ND105-4 + OX20SC00 in 5"x10" long pipe

[mintos]

I finally got around to finish up my XO. The blue line is the modified FRD for the ND105-4 at 88 degrees. I'm going to order the parts today, but I want to see if anyone has some suggestions. tnx

btw... I have lots of those .58 mh inductors around from the sale of those speaker terminals from last year. Thus you see that value a lot

These are the only parts I need. Not to expensive. But those PVC pipes costs a lot @ homedepot.

Item Qty. Price Total Remove
6.8uF 100V Non-Polarized Capacitor
Part Number: 027-336
In Stock
$0.42 $6.72
Dayton DMPC-6.8 6.8uF 250V Polypropylene Capacitor
Part Number: 027-424
In Stock
$2.79 $16.74
Pressfit Speaker Terminal 3-1/8" Satin Nickel Binding Post
Part Number: 260-299
In Stock
$2.25 $4.50
Jantzen 0.15mH 20 AWG Air Core Inductor
Part Number: 255-022
In Stock
$3.27 $6.54

[moosespeaker]

As I recall from previous posts you need to place a 4 ohm resistor in series with the 330 pf cap to eliminate the possibility of an amplifier "short" thru that cap and the 20mf in the leg of the filter. Actually anything between 2 and 6 ohms should work.

George

[Wolf]

That is true. If he had a resistor with the 20uF it would be a non-issue.

Mintos- I know you can't model the resistor in PCD, but if you used speaker workshop it shoud allow that modification. You don't want your amp to oscillate.

Later,
Wolf

[mintos]

I don't understand. Why would my amp short through the cap or oscillate? The small 330 pf cap on the first inductor helps drop the slop a tad more. It seems simple and free since I have some 400v 330 pf bypass caps lying around. I don't see any oscillation in speaker workshop until over 11.5 khz. At that point it's already down to -40 db and spiking only to -18db.

[Wolf]

Okay- picture yourself as an unltrasonic frequency. We might not be able to hear it, but the amp will still pass the signal. If you follow your woofer circuit as the frequency, you can pass through both the 0.33uF and 20uF caps unimpeded, that means you have signal that can go directly from the positive to negative, and not get damped, resisted, impeded, stopped, in the ultrasonic frequency range. If you have a short up high in your crossover, it can lead to oscillation of the amplifier due to instability of the impedance it is presented.

The oscillation you are referring to is frequency response of your system, not amplifier troubles. You need an Oscilloscope on your amp with the speaker as designed hooked up to see it.

Oh- and make sure those 0.33uF caps are non-polar, as speakers run on AC, not DC. That tank-cap is a good solid method, don't get me wrong, but the issues presented elsewhere also need to be addressed.

Later,
Wolf

[mintos]

I see... but I thought most amps or source material for that matter only goes up to 44k. By "ultrasonic", how high does the frequency have to get to create a short?

[Wolf]

"Ultrasonic" refers to frequencies above 20-22kHz or whatever you can hear as a red-blooded human.

Bear that in mind- You can't simulate to 44 kHz, can you? If you can't see what the impedance does that high, how will you know? A lot of amplifiers spec top end at 50kHz, whether it does it or not.

Most simulations stop at 20 kHz. It's just better to compensate for the possibility of it happening and just feel better about it in the long run. There's no use speculating on where the frequencies will cause issues if you can't model it with the tools you currently have.

Maynard said he tried a tank-cap setup, and his amp went into protect, so there is merit here. Just place a 4 ohm resistor in series with the 0.33uF cap, and feel happy you avoided a dilemma.

Later,
Wolf

[mintos]

done tnx