On-board bass preamp need review

[faevry]

Hi !

I try to build a preamp based on two BS170.
I use a zvex SHO for starting point, that way I can use an internal 5k trimpot (the crakle OK pot) to set the gain of each input.
I have some question about my layout :




- a 0,1uf input cap / 10 uf output cap is it ok for bass ?
- can i use a blend pot this way ?
- how can i choose the value for the blend and the volume pot ? Is 100k Ok ?

I find diferent blend section, but I don't know what will be best for my layout :



sorry for my bad english... And thanks a lot for reading / helping me !

[garcho]

great reading here, click this link, it might answer a lot of your questions

why not go this route:


then have the volume control after the op amp for instance. if you have the "mix" pot at 50/50, the currents move through resistance equal to half the pot value, sometimes it can result in audible dips in volume or other problems. it's better to mix then buffer so all the signals have low impedance output from the op amp. what are you building? what's the point of mixing these two buffers/preamps together?

[faevry]

Thanks a lot !

I want to build an my own on board preamp for my Jazz Bass.

If I understand what you said and what I read on your link, the best option to me is to place a volume pot right before each input cap (Fig 6 common collector transistor mixer) I understand that it's a passive mixer but it look okey to me.

Is it OK that way :

[faevry]

And this is the EQ section (based on STM three band semi-parametric EQ)

Did I need anything else ?

[DIY Bass]

If it was me with a J-Bass I would have a passive blend into a passive volume and then into the preamp with EQ, and boost you want and the output impedance reduction. I would then have a push-pull switch on the volume control to bypass the preamp entirely and send the passive directly out.  That way you always have an emergency "my battery died mid-gig" control :-)

[faevry]

I have an emergency protocole for that !
I use a switch that send directly my neck sound to jack out with a little cap on the way.
And I always carry two battery un my case ^^
Yeah I'm kind of haunted by drop dead battery too

[antonis]

IMHO, for Treble boost you could use the "other half" of Baxandall tone control..
(omit C7, caps instead of R13, R16 and resistor instead of C4..)

[faevry]

Do you think it will be more efficient than this layout ?
Did you know how to tune the baxandal for my needs ?
I look for a boost from 3k to max 10k maybe less,  baxandal is up to 20k witch is useless for bass in my opinion.

[antonis]

Baxandall is up to 20kHz due to its full audible frequency spectrum design..

2.2nF caps and 5k6 resistor should be a good starting point..

[faevry]

Ok thank for hour suggestion.
Can you explain me why did you think that a baxandall treble control is me intresting than the STM layout ?
I really open to mod / improve a layout but I prefer to understandwhat I do.

[antonis]

Omiting an item and replacing 2 X 4 holes spacing items with 2 X 3 holes respectively shouldn't be considered as layout improvement..?? 

Additionally, scaling items values results in more "convenient" pole/zero frequency shift..

[faevry]

Yes less is more ^^
I was thinking about improvement on resulted sound, but you made a point it will be easier to fit in the bass body !
Did you think it will be okey to use baxandall bass and treble, with two mid band from my stm inspired layout ?

Also, what did you think about blend pickup then go to only one booster/buffer instead if two un parrallel ?
In fact I'm not shure that having two separate gain control will be such a feature...

[antonis]

Of course you can do it..!! 

IMHO, a single op-amp could manage 3 or 4 band EQ without any issue..
Using a dual op-amp you save space and have handy one op-amp for input buffer or Vref stabilizing (in case of single supply..)