Using the AD844 as an I/V

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
 

Attachments

  • BUF03 buffer fed from TZ (pin5) of AD844.jpg
    BUF03 buffer fed from TZ (pin5) of AD844.jpg
    168.1 KB · Views: 738
Last edited:
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:
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:
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
 
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
 

TNT

Member
Joined 2003
Paid Member
ahhh, brought back to reality.

Thanks!

/

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
 
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
 
Passive 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
Hi George, I am having great luck with using an Edcor XSM 600/600. Put a 4n7 polystyrene across the primary. Makes an excellent filter and no feedback. My Mark II 1541A dac uses this after the Pedja Rogic DDNF discrete current mirror I/V. I never liked Sallen Key filters.... :no:
 
Hi George, I am having great luck with using an Edcor XSM 600/600. Put a 4n7 polystyrene across the primary. Makes an excellent filter and no feedback. My Mark II 1541A dac uses this after the Pedja Rogic DDNF discrete current mirror I/V. I never liked Sallen Key filters.... :no:

Hi, Torchwood,

You may want to use some caution in placing capacitors across transformer primaries. This creates a resonant tank circuit. By my calculation, placing a 4.7nF cap. across the 3H primary of that Edcor transformer may produce response peaking around 1.3KHz or so.
 
Last edited:
Hi, Torchwood,

You may want to use some caution in placing capacitors across transformer primaries. This creates a resonant tank circuit. By my calculation, placing a 4.7nF cap. across the 3H primary of that Edcor transformer may produce response peaking around 1.3KHz or so.
Hi Ken, My cousin did some testing and found that the response was nice and flat to about 20 Khz. I could ask him to look again around 1 Khz. Thanks for the information. He has the smaller Edcors on a CS4397 dac and had no peaks. Worth another look. ;)
 
I have now made up my mind which buffer is better the BUF03 or the AD844's. And it is the BUF03 driven from the TZ of the AD844. I found it gave just as dynamic/detailed sound as the 844 buffer but it was more musical, less highlighted around the edges which makes you want to turn it up higher and higher, it has a more relaxed quality to it, yet can still grab you by the scruff of the neck and demand your attention when needed. It's output impedance is 2ohms with 70mA current so it will drive anything even my Sennheiser HD650's. It is highly biased class A and runs very hot at +-15vdc so it needs those very rare TO-99 heatsinks.


Cheers George
 
Last edited: