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