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Is CMOS really that noisy?

8/18/1996 2:50 PM
Mark Hammer
Is CMOS really that noisy?
Several posters indicated that CMOS-based distortion units are somewhat noisy. My own experience, building a repro of the  
EH-Hot Tubes, in addition to variations on all of Craig Anderton's designs, indicates quite the opposite. Given the gain  
achieved in any of these units, the amount of cumulative hiss is fairly low, and generally easily tamed with a capacitor  
straddling the output level pot.  
That being said, there is always the looming spectre of bad design, and bad layout, not to mention noisy individual CMOS  
chips. I assume that all the respective posters are using the UNbuffered version of 4049's for their projects (i.e., the  
recommended version).  
I might point out that the MXR envelope filter uses nothing BUT CMOS inverters to do its stuff, and is exceptionally  
noise and hiss free. So there you go.  
Mark Hammer
8/30/1996 3:19 PM
Erik Wood

Ok, Since your post, I have seen several related messages about CMOS stating that it is important to use UNbuffered inverters.  
Q1: Why? What does buffering do?  
Q2: Where do I get them? They are not listed in any Mouser or Digikey catalogs that I have found. Perhaps I'm not looking in the right place? Help!  
8/31/1996 9:17 PM
Mark Hammer

Well, generally it's HARDER to find buffered than unbuffered inverters  
in my experience, but since they turn up now and then (I bought some once),  
it's important to mention them. The buffering is to allow CMOS to interface  
with TTL chips. UNbuffered 4049's will end in the suffix UB or UBE.  
Buffered one's will end in B (just after the number 9). Buffered ones will  
not work in the distortion circuits. Since inverters are intended for  
logic purposes, manufacturers tend not to be mindful of tolerances for  
audio purposes. The consequence is that there is variation among 4049's,  
although this tends to be chip to chip rather than manufacturer to  
manufacturer or lot to lot, as far as I can tell. Noise is likely best  
reduced by buying a bubble-pack of 10 of these (or some quantity/form  
where you have enough of them to compare) and try them out until you  
find one you like. Just keep tabs on the static so you don't blow the  
one you DO like.  

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