[sdiy] Yusynth Transistor filter

[Simon Brouwer]

Oakley Sound schreef:
> > In both cases the resistance in the middle setting should be 
> considerably lower than half the maximum resistance.
>
> I don't think this is the case. For a 50K log pot the mid point would 
> give approximately 45K when used as a variable resistor with a 
> decreasing resistance in a clockwise rotation - ie. approx 90% of the 
> track resistance is left to go as you turn it towards minimum resistance.
>
> For a 50K rev-log pot it should be around 5K thus giving us the 
> required sensitivity at high resonances in the Moog ladder.
I see...

It depends on how you are using the pot. In a (reverse) log pot you have 
one pin at the end of the track where there is  lower resistance change 
per degree of rotation (let's call this pin A) and one pin at the end of 
the track where there is higher resistance change per degree of rotation 
(B).

In a log pot that is used for volume control, pin A will be the one 
connected to ground.

A log pot that is used e.g. for attack/decay/release time will be wired 
as a variable resistor using pin A and the wiper, giving a more or less 
constant ratio of resistance (time) change per degree of rotation. A 
reverse log pot could be wired this way for setting speed or frequency, 
for instance that of an LFO. I had this use in mind.

Now that you told me about it I can see why a reverse log pot, with 
wiper and pin B wired, is desirable for filter Q setting in the Moog filter.

Used this way, the characteristic of a (reverse) log pot can be 
approximated using a linear pot and a fixed resistor.
However, that of a (reverse) log pot, used as a variable resistance 
between wiper and pin A, can't.

-- 
Vriendelijke groet, 
Simon Brouwer. 

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