[sdiy] Fixed filter bank questions

Tim Ressel timr at circuitabbey.com
Tue May 14 18:30:49 CEST 2019

Another question to ask: production or one-off? For a one-off I'd match 
the caps, possibly with a simple bridge, and then trim the filter freqs 
with resistors. Matching the caps will make sure the Q of the filters is 
not affected.


On 5/13/2019 10:19 PM, rsdio at audiobanshee.com wrote:
> I recently saw a web page with cross sections of various electronic parts. You could see how the interiors of capacitors are nowhere near as uniform as the external container.
> Just because something is automated doesn’t necessarily mean that the results will always be within 1%. It depends entirely upon the process. There are chemical, optical, mechanical, and many other methods for manufacturing. Some techniques build materials up, some cut them away, and some processes combine the two directions. You can’t even depend upon whether a PCB will be made by etching away existing copper or building up copper on a blank, chemically. This stuff ain’t easy, and if it was then some company would surely have 1% or 0.1% capacitors to blow away all of the competition - it’s not like the market can’t use precise caps.
> As with most technologies, it only seems easy when you have no idea what’s involved. There’s a reason we have the saying, The Devil is in the Details.
> By the way, I think it was an older version of this page - https://twitter.com/i/moments/1112130787694931968?lang=en
> On May 13, 2019, at 5:17 PM, Tim Ressel <timr at circuitabbey.com> wrote:
>> Now we are into the realm of process control. Think about how a film cap is made: two layers of a conductive material sandwiched around a dielectric film, and then rolled up. Any thickness tolerance in any of those layers gets multiplied by the number of wraps. Controlling those thicknesses is possible but probably costs a lot.  You can get 1% caps but there does not seem to be a good selection, suggesting low demand. Also they are pricey. The same goes for SMT ceramic caps.
>> I think we get spoiled by resistors. They can be laser trimmed and tight tolerance parts are common and not too expensive. Even down to 0.1% they seem to be reasonable.
>> When designing we need to determine just how much tightness we need, then ask our wallets how much we can afford, and then balance the two. Just like brothels.
>> --tr
>> On 5/13/2019 4:45 PM, David G Dixon wrote:
>>> When I look at, say, those little blue or yellow film caps, it is simply
>>> inconceivable to me that they could be made 10% different from one another.
>>> If you've ever seen a continuous automated industrial process that cranks
>>> out millions of the same thing, the key feature is repeatability.  In order
>>> for every part of the process to work, and for every piece of the assembly
>>> to fit together, every part has to be identical from one piece to the next.
>>> That is a recipe for 1% tolerance, not 10% or 20%.
>>> Matthew said:
>>> On the contrary, I don't think the basic process of manufacturing
>>> capacitors has changed much since the 1960s - they are still made by the
>>> same kind of machines out of the same kind of materials - so why should we
>>> expect those processes to give much different results today?  Automation
>>> isn't magic, and it's likely to be used to reduce costs rather than to
>>> tighten tolerances, if the market seems to prioritize price.
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--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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