[Tfug] Opine: Bricks, warts or...?

[Bexley Hall]

--- On Tue, 8/5/08, Tim Ottinger <tottinge at gmail.com> wrote:

> On Tue, Aug 5, 2008 at 10:34 PM, Bexley Hall
> <bexley401 at yahoo.com> wrote:
> 
> > Are multi-effect units just N dedicated effect units
> > in a single box?  Or, are they "programmable/configurable"
> > in some way?
> 
> Oh, you can program and configure the heck out of them. 

Ah, OK.  So, they add considerable value... (?)

> You have N banks of N patches which are amazingly configurable.
> And you get a bunch of presets you can edit to taste.  One click
> of a footswitch and you can go from an SRV overdrive/wah/vibe 
> to a U2 overdrive/delay/vox model patch.

SRV =?= "Stevie Ray Vaughn"?  U2 =?= "U2"?
Or, some *other* acronyms?

> Great stuff, though they tend to be solid state, and that means
> less tone than a bunch of analog pedals.

Not sure what you mean by "solid state"  :>  Are the effects
processed *digitally* or just "with op amps"?

In the former case, that would probably explain the special
power supply -- also probably a much higher "load" than one
of your other "dedicated pedals".

> > Understood.  Note that if you could find a *DC* unit with
> > the same sort of plug, this would work as well (unless the
> > unit is using the AC signal as a timebase *or* as part of
> > an active noise reduction (hum) system (unlikely)
> 
> Really?

Yes -- with reservations.  If it is a two conductor 12VAC
plug, then there is a rectifier (probably a bridge) inside the
effects unit that is converting this to DC.  An AC power source
has "no polarity" -- i.e., the inside of the barrel is positive
wrt the outside at one instant... then, milliseconds later,
the reverse is true.

*AS SUCH* ... you can plug in a DC power source of *either*
polarity (i.e., center positive or center negative) and the
internal rectifier will sort this out automatically (i.e.,
the rectifier *in* your DC power source is redundant in this
case).

IMO, all wall warts should be AC output.  Moving the rectifier
into the wall wart doesn't buy you much (this is not true
for bricks, though).  And, since wall warts are AC, DC+ or
DC-, there's just three times as many varieties to MISchoose
from!  :<

In your case, the problem is knowing what the load is like vs.
the characteristics of the source (power supply).

A 12VAC adapter produces a nominal 12V signal but it is really
more like 16V peak.  If the power source has deliberately been
"over specified" wrt the load, then they may be expecting to
have more than 12V available for use in the effects unit.
(e.g., 15V)

OTOH, if the load and source are more closely matched, then
"12V" is really "12V".  In this case, you have to consider
the voltage drop caused by the "extra" rectifier that is
now present.  This can be as much as 1.5V.

I.e., with a very expensive device, it's best to fix the
original power source, if poissible.  Barring that, you
should, ideally, look at the source and load and see
where the "sweet spot" is between the two.

Connecting a power source that is "too stiff" (i.e.,
can handle a very large load) *and* "a bit too high"
can cause more damage -- the rectifier inside the
device is followed by filter capacitors.  These typically
are rated 6.3v, 10V, 16v, 25v, etc.  A robust design
will specify components with lots of "margin" -- i.e.,
use 25V caps on a 12V circuit instead of 16V caps.
(this is called "derating" -- you "de-rate" the specs
of the components you are using and *then* verify that
they work in the design.  I, for example, derate by 40%
which means I pretend a 16V cap is only rated ~10V so
I wouldn't consider it appropriate in a 12V circuit).
If your choice of power source results in the voltage
seen by these devices *increasing* a bit (e.g., 15V
becomes 16V), then reliability suffers.

(capacitors -- and connectors -- are probably the
Achilles Heel of most electronic devices nowadays :< )

[of course, this is more than you wanted to know  :> ]

Maybe Tom W. can clarify/correct/expound upon anything
that I've missed?

> > (sigh)  Do you still have the brick?  (I can probably
> > fix it)
> 
> Yeah, but I'm not anywhere near Tucson.

Ah, well...  :>  Regardless, if this was a "nice unit",
don't give up hope on it.  You can probably find someone
who can fix it (hopefully *without* doing more damage
in the process!)