Piezo pre AND mag pre onboard using Y cable out?

It's rarely an issue. It's done all the time on mixing consoles. Just throw a summing resistor in series with each output.

 

Or blend them with an emg blend pot. Its just a 25K dual gang linear pot. No need for summing resistors here as the in-line resistance is in the pot when the signals are blended. (if you get what i mean.)

It would be nice also, if you can adjust the gain of either of the preamps. Otherwise all the balancing you have is really just the pickup screws.

 

the simplest really would be to replace the matrix pre with the powerchip!

it's designed to do just what you're looking for, right down to automatically splitting the sources with a stereo cable or summing them if you use a mono cable.

 

I saw a a board on the john east website for mixing in a piezo , it might be worth checking out, I have a u retro delux in one of my basses and its a killer pre

 

Most opamp preamps have a series resistor on the output partly for this reason. If you like, try wiring an emg blend pot in a box with three jacks - two inputs and one output. And see if you get a smooth blend from one bass to the other.

Notice that the tone controls in the bass on the right won't effect the signal from the piezo. If you are happy with it this way, then as I mentioned, when you get your fretless routed and fitted with the bart and pre, all you need is that blend pot.

 

The difference between an EMG blend pot and any other blend pot is like, 17 dollars of difference and a little green board.
If you want a blend pot, just get any dual gang pot with an appropriate resistance, like 25k or 50k.

 

Several. Firstly a blend pot is usually MN or log/antilog, and for this you actually need linear, which is what the emg one is. Secondly, that little green board will make it easier for a novice to wire. Thirdly it has a centre click, which a regular linear pot won't have. Fourthly, I noticed your bass is rear routed, and this pot will have a longer than standard threaded portion. And fifthly, IME, they are good quality Alps pots.

Cool. If you get a volume drop or anything weird with volume control interaction, then as line6man said, use a couple of summing resistors.

 

http://www.ebay.com/itm/Dual-25K-Li...251?pt=Guitar_Accessories&hash=item27b92d6fd3

If you want to save a few bucks and don't mind waiting for stuff to come from China, then these should be OK. They look like they have a long thread, but you'll need a shakeproof washer. And I didn't see a mention of centre click.

 

Oh, and be prepared to change the phase of your magnetic pickup. It's hard to predict which way to wire it because of the way the bridge creates voltage when the strings are plucked, so I wire a temporary phase reverse switch to the signal from the magnetic pickup and when all the electronics are up and running, I wind the mag/piezo blend to the middle and flick the phase of the magnetic pickup back and forth to find the best sounding polarity.