Mixing two pickups

[Marko Ursin]

I have two Wal-inspired eight-coil humbuckers on my bass. I have passive vol/vol/tone/tone-controls on it. I don't really like these controls. When only one of the pickups is on it's ok, but when trying to blend them they both begin to sound different from their original sound. I guess that has something to do with passive controls. Is there any good schematics for active buffers/mixers that would let me to blend the pickups without them interacting with each other?
Marko

[fieldwrangler]

A dual opamp (one for each pickup) with acceptable battery drain would be a likely choice, followed by maybe a 25K panpot. Do you use your tone controls?

Also, I seem to recall that Bill Bartolini has a turnkey epoxy-potted solution to this task.

Bob Palmieri

[Marko Ursin]

I'd rather not spend $80 to something that could be done in a fef dollars worth of parts. I've found TL072-based buffer schematics. I'll give them a try.
Marko

[fieldwrangler]

A good policy, and one that often leads to the acquisition of new skills & concepts.

I like the 072 a lot, although the 062 is much kinder to battery life.

Bob Palmieri

[pdf64]

If you turn both pickup passive volume controls down a little, that should greatly reduce pickup interaction. But then you would need a good, low capacitence lead to prevent high end roll off, due to the increased source impedance.
A TL062 may drive you nuts with hiss etc, especially if there's no passive option.
The OPXX series op amps (eg OP27) were the best I could find when I last looked into it, but as pointed out above, battery life becomes an issue as current draw goes much above a milliamp or so.
How do acoustic guitar pickup, eg fishman, manage to keep noise low yet keep button cells running for months, what electronics are they using?

[fieldwrangler]

Don't really know; haven't done any reverse engineering or anything. However, I seem to recall that when I did try optimizing for these conditions many years ago I used to put a FET in front of the opamp, and gain up/pad down somewhat to reduce noise. Seeing as how this was about a quarter-century ago and these applications have been commercialized in a big way since then, I'm sure there must be some cookbook-type solutions to be found (and/or tweaked to taste.)
I also recall feeling that most of the stuff people were using in the early 80's sacrificed adequate headroom for transients, 'specially with the pop-slapping bass thing... Actually, the mashing of the transients in the simpler FET-based onboard preamps became something of a feature for a lot of bass players

Also, voltage doublers can get more headroom out of a low-voltage supply.

Bob Palmieri

[David Schwab]

If you turn both pickup passive volume controls down a little, that should greatly reduce pickup interaction.

It will also load the pickups down so you wont get the real tone.

I've used a simple JFET buffer for each pickup, and then mixed the two together. That works well.

[fieldwrangler]

Haven't actually used JFET's in this application but I'd say it would be the first thing I'd try, 'specially since my recent experiences with the whole passive miasma.

[pdf64]

'It will also load the pickups down so you wont get the real tone.'
I was thinking of the normal guitar type wiring, in which the pickup is connected to the hot track end, and there's a pickup selector switch - are you thinking of the jazz bass wiring in which they are connected to their vol pot wiper, which does without the selector switch?

[David Schwab]

'It will also load the pickups down so you wont get the real tone.'
I was thinking of the normal guitar type wiring, in which the pickup is connected to the hot track end, and there's a pickup selector switch - are you thinking of the jazz bass wiring in which they are connected to their vol pot wiper, which does without the selector switch?

I'm talking the pickup right into the buffer, and the volume pots after the buffer. The volume controls load the pickup, regardless if it's the Jazz or LP style wiring. And then the two pickups load each other. When you buffer them first you actually mix the sound from each pickup without one affecting the other.

[Marko Ursin]

Are you talking about something like the Tillman FET buffer? It sure is simple and as far as I've read it's working fine as long as the J201 is biased correctly.
Marko

[David Schwab]

Are you talking about something like the Tillman FET buffer? It sure is simple and as far as I've read it's working fine as long as the J201 is biased correctly.
Marko

Yes, a circuit like that. I like to use a modified Alembic Stratoblaster, which is a self biasing circuit. I've been using one of those in many of my personal basses since the mid 80's.

[fieldwrangler]

Yes, a circuit like that. I like to use a modified Alembic Stratoblaster, which is a self biasing circuit. I've been using one of those in many of my personal basses since the mid 80's.

David -

I've used this as a starting point for any number of guitar applications.

However, If it were me I'd check the various topologies and available FET types for maximum headroom applications. How much voltage swing can you get from this circuit before things start to squish?

Bob Palmieri

[Mike Sulzer]

When you buffer them first you actually mix the sound from each pickup without one affecting the other.

Actually, that is exactly what happens when you connect them together passively (assuming identical pickups) also, except that the loading by external circuit elements is different. That is, connecting two identical pickups together and buffering that combination gives the same frequency response as buffering each and then adding the results.

You can show this using Thevenin's theorem, which Joe Gwinn introduced on another thread. I would be happy to show you how this works if you are interested.

So the major difference between two pickups in parallel, buffered and non-buffered, is from the cable capacitance. When paralleled, the series inductance drops by a factor of two compared to a single pickup. So an important difference between the sound of a single pickup and two in parallel is the increase in high frequencies from shifting the resonance up. The addition of the two different signals accounts for the rest.

I realize that this might seem non-intuitive, but it really is how it works, and you do have to understand the electronics and do the analysis to understand it.

[fieldwrangler]

Wow.

I gotta look at this when I have time...

Bob Palmieri