Bass preamp design, crit and comments please!!

[Skywalker83]

Oh , here are the 2 schematics for the active and passive circuits for my onboard bass preamp and EQ i'm designing. Feedback welcome people.

[Chuck H]

You may want to repost this over at the "guitar tech" forum. This would be the place to post a schem of the actual active circuitry. Pickup switching is something for the guitar wiring guys. Not that there arent some here that know guitar wiring. But there are people more critical of it over at the other forum.

FWIW your diagram looks fine. There are only two minor rubs I can see. One, I cannot tell from your diagram if there is a ground reference on the input for the preamp. Without one you will get a 'POP' when you flip the switch. Two, By placing each pickup on it's own volume control and then sending them to a "master" volume coltrol you have effectively doubled the amount of loading to the pickups. Most guitar wiring schematics I've seen use only one ground reference on a voltage divider for loading. On a bass guitar I don't think it will be a problem, but it's probably not ideal. You could switch to 1M pots and that would put your total load on the pickups right where it would be if you only used one volume control. But the taper might be funky.

Chuck

[Skywalker83]

Thanks for your reply.

I think you may have missunderstood the diagrams slightly.

Each pickup doesn't have its own volume.But each circuit does. So there are 2 volumes, one for the Passive circuit and one for the Active circuit. (This was done out of recommendation).

I think you are right about the ground for the active circuit input. Where would the ground go?? feel free to make any changes to the circuits and post it back up.

[Chuck H]

Ok. Since you asked for it...

Your basic circuitry looks good in a macro sense. But there are some things that can be idealized. I did find a couple more "rubs" with the original schem you provided. So overall the problems that need addressing are:

1) No ground reference to the active preamp side of the switch can 'pop'. 2) The way your volume control and switching is wired your 'active' preamp will also be effected by the 'passive' volume control. Essentially making it so you must adjust two volume controls that do the same thing. That is, unless you will not have the active preamp volume control on the front of the guitar so you intend to use the master volume for the active output also. I would have to re-draw the schem. 3) Your tone control is cattywhompus (a technical term ). The way it is now just adds series resistance to the signal. the way I have drawn it is "correct" as far as a typical circuit goes. 4) The output from the 'active' preamp is always connected to the guitar output even when not in use. This can cause extra hiss and noise that is easily avoided, and should be. It involves using a DPDT switch instead of a SPDT switch.

I've re-drawn a schem that should "work" in the technical sense. But I want to mention something about the 'blend' control.

That control makes it so that when both pickups are equal in volume they are both at half volume. Because the pickups are in parallel (which means they share the load) whenever you have both pickups in use you can never be above half volume. If your guitar has 3 pots for the passive circuit (as the wiring suggests) I think you would be much better off with one volume control for each pickup and a master tone control. That way you can dial each pickup to any output you want (including both up full) and you would have infinite blending capabilities. JMHO though. It's your guitar. You know how you like to play.

If your interested I will redraw it again. You would have to decide which pots control what for the most ergonomic approach. The dual gang pot would be replaced with a single pot.

HTH

Chuck

[Steve Conner]

Connecting the wipers of the two volume controls like that isn't going to work. If you turn one down to zero, it'll short the other one, and you'll get no output at all.

[Skywalker83]

ok i've re-done the schematic (again).

i want the input op amp stage confugured so i can adjust the input gain via trim pot.

How do i go about doing this? Or does anyone know of a suitable circuit that could be altered (if need be)and put into the schematic?

Also , where to the 2 leads that come out of the battery section go to?