Analog filter

Prune · 16th February 2004, 07:07 AM

The CS43122 only needs a simple analog filter at the differential outputs. In the datasheet, it's two 300R in series with each output followed by 2200pf between + and -. When I put this in LTSPice, it shows a pretty big dB dip and phase shift at the high end.
Would it be allright to change this by say using a lower capacitance, or otherwise modifying the values? Or is there a reason not to do this?

Prune · 19th February 2004, 07:20 PM

Still hoping for a reply

Elso Kwak · 19th February 2004, 09:31 PM

Quote:

Originally posted by Prune
Still hoping for a reply

Hi Prune,
On the Analog Devices website or that of Texas Instruments is a program to calculate balanced filters.
www.analog.com
http://www.analog.com/Analog_Root/st...er/filter.html
www.ti.com
Search for FilterPro(R)

Prune · 19th February 2004, 10:29 PM

I'm using jwb's desing:
http://atari.saturn5.com/~jwb/43122.png
There are no opamps. The CS43122 has voltage output. jwb used the same passive filter as in the CS43122 datasheet (2 resistors and a capacitor), followed by a voltage follower.
When running this output stage in LTSpice, lowering the capacitor value slows falloff in the high frequencies. I was just wondering if there would be negative side effects in doing that (I want to even out the frequency response).

Lyra · 20th February 2004, 01:01 AM

Maybe this

site could be of help.......

Lyra

hjelm · 20th February 2004, 11:39 AM

I do not think it is that bad. It falls less than a dB and 9degrees phase shift, or i might have done something wrong.

I am also kind of interested to see what people think of these low pass filters. If i look at the CS43122 spec it seems that if you are running it in mode0(whatever that is) the attenuation is at least 100dB in the stopband so i would guess that this circuit would not cause any trouble in any amplifier that follows it even without filter.

Is it important sounwise to have absolutely flat frequency response 20-20k?

Prune · 20th February 2004, 08:44 PM

Yes, it falls just under a dB; however, I'm getting close to 24 degrees phase shift. Considering that the DC blocking output capacitors shift the low end phase to the other direction, that's pretty large.

I tried without a filter with my tube headphone amp, and didn't notice a problem. I'll be making the Aleph-X, so I wonder if that might be a problem.

As I don't have any smaller SMD caps for this, I wonder if it would be OK to just put two of these 2200pF in series.

hjelm · 21st February 2004, 09:44 PM

Just a gut feeling i have but i would not put two caps in series. You decrease the resonant frequency of the cap by four, might still be ok though.

Could you decrease the value of the resistors instead or would that load the dac too much?

Prune · 21st February 2004, 11:19 PM

Well I did it anyways, and it worked fine.
Capacitors in series halves the capacitance. But I don't know about resonant frequency. Can you point me to some info? I had trouble finding what you were talking about on Google.

Prune · 22nd February 2004, 03:57 AM

BTW will the CS43122 be able to drive a potentiometer? I need a volume control and I want to put it before the voltage follower, else I'm limiting what kinds of loads I can drive. And what value pot would be optimal?

16th February 2004, 07:07 AM	#1
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	Analog filter The CS43122 only needs a simple analog filter at the differential outputs. In the datasheet, it's two 300R in series with each output followed by 2200pf between + and -. When I put this in LTSPice, it shows a pretty big dB dip and phase shift at the high end. Would it be allright to change this by say using a lower capacitance, or otherwise modifying the values? Or is there a reason not to do this?

20th February 2004, 11:39 AM	#6
hjelm diyAudio Member Join Date: Nov 2003 Location: Västerås	I do not think it is that bad. It falls less than a dB and 9degrees phase shift, or i might have done something wrong. I am also kind of interested to see what people think of these low pass filters. If i look at the CS43122 spec it seems that if you are running it in mode0(whatever that is) the attenuation is at least 100dB in the stopband so i would guess that this circuit would not cause any trouble in any amplifier that follows it even without filter. Is it important sounwise to have absolutely flat frequency response 20-20k? __________________ Hjelm

21st February 2004, 09:44 PM	#8
hjelm diyAudio Member Join Date: Nov 2003 Location: Västerås	Just a gut feeling i have but i would not put two caps in series. You decrease the resonant frequency of the cap by four, might still be ok though. Could you decrease the value of the resistors instead or would that load the dac too much? __________________ Hjelm

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19th February 2004, 07:20 PM	#2
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	Still hoping for a reply

19th February 2004, 10:29 PM	#4
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	I'm using jwb's desing: http://atari.saturn5.com/~jwb/43122.png There are no opamps. The CS43122 has voltage output. jwb used the same passive filter as in the CS43122 datasheet (2 resistors and a capacitor), followed by a voltage follower. When running this output stage in LTSpice, lowering the capacitor value slows falloff in the high frequencies. I was just wondering if there would be negative side effects in doing that (I want to even out the frequency response).

20th February 2004, 01:01 AM	#5
Lyra diyAudio Member Join Date: Nov 2003 Location: Norway	Maybe this site could be of help....... Lyra

20th February 2004, 08:44 PM	#7
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	Yes, it falls just under a dB; however, I'm getting close to 24 degrees phase shift. Considering that the DC blocking output capacitors shift the low end phase to the other direction, that's pretty large. I tried without a filter with my tube headphone amp, and didn't notice a problem. I'll be making the Aleph-X, so I wonder if that might be a problem. As I don't have any smaller SMD caps for this, I wonder if it would be OK to just put two of these 2200pF in series.

21st February 2004, 11:19 PM	#9
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	Well I did it anyways, and it worked fine. Capacitors in series halves the capacitance. But I don't know about resonant frequency. Can you point me to some info? I had trouble finding what you were talking about on Google.

22nd February 2004, 03:57 AM	#10
Prune Account Disabled Join Date: Mar 2003 Location: Vancouver	BTW will the CS43122 be able to drive a potentiometer? I need a volume control and I want to put it before the voltage follower, else I'm limiting what kinds of loads I can drive. And what value pot would be optimal?

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