Circuit alteration for preamp

[Skywalker83]

Ok in my search for some circuits for help with my preamp design i came across the circuit shown below.

My preamp is going to have

Passive/Active DPDT switch
Tone (pasive and active)
Bass and treble stacked pot (Active)
Mid with mid sweep stacked pot (Active)
Volume
Blend

I plan on taking the pickups into the blend pot then to the input (im going to change out the boost switch in the circuit for a trim pot that will go on the board) then throught to the preamp and EQ.

Where would i add the tone pot and the passive/active switch in this circuit?

How would i alter the "Mid" section to include a mid sweep?

[BASSnSAX]

I'm not sure how do a mid-sweep, but I was looking to do something very similar to what you were going to do, except to only have the tone control work for passive mode.

Here's my diagram:




Sorry for the poor diagram, I'm not an electrical engineer. I've got a Bartolini NTMB HR 4.6 harness: push/pull volume, blend, push/pull mid for 250/800 MHz freq selector, stacked bass/treble with side mounted jack.

I'd like to redo the plate, and have the layout be as follows: stacked 250k volume/tone, active/passive 2 pos mini switch, blend, mid-freq 3 pos selector switch with 250/500/800 MHz, 50k Mid control, stacked bass/treble.

The one thing that I was thinking about was if it would really pay to have this feature on one bass, or if I should just get a second bass that's passive. I was also thinking, for a solution with volume issues switching back and forth between active and passive, I came up with two solutions. 1) Use a volume pedal, and set the minimum volume setting for the active setting and max volume for passive, or 2) plug the output of the bass into an AB or ABY switch, and plug both outputs into the active and passive inputs on my amp.

Thoughts?

[slyjoe]

For mid sweep, put a dual ganged 50K pot in series with the 2 47K resistors at the mid control. Good luck finding that pot.

Not sure why you would want a tone pot in both passive and active modes.

The active/passive for this circuit is pretty straightforward if you don't need a tone pot. I don't have time right now - maybe someone else will contribute, or I can get to it later. Good luck!

[fdeck]

slyjoe has it. Some interesting diagrams here:

http://www.all-electric.com/schematic/mix-eq.htm

[Skywalker83]

yeah the tone is just for passive, dunno why i wrote "active" there.

Thanks for those schematics fdeck, one of those looks like a good semi-parametric version (the 3 band with the mid shift).


I think for the volume difference in the active and passive circuits can be balanced by adding a trim pot it the feedback loop of the active preamp stage. I'm not sure is that would do it, but i've seen this in quite a few preamps as an input gain adjust. The John East preamps use them, and can be seen on his website.

[BASSnSAX]

Originally Posted by slyjoe

Not sure why you would want a tone pot in both passive and active modes.

The passive tone comes in handy if you have a boost-only preamp, like the Sadowsky preamp/VTC. I don't particularly enjoy the treble frequency setting (a little too clanky) on the NTMB preamp, so I think a passive tone control for both active and passive would work great. I think it would round out the sound and make up for the huge gap between the mids and treble frequencies.

And yes, the internal trim pot would work too, I just thought of that while I was going to sleep last night.

[slyjoe]

To disconnect the battery you would need a 3pdt switch. This may not sound good with a 10K volume pot (passive) either, but not sure how to get around that without adding another pot.

[Skywalker83]

i was going to have the battery conected throught the jack so that when the jack is plugged in it completes the circuit. It would mean it wa son all the time, but would have less drain when the bass was in passive mode.


Heres my wiring schematic.

[slyjoe]

That's fine - but the problem is the volume pot - 10K for the active side. That is way too low for the passive side.

Even when the preamp is out of the circuit, it's drain is pretty much the same as when it's in the circuit. Unless you are talking clocked circuits, the difference in current consumption isn't that different between when the preamp is being used and when it is not.

I still think the major problem will be the value of the volume pot in the passive case.

[Skywalker83]

Yes , the volume knob would have to have 2 sections to it.
A passive section that allowed a high impedence output, and an active section that allowed a low impedance output. The john east preamps have customed designed pots (from ALPS)that do just this.

[Skywalker83]

Oh hang on , what about adding a buffer to the passive circuit?? Wouldn't that work?

[Rick Auricchio]

Originally Posted by Skywalker83

Oh hang on , what about adding a buffer to the passive circuit?? Wouldn't that work?

Yes, it would now be an active circuit. Without passive tone controls.

It seems you're overcomplicating this.

[Skywalker83]

It seems you're overcomplicating this.

well then what would you suggest doing?

if i put a buffer in between the tone and volume pot , it would be a passive tone going to an active volume.

The easy way to do it would just be to have a volume pot for each circuit, but i dont want that.

[slyjoe]

In my opinion it would be better to have 2 volumes. If you don't want another hole, use a stacked volume/tone for the passive side.

I guess it depends on what the passive switch is for. If you plan on using it a lot, I don't think there is any way to switch between passive and active without adjusting the single volume pot. And, like I said before, the 10K for the active side will make a terrible volume control for the passive side.

If you have two volume pots, you could punch the switch and get the passive and active gains to be close together, with no need to adjust volume controls when switching.

[Skywalker83]

Thanks for all the input guys, i'll draw out some new set ups this weekend , and hopefully do a full schematic as well and post them up.

[slyjoe]

Well get to it - we don't have all day.




Just kidding. I know how RL can get in the way.

[Skywalker83]

OK,

Here is version 1 of the active circuit.

I took parts from a couple of different circuits and sort of joined them together.
The Mid control section is missing, i dont have clue how to set out the double gang pot Mid and mid sweep part.

All input appreciated as always.
I did the schematic using EXPRESS SCH.

[Skywalker83]

BUMPED

[slyjoe]

OK...is there a question?

[Skywalker83]

How does the schematic look? (apart from incomplete)