Altec 605a duplex crossover upgrade

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I’ve been listening to these for about a year, they have the original crossover, I think it’s a n-1600. I’ve been really curious about the Markwart crossover. Does anyone have before and after experience? They are in 1620 style caps, right at 9 cubic feet.
 
Someday I’ll even finish the cabs...
 

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The 2 crossover options for the 605 on Jeff Markwart's site are very good. Used one of the (slightly modified) for an Open Baffle design I presented at the Lone Star Audio Fest a few years back. The systems performance was very well received. Sounded wonderful. Coherent. Smooth. Well resolved.

For the Open Baffle application, I used the simpler of the two presented.

The only caveat is you need the best low pass inductor you can afford / find. With that 15" woofer section playing well into the midrange, a crappy inductor really degrades the system performance.
 
I just built the Markwart crossover for my 604-8Gs in 9 cubic ft. cabs and it added a sense of spaciousness and clarity to the speakers. However it didn't improve the bass as I had hoped it would and it didn't completely do away with the annoying congestion and peaks that occur in the crossover region.

I'm beginning to think my room is too small for these to really shine. That said, dotneck is absolutely correct. The best sound I've gotten out of these was when I bi-amped them when they first arrived. But it still wasn't as exciting as I'd hoped when I bought them.

If you decide to build Jeff's crossover the best price I found on copper foil air core inductors was here at Erse.
 
No, I started with GPA's crossover, then built a Markwart crossover and currently use a modified version of that Markwart without the shelving filter and an autoformer to attenuate the compression driver.
I also took out the loading cap of the compression driver which didn't change anything in my perception of the mids/highs but did a lot for bass. Less "muddy, one-note" and more distinction. In less audiophile speak: the overtones that make the specific sound of an instrument did more of their thing.
 
Thanks for all the feedback! I hadn’t ever thought of an active crossover and bi-amping, I might have to try that after I have some extra amps kicking around. I think I will probably go the passive route first. To my ears they sound pretty good now, but I can’t get it out of my head that they could be better....
 
No, I started with GPA's crossover, then built a Markwart crossover and currently use a modified version of that Markwart without the shelving filter and an autoformer to attenuate the compression driver.
I also took out the loading cap of the compression driver which didn't change anything in my perception of the mids/highs but did a lot for bass. Less "muddy, one-note" and more distinction. In less audiophile speak: the overtones that make the specific sound of an instrument did more of their thing.

Thanks for sharing your experience. I'm currently trying to overcome the muddy bass you referenced as well as fatiguing mids. Would you be willing to share your schematic?

By loading cap do you mean the 6uF C4 in the Markwart schematic? Did you keep all resistors as designed? Finally, where did you insert the autoformer? and what are its specs?

Ultimately I plan to bi-amp when I complete my Pass VFET kits but that will take a while. Until then I'd love to optimize the performance of the 604s with a passive crossover that will make them sing.
 
To my ears they sound pretty good now, but I can’t get it out of my head that they could be better....

I agree. The imaging is spooky at times! I can't believe the way you can pinpoint instruments on the sound stage. I've read about it, but never really experienced it this way until I got these things. Hearing Artur Rubinstein's piano bench creaking kind of freaked me out at first! :D
 
Thanks for sharing your experience. I'm currently trying to overcome the muddy bass you referenced as well as fatiguing mids. Would you be willing to share your schematic?

By loading cap do you mean the 6uF C4 in the Markwart schematic? Did you keep all resistors as designed? Finally, where did you insert the autoformer? and what are its specs?

It's bedtime, I'll draw up the crossover schematic tomorrow.
The loading cap is an actual plastic cap at the back of the compression driver, limiting the space behind the diaphragm. I have never taken apart a 605 but the 604s and 802s have these. They are necessary to protect the diaphragm in pa use with high power. On a 604 you take the back cover of the driver and there is a plastic cap. Remove that and you have access to the diaphragm. Line the outer cap with thick felt and put it back on with shorter screws, leaving out the inner cap.
No idea if the 605s have that inner plastic cap, but it's easy to check.
 
605s (mine are B's) have the caps in the back. I took mine out and made and exit horn-like back piece from wood and now use the compression driver in dipole mode with low powered amps. You need to get shorter screws to hold the diaphragms in place. I got stainless screws.

I'm curious about your autoformer attenuators on the tweeters. Did you have to do much work/experimentation to get the right loading?

Skip
 
Okay, now I remember what the loading cap is and reading Jeff's comments about removing it. I'll look around for some thick felt. Michael's Stores maybe?

Skip Pack, do you have a picture of the wood piece you mentioned? It sounds interesting. How do you have your drivers mounted?
 
GPA604.jpeg

Here's the schematic of my current crossover. Capacitors are relatively uncritical, low ESR polypropylene is a plus. L4 is cored inductor, DCR 0.07 Ohm.
The autoformer L5/L6 is set to -12dB, R3 is the shunt resistor setting the impedance seen by the high pass filter. R3 lets you pick an impedance to play around with smaller C, which then has to be made up with larger L of course.

Picking R3: it can be calculated easily. I did that as a start, then checked behaviour of the crossover on a scope and lastly put a rheostat in that position and measured different settings.
As what worked for me here is for different drivers in a different cabinet it can only be used as an inspiration, you will have to measure anyways.
 
Thanks for all the feedback! I hadn’t ever thought of an active crossover and bi-amping, I might have to try that after I have some extra amps kicking around.
Dotneck is absolutely right. Hard for me to imagine reasons why anybody would settle on the crudeness of crossovers today. Makes no sense. The only reason people say nice things about crossovers is their adaptation to it. Once you cobble together those crude components, you are stuck with whatever you've got, room acoustics be damned.

On eBay, you can get a Behringer CX3500 (?) electronic crossover* or much, much, much more wonderful, the DCX2496 DSP.

B.
*and add a fried-egg dome tweeter on top for a few pennies
 
"fried-egg" ? Does that mean it's facing up?
Not ordinarily. Few elements downstream of the amp that fulfil their appointed role as well as dome tweeters.

Before going electrostatic, while at Bells Labs, some audio buddies tested a couple and we bought a carton of the best* of the dome tweeters. I made a 3x3 (semi-cylinder) array using 9 units. Great tweeting power.

B.
*sorry, it was long ago; they were marked Magnavox if I recall
 
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