[sdiy] wall wart confusion
amokan
amokan at gmail.com
Mon Feb 27 22:45:55 CET 2006
Thanks Bob. I guess Laurie only replied to me and not the list. Sorry to
confuse anyone and drag this out.
On 2/27/06, Bob Weigel <sounddoctorin at imt.net> wrote:
>
> Who is Laurie? THought that's what I said. Must have missed someone
> else saying the same thing... anyway there are several types of
> 'ground'. Please endure through this tutorial as it contains some hints
> that will probably revolutionize your understanding on this matter.
> This little journey contains many useful tidbits and is well worth the
> time to absorb for people who don't have a fairly high level
> understanding of these things even.
>
> Earth ground means there is a connection to earth potential.
> "Common ground" could be earthed but not necessarily. In fact it could
> be totally isolated from earth and there may or may not be an earth
> ground also that for intsance shields the casing for safety reasons or
> for ES shielding.
>
> For instance old fender amps and the like often have a ground switch.
> Yet..they had a two prong cord. ie.. not directly earth grounded at
> all. The switch let you select which ac line coming in was touching a
> coupling capacitor (usually .022 - .068uF with .047 the most common I
> believe) whose other leg was touching the case ground. Case ground was
> used as a common tie point then for all lowest potential points in the
> amp.... since THESE amps used only single sided voltages except for bias
> voltage I guess you could say.
>
> In other words the power supply would have 440V- 490V to feed the high
> voltage needs of the power tubes. This was often filtered again after a
> choke to feed the screen grids of the power tubes.. This was dropped
> through resistance to attain another refiltered 350V ish supply for the
> plate resistors of the inverter tube. Then dropped some more and
> refiltered again for an even lower 200 some voltage supply for the
> pre-amp tubes.
>
> Meanwhile a separate winding fed the heaters of the 6V tubes, and a
> separate winding for trhe 5V rectifier in amps that used that method of
> rectifying. This winding had HV insulation since the DC voltage would
> accumulate right on that heater! (as electrons rushed from the heater
> to the plates powered by the HV windings of the power transformer; the
> center tap of which is case grounded!)
>
> Anyway these 6.3V windings were, in the old days just left to float
> often. (Eg. early Gibson EH-150 guitar amp). Today this is a source of
> GREAT NOISE because we have so much more 60hz floating around than back
> then. Quickly mfgs figured out that they need to put a low impedence
> from this winding to ground. However, in event of power tube failure
> it's not a great idea to totally ground them as it can destroy the HV
> windings of the transformer possibly especially if someone sticks the
> wrong fuse in. :-) So Fender put a 100ohm 1/2W resistor from each leg
> of the heater and in case the HV shorts, they will actually burn up
> before damage occurse to more expensive stuff. (100ma even going through
> 100 ohms, which all power transformers can supply, =1W of power.)
>
> Anyway, THE BIAS voltage in the bigger fender amps is designed to feed
> a small amount of current BELOW CASE GROUND to the control grids of the
> power tubes. This allows operation with the cathodes grounded directly;
> rather than running through a resistance to create a positive voltage
> there (which limits the clean headroom of the amp even with a paralell
> capacitance to feed signal by the resistor; a common failing part always
> also..). TUBES..work this way. If you allow the grid and cathode to
> be at the same voltage, they run pretty much wide open and the tube
> glows and burns out quickly. (Not to mention it's already at saturation
> so no signal dynamics are possible..) So the control grid must have a
> voltage below that of the cathode. ~-14V for EL84 tubes... ~-40V for
> EL34 tubes and ~-50V for 6L6 tubes... etc.
>
> To produce this voltage fender used an appropriate voltaged AC winding
> and they fed it through a diode that points away from a filter
> capacitance whose POSITIVE terminal..touches common case ground and
> whose negative terminal of course attaches to the diodes anode. The
> other leg of the winding is grounded creating a charge pump that will
> give a negative voltage buildup on that capacitance.
>
> So anyway... I dont' know what the circuit looks like that you are
> working with. But if it's designed for a + and - voltage rail with a
> common ground that is HALF WAY BETWEEN those voltages ...and you have
> available something with +18, 9, and 0V...then yes treat the 9V leg as
> the common ground and attach the other two with 18V on the + and 0V on
> the - and you should be ok. -Bob
>
> amokan wrote:
>
> > Thanks. Would I just tie all grounds on the circuit to a common point
> > in the case?
> >
> > On 2/27/06, *Aleks Clark* <aleks.clark at gmail.com
> > <mailto:aleks.clark at gmail.com>> wrote:
> >
> > yea, just use an 18v wall wart. what would have been 0 would be
> > -9, and what would have been +18 would be 9. Like Laurie said,
> > it's a matter of relative potential.
> >
> > On 2/27/06, * amokan* <amokan at gmail.com <mailto:amokan at gmail.com>>
> > wrote:
> >
> > So, are there wallwarts that will do +/- ? Am I better off
> > using an AC wallwart and then building one of the many power
> > supply circuits available online?
> >
> > I really just want something cheap and that only involves 1
> > plug. I also want to avoid anything much more than a regulator
> > in my circuit to keep it small.
> >
> > I've never really put much thought into this until now. All of
> > my circuits are done on a breadboard with a bench supply and
> > by the time I get them in a frac, they just plug into my main
> > supply that "magically" works :)
> >
> > Thanks again everyone. Sorry to drag this thread on.
> >
> >
> >
> > On 2/27/06, *Laurie Biddulph* < elby_designs at ozemail.com.au
> > <mailto:elby_designs at ozemail.com.au>> wrote:
> >
> > If the outputs are truly isolated from the mains power
> > (specifically that is not connected to the Earth pin -
> > which they should be) then you can connect the positive of
> > one wallwart to the 0V of the other. This (double) 0V
> > connection is connected to the 0V connection of your
> > module and you then have +ve (from the free positive lead)
> > and -V (from the free 0V lead).
> >
> > What you won't get is tracking between the outputs so one
> > might go up while the other droops etc. If you are using
> > these to power regulators in your circuit then this will
> > be less of a concern. You will also need to power outlets
> > and if your wallwarts are large then it may not be
> > possible to get 2 on to them (double) outlet.
> >
> > The size of a wallwart is a possible indication of wether
> > it is a transformer or switching design but not always. A
> > low power transformer unit will be quite small. Usually
> > there is next to no weight in the switching designs. Above
> > should hol true for these as well as the outputs should be
> > totally floating.
> >
> > Best Regards
> >
> > (Mr) Laurie Biddulph
> > Mobile: 0404 846 943
> >
> > Elby Designs
> > ABN: 70 022 727 605
> > http://www.elby-designs.com
> >
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> >
> > ----- Original Message -----
> > *From:* amokan <mailto:amokan at gmail.com>
> > *To:* synth-diy <mailto:synth-diy at dropmix.xs4all.nl>
> > *Sent:* Monday, February 27, 2006 5:20 PM
> > *Subject:* [sdiy] wall wart confusion
> >
> > I didn't pay attention back in high school electronics
> > when it came to power supplies so please forgive me if
> > this is a newbie question...
> >
> > I've built bipolar supply kits for modular use but
> > never really understood _how_ they work. Negative
> > supplies are a mystery to me. I know there is a
> > difference between ground and the negative rail, but I
> > don't know what the difference is. A 9v battery is
> > in-fact bipolar, yes?
> >
> > Long story short, I'm working on modules that will run
> > in a stomp box format and I'm trying to figure out the
> > cheapest & easiest way to power them. I know I could
> > use the dual 9v battery solution to power them
> > (assuming the circuits could run at 9v) but I'd really
> > like to just use a standard 9v or 12v dc wallwart that
> > I have laying around the house. How do I get a
> > negative voltage off of a standard 12v wall wart? Is
> > it already there normally, or just a positive and a
> > ground?
> >
> > Sorry for rambling. If there is a web resource
> > explaining this, please let me know.
> >
> > Thanks!
> >
> >
> >
> >
> >
> > --
> > http://www.nomesh.net <http://www.nomesh.net>
> > http://www.radioresponse.org
> > http://www.freebsd.org <http://www.freebsd.org>
> >
> >
>
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