View Full Version : Crossover for Altec-System

06-14-2005, 05:55 AM

sorry for crossposting, i posted in the Altec-Hostboard, but there comes no answer.
Here is my last selfmade Altec system:


The Sub includes two 416-8C in a vented cabinet. The top includes the 288-8K driver with MRII-594 horn and a 515-8GHP. The cabinet is similar to the A700/A800 cabinet, but home made.
I drive these speaker active with Ashly crossover, crossing at about 70Hz (Sub/Top) and 500Hz (515-8GHP/288-8K). The sound is really great!
Now i am looking for a passive crossover, because i will have the possibility to drive the tops passive as fullrange with only one power amp in little clubs. Because i know the better way is the active way i want to switch between active and passive.
What do you think about the Altec 1285-8B? Is this the right crossover or should i build one for myself? Does anybody have the circuit diagram of the 1285-8B?
Thank you
Wolf More of my cabinets:
www.vallieur.de (http://www.vallieur.de/) -> Wolf -> Lautsprecher -> Altec A700-WV

Earl K
06-14-2005, 06:38 AM
Hi Wolf

**** Nice System**** ( I guess now you want to make some money with it ) :p

(A) Just so I understand your questions correctly;

(1) Do you want to only passively crossover just the components in the top box ( that holds the 288-8K and the 515HP ) ?

(2) What about the subwoofer portion with those nice twin 416-8C(s) ?
Are the subs going to remain active ? ( Or are the subs not going out to the club ? )
- FWIW ; I would not take those 416-8C out of the home and into a club environment / you'll smoke those voice coils in no time .
- For Sub Duty ; I'd advise using more robust woofers ( 250 watts plus ) for your club dates . ( Just my two cents - since Altec parts are hugely difficult to get in Europe , apparently )
- And just in case there's a language barrier here; you should still count on using some sort of sub below that 515HP in the top box / "especially when used" in the club environment. I suggest that you get used to the idea of using that 515HP as a dedicated lower mid / midrange device .

(B) Anyways;
- If they were mine ( & only if I was forced to go this direction since I prefer active setups ) , I would make my own passive crossover before buying the mentioned 1285-8B ( unless you can get a pair dirt cheap ) . Actually, I would make a passive crossover just about always before paying any money for an old Altec crossover . The original equipment Altec coils/inductors are easily outlclassed by todays available offerings. As well the old style capacitors are all in the process of drying out since they were usually a PIO type ( Paper in Oil ) .

(1) So, how handy are you with electronics style math ?
(2) Are you a DIY design & build sort ?
(3) Are you looking for design ideas to implement - or - are you looking for something that is proven, test driven & essentially ready to go, "off the shelf" ?


06-14-2005, 07:15 AM
Hello Earl K,

sorry for that confusion in my posting.

1) Yes, i am looking only for a crossover for the top.
2) You are completely right about the 416; i don't use them out of my home because they will not stand that for long. I have (at the moment) vented cabinets with Eminence Kappa 15lf, but i don't like that much. In future i will build sub-horns (i think about BEC1 - California).
The vented cabinet is tuned to 55Hz. The lower bass of the 515 is surprising, so that i had the idea for fullrange use, but only for little rooms.
I don't earn money with that equipment, i do it because i like it. It's especially for gigs with my little band.

I agree with you concerning the active crossover. But for little gigs in little pubs i will drive the tops fullrange and for that i am thinking about a passive crossover, because the amp rack with an additional amp for monitor is really heavy enough (!).
I hoped, Altec used a crossover with proven and good quality. If the 1285 is the same simple way as the older ones, i can build this for myself; i have no problem with the electronic of an passive crossover.
1) I think i have no problem to design a passive crossover; the problem is the testing later with different designs.
2) No, i do it for myself
3) I hoped to find the 1285-design to build it for myself with newer and better parts.

Thank you for your advice

Earl K
06-14-2005, 07:38 AM
Hi Wolf,

- IMO, The only attraction the 1285 might have , is that it has 3 selectable points ( 500/800/1200 hz ). It's rated at 200 watts. It's just about guaranteed to be 12 db per octave .

- You should build something that has a power rating of at least 400 watts / just to maintain some system headroom. Personally when crossing over so low ( 500hz ), I'd use at least an 18db HiPass with a 12 db per octave LowPass .

- What slopes does your Ashly have ? ( their ancient crossovers started with 12 db Butterworths back in the early 80s / then 18 db / then 24 db per octave. So just what model do you have ?

- Regarding Subs ; even though you like what you hear from your 515-8HP woofs in your home, do you really want to risk damaging these just about irreplacable components ? It's a given in the live environment that the unexpected will happen; like some drummer that gets out of control while "giving-her" in a slightly large room than was anticiated ..
- I'd give the 515 some relief by adding subs / any subs .


06-14-2005, 07:54 AM
Hello Earl K,
i think, i would cross at least at 800Hz with passiv crossover, better may be at 1200Hz with 12dB for low and high.

The Ashly is the XR2000 with 24dB/Oct.
I understand what you mean with the 515. I thought to use this top with a HP at about 50Hz. But looking at the Xmax of the 515 i see there is no reserve. And the speaker ain't as cheap as the Kappa. OK, not fullrange, only with a sub.

Next idea for live-use: sub/top active at about 100Hz/24dB; top passive as described at 1200Hz. Do you have any experience with the passive equalization of the 288-8K?

But: i like the active way and as often as i can i would drive the top active with the Ashly.

PS: :blink: I am the drummer in our band!

Earl K
06-14-2005, 08:50 AM

- That large horn is just a waste of space if you crossover as high as 1200 hz ( IMO ).
- Using an 800hz point preserves its usefulness in creating some directivity .
So I would go with at least 800 hz / maybe 700 or 750 .

Do you have any experience with the passive equalization of the 288-8K?

It's quite simplistic to create an HF EQ circuit . It need only consist of an appropriate valued capacitor paralleled to an appropriate valued resistor / with this mated pair put in series with the compression driver / after the main crossover elements ( these LC values will need to account for this resistors value ) .

(1) Capacitor Value needs to be determined by looking at the nonequalized response of the compression driver / horn combo . So what test gear do you have ? Get an RTA ( like a Behringer 8024 or 2496 ? EQ with test mic ) or some software package that does the same thing . That big Altec horn likely needs EQ starting at the classical breakpoint of @ 3 or 3.5 K.

S0 ; 159155 /[ 8 (ohms, the driver load)*3500 ( in hz) ] gives a capacitor value around 5.7uf . This looks about right . I'd play around with values from 4 to 6.5 uf to find the best results. Remember ; you can parallel capacitors together to get a new value, that is the sum of the smaller values ( in uf or mfd ) .

(2) Resistor value is usually around 8 ohms for an 8 ohm Altec driver . Give this resistor a 40 watt value for live work . Parallel (4) 32 ohm , 10 watt resistors together to create this part .

(B) One Classical Method for determining LC component values ;

(1) You should obtain real AC Impedance graphs for your components in your boxes and on your horns. One can't accurately predict the correct LC values for a crossover without a good idea of what the AC impedances are of the drivers to be used.
- I believe the best product right now for this work is "Woofer Tester Two" available from "Parts Express" . Ask Zilch about this product and learn to use the "Search" function while you are looking for Zilches info on his "Quick & Dirty" 4430 thread. :p

(2) Some Quick & Dirty Figuring ; ( for a 12 db Butterworth slope / transform )

(i) Inductor Value ; ( just for the woofer, stated in mH)
- [ 159*impedance ( in ohm)]/desired crossover point ( in Hz ) = L ( this inductor value in mH ).
- The derived value needs to be scaled into a 12 db ButterWorth Transform so multiply the above by 1.41 .

Example : [ 159*8 (this is not accurate, but will suffice )] /800 = 1.59 mH
,,,, 1.59mH times 1.41 = 2.2419 mH ,,, rounded = 2.25 mH for the woofer and twice that for the compression driver ( since that 8 ohm series resistor for the EQ circuit comes into these equations ) .

(ii) Capacitor Value ; ( just for the woofer, stated in uf )

- 159155 /[ impedance ( in ohm)*desired crossover point ( stated in Hz )] = C ( the capacitor value in uf ).
- The derived value needs to be scaled into a 12 db ButterWorth Transform so multiply the above answer by .71 .

Example : 159155 / [ 8 (this is not accurate, but will suffice )* 800 ] = 24.87 uf (mfd)
,,,, 24.87 uf times .71 = 17.66 uf ,,, rounded up = 18 uf for the woofer and half that for the compression driver ( since that 8 ohm series resistor for the EQ circuit comes into these equations ) .

(3) These above LC values are theoretical ballparks for "flat-line" 8 ohm loads and as such , discount a bunch of other relevant factors that must be considered . In real life , no transducer has a perfectly flat impedance curve. These derived LC values are for demonstration purpose only ( though they'll be a lot closer/ more usable for Altec transducers than JBL transducers ) .

(4) Have you visited Jeff Markwarts WebSite ? (http://home.earthlink.net/~jmarkwart/) . There's lots of good Altec crossover info there. If you feel this passive network design stuff is beyond you / then you might consider hiring his help to create a custom network for your boxes . :p


06-14-2005, 09:35 AM
Hello Earl K.,

thank you for your great advice.
1) The top ain't only used for PA, it's my HiFi at home in my little music studio too. So i wanted that big horn, i formed using the same horn parameters than the A7 (i use in my little home cinema). To make that big (for me) sound (A7) portable, i built these cabinets.
2) The directivity may be the problem with 1200Hz. I have to find out. But with 12dB at 1200Hz i have more reserve at the 288.

A) At the moment i use in active equipment a circuit with a 3,9mF capacitor paralleled by a 4,7Ohm resistor, in series with the 288. This sounds good to me in my room.
1) I use the Behringer 8024 with the suitable mic. But i will take the test outside of the room and the cabinets are 1) new and 2) really heavy. I need my sons to take it outside. In the next weeks i will do it (when my sons are at home...).

B) I already designed some passive crossovers, especially in 12dB butterworth. So your advice is the control for me that i did right the last years. Thank you again for that work.

OK, the (passive) way will be: finding the equalisation of the 288 in active equipment (to choose the resistor) and than add the choosen resistor to the theoretical impedance of the high channel in the crossover. Simple, i understood.

3) That was my idea: i thought to go the easy way by copieng the 1285. But i see, that's too simple. I expected it anyway.
4) I will visit this website. Thank you!


PS: Sorry for my broken english, i hope you understand me the right way.