Two Si NPN bass preamp

[Gus]

green input, blue output


Not built yet simmed in LT spice.

Gain for each stage was selected for 9VDC operation
Tone section uses Fender value caps and potentiometers
Bootstrapped Si gain stages
Graph is as the controls are set in the sim screen shot
Capacitor type can be what you like.
If I was to build this I would use films where I could and a ceramic disk at C4
You can adjust C3 value up and down to taste.  I liked how .047uf simmed

You can adjust the tone section values to taste or use this for guitar etc.

check this thread
http://www.diystompboxes.com/smfforum/index.php?topic=96465.0
also look at figure 7 and figure 8 in a link I posted in that thread, compare emitter feedback to single JFET and single triode graphs

[nobodysweasel]

This looks interesting...I might have to put it on the breadboard when I get some time (which probably won't be for a while).  I'm not too familiar with how LTspice sets things up.  So is V1 the input?  And R8 would be a pot wired to the output?

[Gus]

Input is the left side of C6.  The other parts to the left of C6 are the input cable and bass sim.
R8 is a fixed resistor
The volume control is (R17,R19) the wiper is the center node.
9VDC and silicon transistors instead of tubes or jfets
250K treble and bass controls 10K mid control


Compare to the schematic in this thread
http://www.diystompboxes.com/smfforum/index.php?topic=96465.0

[Quackzed]

i've been doing a layout to build an fet 2b type circuit, and have been playing around with single jfet stages, fetzer stages etc, might have to try this out.

[joegagan]

thanks gus, this looks cool. my laptop went down, i am on a dumb ipad. only the left side of the schem image is showing with no way to to scroll right.  can't wait to see the rest of the schem!

[ubersam]

Nice... I just gutted my bass in preparation for replacing the onboard preamp with a circuit resembling a Bassman but with JFETs (Fetzer). I might just have to build this one too and see which one I dig more: JFETs or Si's. One question, would scaling the tone stack (like what was done in the Dr. Boogie) have any benefit in this circuit?

[B Tremblay]

thanks gus, this looks cool. my laptop went down, i am on a dumb ipad. only the left side of the schem image is showing with no way to to scroll right.  can't wait to see the rest of the schem!

Weird, I was able to scroll across on my iPad.  With one finger!

[Gus]

According to the sim it draws about 1.1ma
If you want to scale the values of the tone stack be aware of the 39K collector value at the first stage.

If you use
http://www.duncanamps.com/tsc/
note it includes the source resistance feeding the tone stack  39k is close to 38k.  So you can make a sim or use TSC and use 39K as the source resistance.

So if you want scale exactly you will need to change the source resistance to match the new tone stack resistor and cap values.

1.1 ma might be an OK load for a battery powered version in a bass however you might want to add a cap to sim the cable cap if you use the circuit inside a bass or you can make it switchable.

I have not built this circuit yet.  I do have more variations on it with another 9VDC and 18VDC circuit.  One can add the bright cap and switchable tone caps.   I kept the same tone section so one could use the same tone control values and try different types of gain stages.  The 4.7K and 470 ohm should allow the driving of longer cables.

[Quackzed]

i just ran the npn stage w/out tonestack; in falsteds sim. i like it. a 100hz signal at .4v just starts to clip on the bottom. at .5v the tops are also clipped, but softer clipping... the bottom half of the wave seems to stay 'just barely clipped, as the top wave gets progressively more clipped...
to me that looks like 2nd harmonic at onset,where only one side of the wave is clipped-
followed by 3rd,as the other side begins to clip-
then mostly third as the clipping on both sides reaches symmetry,
then 2nd increases with gain and 3rd less so... that is as gain goes up 2nd order goes up at a higher rate than 3rd..
at around 1 volt theres a good amount of clipping going on.

[Gus]

Quackzed

Thanks for the information from the falsted sim.  I like the "sound" of bootstrapped gain stages like in the bass sim.  I set the gain for the "headroom" limits of 9VDC power.   Simmed for a passive bass, for a active one might want to adjust the gain lower or use an input pad.   One can adjust the gain with  R3(keep in mind output resistance) and/or R4 and then adjusting R15 for the desired operating point.

R18,R23 and R24 are for stability

[Renegadrian]

Resurrect this thread as you Gus suggested to take a look at your schem - now, for the little I read, bootstrapping is a technique used in the design of transistor amplifier circuits to increase the input impedance and thereby reduce the loading effects on the input source.
now, would the circuit act the same putting a regular bjt stage before or after the TS?! which one would in theory make the TS behave better!?
Also, bootstrapping is only about inpedance or does it apport some changes in overall gain!?