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Old 22nd July 2013, 03:35 AM   #501
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Quote:
Originally Posted by jcx View Post
increase the filter order, Butterworth is much flatter in the passband
should be inaudible at a much lower fc

since you need a buffer anyway why not make it a 2nd order LPF with Q, fc coordinated with the RC for a 3rd order Butterworth @ ~ 40-50 kHz
Nice of you to chime in JCX, can you give details on how you would do this on the BUF03 buffer without changing it's nice low 2ohm output impedance (attached) I am using?

Cheers George
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Last edited by georgehifi; 22nd July 2013 at 03:54 AM.
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Old 22nd July 2013, 04:02 AM   #502
jcx is offline jcx  United States
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in the past I'd just fire up MathCad and derive the symbolic equations myself except its expired again and I'd have to call in to reactivate my version 11 - and it still needs XP

but its not that uncommon a circuit

Google

with a dedicated unity buffer chip you're stuck with the unity gain equations unless you add another op amp for the 2nd order section -which does simplify/separate the design equations

just use the same 1st real pole RC values with the R gnded, I into the 1st RC node == Norton equivalent

but with free choice of LPF I'd use the multiple feedback filter with good VFA - for lower distortion

Last edited by jcx; 22nd July 2013 at 04:22 AM.
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Old 22nd July 2013, 04:22 AM   #503
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George -- to get high enough Q to do what jcx is suggesting with only Rs and Cs you'll need to use feedback. Alternatively, embrace inductors....
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Old 22nd July 2013, 05:08 AM   #504
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To do feeback around the BUF03, I'd need another opamp not for me, I have at the moment no global feeback from the output of the PCM1704's all the way to the speakers I think I'll keep it that way, I'll just settle for the 1st order filter at TZ of -3db @125khz

Cheers George
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Old 22nd July 2013, 05:13 AM   #505
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The standard Sallen-Key style 3rd order filter uses a unity gain buffer so you could do it with the BUF03 you already have. But if you prefer to keep things feedback free, then inductors are the only solution I'm aware of.
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Old 22nd July 2013, 05:17 AM   #506
jcx is offline jcx  United States
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if you use bigger than 4:1 unequal C in the unity gain buffer Sallen Key then Q>1 is possible with equal R in the 2nd order section considered alone

values have to be adjusted due to the interaction of the added 1st RC section for 3rd order Butterworth

less interaction can be had with further large impedance ratio between 1st section and the 2nd order part


as far as "avoiding feedback" is concerned - why do you think the BUF03 has unity gain in the 1st place? (hint: because it uses 100% voltage feedback internally)

and if you just don't like negative feedback then you may be entertained to know the Sallen Key is actually using positive feedback to boost Q

Last edited by jcx; 22nd July 2013 at 05:34 AM.
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Old 22nd July 2013, 07:13 AM   #507
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It sounds magnificent as it is, I think I'll just keep it at the simple 1st order at -3db @125khz on the TZ filter. With this there is just a couple of degrees of phase shift at 20khz, and the vhf noise on the output of the BUF03 is only a couple of mV at more than >100mhz, I don't think this is detectable at all. And then this way it's very simple with no global feedback anywhere in my system.

Cheers George
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Old 22nd July 2013, 04:48 PM   #508
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the worry about “phase shift” is also misplaced – how long do you think the digital filter is – typically low double digits of the sample rate, 100s of degrees

so “phase shift” is not so useful a quality metric, the player already has lots – if linear with frequency it is simply a pure delay

the real “time distortion” is the variation in group delay over the audio range

for a 50 kHz 3rd order Butterworth that's ~ 700 ns of delay change from 20 to 20 kHz

or about 0.2mm (0.008”) of air path difference – how's you speaker driver alignment, what's you're driver x_max?, any baffle diffraction, room reflections? what's the radius of your tweeter diaphragm?


Cary Audio uses 3rd order analog filter
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Old 22nd July 2013, 08:31 PM   #509
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ahhh, brought back to reality.

Thanks!

/

Quote:
Originally Posted by jcx View Post
the worry about “phase shift” is also misplaced – how long do you think the digital filter is – typically low double digits of the sample rate, 100s of degrees

so “phase shift” is not so useful a quality metric, the player already has lots – if linear with frequency it is simply a pure delay

the real “time distortion” is the variation in group delay over the audio range

for a 50 kHz 3rd order Butterworth that's ~ 700 ns of delay change from 20 to 20 kHz

or about 0.2mm (0.008”) of air path difference – how's you speaker driver alignment, what's you're driver x_max?, any baffle diffraction, room reflections? what's the radius of your tweeter diaphragm?


Cary Audio uses 3rd order analog filter
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Old 22nd July 2013, 09:55 PM   #510
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Quote:
Originally Posted by jcx View Post

Cary Audio uses 3rd order analog filter
Yes they did it was based on a opa134, every time I used it between the TZ and whichever of the two madeup output buffers I was using. The sound quality suffered everytime it was inserted. Sure the output was clean down to the baseline noise of the Tek scope, but the music just wasn't there anymore.
This is why I have been so happy with the 1st order at 125khz even though there a little bit of vhf **** now seen on the Tek scope on the outputs, the music sounds real. As Zenelectro once said to me "sometimes don't let you eyes decieve you" listen to it.

Cheers George
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