Smash Drive input cap - why so small?

[gigimarga]

Hello,

I am curious why Smash Drive (and other LM386 designs) uses for the input cap only small values from 4.7nF to 22nF?
In this way, doesn't it lost a lot of the low frequencies?
I replaced the input cap with a 33nF one, and, after, with a 100nF and the difference was subtle...why??

Thx a lot all!

[dschwartz]

I DONT KNOW.. CAUSE THEY ARE CHEAPER? 

but the reasons to use small caps as input caps are two:

- increase bass rolloff to get rid of "farty" sounds..
- the input impedance of the lm386 is high enough and make a RC filter with the 4.7nF cap with low enough bass rolloff..

bassically it´s a design choice..some design choices come from issues as simple as "i used that value cause i had a bunch of them and i wanted to get rid of them"

[aron]

It just sounded better with that capacitor value. I tried others but the simple answer is that it sounded good with it.

[dschwartz]

ohh.. i forgot that desigh choice.....it sounds good with it..

[gigimarga]

Thx a lot both!!

I see that i must to study more on impedances

[Electron Tornado]

Question for Aron,

I've got a smash drive on the bench right now. I really enjoy the sound with the drive at max. For the drive pot I tried both 5k and 1k pots and the drive seems to roll off abruptly when I back off the drive pot by about 1/3. Should it roll off that abruptly, or is there something I can try for a smoother decrease in drive?

The circuit right now uses an input buffer from the Grace Overdrive from Runoff Groove. This evening I'm going to change it and try a gain stage in front like the Big Daddy and see how it sounds:

http://runoffgroove.com/grace.html

[aron]

>Should it roll off that abruptly, or is there something I can try for a smoother decrease in drive?

I honestly can't remember because I have a tendency to run the drive on my distortions at a high level :-)
What you can do is is to use a smaller value pot and put a resistor in series with the pot. That way the pot is restricted to a narrow area of resistance. I would figure out the usable resistance by turning the pot and then measuring. Figure out the usable range and then use a close value pot for the range and a fixed resistor in series.