won't be an easy job.

thetragichero · Post by **thetragichero** » 11 Jan 2019, 03:20

hey it works! and it's LOUD!
bass control isn't doing anything but the bass is in a state where I'm okay with that. treble control has a lot of range, from slightly dull to almost shrill - I like it!

I don't have a 'clean' bass amp (not really my style), so I will have to wait to hear it through the DI at church

but this old has some new life!

marcao_cfh · Post by **marcao_cfh** » 11 Jan 2019, 03:32

Nice you got it working! What was the problem? Now it's time to make the bass pot work.
I'm gonna try it soon, just need to etch the board (I'll be using Jarno's layout) and wait for some parts to arrive by mail.

Baja low noise low current drain bass guitar preamp 121218 [documentation]