Bessel filterdesign

[tubee]

Hi all

I modified a Philips CDP304mk2 with Kwak clock, OPA2132 opamps and a lot more. Sounds good, but the treble has sometimes a sharp edge. I wondered if it is possible to exchange the standard (butterworth?) filter around opamps to a more phase-lineair Bessel type.

The original Philips schematic shows a 2N cap paralleled with 1k8 in first feedback loop/opamp. Two 2k4 resistors to non-inverting input with inbetween 2.2N and 470uH in 2nd feedback loop of 2nd opamp.

I have been looking on some sites, eg: http://www.analog.com/Wizard/filter/filterUserEntry/#responseType

Filter free offer a download too. Too complicated for me. My electronics education was a very long time ago,(though having a good grade for it 😉
Has someone a suggestion and hints to calculate the right part values?

Thanks.

 

[poynton]

you could try mailing 6H5c. He has done a lot of modelling on the outputs of the CD63 range.

 

[jackinnj]

try this;

http://www-users.cs.york.ac.uk/~fisher/mkfilter/trad.html

and there is a great freeware program from NuHertz -- Filter Free -- http://www.nuhertz.com/

 

[tubee]

Thanks Poynton and Jackinnj for helpfull answerings.

Will try & calculate Jackinnj's reffered website first, if i cannot succeed search an answer on 6H5c.

 

[poobah]

tubee,

Keep in mind a Bessel filter is not as efficient as a Butterworth. You will need more stages of filtering to accomplish the same task. Bessel will give better phase response... there is much debate as to whether this can be heard though.

🙂

 

[tubee]

Hi Poobah, thanks for the info. Because i kept the SAA7220 on its place(this time), the sound has a once in a while a sharp edge, also because of the opamp upgrade: the OP2132 sounds some laid back, this is discussed here on diyaudio more often. But i discovered DVD has an sharp edge sometimes too (but some more detailed because of 24/96 resolution). This could be a digital phenomenon then.

Maybe the sharp treble can be some sweetened by a roll-off of the less steep bessel. My pre amp and fet-tube poweramp have tubestages, can take some HF i think.

 

[poobah]

Hmm,

You have to be your own judge of course. After 35 years of playing in bands (off and on), I can tell you many harsh sounds are very real. Listen to an 8 year old with a violin!

Whether you can change to Bessel may depend on how your filter is layed out now. Can you post the schem of your Butterworth?

🙂

 

[tubee]

There is something else going on: i have a small 30-40 dB dip in my hearing on +/- 4000hz (my father had also) And some attenuation again to 8 / 10 K. So allmost all "straight level" tweeters sound harsh to me, even sitting sideways(eg coaxial Kef speakers from my friend) But at this moment hear classical music with a Kenwood tuner: no harsh sound, but nice "articulated" treble.

The schematic, yes ehm, threw away the scanner, so have to find a way to show.
Filtering is standard like most 16bit 1541 players with a dual opamp.

 

[tubee]

I just added a small 4uH ferrite coil between TDA1541 and the first stage of IV. This helped allready, some sharpness is gone. The filter itself is left as it is.
I don't want to get rid of the highs, but only the"S" sounds in voices.

 

[tubee]

Very long time later, but here is the schematic of outputstage from CD304MK2 (Have att last that needed serv. manual)

Can somebody confirm what filter is used? (bessel or butterworth)

Thanks!