Audio circuits

David Betz · 2012-08-06 19:15

I know this has been discussed here at least once before but I want to build an audio device using SIDcog and I'm wondering what good choices are for the filter components on the audio output pins? I know there was some discussion about whether the C3 audio output filter was designed correctly for decent audio. What are the recommended values for the filter components for line level audio output?

Thanks,
David

Phil Pilgrim (PhiPi) · 2012-08-06 19:44

It all depends on what frequency response you want to achieve. Here's a link to an online Sallen and Key audio filter design tool:

http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

-Phil

average joe · 2012-08-06 19:47

Hi David,

I use a 1k resistor and .01uf cap. Ceramic caps work fine. I also recommend a 1uf in addition to parallax's stock filter. See the bottom middle of the touchburger schematic for example.

*edit*
Only because I didn't look for the schematic!

Dr_Acula · 2012-08-06 19:48

I'm using 1k, 0.01uF lowpass and then a 1uF polyester blocking capacitor. 15khz cutoff if I recall correctly. Can drive headphones directly and sounds pretty good to me.

addit - averagejoe beat me by 1 minute!

David Betz · 2012-08-06 19:50

Thanks everyone!

David Betz · 2012-08-06 19:56

average joe wrote: »

Hi David,

I use a 1k resistor and .01uf cap. Ceramic caps work fine. I also recommend a 1uf in addition to parallax's stock filter. See the bottom middle of the touchburger schematic for example.

*edit*
Only because I didn't look for the schematic!

Thanks again. I found it myself in your thread. Are these boards available yet?

average joe · 2012-08-06 20:02

Yep! I've tested mine and can give it 2 thumbs up! I have 2 extras in my possession, and most parts for 2 more. There will be a second version without debugging leds and an expansion port. I'm holding off sending the "pre-sold" kit for the expansion. Doc will have them soon I'm sure!

David Betz · 2012-08-06 20:08

average joe wrote: »

Yep! I've tested mine and can give it 2 thumbs up! I have 2 extras in my possession, and most parts for 2 more. There will be a second version without debugging leds and an expansion port. I'm holding off sending the "pre-sold" kit for the expansion. Doc will have them soon I'm sure!

Cool! It sounds like a nice board.

Dr_Acula · 2012-08-06 21:30

Yes getting 30 boards made at the moment. Minor change from the one averagejoe has - now that it is working we removed the 10 diagnostic leds and replaced this with a 34 way expansion header. I think averagejoe is planning on doing some kits.

zappman · 2012-10-26 16:08

Dr_Acula wrote: »

I'm using 1k, 0.01uF lowpass and then a 1uF polyester blocking capacitor. 15khz cutoff if I recall correctly. Can drive headphones directly and sounds pretty good to me.

addit - averagejoe beat me by 1 minute!

I am using the 1K, 0.01uF lowpass and a 1uF Electrolytic blocking capacitor (+ hooked to lowpass filter, - to the center conductor of my audio jack), the filter sounds great.

Thanks,

zappman

Mark_T · 2012-10-26 18:20

Your amplified speakers are likely built for an earphone drive level - ie 16 or 32 ohms. Try changing the 1k for 120 and 0.01uF for 0.1uF, and the 1uF electrolytic for 330uF - won't be that loud, but 18dB better I think.

ManAtWork · 2012-10-30 02:33

Phil Pilgrim (PhiPi) wrote: »

It all depends on what frequency response you want to achieve. Here's a link to an online Sallen and Key audio filter design tool:
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

Hello Phil,

I found out that sallen key filters don't work as well as expected with the prop. The reason is that the feedback capacitor conducts HF noise to the output and its bandwidth is way above what a cheap opamp can handle. A second order passive RC low pass followed by a simple non-inverting buffer works better (no feedback connected to the input = no HF bypass).

Audio circuits

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