Stereo Bass with Dedicated Box and other Circuits

[Opher]

So, reading plenty of forums and tutorials I have a basic understanding of 1,000 different ways to reach my goal, but any input from some more experienced builders would be helpful. I've wired and rewired plenty of tone control and switching circuits so I am not a complete beginner but this may be my first foray into building anything other than passive circuits.  So without too muchuck, here's the basic idea I have and the questions.

Passive Bass vi, wired for parallel pickup outputs to one volume to a stereo jack, usually this will be bridge only, series connected pickups to another volume to the other leg of the stereo jack, usually will be neck and middle in series, common ground throughout bass to ground on stereo jack
- usually when people talk about doing this on guitars they use two mono jacks, I don't understand why if you've got your pickup selection on board why you don't just go for a stereo output (if you're all passive like I am and don't need the jack to be a battery switch)

Stereo output from bass gets split back to two mono channels on a "bass control pedal". The pedal will start off simple and expand as I breadboard some of the simple circuits I've been dying to make

Series pickups enter a passive low pass filter for more bass like signal path. 
Parallel pickups enter a passive high pass filter and on to a more guitar like signal path

Part of the idea is to add a small delay to the "guitar" signal path so it sounds like there is a guitar and bass playing the same part.  The bass vi is especially good at creating both tones

So, when I split these signals back out, do I need to go through a buffer or transformer isolation stage to minimize noise and do the grounds need to be isolated for some reason I'm unaware of when I hit this stage. 

The next stage in the signal chain will probably be a minimalist active eq or a passive tone stack, most like because I've already basically created a "crossover" I will go active anyway on the eq section to minimize signal loss

From there I would have any other effects I deem necessary, I am just concerned about having too many "buffer" stages in this design because it will probably expand into two desperate compressors, one for each circuit. Which brings me too...

I've been looking at some designs on Elliot sounds website on line level crossovers and other stuff to reach other end goals in my signal chain.  On eqs, compressors, etc, if the design is for line level is it possible to just go do circuit into line level device back into "revamp" circuit (if necessary to get back to hi Z) or can these designs generally crossover anyway. Do all the resistor/cap values change or do I just need to change my input stage to match impedance.  Its one of the details in circuit design that I can never seem to find an answer for or get my head around. I know the impedance and output voltages are different in these circuits, but I don't quite get the differences in the holistic circuit design when it cones to these factors.

I'll leave the post at this for now. Its already too long winded, but if I get some good feedback I may delve into my other ideas.  Thanks

[ashcat_lt]

nm - didn't read.  sorry

[anotherjim]

So, this Bass has 4 pickups? If not, I can't get my head around 2 p/u's in series AND in parallel at the same time then isolated to 2 outputs. Do you have a drawing of that you can post?

Why 2 mono jacks? Try getting a proper screened stereo cable that's as tough as a normal guitar one. Your only real option is balanced or mic cable. These have the hot & cold wires twisted together throughout, which is a noise cancelling feature in it's intended use, but will mean crosstalk between channels if used for a dual mono role.

That said, I don't believe crosstalk will be a problem in your application. You plan to low pass the "full bass" channel and high pass the "lead", so that means the only crosstalk audible will be from the unfiltered bass channel into the lead channel. That's assuming the filter's in the box, not the guitar.

I would have buffers as the first thing the cable meets. Then filtering, which could be built into the buffers.

[Opher]

Thanks for the input. No, its 3 pups and the onboard series parallel switching is really immaterial but any given moment I'll have up to three pickups on one line in series on 1 feed, 3 in parallel on the other, or something in between.  Generally though, two on series on one and onestand alone on the other.

SO the immediate buffer will most likely be my first eq stage. Which brings me back to my other question which is, when looking at those designs, if they were designed as line input or mic level eq's, is that going to extensively change the circuitry. I probably just have some more studying to do....

[ashcat_lt]

I mean, it's not really immaterial if your entire idea is based on a false premise.  You just plain can't have two or more pickups in series and in parallel at the same time.  You can't have one of a series set also in parallel with another without having them all on both sides.  But ok.  Let's assume that you've got that all worked out.

Anything designed for line or mic use will almost certainly have an input impedance too low for guitar work, which will cause some pretty severe low-pass action before you try to do anything else.  That is usually pretty easy to fix, but we'd need a specific schematic to identify the appropriate changes.

[anotherjim]

It think I get it.
So 3 p/us - call A, B & C
2 channels H & L (Hi pass & Lo pass).
Selections could be
H = A + B +C (3 in series)
L = non
or
H = A||B||C (3 in parallel)
L = non
or
H = A + B
L = C
or
H = A||C
L = B
or
H = C
L = A + B
etc...

Plenty of simple guitar buffer designs out there - search.
A low input impedance may suit your bass channel (automatic low pass filtering!), but not the lead.

[ashcat_lt]

A low input impedance may suit your bass channel (automatic low pass filtering!), but not the lead.

The LPF action would change depending on which/how many pickups are selected.  If you want predictable, consistent results, you'd be better off with a high impedance there.

[Opher]

Anotherjim you're exactly right. Although to be honest I haven't perfectly figured that arrangement out with 3 dpdt switches. Its still flawed and I may have to use my fourth which I was trying to avoid

Ash cat youre getting to the heart of my last question which I am starting to understand as I keep reading more on buffers. I guess no matter what I should hit for around a 10k ohm input at least to keep everything moving. I am still reading some suggestions at other sites that state transformer isolation on the stereo signal should be done, at least at the end of the circuit,  and at least on one channel, to minimize ground loops, I may go that route for safety's sake, but I just wanted to be sure it didn't need to be at the beginning.

I regard less, if someone will entertain me a little more before I start drawing or linking schematics, if I were to utilize a circuit, again say an eq, for a mic level or line level input, would another buffer in front of that circuit, presume in the 10 to 100 ohm output range, be sufficient to ensure that circuit works properly?  Then end with another buffer in the 1kohm range to drive remaining effects before a di/pre

I'm getting there. Need to take some more notes then I may be able to draw a legit schematic for scrutiny

[ashcat_lt]

Errr...  Guitar pickups have typical DC resistances from like 3K up to even 15K.  Just that into a 10K input makes a voltage divider that causes significant broadband losses (compromising the already piss-poor s/n ratio).  Much worse is the huge amount of inductance in the pickup coil, which means that for higher frequencies the pickup's impedance gets really big really fast, so that higher frequencies are divided down even further.  Pickups in series just make everything that much worse.  10K input impedance is very much like turning a typical T pot down to like 5 or lower.  You won't like that. 

We normally go for a 10:1 ratio of load impedance to source impedance, so that might indicate somewhere between 30K and 150K (based on my numbers above), but this is in parallel with the V and T pots in the guitar, so what we really want is a value bigger enough than those pots to not significantly impact their value.  We usually shoot for 500K-1M for pedals that aren't deliberately destroying the treble.  That's a couple orders of magnitude bigger than you've suggested.

Yes, a buffer is exactly what we use to connect a high impedance source to a circuit with a low load impedance.  Like I said, though, it is sometimes pretty easy to change the in-Z of a circuit.  It could be as easy as swapping out one or two resistors.  We need to see the scheme to say for sure.

Something to keep in mind is that a circuit designed for mic or line work is likely to be set up for balanced input or output or both, which will want some thinking when you go to connect it to the unbalanced world of guitars their amps.

[anotherjim]

Go for a non-inverting op-amp buffer. TL07x op-amp would be fine. The input impedance can easily be made switch-able if it's any use, because it's set by a single resistance that has no effect on the gain of the buffer (which would be unity).
This...

R3 sets the input impedance, so 1Meg which is considered standard for passive pickups.

[Opher]

Whoa, that's the Plon Fentar buffer .  . I was seriously thinking about breadboarding that one first. Any experience with that with other opamps?  I was considering going with one with some better specs just for peace of mind.  Maybe that's overkill, all of the authorities seem to state that type of transistor/opamp isnt that mportant (aside from Germanium)

Yeah, I apologize for screwing up my numbers, I've read them a hundred times in the past week and am just now getting them to stay in memory.

I also found a schematic on a high impedance output buffer. I really want to build a variation of the bazz fuss to go with this project, but didn't know if I should throw that in a bypass before these buffers or build a high impedance output buffer before it so it has flexibility in the effects chain?  Thoughts, concerns, criticisms, recommended schematics?

Thank you all for bearing with me