help on baxandall onboard bass preamp

[Renegadrian]

Hi guys, I wanted to replace the onboard standard 3 band preamp on my ibby sr300 with something simpler, so I went developing my version of it.
it basically is a buffer/gain stage (first part of parasit studio, as linked below) and the bax found in ac/rc ecc, with some values changed.

Now, the bass does its job and am quite happy about it, the boost is good.
On the other hand, treble pot seems to do nothing! Also, the trimmer at the first op amp dos nothing...can you give me some clues about it!?

my schematic

and my vero (c1 c2 and c out are 10µ non pol. trimmer is 250k)


active bax by parasit studio
https://www.parasitstudio.se/uploads/2/4/4/9/2449159/active_bax_eq_pcb.pdf

rc bass booster
https://aionfx.com/app/files/schematics/xotic-bass-rc-booster-trace-schematic.png

[antonis]

The trimmer at the first amp can be omitted..
(replaced by a jumper between pins 1 & 2..)

[Renegadrian]

The trimmer at the first amp can be omitted..
(replaced by a jumper between pins 1 & 2..)

Yes I know that, just jumpering them gives 0 gain, but is it intended to add some gain with a trimmer/resistance? I assumed that, seeing it in the parasit schem I posted.

[Renegadrian]

ok i got the feedback trimmer error. Think I am gonna take out trimmer for a jumper, it is good anyway. Tried a resistor from the -in to vref. it's good to set the gain.

[Renegadrian]

[Renegadrian]

now i am gonna work on the treble side...still doesnt work properly

EDIT: it works but not so evident as the bass cut/boost, how can I enhance it?

[antonis]

Don't know if you've noticed (or you mind) but in your mods you have IN-OUT phase inversion..

If you really mind for high input impedance, you can modify Freppo's input stage as follows:

[Renegadrian]

again, i studied/compared/copypasted from several bax schems, fwik phase shouldnt be a problem actually i didnt consider it but its not a problem

[antonis]

but its not a problem

Not by its own, of course..

[Renegadrian]

treble works but not so evident as the bass cut/boost, how can I enhance it?

[antonis]

pages 2.46 to 2.53

[Renegadrian]

thx but it didnt help, as the treble side (pot pins 1 and 3) has capacitors not resistors.

[PRR]

pages 2.46 to 2.53

That's a James ("passive").

The Bax starts page 2-50 section 2.14.7.   

> the treble side (pot pins 1 and 3) has capacitors not resistors.

There are MANY ways to draw a Bax. Some work better than others. I don't recognize the one you show and suspect it has bad leverage on treble. Bohn's 2.14.13a is a known-good plan. Scale all parts for the pots in your hand.

....Ah, it does help to have a higher-R treb pot, and Bohn used a fancy-pants opamp needing special treatment. Also his is bipolar supply and I assume yours has half-supply bias?

[Rob Strand]

treble works but not so evident as the bass cut/boost, how can I enhance it?

Your treble caps are too small.  Try 10nF maybe upto about 15nF.
(Some people might even like upto 33nF.)

Your original circuit is OK except for the treble caps.
You know you can change a the bass and treble caps on the original preamp.   It's a lot easier than building a new one!  Off hand I'm not sure which Ibanez preamp is in the SR300 - I'd have to look through old notes.   Just skimming the web I'd say try using bass caps twice what's there, maybe even three times larger.  Leave the treble caps.    Unfortunately without know which preamp it is or having a schematic it's a bit hit and miss.    Some three band preamps have a separate mid circuit and these are easy to modify.   However ibanez have three-band EQs packed into one opamp and gyrators based circuits which need more care to mod.

[Renegadrian]

There are MANY ways to draw a Bax. Some work better than others. I don't recognize the one you show and suspect it has bad leverage on treble.

Yep I did some study on different baxes, the one I used is directly taken from xotic schem, as I linked in the first post.

[Renegadrian]

treble works but not so evident as the bass cut/boost, how can I enhance it?

Your treble caps are too small.  Try 10nF maybe upto about 15nF.
(Some people might even like upto 33nF.)

Your original circuit is OK except for the treble caps.
You know you can change a the bass and treble caps on the original preamp.   It's a lot easier than building a new one!  Off hand I'm not sure which Ibanez preamp is in the SR300 - I'd have to look through old notes.   Just skimming the web I'd say try using bass caps twice what's there, maybe even three times larger.  Leave the treble caps.    Unfortunately without know which preamp it is or having a schematic it's a bit hit and miss.    Some three band preamps have a separate mid circuit and these are easy to modify.   However ibanez hand three-band EQ pack into one opamp and gyrators based circuits which need more care to mod.

yep gonna try 10 or 15n, after all the aion schem I posted has 10s.
The original preamp was good, I just wanted to sub it for a 2 band for simplicity and just for fun.

[Rob Strand]

yep gonna try 10 or 15n, after all the aion schem I posted has 10s.
The original preamp was good, I just wanted to sub it for a 2 band for simplicity and just for fun.

Yep, nothing wrong with tinkering.

Your circuit values with 12nF is roughly like the Xotic Bass RC with 10nF.   So that 10nF to 15nF caps range should be close to the mark.

Some of the Ibanez preamps roll-off the high frequencies to take the edge off.  Not sure about the SR300.  Here's an example,
https://www.talkbass.com/attachments/sr480_preamp-jpg.2287980/

Components R12, R13, C8.