FA-1 preamp with some issues

[bluelagoon]

Have this FA-1 preamp circuit, It works fine at 9 Volt power, but when upped to 18V power in, it motorboats, very fast motorboating, not real loud, but noticably annoying all the same. Would there be a reason for this and a means to ammend the circuit to be rid of the issue.
Also it has a mid range control added to the original  Boss circuit design, While it adds a decent mid frequency boost when switched in, it really only becomes effective on the mid pot control as a mid boost on the last 1/3 range of the pot. It also does tend to detract the variable range of the Treble and Bass controls while switched in. Was wondering would there be some tweaks for different component values and or a mid circuit variation to what is present.? Thanks

[antonis]

I'd place a 100R to 220R resistor right after +9V and move C12 right after that resistor (making its value 470μF to 220μF respectively..)


8
Also, I'd place 100nF ceramic caps as close as physically possible to IC's pins 8..


As for the MIDs control:

[duck_arse]

[ I'd draw SW1 properly. ]

[bluelagoon]

Thanks Duck, but its just 3 separate SPDT all part of a 3PDT, thus named 1A 1B 1C, all same switch just used in different circuit locations, Dont know how I would simplify it any better.

[bluelagoon]

Thanks Antonis, I already have the 100nF across the opamp + to Gnd, But the small resistor at the +V in and upping the C12 cap to higher value makes sense to hopefully curb the motor boat issue. And that Midrange control article you posted, I think is where I got the general ball park figures for the mid control circuit I used, its really not that bad, it may be better if I changed specifically to all same values for the complete Baxandall as in that article, may give that a try sometime, Thanks again for the support. Cheers.

[antonis]

But the small resistor at the +V in and upping the C12 cap to higher value makes sense to hopefully curb the motor boat issue.

Motorboating might occur due to power supply fluctuations caught by resistive divider for VR..

If the above proposed +9V LPF reduce it but don't eliminate it, increase VR LPF RC(*) time constant..

(*) R13 // (R14+R15) X C11..

[Rob Strand]

When you add the Mid control it stuffs up the bass and treble controls.   Theses things very fiddly to get right and you can never really get a three-band to sound like a two-band even with the mid control flat.

What happens is the Mid circuit interacts with the bass and treble circuits in a way that cannot be separated.   Because of that,  there's a limit how high you can set the bass frequencies and how low you can set the treble frequencies without it acting weird.

I think the G&L Bass preamp is a good example of tweaking these circuits.   Although, as is, the frequencies may or may not suite what you want.
https://www.bassesbyleo.com/forum/viewtopic.php?f=28&t=2342

The 3-bands EQ's tend to use the single cap treble and single cap bass forms of the Baxandall EQ - completely opposite of the FA-1 2-band!

[bluelagoon]

Hey Rob, Thanks for the link to the G&L Bass Pre Amp, Looks interesting, Hope to give it a try sometime,
Cheers,

[duck_arse]

Thanks Duck, but its just 3 separate SPDT all part of a 3PDT, thus named 1A 1B 1C, all same switch just used in different circuit locations, Dont know how I would simplify it any better.

[quite so, but, all three throw the same direction at the same time, so one pole can't be (shown) up whilst the other two are (shown) down. techs and builders are simple souls, easilly confused by transpositions.]