--- In motm@y..., "Paul Schreiber" <synth1@a...> wrote:
> I offer replacement pots (both Spectrol and Bourns) for $6.50ea.
> And, if you need a manual for a purchased module, send me an
> email. [...]
Some of you are probably confused by this response and are thinking,
buy replacement pots? Huh? And as you might expect, there is a little
more to this story!
Right after posting the "why not just use one brand" question, I
called Paul expecting to ask a completely different question.
However, "Spectrol vs Bourns" still seemed to be on his mind - so here
is my summary of potentiometers 101:
There are essentially two base technologies: cermet and conductive
plastic. (This refers to the type of electrically resistive element
used inside the potentiometer.) Conductive plastic has the longest
life so, all else being equal, it is the technology of choice!
However, all else is almost never equal. Cermet offers significantly
lower electrical noise (Paul claims 10x) and, as such, it is preferred
in a few critical applications such as tuning the frequency of an
oscillator or filter. (But you already knew this based on other posts
to this thread.)
So, why choose one ∗brand∗ over the other? Here are the details:
Spectrol offers conductive plastic elements (148 series) and cermet
elements (149 series) in an otherwise equal form factor. These
Spectrol pots are only available with PC board "pins" - that is,
intended to mount directly to a printed circuit board. The standard
Spectrol 148 and 149 series pots are linear taper. Log taper is only
available by special order and you must commit to quantity.
Bourns only offers conductive plastic elements. Bourns pots are
available with PC board "pins" AND with solder tabs for termination to
wire. Bourns pots are always "special order" (no distributor stock)
but are available in both linear and log taper configurations.
The form factor (specifically, the distance from the centerline of the
shaft to the base of the pot) is different between the Spectrol and
Bourns pots and, as such, it is not possible to mix and match the two
brands of pots on the same printed circuit.
So as it turns out, there does seem to be valid reasons why you might
choose one brand over the other based on the requirements of a
particular module. (A specific example from Paul - EGs need large
value log taper pots and it is expected that these pots will get a lot
of use. In this case, Bourns is the preferred solution as these are
the long life conductive plastic and log taper is available as a
standard configuration. Sure enough, there are board mounted Bourns
pots in the 800 EG! In other modules where there is a mix of cermet
and conductive plastic types mounted to the circuit board, the
Spectrol types are used.)
Fine - but you are probably still wondering about the replacement pot
comment? Notice in my original question that I mention the Spectrol
248 series - specifically, that I found the 248 series to be
particularly stiff. Wait a minute, what is the 248 series??? Well,
it turns out that very early on (before Paul managed to convince
Spectrol to build 148 series 100k log pots) some modules were built
with this 248 series. This Spectrol 248 series has a plastic busing
around the shaft where all the other types have a metal busing -
logically, this explains the difference in feel. The 148 and 248 seem
to be interchangeable otherwise.
Because the 148 series log pots are special order, I asked Paul if I
could purchase a few of these from him should I decide that I really
can't stand the feel of the 248 series. He agreed, which I think is
pretty damn nice of him!
Paul also agreed to provide me with a copy of the MOTM 300 assembly /
calibration / operation manual - which was the real reason that I
called him. My MOTM setup doesn't have any VCOs yet and, as Cuari has
put his MOTM system up for sale, I offered to buy a bunch of VCOs from
him. (He didn't have the module documentation - and I wanted to be
sure I could get a copy just in case.) So, now that I have some
VCOs on the way I can really make some noise! Oh wait, I still need
to work on that patch cable deficiency... ;)
Seth