The importance of the lead spacing of caps and ppm value of resistors

[hetsch]

I'm currently stocking up components and I would have some questions regarding

• the lead spacing of capacitors: Reading through the forum posts and some BOMs, I think that I've read out the following rules. For values above 1uF buy aluminum/tantalum caps with a lead spacing around 2.5mm. For values between 0.01uF and 1uF, buy film caps with a lead spacing of 5mm. For pF values, one could use ceramic caps with a lead spacing of 2.5mm. The pF values are the thing that troubles me. My plan is to buy MLCCs for these values and they come with 2.5mm or 5mm lead spacing at Mouser. Some PCBs (http://www.rullywow.com/wp-content/uploads/2013/12/What-Components-do-I-need.pdf) that I've come across use 5mm lead spacing for ceramics, for other's I could not find specifications or they don't provide one. While I'm quite sure that I can bend their leads that they fit most PCBs with either 2.5 or 5mm spaced holes, I don't know what makes more sense to buy, 2.5mm or 5mm pF caps?
  (By the way: Some people prefer to use poly (boxed) film caps for values between 100pF and 1nF - don't these usually have 5mm lead spacing?)


• the ppm value of resistors: I've found some cheaper KOA MF1 100ppm resistors at Mouser whose price - if bought at a quantity > 100 - is really good. The thing is that these resistors have a temperature coefficient of 100ppm, while the often recommended Xicon 271 resistors have 50ppm, but are more expensive. While I've found a post by Paul (http://www.madbeanpedals.com/forum/index.php?topic=13939.msg129278#msg129278) that recommends to not worry about ppm, I would be very interested in your opinion about the ppm and the KOAs.

Thank's again for your help!

[niektb]

Well, you're going to have a tough time with fitting a 5mm boxed film cap into a 2.5mm footprint, that's for sure. Other than that it doesn't really matter (you'll probably have to bend the leads a bit)

Verstuurd vanaf mijn Lenovo P2a42 met Tapatalk

[EBK]

Isn't it wonderful to have so many choices of parts available that we can discriminate among them down to the most minor details?  You could probably even specify what lead thickness you wanted.

(Not trying to play down your concerns.  I actually do respect what you are doing.)

[hetsch]

Hehe , yes too much choices without having a understanding what they really mean. I think that means I should not care too much ...

[amptramp]

Lead spacing of capacitors is usually some multiple of 0.1 inch = 2.54 mm so you should see what multiple you need.  (Yes, the US is still mired in the foot/pound system rather than metric, but that gives them an advantage in trade with the other nations using the same system, namely Myanmar and Liberia.)  These work perfectly with veroboard which is also 0.1 inch spacing.  You can always bend leads and I have seen box capacitors with greater than 5 mm / 0.2 inch spacing - 0.3 is common and other larger ones up to and beyond one inch exist.  I have a bag of tropical fish capacitors rated at 0.15 µF 400V sitting beside the keyboard with 0.8 inch or 20 mm lead spacing.

Resistor temperature coefficients are only important where you are trying to balance something (like the phase splitter in a Green Ringer) or where you have precise non-adjustable tuning like in a phaser.  Otherwise you will not see much effect and in these two cases, you still only have an effect that is measurable but not usually audible.  Let's face it, if people can use a Fuzz Face which has wild variations in response with temperature, resistor drift is usually a miniscule problem.

[reddesert]

It's usually easier to bend leads out to fit a 2.5mm capacitor into 5mm spacing than to bend them in the other way around.

Think about the temperature coefficient quantitatively.  You can easily build nearly any guitar pedal circuit with 5% tolerance resistors, let alone 1% tolerance.  For a resistor with a temp coeff of 100ppm/degree C, in order to change its value by 1%, the temperature has to change by 0.01 / 100ppm = 0.01 / (100x 10^-6) = 100 degrees C.  Your circuit is unlikely to be used outside a very generous ambient range of say -20 to 40 deg C.  In principle, if you built something that dissipated a lot of heat, the internal temp could get over 100 deg C, but this is unlikely for pedals, and we're still only talking abut a 1% shift. It really doesn't matter.

In practice, the only DIY audio circuit I've noticed so far with temperature compensation is in tuning stability for DIY synthesizers.

[PRR]

Don't even look at tempco. All modern passive parts are MUCH better than audio needs.

{EDIT - reddesert said the same thing, faster} 100PPM is 0.1% per degree C. Many old products were built with 10% base tolerance parts. For a resistor tempco to exceed this range, the temperature has to change 100 degrees C. The *player's* performance will degrade long before the thermometer hits -75C or +125C. (-103F or 257F)

> where you are trying to balance something

If you need a "matched pair", you get two of the same part. The differential error will be much smaller than the absolute error or rated tempco. If one of the pair of "6.8K" drifts to 6,750 ohms, the other will probably be 6,730 or 6,770 Ohms, far better than 1% error and close-enough for almost any audio.

> tuning stability for DIY synthesizers.

Because the log converter junctions have quite huge tempco. (And because the ear is very sensitive to relative pitch.) You put junctions in pairs for a first correction. Then you run into minor errors and start tying to balance-out those. This IS a specialized field, quite apart from most audio, and certainly stompboxes.

[GGBB]

As mentioned already - it's easier to fit a 2.5mm spaced ceramic into a 5mm spaced board slot than vice-versa. That goes for just about any radial leaded component - but pay attention to the lead length as well as spacing. Ceramic and film caps can be bought with either short (PCB mount length) or long leads - short ones will only fit into their designed spacing.

Box films - especially smaller ones - can be found in 2.5mm spacing, but I don't see those used a lot in pedal layouts - probably best to stick with 5mm.

With electrolytic caps you'll find that lead spacing is often not matched to the 0.1" increment. Aluminum electrolytics in small values often have 1.5mm spacing, and as they get larger you often see 3.5mm spacing. This is in addition to 2.5mm and 5.0mm spacing etc. Try for 2.5mm but 1.5mm is fine, and even 3.5mm can be made to fit into both 2.5mm and 5.0mm positions if necessary.

And do not forget about physical size - this is just as important as lead spacing as layouts are sometimes tight. You may find that film and ceramic caps wider than 2.5mm won't fit beside each other properly. Bigger film caps (>220n) usually are given more room since they are difficult to obtain at widths <=2.5mm. Small aluminum electrolytics often need to be <=5mm in diameter. A tight layout will often push to the edge of these bounds to fit all the components in a small space so you really need the right sized components in addition to the right lead spacing.

And watch out for component height too (large electrolytics mostly) if you want to use shallow enclosures like the 1590B and 1590BB.

[hetsch]

Wow... Thank you all for these thoughtful and insightful answers, I really appreciate that. I for sure would not have grabbed the 100ppm resistors and I was biased towards the 5mm ceramic caps. Saves me some $ and pain! Thank's!

[mg.audio]

By far the most common capacitor spacing I've encountered is 5mm. I use TDK MLCC's from Mouser, the little blue ones. They come in both 2.5 and 5mm spacings, and the 5mm is suuuper easy to straighten out the leads to 2.5mm.

Check it:

[GGBB]

Good point about the ease of bending the leads. Likewise, the 2.5mm are quite easily bent out to 5mm:



I typically buy 5mm myself, but either way can be made to work when you have these styles of leads.