Floating Ground

[smoguzbenjamin]

Hi all

Still working on the ole woody here (haven't had much time, lately), and I didn't want to use 2 9v batteries, but a wallwart, as with the other 6 pedals I have

Anyway I decided to halve the voltage and use tha 4.5v as a flaoting ground, 'tricking' the opamps into thinking they had +/- 4.5v, which should be OK. But doesn't having 4.5v (not being true ground) mess up the ground for the other pedals?

I was wondering about that, I don't wanna let the blue smoke outta my BOSS pedals

[gez]

Anyway I decided to halve the voltage and use tha 4.5v as a flaoting ground, 'tricking' the opamps into thinking they had +/- 4.5v, which should be OK. But doesn't having 4.5v (not being true ground) mess up the ground for the other pedals?

I'm not familiar with this schematic, but I believe you're using a single 9V supply and are referencing all the ground connections to 4.5V using a divider?  If that's the case then you'll need an output cap to block DC.  If you exclude this most effects would be all right but I can think of a few FET based circuits (not to mention amps without an input cap) that might not be too happy!

edit: plus you'd get a few pops when switching!

[smoguzbenjamin]

Yeah, I'm referencing ground to 4.5v. however, putting an input cap in, where? The signal path or ground? I always put a 1uF cap before & after an effect if there isn't one allready, so that's covered. And popping when switching, I don't think it'll be a problem... Just add pulldown resistors.

But having 4.5v on the woody ground won't interfere with other effects in the chain, won't it?

[gez]

If you have input and output caps on the effect then it's not going to be a problem.  Whatever follows/is before will just see 0V DC and won't be affected by your effect's biasing (that's what the caps are there for, they block DC).

[gez]

But having 4.5v on the woody ground won't interfere with other effects in the chain, won't it?

It would help if you posted a schematic (edit: or a link to the original), but if you're doing what I think you're doing then 4.5V isn't ground, the negative rail still acts as ground; you're just referencing the op-amp's connections that normally go to ground in a dual supply to the middle of the divider in a single supply.

[Mike Burgundy]

ground still really *is* ground on input/output when executed correctly. It's just that the opamps "see" 4.5V as their own personal ground, which only means you set the + input at 4.5VDC, and the opamp will force the output to that level as well. Stick in coupling caps before and after, and there's no problem with weird DC levels. Offcourse if an opamp is feeding directly into a next one without a coupling cap (pos. input) you don't have to hook up Vref because the first output already is at that DC-level.
Have a look at almost any opamp circuit to see how this is done.
Any discreet circuitry (such as FETs) you can still run off 9V/ground.

[smoguzbenjamin]

http://ampage.org/hammer/files/Woody.zip from http://hammer.ampage.org

I was thinking of putting 4.5v on ground (everything with the ground symbol) and 0v on the opamp grounds. Or should I do it the other way round?

[Mike Burgundy]

this will leave 4.5V DC on the in-and output grounds!! This will possibly screw with other effects, and make for a wonderful BANG when switching. Also, you'll have DC on some pots which makes for nice scratching sounds.
Correction: I checked the schem. In-and out should be fine.
It's nice to hook up to Vref:
IC1a +
IC1b +
R12
IC2c +
R19
and stick in 1u caps
between IC2a out and P1a
between IC2d and P1b

Your idea will also work I think, as long as you make sure you R1 and P2, plus in/out ground to *real* ground (0V)

[smoguzbenjamin]

So it's jackplug rings to ground, pulldown resistors to ground. Well, I'll try both and see what sounds best. I still haven't got round to finishing it up on the breadboard Thanks Mike!