A Different Grounding Question

[Mobius]

I need to build a couple direct-to-soundcard guitar preamps that will use bipolar supplies. I want to power each of them from their own AC wallwart.

I have a 24V/800mA wart that puts out 28.4VAC unloaded. Neither of these preamps would draw much, so I should expect to be able to have as much as 28V to use. (Never mind that the wall outlets at my apartment are at 129VAC and I can’t expect that everywhere.)

28V will rectify to almost 40V. I’ll split this in half with a TLE2426. Let’s assume that I end up with -18V/0V/+18V. Cool.

BUT:

I will have (at least) three jacks connected to each box: Guitar in, Line out, and Headphone out.

Where do I connect the grounds of these jacks? Only to each other? If so, will I be able to hear anything when only the guitar and headphones are plugged in, with no hard ground? I can’t connect them to the 0V of my rail splitter because it’s +18V to the outside world, right?

Thanks,

Matthew

 

[nickt]

[quote author="Mobius"]I need to build a couple direct-to-soundcard guitar preamps that will use bipolar supplies. I want to power each of them from their own AC wallwart.

I have a 24V/800mA wart that puts out 28.4VAC unloaded. Neither of these preamps would draw much, so I should expect to be able to have as much as 28V to use. (Never mind that the wall outlets at my apartment are at 129VAC and I can’t expect that everywhere.)

28V will rectify to almost 40V. I’ll split this in half with a TLE2426. Let’s assume that I end up with -18V/0V/+18V. Cool.

BUT:

I will have (at least) three jacks connected to each box: Guitar in, Line out, and Headphone out.

Where do I connect the grounds of these jacks? Only to each other? If so, will I be able to hear anything when only the guitar and headphones are plugged in, with no hard ground? I can’t connect them to the 0V of my rail splitter because it’s +18V to the outside world, right?

Thanks,

Matthew[/quote]
Hi Matt,

You're thinking of it the wrong way. The 28V in is just AC and is not referenced to ground. If you rectify it and split it in half there is no reason the half point (0V) can't be set as ground. Hope this helps.

cheers
Nick

 

[Mobius]

Mike, thanks for your reply.

What threw me off was that the TLE2426 data sheet calls one of the inputs “common.” Yeah, I know that common does not necessarily mean 0V or ground, but in building wiring, (where I have the most experience,) it does.

Matthew

 

[BradAvenson]

You shouldn't need the TLE2426. A voltage doubler will give you +/- supplies just as easily.
http://www.play-hookey.com/ac_theory/ps_v_multipliers.html

 

[Mobius]

Thanks, Brad. I've seen that before, and others like it and it was another route I was considering. Is there any reason why it can't be done with dual full wave rectifiers?

 

[nickt]

[quote author="Mobius"]Thanks, Brad. I've seen that before, and others like it and it was another route I was considering. Is there any reason why it can't be done with dual full wave rectifiers?[/quote]

Hi Mobius - two full wave rectifiers will cause a nice fireworks display :twisted: but won't give you what you're after.

ONE full wave rectifier will give you 28V * 1.414 = ~40V between the +ve and -ve outputs.

Put two 10K resistors in series between the +ve and -ve outputs. Then parallel each resistor with a biggish electrolytic - say 1000uF 63V - keeping +ve towards +ve.

The mid point of the two resistors (and capacitors) is your 0V.

Connect all your Grounds to this 0V and you're done.

 

[Mobius]

two full wave rectifiers will cause a nice fireworks display

So this won't work?

http://twin-x.com/groupdiy/displayimage.php?pos=-2607

Thanks,

Matthew

 

[nickt]

[quote author="Mobius"]

two full wave rectifiers will cause a nice fireworks display

So this won't work?

http://twin-x.com/groupdiy/displayimage.php?pos=-2607

Thanks,

Matthew[/quote]

Matt - the way you've drawn it it won't even work for one full wave :grin:

Pick the top secondary of the trannie. then trace it through the diodes that conduct when... bugger it... here's what I'm talking about.

http://twin-x.com/groupdiy/displayimage.php?pos=-2610

What you want is...

http://twin-x.com/groupdiy/displayimage.php?pos=-2609

cheers
Nick

 

[Mobius]

Confounded computers! I was in a hurry and cut and pasted wrong. :evil:

This http://twin-x.com/groupdiy/displayimage.php?album=lastup&cat=0&pos=0 is what I meant.

Will this work?

Thanks,

Matthew

(I see I made this mistake elsewhere on this computer. I probably shouldn't be doing this while at work.) :roll:

 

[nickt]

No it won't