DIY Altec 2 Way Build ala JELabs - 802/32/414

[J-ROB]

I don't know if I should explore TVCs on the preamp or speaker end.

BOTH!

Well, autoformers do two things, they change voltage and transform impedance.

In the case of the N2 xover, the autoformers are used to maintain the same filter impedance with either one or two drivers hooked up.

If the impedance changes, the crossover frequency changes. The autoformer allows the crossover to maintain desired response with different "loads"--in this case known 4 ohm WE speakers.

Incidentally, these speakers have a very flat impedance curve, as do 755As. They are 4 ohms, I think, to keep the inductance of the voice coil down. VC inductance is what cause the typical HF impedance rise in dynamic speakers.

For general hifi applications, you can terminate the autoformer on either primary or secondary with a resistor, which will give you a somewhat stable design impedance for the crossover filter, independent of speaker impedance variations. You could use a higher value than 4/8/16 ohms if desired. The speaker won't short out a primary-connected termination resistor because its impedance is stepped up to a high value at the input terminal of the autoformer.

That done, you pick an output tap for the desired step down factor/level control and it won't affect the crossover frequency.

I'm using the German autoformers from ebay.de right now, pictured above with a 3000 tweeter, but I have been trying all sorts of ghetto hookups. The German ones are nice...fourteen -1.5db steps I think.

 

[Redboy]

...and it just keeps getting better!

@je2a3, you definitely need more iron in your diet. Or in this case, nickel, but you know what I mean! :-)

I have been curious about the Finemet TVCs for a long time now and I really want to hear them! I loaned my Slagle preamp to Kenji for comparison a while back... He gave the edge to Teramoto san's work, of course.

If you had to choose, get some volume controls first.

 

[Salectric]

As requested, a photo of the rear of one of my open baffle speakers. The horizontal shelf is left over from an experiment where I tried an enclosed cabinet behind the woofer. As it turned out, the open baffle sounds better but the shelf improves the sound even with the OB, maybe due to the bracing between the sides. In any event, I will eventually cut a proper size shelf to use instead of this scrap wood, but not today.

 

[Redboy]

That's just cool, @Salectric!

 

[Salectric]

Regarding autoformers---I have an Intact Audio Autoformer Volume Control which I like very much. Based on that experience, I ordered a pair of autoformer attenuators (also Intact Audio) to use in the crossovers, but those didn't work out as well. I will try them again in the future but for now I prefer Mills 12w wirewound resistors for attenuating the midrange and tweeter.

 

[Salectric]

Thanks again Redboy. Looking at the photo full-size I see I should have picked up the room before snapping that pic. Oh well.....

 

[J-ROB]

Very cool speaker! Vu had a good sounding Jensen woofer box at DC Audiofest a couple years ago. The next year it was a vintage mystery woofer that they weren't saying what it was!

I'm real tempted to try aperiodic loading behind an OB woofer like that, maybe using a felt acoustic resistance panel.

(Yeah, I have a stash of felt)

This would also cut back on rear wall reflections somewhat. Even a curtain of thin 2mm felt would probably tone down the rear wave a lot and give a little bit of something for the speaker to work against.

Does the back wave add anything in an enclosed reflective space like most of our rooms? I don't know from experience since I am not an OB guy. Western Electric took measures to kill the bounce off the rear stage wall with their open back woofer systems. Maybe a small degree of waveshaping could help? All the other drivers are totally directional.

Another option would be linen louvers like window shades, seen on the TA-7331 baffle, what would be called an H open baffle these days. Does not impede airflow but it does break up the backwards-firing slap wave. Nobody is sitting back there. Nothing but a wall, right?

http://cfile202.uf.daum.net/image/122CF84E4F144F6C411012

Many people get bored with talk of all this old stuff but there are a lot of good reusable ideas in these antique systems, even if the original devices are out of reach. I mostly downgrade the great WE ideas to Altec applications that I can actually afford.

 

[Salectric]

I don't claim to be any expert on open baffles since these are the only OB speakers I have ever had at home. But I have tried damping the rear wave with various things including foam and different thicknesses of fabric. So far, however, I haven't found one that I prefer over the open back.

I do want to try some louvers behind the cabinet that would operate similar to the TA-7331. My guess is that those louvers (which WE covered with some kind of thin damping material) operate to diffuse, redirect, and maybe attenuate the rear wave especially higher frequencies without introducing as many colorations as a fabric covering the entire back opening. My plan is to do this with a contraption that sits on the floor behind the speaker but does not touch the wood frame of the speaker. The OB cabinet is surprisingly sensitive to other materials touching it. There must be some significant vibrations in the plywood even though I hardly feel anything if I put my hand on the sides while music is playing.

 

[J-ROB]

Out of three TA7331 cabinets I inspected, two did not have any material except the linen slats, but one had a piece of quilted material, looking sort of like an old-fashioned cotton mattress cover behind the slats. It was an old relic looking piece of cloth so if not factory, at least contemporary.

I would say that "stock" was no cloth damper.

I believe that WE was playing with a higher impedance airmass on the back chamber of the 7331 by virtue of the different sized enclosures, front and back. There is no documentation on this stuff, you just have to figure it out. They were not just winging it, whatever they were doing. Intriguing box.

Sadly, that cabinet is not great use of a 13.5" WE woofer. It is basically a minimum size, somewhat portable solution. In a huge horn baffle like a TA-7395 horn built into a big flat baffle, that woofer is the funkiest thing you ever heard.

 

[mhardy6647]

In a huge horn baffle like a TA-7395 horn built into a big flat baffle, that woofer is the funkiest thing you ever heard.

In a good way or just a funky way?
Funky is a funny word*, you know?

_______________
* I was tempted to write "Funky is a funky word..." but that would be too precious.

 

[TubeHiFiNut]

I don't claim to be any expert on open baffles since these are the only OB speakers I have ever had at home. But I have tried damping the rear wave with various things including foam and different thicknesses of fabric. So far, however, I haven't found one that I prefer over the open back.

I do want to try some louvers behind the cabinet that would operate similar to the TA-7331. My guess is that those louvers (which WE covered with some kind of thin damping material) operate to diffuse, redirect, and maybe attenuate the rear wave especially higher frequencies without introducing as many colorations as a fabric covering the entire back opening. My plan is to do this with a contraption that sits on the floor behind the speaker but does not touch the wood frame of the speaker. The OB cabinet is surprisingly sensitive to other materials touching it. There must be some significant vibrations in the plywood even though I hardly feel anything if I put my hand on the sides while music is playing.

Not OB but I did get better balance, top to bottom, when I felted my Lowther/Medallions.

 

[J-ROB]

I felt covered the baffle for that 753 cabinet. Why not? I had the felt.

 

[Audionut]

I felt covered the baffle for that 753 cabinet. Why not? I had the felt.

View attachment 5097

Or you felt like it?...what would say are the benefits? I am still debating on what material to line my 604e cabs.

 

[J-ROB]

The idea of felt or other absorptive material on baffles is to absorb reflections off of the baffle of frequencies shorter than the dimensions of the baffle. The WE757A pictured above uses white acoustical tile (asbestos?) for the front baffle face, for example, to achieve this end.

Since the jail cell bars on a 753 style cabinet appear to be a diffraction nightmare, using some felt helps put my mind at ease. I'll never know for sure if it helps or not because I am not going to take it off and A/B with/without felt!

Felt diffraction treatments are totally old hat. Remember the Spica TC-50, Dunleavys, and others from the 80s? I'm sure some manufacturers and DIYers are still doing this today but I cant name any commercial examples, because I'm not really paying attention.

I have some antique DIY cabinets lined with thick felt for internal damping. If you can score a deal on felt, this might be a path to follow. Otherwise, denim insulation from homedepot.com is easy to work with and seems quite effective

 

[marantzfan]

The idea of felt or other absorptive material on baffles is to absorb reflections off of the baffle of frequencies shorter than the dimensions of the baffle. The WE757A pictured above uses white acoustical tile (asbestos?) for the front baffle face, for example, to achieve this end.

Since the jail cell bars on a 753 style cabinet appear to be a diffraction nightmare, using some felt helps put my mind at ease. I'll never know for sure if it helps or not because I am not going to take it off and A/B with/without felt!

Felt diffraction treatments are totally old hat. Remember the Spica TC-50, Dunleavys, and others from the 80s? I'm sure some manufacturers and DIYers are still doing this today but I cant name any commercial examples, because I'm not really paying attention.

I have some antique DIY cabinets lined with thick felt for internal damping. If you can score a deal on felt, this might be a path to follow. Otherwise, denim insulation from homedepot.com is easy to work with and seems quite effective

Yep, that denim is exactly what I ended up using in my 604-8G/620A build I dud a few years ago and I swear by the stuff now. It’s cheap, available locally, healthier for you and works very well.

That’s exactly what I’m planning to use in the 614 cabinets.

Btw, I got a call from Bill at Great Plains Audio yesterday; my 414z’s gave been reconed and will be here Tuesday!

One step closer.

 

[J-ROB]

berncrecined

Is that a word?

 

[je2a3]

If memory serves me right the external dimensions of the WE757A cabinet may have had an internal volume of approximately 3.5 cf? Correct me if I am wrong Joe. If that is so, it's pretty much the equivalent of an Altec 614 in terms of size. With a little bit of juggling, a front baffle with cutouts for a 414, 32 horn and port is a possibility. But then, there's already the Altec 9849 I posted earlier.

 

[marantzfan]

Is that a word?

Fixed it. That’ll learn me to post something that long from my phone.

 

[J-ROB]

With a little bit of juggling, a front baffle with cutouts for a 414, 32 horn and port is a possibility

But the box will not miraculously change the Altec parts into Western Electrics!

The 757A was the apical ancestor of all the Altec, JBL, Westlake, etc 12" and horn studio monitors

 

[ICTWoody]

Getting rolling. A friend of mine should be starting on the 614 cabinets this week when the baltic birch arrives. And my first batch of caps and resistors arrived today. I ordered enough caps and stuff to build 3 different versions of the JE network and 2 different types of 1.5uf caps for a J-Rob style rig.

I've never rolled networks before. Should be fun.

What about stuffing the box? Just 2in of OC703 on 3 of the 6 interior faces? Or something else?

- Woody