Obscure Part

[Deep Blue]

According to JD Sleep's bill of materials for a DOD 440 Envelope Follower, I need something called a "VTL5C4/2 dual opto resistor unit."  I'm not seeing on a Small Bear, but I might be missing it.  Anybody have any idea what it is, or where I could get it?

(stupid random question) - Do I want to be buying audio or linear tapered pots?  Somehow I knew which to buy the last 3 times I started a project, but for some reason I can't recall.

[Dan N]

Look again. Smallbear has them listed in Photocells and Photocouplers (Vactec VTL5C4/2 Dual-cell Photocoupler).

Good luck!

[Mark Hammer]

About pot tapers....

Log taper pots exist largely because of the properties of our ears.  Look at any intro psych text and you'll see something in the chapter on perception about the non-linear nature of our senses.  Take a glass of pure distilled water, and it is easy to taste the difference between that and the same glass of water with, say 20 grains of salt in it.  Of course, put a level teaspoon of salt into that glass and it takes a whole lot more than 20 more grains for you to taste it as "a little bit more salty".  In general, across all our senses, as something gets more intense, it takes much more of it to result in the sensation of it being more intense.  

This is also true of hearing, so volume controls have a taper such that from the midpoint of rotation on up, there is a much bigger change in resistance, than from the midpoint down.  The intent is to have small changes in resistance for the first bit (because these are easy for the ear to detect), and increasingly larger changes in resistance as you move along (because they result in the signal amplitude changes the ear needs to detect sound level change).

The same thing generally goes for frequency too.  It is easier  to detect the difference between a tremolo speed of 0.5hz vs 1hz than it is to detect the difference between 7hz and 9hz.  As a result, LFO rate controls are often logarithmic for the same reason volume controls are.

So why linear controls?  There are two reasons.  One is that sometimes you need to be able to accurately define the "middle".  For instance a balance control that allows you to combine dry and wet signals needs to be able to graphically represent an even balance so having a control with a visible midpoint (i.e., same resistance on one side of the midpoint and the other) is handy.  Same thing goes for shelving or resonant equalizer cut/boost controls: the visible midpoint conveys that there is no boost OR cut at that setting (i.e., flat).  There ARE log-antilog pots with a true midpoint such that the resistive element has the same overall value on either side of the midpoint but the amount of resistive change per degree rotation does not stay the same.  These are pretty rare, though.

The second reason to use a linear pot is when you need to spread out the change over enough of the pot's rotation that it is physically easier to dial in what you want more accurately.  So, even though it may be easier to *detect* smaller changes in LFO rate at lower frequencies, it is often more critical for the feel of a song to dial in speed more accurately at faster speed settings, in which case you want a bit more of the pot's rotation to be devoted to that range from, say, 4hz to 8hz.  Here, a linear pot can be helpful.

So, the general rule of thumb is that if you really need to be able to easily identify some sort of "middle", go for linear.  If it is volume-related, go for log.  And if you need to spread out some portion of the range of a control to let you zero in on a paremeter with more accuracy, you can either use log or linear depending on what it is you are controlling.  The first step, though, is always to think through the characteristics of what it is the pot is being used to control.  That will usually tip you off as to the needs.  There is generally no physical or electronic risk to using one when you don't have the other, merely an inconvenience from a user standpoint, so feel free to use log for linear or vice versa if you go to the parts bin and that's all you have at the moment.

[Adam Shame]

That is probably one of the best posts I have ever seen on this board...  Thanks very much Mark for explaining that in the simplest terms possible!