Point-to-Point Wiring (interesting article)
michael p mccullough
Point-to-Point Wiring
hello
here is an interesting article i found on the mesa homepage that i
thought i would post here. it is written by randall smith. the
original URL is:
http://www.mesaboogie.com/US/Smith/ptp.shtml
please note his comments on repairing old fenders.
take care,
michael
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Point-to-Point Wiring
Myth VS. Reality
In the course of a recent interview, I was asked how the Trem-o-Verb
gets its great vintage
performance--without being wired point-to-point, and "would it be even
better if it were?"
Perhaps this wasn't such a surprising question since the author had
recently reviewed some pricey
amplifiers whose manufacturers emphasized that their amps were indeed
wired point-to-point, as if
that mere fact would indicate (to those "in the know" at least) a
product of precious distinction.
What Is It?
In point-to-point, the resistors and capacitors are joined with wires
to solder points laid out on an
insulating tag board. Wires from each solder joint then run off to the
tube sockets, switches, and
controls. One manufacturer has even rejected the tag board and uses
phenolic terminal strips
screwed to the chassis instead. This method deserves recognition for
being the slowest, most
labor-intensive and most error-prone wiring method of all.
Point-to-point is probably about the
oldest construction style and it's still appropriate for making a
"one-off" piece of elecrronics. But
does it produce a better sounding amp?
My job as designer has always been to focus on the "black magic" of
amplification: first, how to get
it and, second, how to get it consistently. And there, my friends, is
the biggest disadvantage with
point-to-point, consistency is very hard to maintain.
What We Do
From the first, Mesa/Boogies have used a combination of point-to-point
and printed circuit board
methods in order to ensure absolute consistent placement of critical
parts and conductors. It is easy
to demonstrate how moving some parts or lead wires as little as 1/4"
can make a huge difference in
the top end "transparency" of the sound--exactly where a lot of the
magic lives--or dies.
Inside a typical point-to-point amplifier are signal wires leading
from the component board to the
preamp tube sockets. Because many of these can be quite critical, we
always locate our tubes along
the center line of a printed circuit board so the lengths of traces
can be extremely short and perfectly
consistent.
Great care and many scrutinized revisions result in a circuit board
layout that avoids any unwanted
"stray couplings" that can rob tone, or may even include a few nuances
of intentional interaction
(black magic) which cannot be reliably duplicated in the
point-to-point style.
And Why We Do It
For example, the great top end of the Dual Rectifier has an aggressive
bite but avoids the harshness
by the way two very important traces are laid out on the board. One
trace is on the top and the
other runs right beneath it on the bottom side of the board. Thus the
very small--but
significant-amount of capacitance coupling these two together through
the board performs a subtle
filtering kind of negative feedback in a critical region where the
harmonics can be made to line up
just right. Obviously the alignment of the traces on both sides of the
board must be precise and
consistent for this to work.
Here's an illustration of the different wiring techniques. Think of
printing books versus writing them
out longhand. Once the type is properly set each printed page will be
the same--no errors. Now go
one step further and imagine trying to line up the letters on both the
top and bottom sides of a sheet
of paper--this time by writing longhand. It's all but impossible. Yet
such precise alignment is critical
to achieving the performance we're after--each and every time. With a
printing press or a printed
circuit board it's easy.
Where do we still wire point-to-point? Anywhere it makes a better
amplifier is the brief answer.
Many manufacturers try to put everything on printed circuitry, but we
individually mount and
point-to-point wire all jacks, switches, transformers and 8-pin power
tubes. The reasons are
increased reliability and total ease of replacement. If those parts
are PC mounted, major disassembly
is required to get at them. Even worse, they can cause the whole PC to
fracture if they're bashed in
hard enough, then the amp is practically non-repairable.
Our commitment is to provide a professional instrument which, with
minimal maintenance, can last a
lifetime or more. All our circuit boards are double-sided and "plated
through" meaning that each and
every hole has a platinum and copper sleeve formed inside of it which
is integral with the copper
traces on the top and bottom. This way each component lead is soldered
three times: top, bottom
and inside the hole.
Most manufacturers use singlesided boards where there is only one
solder connection per part on
thin foil glued to the boards surface. Moreover, these boards, once
installed can't be hand-soldered
for repair or replacement without totally dismantling the amplifier.
Let's Talk History
I must have repaired thousands of old Fenders--and I was the guy who
could usually get rid of the
stray noises--but not always. I finally found out why some of these
noisy amps seemed incurable.
The noise was originating in the point-to-point tag board itself! The
board material can absorb
moisture and become slightly conductive. The factory's recommendation
was to bake the boards in a
drying oven then dip them in melted wax to seal out the moisture!
Here's another: Tag boards warp, and the warpage increases the space
between eyelets. Years ago,
a guy phoned me, freaking out that his Fender had gone down before a
big gig. I listened to his
description of the problem, then advised him to pull out the chassis
and look for the
brown-black-brown-silver resistor running lengthwise across the board.
"Give it a little tug and you'll
probably find it's broken loose at the solder joint," I told him. He
called back relieved and
ecstatic--said that he'd never fixed anything before in his life. No
sweat. I must have fixed a hundred
black face Fenders where point-to-point board warpage had caused this
problem. Don't get me
wrong, I love Fenders. Without them, it's safe to say, none of us
would be here!
Conclusion
So there you have it, four specific advantages of the printed circuit
board: 1.Consistency of location,
2.Ability to use the board for intentional coupling, 3. Immunity to
noise-causing moisture, and 4.It
does not warp.
Now what about the way in which a conductor affects the sound? Is a
round copper wire better
than a flat copper trace on a circuit board? Not according to Randall
Research (no relation to me!)
who has studied audio conductors under all conditions, including at
the molecular level under an
electron microscope. His opinion is that if there is any advantage
either way, it would be with the
circuit board trace whose profile offers far greater surface area. It
is well established that audio
signals (especially the higher frequencies) tend to flow on the
surface of the conductor, not through
its core.
All this is not to say that there aren't great sounding point-to-point
amplifiers-obviously there are. But
having built lots of amplifiers both ways myself, the mere fact that
an amplifier has been wired this
way is, as far as I can see, a guarantee only that it will cost more.
If there was an intrinsic sonic
benefit to using point-to-point throughout, you can bet that we would
do it!
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michael p mccullough ++++ Klamath Falls OR # 1.541.884.3278
mic...@magick.net mich...@cdsnet.net
all moanday, tearsday, wailsday, thumpsday, frightday, shatterday (joyce/FW)
http://www.magick.net/~michael/ http://www.magick.net/~michael/music.html