[Gordy]

Thank you Jens and Andy!

It certainly wasn't my idea though, it was suggested by George Short, the designer of my North Creek speakers and sub.   It was intended as a 80 hz (?, been a couple of yrs.) x-over to my monitors, the other tail of the 'Y' adaptor would feed a powered sub.  I though I could use this as a feed to the AKSA, to save it some work.  Perhaps a simular rig could be used as a low pass to feed a Carver switching amp to feed two nonpowered sub units?   What is the formula I would use to calc. the cap sizes?  

Thanks again for your feedback
Gordy

[AKSA]

Hi Gordy,

Formula is C (in uF) = 159000/Xc.f

Xc is impedance desired in ohms
f is frequency in Hertz.

Example:

Let's say you want your coupling cap to be 6dB down (that's half voltage) at 80 Hertz.  The input impedance of an AKSA is around 43K ohms, so the equation give us:

C = 159000/43000 x 80
This is 0.04622uF.

You'd likely go up to the nearest preferred value to further reduce LF phase shift, and this 0.047uF, or 47nF.

Recalculating for the preferred value gives us 78.6Hz, close enough.

This simple first order filter is flawed, however.  The Zin of the AKSA is not a constant 43K;  it varies very slightly with input amplitude because of the base admittance on the semiconductor, T1.  But this is likely only about 10%, so it's not all bad.  Furthermore, there will be considerable phase shift at the corner frequency, extending upwards to about 300Hz, which is significant.  The only way around this is to increase C1 to large levels, and the stock value is 0.47uF, ten times larger.  But for the frequencies at which you are using the amp, it's unlikely all this is too important, because it beats using an active filter, with all the attendant complexity, to secure just a first order function.

There is a swathe of math around filter design, something I don't fully grasp.  Others might know the fine detail;  but I can assure you that this will be around 6dB down at 80Hz, and 12dB down at 40Hz, and 18dB down at 20Hz.  This is useful attenuation, as it reduces out of band operation of the midbass driver.

Cheers,

Hugh

[andyr]

Hi Gordy,

Let's say you want your coupling cap to be 6dB down (that's half voltage) at 80 Hertz.  ...

Sorry Hugh but a series cap is a 1st order passive HP Butterworth filter and I believe the equation you quoted gives the 'customary' -3dB (70%) point ... not the -6dB (50%) point.

This simple first order filter is flawed, however. The Zin of the AKSA is not a constant 43K; it varies very slightly with input amplitude because of the base admittance on the semiconductor, T1. But this is likely only about 10%, so it's not all bad. Furthermore, there will be considerable phase shift at the corner frequency, extending upwards to about 300Hz, which is significant. The only way around this is to increase C1 to large levels, and the stock value is 0.47uF, ten times larger.  ...

Shirley, if you increase the C by 10, you reduce the resulting f by 10 ... so you get a 7.86Hz -3dB point ... which ain't much use to Gordy?

Now, I've heard many "experts" proclaim the so-called "advantages" of PLLCs over "conventional" PHLCs but no-one has ever mentioned potential changes in the amp's Zin before ... which, even though it might be only 10%, seems to me like a bad thing.

Do all ss power amps have this attribute?  Wot about toob amps?

Regards,

Andy

[Gordy]

Thanks Hugh and Andy,

I thought I was following you there for a second   I may just try it anyway as I never learn anything unless it's the hard way!  Then I'll just go back to running them full range and bring the subs in from below...

Appreciate all the help
Gordy

[Joules]

This is what I have been doing for a long time (at 130 Hz)- very clean, very inexpencive !! I still get a lot of bass in the mid bass driver tho. I supose one could cascade multible RC filter in series to get faster rolloffs.
I just wish LP was just as easy !!

[Jens]

It is possible to help the low order roll-off you get with just a single cap by constructing your speakers in such a manner that the acoustic roll-off of the midrange coincides with the cap roll-off. Very efficient!

This is obviously difficult to do if you don't build your own speakers, but I thought I might offer this advice for those who do.

In my case, the line level cap will replace a capacitor bank in the speaker x-over of some 150 uF! Absolutely worthwhile. And it's possible to use very high quality caps because they are so small. Much cheaper than the passive x-over components.

Cheers,

Jens

[Joules]

"In my case, the line level cap will replace a capacitor bank in the speaker x-over of some 150 uF! Absolutely worthwhile. And it's possible to use very high quality caps because they are so small. Much cheaper than the passive x-over components."

  EXACTLY !!!
Much cheaper than a Active x over too!
and cleaner sounding.

[Jens]

"In my case, the line level cap will replace a capacitor bank in the speaker x-over of some 150 uF! Absolutely worthwhile. And it's possible to use very high quality caps because they are so small. Much cheaper than the passive x-over components."

  EXACTLY !!!
Much cheaper than a Active x over too!
and cleaner sounding.

No doubt about this!

But - what type of cap is best suited for this purpose? I would have gone for Black Gate, but since 15 nF (which is what I need) is not available, what type of cap should you use? I've had some luck with Wima MKS/MKT caps, which seem to work well for this application, but there might be better things out there.

Suggestions, guys?

Cheers,

Jens

[kyrill]

Hi Jens

You can create a value by paralelling it. I am sure you know

[Gordy]

This is what I have been doing for a long time (at 130 Hz)
I just wish LP was just as easy !!

I found these at Parts Express, http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=266-252    and wonder what they are comprised of, worth pursuing?

[Davey]

Hope you fellas don't mind me butting into this thread.

Those Fmods are line-level passive crossovers implemented with internal RC combinations.  They're not insensitive to source and load resistance so the actual roll-offs and slopes may vary a bit from the advertised numbers.  In all cases you would need a source with fairly low output impedance.

Low-pass RC filters are harder to implement because there needs to be a resistor in series with the load.  This creates a voltage divider with the load so you get some insertion loss.  However, if the source impedance is low enough and the load impedance high enough the component values can be optimized to reduce the loss.

Here's a reference with some basic information:

http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

15nF is a standard capacitor value.  A good quality polypropylene capacitor can be sourced from folks like Digikey for less than a dollar.

Cheers,

Davey.

[Joules]

Marchand has a passive line level XO using ferrite coils as part of an LC filter. Any one have any experience with these? It all boils down to the sound of the ferrite coils vs. a high quality buffer amp in an active XO. Come to think of it the widly acclaimed ribbon tweeters use transformers to match imp. and no one seems to complain to much about them. The same could be said for the output Xformer in tube amps. And then there is the Beringer digital XO. Iwish I had the money and time to try them all.