6 dB per Octave Passive Crossovers

[LowOhms]

6 dB per Octave A7 Passive Crossovers :

Hello,

I recently joined this Forum and have been getting prompts, every time I log in, to make my first post and introduce myself. O.K. !! My name is Jeff, I live in a small college town with rolling farmland all around, fifty miles outside of Kansas City, Missouri. I am a life-long audio and music enthusiast.

Sixty years ago, very late one night, my Father sneaked a brand new Altec 604B into our home, while my Mom was sleeping. Dad sawed a hole in Mom's Laundry Room’s wall, in our finished basement, and mounted the 604B to "fire" into our family’s Recreation Room.

We had stereo. Dad had a second ALTEC coaxial in a Karlson enclosure, and so, I was weaned at a tender age on home audio and music. This was how my Dad handled “WAF” in the early 1950’s.

Fast forward, it is 2013, ( Happy New Year ), and I am about to put together my first Altec A7 system, for my home use, powered by low Watt directly heated triode amps, ( you know - 2A3s ). I sense this ALTEC community is vast, so I would like to ask for some help from those who are ( or have ) used 6 dB per octave crossovers.

So far, I have found two ALTEC write ups which gave a good run down, one from GM and one from Larry D. Moore, a gentleman in Ohio who runs “ UltraFi “.

GM posted this, on 4-29-2003 :

"The 416 has fairly benign breakup modes and in the good ol' days a lot of the guys ran them straight up, with just a shunt resistor across the HF capped off at 1.8kHz for the 811 and 1.2kHz for the 511 IIRC. Experiment with a series resistor to shelve it down some and you're done. If you find the breakup modes annoying, then roll it off with a 3mH big wire inductor, which may require swapping out the series resistor with something smaller."

GM was asked :

"I don't quite follow the "shunt resistor" idea. Can you explain a bit more to a novice?" tdc

GM responded :

"A shunt is any device wired in parallel to a load, so in this case it gets wired in parallel to the driver's VC. Some folks call them damping resistors. Here it serves two purposes, one to damp the horn, and with the series resistor shelves the horn's entire BW to whatever sounds best in your room. If the highs are too rolled off for you, put a cap in parallel with the series resistor to taste to keep the extreme highs from being attenuated."

-------------------------------------

Larry D. Moore, a decade or so ago, preferred 16 Ohm 515B woofers, or 416-8Bs in his A7s. He liked either no inductor or a 2 mHY low DCR air core foil inductor. He preferred a 511B horn with a 802D and 16 Ohm early aluminum diaphragms . No bug screens, no vanes in the 511B .

He rolled off the tweeter with a 20 uF film cap, ( seems sorta large to me ) and padded the tweeter's output with 8 to 16 Ohms of series resistance, ( Mills MRA-12) bypassed with a 0.5 uF to 2 uF film cap, value selected by ear, to regain some of the highs. He used SET amps. If anyone wants to see Larry’s entire A7 write-up in UltraFiTimes, Issue 6, email me and I will promptly send it as an attachment.

OK, I bet there is some of you on this Forum who use simple first order passive crossovers for home HIFI, and maybe even SET amps. What can you share with us, as to what you ended up doing, and what you tried that worked best ?? Please don’t let the many Forum members, with huge listening rooms, solid state amps and active equalizers scare you off !! Allow your minority voice to be heard.

I pick-up 825 plywood enclosures, later this month. Hooray . ‘Have on-hand ALTEC 515B woofers, 802 tweeters , 511B horns, and parts to build a home brew crossover. The tweeters go to GPA soon - for a redo, remag, and GPA's “low powered - critical listening” 16 Ohms diaphragms installed. I am considering configuring a passive series crossover, rather than build a typical passive parallel. Yes I will brace the enclosures, and add smallish wings, similar to Per Hasling’s.

Can I gain from your expert arcane experiences ?? I am a BIG believer in KISS and not messing much with the signal. My amps are KISS, two tubes direct coupled and a Magnequest output transformer.

Treat me gently, this is my first post !! Isn’t audio fun, and music sublime ?? Thanks in advance.

Jeff

[Old Guy]

20 is pretty big ie 500 Hz at 16 ohms.

The bypass cap does not 'regain' highs. It essentially is an EQ circuit, the result is to lower the mids relatively. The cap conducts across the resistor at higher frequencies. You can't consider components like that- you must view the whole circuit.

There are a few who advocate 6 db/octave, bear in mind it lowers power handling.

But best is still active crossovers and separate amps. Just my 2 cents.

[just me now]

I've played around with very similar components for a few years...

I ended up going active, as OG above mentions...

when running the VOTTs biamped
I liked a push pull 6BQ5 I came across for not too much coin on the 802g
and a solid state amplifier with a bit more power/control on the woofer


the most revealing experiments for me personally, were toying around with a Pioneer sf850... very easy to vary frequency/slope of the xo for comparison purposes....

I ended up with a preference for 12db slopes, generally....
at my chosen frequencies...


I will hopefully be putting an A5-ish rig back together next October.

I do have to admit, if I listened to just my jazz, cajun, and creole records... I could have enjoyed just the tube amp and a passive xo...

but it just didn't cut it with KISS, 2 live crew, or Ministry... IMO

[LowOhms]

Hi Old Guy,

Thanks for posting . Some comments :

You state : "20 is pretty big ie 500 Hz at 16 ohms."

I agree, and brought it up in my original post.

I was amazed that Larry Moore came to a 20 uF ( 500 Hz ) value by ear. It makes me want to do the determination myself. I wonder what he was hearing?? I would start with approximately what GM posted in 2003, ( on 511B horns with 515B woofers ), 2 mHY and 12 uF, which for 16 Ohms is 1250 Hz. I would tend to vary values around that start point.

"There are a few who advocate 6 db/octave, bear in mind it lowers power handling."


The few people who advocate 6 dB per octave are THE ONES I hope to hear from.


You, and many other people, would like to tell me otherwise ( than to use 6 dB ) in a good natured and well-meaning way. "Power handling" you mention is of NO concern when I use 1.3 Watt to 3 Watt audio amps.

OG, my 1.3 Watt SE amplifier is really about a 500 Watt power supply, with the audio amp gated to 1.3 watts. The amp's custom power transformer weighs 18 pounds, and the DCR of the power transformer and two supply chokes has less than 15 Ohms of series resistances. See my moniker, ... LowOhms.

"But best is still active crossovers and separate amps. Just my 2 cents."

Not me. no thanks !! I have NO interest in biamping or using things like active equalizers. I have been there, done that, reject it fully now. I owned two-way horn systems previously, RCA Ubangies ( two 15s) and RCA MI-1428B field coil tweeters on RCA 90 by 40 horns, biamped. Drove me crazy. I can hear the timing differences and signatures of two different amps, on the system. Each different circuit has a different time constant through the amp. No, not for me.

IF ( and "if" is a big word ) I biamped , I would ONLY use exactly the same amps and exactly the same speaker wire and wire lengths, so that the music came to me as one, a similar time constant through the electronics.

However, as soon as you biamp, you need to add an extra active amplification stage for the biamp crossover, and the system loses transparency and information ( if its a good one), just like it does when you plug an active equalizer into a great audio systrem. Not for me, thank you !! Extra interconnect wires, extra stages, all add up to LESS IF you have GREAT basic electronics and and a GREAT front end. JMHO. YMMV.

Heck, ideal may possibly be NO inductor on the woofer, as Larry Moore suggested and GM mentioned. Maybe I need to try that, on the 515B and on a wider band width 416-8B.

I try to respect all people, and I hope we will tolerate diversity of opinion. " 6 dB ers ", I think you are out there. Lets hear your experiences, comments, please. Thanks.

Jeff

[mah]

I will tolerate your opinion as unsubstantiated impressions, no more or no less. If it can be substantiated as fact then we will all benefit from your experience. Interesting thoughts you put forward - just the sort of input that promotes discussion.

[Old Guy]

If you could hear timing difference you didn't do it RIGHT.

Rejecting something after doing it wrong is kinda silly.

Honestly, if you just want crossover info a general audio forum might be better. this forum is Altec specific.

And if you like to argue, try Romy the cat's forum...

[rontec]

Well, would have to side with OG somewhat.....very subjective maybe OT in the strictest sense. However I did this same route some years back and here's my... as Mah put it..."unsubstantiated impressions". If memory serves I used 511s/808s/416s. 10-15ohm resistor to pad down the HF drivers along with a 8-10uf cap and a 1uf bypass cap. The 416 was hooked up straight with no inductor. The mid and HF sounded fairly opened and detailed. However the bass for many would probably be considered thin, anemic. When I supplimented the bass with 18" B52's subs, everything seemed to (suprisingly) come together. Again...totally unsubstantiated and subjective to personal opinion , room acoustics , program material to name a few! I do think it is instructive to listen to individual drivers at times with minimal crossover components connected though. Especially for (DIY) matching/diagnostic purposes. BTW..wecome to the forum Jeff!

[LowOhms]

If you could hear timing difference you didn't do it RIGHT.

Rejecting something after doing it wrong is kinda silly.

Honestly, if you just want crossover info a general audio forum might be better. this forum is Altec specific.

And if you like to argue, try Romy the cat's forum...

Hi OG,

Thanks, that is pretty funny, you have assumed I did it "wrong" and I am silly. Then you suggest I go to another Forum !!

How can I get into so much trouble in just two posts on this Forum??

Goodness, I would have thought just the opposite : " If I can readily sense the sonic signatures of two different amps, with different time constants through each, I did it very RIGHT ".

I happen to think that is pretty easy to sense OG.

Also, I don't have to prove this to anyone, other than me,.... myself being satisfied. YMMV as I originally posted, fine !! I do understand and respect that. No problem for me. Just don't tell me I am automatically wrong, because we disagree.

Finally, since my original post is specifically about Altec A7s, with Altec components, this should be the best Forum to ask others to share their experiences with 6 dB. crossovers.

Your posts, and you, are appreciated by me. Lets just say we have different listening experiences and disagree, and leave it civilly like that. Thanks for contributing !! .

Jeff

[Altec Best]

No Mah, I didn't read the whole thread..But I will now.I was alerted to the fact that I may have to simmer this down that's all...

I also agree that this is an Altec board so I would like it to get back to that.

[Altec Best]

This pissing contest has gone on long enough gentlemen ! Jeff may have gotten off on the wrong foot and possibly offended some of our longtime members,but we would really like to see some tolerance for a new/young member who may not know the ins and outs of our board... Please lets get back to audio and more to the point "Altec Lansing" Thanks..

Or thread will have to be closed.