Audio filter values swapped?

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alastair
Posts: 17
Joined: 10 Oct 2019, 14:28

Audio filter values swapped?

Post by alastair » 19 Dec 2019, 15:36

I've been working on the audio amp/filters on my own project and investigating if the R2R DAC could be rolled in to the low pass filter as the input resistor. It seems to work and this is also the approach taken by the Gigatron. On closer inspection though I'm not sure the filter values look correct. The Gigatron low pass filter would have a cutoff frequency of 160 Hz and the high pass around 720 Hz. This is the opposite of what is shown in the note on the schematic.

I might have missed something here though, so just wanted to clarify. I did calculate what I think the correct values should be (based on the schematic note) and came up with a value of 22nF for the low pass filter capacitor. You could use the same value in the high pass filter, but the resistor would need to be 47k. This represents a very high output impedance, so it might be better to use a larger capacitor like 1uF and drop the resistor down to 1k.

alastair
Posts: 17
Joined: 10 Oct 2019, 14:28

Re: Audio filter values swapped?

Post by alastair » 19 Dec 2019, 18:16

It looks like the frequency response diagram is correct given the Gigatron component values. I think it's just the text that's swapped over. It should probably read "f_H (700Hz) > f_L (160Hz)"

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marcelk
Posts: 490
Joined: 13 May 2018, 08:26

Re: Audio filter values swapped?

Post by marcelk » 19 Dec 2019, 18:42

I think so. The text can also be removed :-)

I remember we were delighted when we discovered that 3 functions (HPF, LPF, line level) could be made from 3 passives. And not just that, but only values already used elsewhere in the design.

Here's the prototype:

IMG_2626.jpg
IMG_2626.jpg (161.11 KiB) Viewed 1532 times

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