Calculating Cut-off frequency of Low Pass Filter for synth project

Thread Starter

Jack Tranckle

Joined Jan 20, 2016
73
So I am designing my own midi to CV converter. my design is similar to the one below, and will be connected to a sawtooth oscillator. My design is basic and will only require a range of 5 octaves, highest being just over 2000Hz. Therefore from my knowledge my cut off frequency will not need to be much more than that.

However, taking the below circuit as an example , lets say that it is designed to output the highest human range of 20,000Hz! Using the equation:

upload_2018-3-10_19-15-12.png

R = 6k8 & C = 270p

Which calculates cut-off frequency to be 86685 Hz!! why is it so high? There are a few designs that produce similar results online too. Am i missing something? Do i need to factor something else into the equation?

 

Thread Starter

Jack Tranckle

Joined Jan 20, 2016
73
I thought that is was used to smooth the DAC output? Also, is that 50 HZ sharp notch the same in all DACs? as I was looking into a newer 8 bit DAC.
 

ericgibbs

Joined Jan 29, 2010
16,364
hi,
This PDF has some useful info on DAC noise.
I will look around for any upto date DAC data, let you know if I find any useful data.
E
 

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Thread Starter

Jack Tranckle

Joined Jan 20, 2016
73
I wonder if this applies to modern day DAC's. maybe I should stick to what I know and use a DAC0808 with a 80Khz LPF. but it would be nice to use a more modern IC since 0808's seem to be more expensive due to their lack of use.
 
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