Misc. Filter Questions

[Marzen]

When determining a xo point for a woofer in a 3 or 4 way design, should I take into account the A/V rcvr's built in xo point of 80hz when the spkr setting of 'small' is selected? Would the rcvr's HP filter cascade with the speakers xo?
Thanks-
Ward

[Hdale85]

I dont think it works that way. A crossover regardless of a preset one is just limiting the sound's range and lets say the reciever is only letting through up to 100hz for the sub but your sub is crossed at 80hz then it will be crossed at 80 hz regardless of the recievers preset crossover. The same is true for vice versa. This is what i believe is true anyways. Somone be sure to correct me if im wrong

[BobEllis]

Yes, the speaker and receiver HP filters are cascaded, along with the speaker's own response. So if your receiver has a second order electrical roll off at 80 Hz and you have a sealed speaker with an F3 of 80 Hz (second order roll off) your net effect is a 4th order roll off at 80 Hz. To make a L-R XO, you'd want fourth order electrical filter on the sub low pass. If your speaker's response extends well below the XO point you would choose identical electrical slopes.

Dougie085's example is almost correct. If your receiver LP is higher than the electrical filter you use on your sub the initial roll off will be determined by the sub's filter (80 Hz in his example) However, above the 100 Hz receiver XO, the roll off will be additive. If both filters are fourth order, you'll have an 8th order roll off. Also not that the second filter will affect the corner shape - the 100 Hz filter is already rolling off below 80 Hz, so the net response will be a bit of sag before 80 Hz.

Now the question is why go to the trouble of building a 3 or 4 way and set the receiver to small?

[Hdale85]

That is a good question

Now another question do most receivers/pre pro's have the option to turn off the built in XO? I'm concidering building my own amp and for now will probably be played off of my computer or somthing maybe a cheap reciever until i can get the outlaw 970. I've also been concidering buying the Parasound new classic 2125. Anyone ever seen this amp? I'm not sure which would be better the UCD180 amp i was planning on building or the Parasound amp i could probably get both for about the same price.

[Marzen]

Now the question is why go to the trouble of building a 3 or 4 way and set the receiver to small?

Short answer: low powered A/V rcvr in multi-channel mode.
Long answer: At this point in the design process I tend to think as I do at work - which is factoring in associated equip & possible usage with same. This results in either a compromise (altered xo point & slope) or a specific usage (full range only).
Thanks Bob. I'll take the easy way out & ignore the subwoofer for now.
I'm bogged down at the moment with overhauling the two BP filters using the bandpass freq ratio {S=Fh/Fl} in an all pass LR filter. I had some problems with the previous CPC 8th order versions due to Zmin, Zph & BP Vg issues.
-Ward

[BobEllis]

That is a good question

Now another question do most receivers/pre pro's have the option to turn off the built in XO? ...

Most receivers will send a full range signal to the channels selected as "Large" If you set subwoofer to "None" you'll also get the LFE sent to "Large" speakers, too.

I haven't heard either of the amps you are considering, but my preference is for class A and DIY (Love my modded Pass/Thagard A75s). The Leach amp is pretty darned good, and there are a few updated versions out there. I also have a 4 channel Leach.

[Dennis H]

if your receiver has a second order electrical roll off at 80 Hz and you have a sealed speaker with an F3 of 80 Hz (second order roll off) your net effect is a 4th order roll off at 80 Hz. To make a L-R XO, you'd want fourth order electrical filter on the sub low pass.

And that's how most receivers work these days -- 2nd order Butterworth highpass and 4th order LR lowpass. People with true fullrange speakers are kinda screwed if they want to run them as small. It all goes back to THX certification where that's the XO configuration they demanded. Fortunately, most current receivers let you select a frequency other than 80 Hz. So, if you want to run your speakers as small, you'll get the best results with a sealed Butterworth box (Q = 0.7) with F3 at a frequency your receiver can set. The 3-ways in the stickies, using the Dayton drivers, would be pretty easy to tune to a Butterworth alignment at 50 Hz so a 50 Hz XO in the receiver would be perfect.

[Hdale85]

What could you build a 2 channel leach amp for? cause i deffinately dont want to spend anymore then like 700 bucks for a 2 channel amp. And less is certanly better im still trying to scrape money together for the MTMWW's i want to build

[kgveteran]

I wish my Outlaw 990 let me choose filter slopes.it would be nice to be able to match the usual 24db/oct 80hz on the sub, with a 24db/oct on the mains.They always use 12db/oct for the mains.

Assuming that the acoustic rolloff of the mains will be @80hz and a sealed 12db/oct slope.Not many speakers are designed this way.

it's a Lucas thing,I'm sure it dates back to THX in some way shape or form.

[BobEllis]

Amp cost depends on a lot of factors, elaborate metalwork is a big one. With careful/lucky shopping surplus stores for heat sinks, power transfransformer and filter caps I built 4 channels for just under $700 excluding the case. Right now ApexJr has suitable heat sinks for $12 ea (buy 2 - tell Steve I sent you for the gold flatbacks not shown on the website) and 10,000 uF/63V caps for $1.50 (you'll want at least four, 8 would be better) Parts Express carries Avel toroids for reasonable prices. Figure $55-70 depending on your degree of overkill. Boards and parts for two channels are another $200 or so. You're probably still around $350 for two channels until you decide that it would look better in a case than sitting on a slab of MDF.

The case can be something fancy, a ready made like a Par-Metal enclosure (their 20-16125 works for those heat sinks at $80) or a painted MDF box, depending on your budget.

KG - you could add an 80 Hz 2nd order high pass filter between the Outlaw and your amps. I have some filter boards leftover from the diyaudio group buy. (power supply boards and parts too) Drop me a line if you want to try that.

[Hdale85]

Well 350 bucks before the enclosure isnt bad at all. Whats it sound like though? How many WPC? Will it be able to push the MTMWW 3 ways ok? thats why i was concidering the Parasound amp as its at the perfect wattage for the RS 3 Ways. Thanks fo the info though Oh and could you PM me the links for the Leach amp stuff and what not, and possibly for those parts just so i know they are the right ones. Thanks! :B

Doug

[BobEllis]

Doug, YGM.

[Marzen]

People with true fullrange speakers are kinda screwed if they want to run them as small. It all goes back to THX certification where that's the XO configuration they demanded. Fortunately, most current receivers let you select a frequency other than 80 Hz. So, if you want to run your speakers as small, you'll get the best results with a sealed Butterworth box (Q = 0.7) with F3 at a frequency your receiver can set. The 3-ways in the stickies, using the Dayton drivers, would be pretty easy to tune to a Butterworth alignment at 50 Hz so a 50 Hz XO in the receiver would be perfect.

Dennis, thanks for the detailed info on how the rcvr xo is configured for THX specs, I have a coworker that may be interested in the DIY route & maybe we could implement that info into a TM+Sub. I reread Dickason's chapter on this & he seems to be of the same opinion that the THX xo freqs chosen were less than ideal.
Speaking of Prof Leach, I reread his excerpt regarding zobels & even order networks with closed box mids & tweets. I have to say I think I'm much happier with my APC bandpass filter compared to the CPC version for Z and Z phase response. I do wish group delay was lower using the +/- 48dB slope, but it's under 9ms & ripple is still below 50dB.
BP gain brings the RS180 up to the RS52 level nicely; but I suspect I may run into problems with level matching when I get down to the RSS315 and its LP.
-Ward

[BobEllis]

... but I suspect I may run into problems with level matching when I get down to the RSS315 and its LP.
-Ward

That's one of the reasons I like active filters. Full range with a 94 dB/2.83V woofer, 89 dB midrange and 95 dB tweeter is a piece of cake.

[Marzen]

I got me sum problems here <sic>

Well, my BP's on the RS180 & RS52 looked ideal as stand alones. However, connecting them in parallel led to a ~6dB peak at the xo freq; reverse phase on either driver had the same result. Hopefully someone can point me in the right direction. Here's some details:
RS180S-8, Allpass 4th order BP +/-24dB (90-750hz) with notch filters added for 48dB attenuation (ala Jon's Modula MTM xo).
RS52AN-8, same topology as above (750-300hz).
-Some thoughts: I avoided using the Butterworth topology because I couldn't factor the LCR notch filters correctly; and I avoided the LR filter because I couldn't attenuate the peak on the low pass section without affecting overall shape. That left me with the 'Butterworth Type' and the Allpass, both of which have the same cascaded 4th order topology as the LR. The Butterworth Type yielded what appeared to be maximally flat response, but gave me some problems with Z and Z phase.
-Ward

[Dennis H]

Well, my BP's on the RS180 & RS52 looked ideal as stand alones. However, connecting them in parallel led to a ~6dB peak at the xo freq; reverse phase on either driver had the same result.

Most likely time alignment. If you're going active, you can delay one driver with an allpass ala SL. If you're going passive, you probably need to adjust filter slopes to get them in phase at the XO frequency.

[cjd]

That's one of the reasons I like active filters. Full range with a 94 dB/2.83V woofer, 89 dB midrange and 95 dB tweeter is a piece of cake.

That has nothing to do with active filtering. Not only that, I still say it's a piece of cake all passive and with a single amp. :P

C

[BobEllis]

OK, C, we can agree to disagree on that one. I should have made it a bit harder, with the woofer - mid sensitivity difference bigger than baffle step and maybe the tweeter less sensitive than the mid. Besides, I enjoy building amps as much as I enjoy building speakers. Dust getting to you? Go breathe some solder fumes for a while, and vice versa.

[joetama]

That has nothing to do with active filtering. Not only that, I still say it's a piece of cake all passive and with a single amp. :P

C

Yea, I would have to agree. Why burn through money you don't need to....

[cjd]

OK, C, we can agree to disagree on that one.

Oh, you can tri-amp passive and adjust output levels still. THAT part has nothing to do with being actively filtered. It's all I meant.

The other half was in good fun, since it's one of those debates that has good points all around.

C

[Marzen]

Here's a link for the RS180 filter. I left my dongle at work so I don't have access to the remainder, I'll get that up tommorrow.
4way XO
Thanks-
Ward

[ThomasW]

I left my dongle at work

We really don't need that kind of personal information....:wink:

[Marzen]

I was unable to duplicate the 6db rise today, I may have enabled some option I was unaware of. After starting over from scratch I now get some ripple in the bandpass region & a 3dB peak at Fb (90hz) on the RS180. Not to bad looking at this point. Phase & imp look very kind as well. I should have the test enclosures for the 315 & 180 done sometime next week. Hopefully there won't be any suprises hidden there.

[Marzen]

I finally got some data posted just now...I have some questions regarding polar plots in Point & Piston Mode...perhaps later when I have more time.
Ward
Dayton 4 way
OK, now I've got time...
1) What's the difference basically between point & piston mode?
A: Radiation modes

2) Do you guys model your initial simulation with all drivers aligned on the Z axis - rough out a xo - then spread them out & adjust with the mic on axis with the tweeter?
A: Doesn't matter, I already did it that way...

3) Since the polar plots are rendered at one specific frequency; which freq's should I be looking at? So far, I'm just looking at xo points.
A: Coudn't locate an answer for this one..

4) Is any of this covered in D'Appolitto's book?
A: I'll find out in a few days when it arrives

Thanks,
Ward

[Marzen]

3) Since the polar plots are rendered at one specific frequency; which freq's should I be looking at? So far, I'm just looking at xo points.
A: Coudn't locate an answer for this one..

Can anyone help with this question?

Ward

[JonMarsh]

I'd be looking at the crossover point, half an octave above, and half an octave below.

Much of this is determined by the driver piston radiating size, and it's breakup or absence of breakup. Generally you only have the choice to enter the radiating area.

I'm not a SoundEasy user anymore, so I'm not familiar with setup and use of the last few versions.

[Marzen]

Thanks Jon,
Overall I'd say it looks pretty good at this point. Well enough to go ahead with a test mule for an enclosure. Once I get some actual measurements I hope to smooth out some areas. Specifically the 3Khz response for the vertical polar plot; and the SPL/Volt vs. Time response when compared to a square test pulse looks a bit loose just in this range.
Here's the short specs for the model:
Impedance: 4Ω
Sensitivity: 87dB @ 1W
FR: +/- 2.7dB 40-18Khz
RSS315HF: Sealed w/ Q = .51
RS180AS-8: Sealed w/ Q = .7 (due to power/excursion limits)
Edit: I changed the RS180 Vb to Q=.5 as I didn't take into acct xmax was reached below the xo point (so many things to keep track of...); and I got the test mule built from scrap wood - upper box houses the top 3 with a 9" wide baffle & lower box for just the RSS315.
--time to make some dust :g>
Ward

[Marzen]

Overall I'd say it looks pretty good at this point.

Looks can be decieving.....

Impedance: 4Ω
FR: +/- 2.7dB 40-18Khz

Pure rubbish...perhaps if measured on the moon... :rofl:
The narrow bandpass on the RS180 really lowers the impedance throughout the passband, and the measurements I took at 1 meter bely the simulated freq response I generated. Although, it does sound nice, and at 20 watts puts out very clean, tight bass response ( I was worried a little as my group delays were getting up there due to using the 4th order notched filters).
I'm going to cut a new baffle for the top 3 drivers and align them vertically on their center axis, and move the whole group away from the center line of the cabinet vs moving just the tweeter out of alignment.
-Ward

[Marzen]

Isolating inductors with steel cans?

Assuming all my inductors needed to lay flat on a surface, would a steel can placed over it reduce any interaction with similarly shielded adjacent coils?
-Oh, and I revamped the BP filter using a transformation topology vs the orig cascaded type, that brought the impendance up a bit where it hovers ~ 4 ohm.
Ward

[JonP]

Assuming all my inductors needed to lay flat on a surface, would a steel can placed over it reduce any interaction with similarly shielded adjacent coils?
-Oh, and I revamped the BP filter using a transformation topology vs the orig cascaded type, that brought the impendance up a bit where it hovers ~ 4 ohm.
Ward

Sounds like a good idea, shielding the magnetic fields... BUT, that nearby iron will also change your inductor values if it's near enough.

And, that would be bad....

See if you can arrange the boards with standoffs, or something. If you can get enough space between the coils it becomes a non issue.