The Crossover Preamp

[WaveshapeIllusions]

A friend was looking for a preamp that had a built in crossover. He wanted one lowpassed pickup and one highpassed pickup, with the crossover at 250 Hz. I got about 234 Hz with standard values, close enough.

Each pickup is buffered and the sent to the filter sections. After that they go to the balance pot and a summing amplifier. Finally, there is a tone pot that sweeps from 15 kHz to 1.5 kHz and a volume.



So, what do y'all think? It looks pretty good to me. Any additions or improvements? I wanted to use a +/- supply, but that wouldn't work out too well if I wanted to use the ring connection on a jack to switch it on and off.

If you like, please feel free to use it in your instruments. I thought his idea was pretty cool, he said he was looking at it like a two-way speaker system. You have the bridge pickup that excels at treble and the neck that excels at bass. A crossover to mix them, which would also help eliminate any phase issues between the pickups. The balance control can kind of act like a tone control in a way.

[PRR]

Have you built it?

From the drawing:

Needs input capacitors.

R6 R5 R16 volume pots are backward (as drawn, at low setting  they short-out the opamp).

Vref needs a hefty bypass cap or signals will be leaking around your filters.

U4 could be re-designed with 680pFd and 1Meg, and then you would not really need the input buffer.

The U2 low-pass could 'probaby' live without input buffering since you don't *want* any treble from it.

(Both these hacks should be checked before rushing to mass production.)

Equal R equal C unity-gain 2-pole filter is not the sharpest knife in the drawer, though this is probably good for this aplication.

There WILL be phase issues. Maybe benign, though 240Hz is a kinda-sensitive area.

[Gus]

To add a little to PRRs post.   Do you want R7 and R8 to be 1meg?  

FWIW
I built a crossover box for a friend that played Jazz with a 7 string, one guitar to the box then a HP to the guitar amp and a LP to a bass amp.  IIRC we selected a D for the crossover frequency.  The amps used were, a bass amp closed back speaker cab and a Musicman amp open back speaker cab.

Some findings
Like PPR posted phase not only inverted speaker phase but distance between the amps, EQ settings etc.  The Musicman amp sounded best turned around
The HP and LP having the same frequency did not sound good
I changed the circuit to be more of an overlap circuit using first order filters.  Set the high pass at the D we first picked and I moved the lowpass frequency up.

I don't know were my schematic and notes are if I find them I will post more details.

My point is maybe experiment with the frequency and don't get stuck at setting the HP and LP at the same frequency.  
Your circuit is different than the one I built.  
I would add a dpdt switch to one of the pickups to invert the signal from one pickup to hear how the sound changes.
You will be picking up different parts of the strings with the pickup placement.

[WaveshapeIllusions]

Ah, I didn't notice that issue Paul. Probably need at least 10uF on Vref, maybe 47. I'll switch the balance pots ground to Vref, or maybe just series resistance. I've found there's a bit of a volume drop in the center position if I wire a balance like a standard volume. The last volume pot is after the coupling cap, so I think ground-referencing it will be okay. I normally ignore the input cap when it's meant to be inside a guitar, since pickups don't add any DC offset, and shielding blocks enough RF. In this case, I agree they're needed; don't want 4.5 V across the coils. Can you tell I don't mess with virtual ground much?

I wanted to buffer them to make sure the crossover frequency is accurate mainly. I'm not sure if I should be too bothered by the phase issues. It's one of those unavoidable things.

[WaveshapeIllusions]

Gus, I like your idea. I figure since they are separate pickups, the phase cancellations might be less of an issue. If they're mostly passing different frequencies, there may be less peaks and dips. I have to test it out. I'll add a DIP switch to invert one pickup though. The input resistors are 1meg.

My test rig might not match up with my friend's though. His bass will have a mudbucker at the neck and a jazz pickup (split coil humcancelling I think) at the bridge. I've got an EB3, so at least I have a mudbucker too to see how it sounds. My bridge pickup will sound different though.

He suggested adding variable crossover frequency to it. Implementing that would be a tad difficult. I could use a dual pot for each. Or I could take a page from some envelope filters and use a pot to vary the voltage on some jfer gates. I have a feeling that wouldn't be too accurate though... matching would take some time.