Infinite Baffle System Questions

[Guest baumgrenze]

I need help from the electrical engineers following this board.

Below I have pasted a photo of the circuit diagram for an infinite baffle system which was installed by the previous owner of my open-plan post-and-beam house. The speakers are mounted on 1" plywood and back onto clothes closets on a hall adjacent to the living room into which they project. The previous owner worked at SRI (a.k.a. Stanford Research Institute, Menlo Park, CA). He copied a system built by a colleague. The story I was given is that the colleague was a friend of Paul Klipsch, who, in turn designed the system for him.

The LPads on the system are noisy and unreliable. Their ohm rating is not specified on the diagram.

Can someone knowledgeable tell me what the rating should be?

On the AR forum, I've seen comments that suggest that it is possible to disassemble an LPad and clean it up (Deoxit red and blue?) and then put it back together. Should I invest the time in this approach first?

Thanks,

Baumgrenze

[soundminded]

This is a most unique and unusual design in many ways. In the words of people on the Antique Road Show, thanks for sharing it with us. Paul Klipsch was best known for his work developing the folded corner horn embodied in his Klipschorn design still available and having a cult following today but designing an infinite baffle loudspeaker system was certainly well within his capabilities.

The L-pad for the midrange drivers is almost certainly 16 ohms. That for the tweeter is unknown but probably the same as the tweeter itself and also probably 16 ohms. Ordinarily I'd say just replace them but 16 ohm L-pads are difficult to find today since most drivers are designed for 4 or 8 ohms to take advantage of solid state amplifiers' higher output capabilities at those impedences. (BTW, the speakers are probably not wired to the L-pads as shown either--I think--that is not the way L-pads are normally wired, they are three terminal devices.) I'd try de-Oxit and several others cleaner solutions before anything else. Use the thin plastic tube and try to get into the L-pad with as far as you can through any opening before spraying the liquid. The working part probably consists of a fixed semicircular conductor and a rotating contactor. A film of oxide or other non conductive substance on either or both is the problem and the solvent should chemically disolve it. Work the L-pads back and forth after applying, the force between the two will help removing the oxide. Disassembling them is something you want to avoid if at all possible and use only as a last resort. Some types can be disassembled and some may be difficult or impossible without destroying them. If you must in order to clean them, be careful so that they can be properly reassembled. Good luck.

[Guest baumgrenze]

Thank you Soundminded.

I can confirm that the LPads are more complex than is indicated in the sketch. Each LPad has 6 terminals, probably the wiper and both ends of the resistance coil for each of two coils. I should have dug deeper. The backside of the devices shows that both are 16 Ohm with a logo that looks like WL-16.

Since you suggested that the design is unusual I took a few digital photos to share. I could not read all of the hand written notes on the capacitors. Clearly each required capacitance was achieved by connecting several capacitors. Do they really need to be this huge? The inductors are hand wound. The overall size of each crossover network is 7"x15"x20"; they are wall mounted over the top shelf of the closet. I thought they might amuse someone who is accustomed to looking at crossovers that fit in a speaker cabinet.

I took a couple of photos of the backside of the speakers to show just how the LPads are wired to the crossover and speakers.

I'll try seeing if I can get the schematic at the bottom of this post as an embedded graphic.

Thanks for your interest.

Baumgrenze

http://www.classicspeakerpages.net/dc/user_files/924.jpg

http://www.classicspeakerpages.net/dc/user_files/925.jpg

http://www.classicspeakerpages.net/dc/user_files/926.jpg

http://www.classicspeakerpages.net/dc/user_files/927.jpg

http://www.classicspeakerpages.net/dc/user_files/928.jpg

http://www.classicspeakerpages.net/dc/user_files/929.jpg

[soundminded]

Due to the age of these capacitors, you might try replacing them with modern polypropylene types such as those available at Parts Express. I don't subscribe to buying expensive audiophile types as I never thought they would make much difference if they made any difference at all but these must surely be at the end of their lives being 50 or 60 years old. Replacement should restore the performance to new or better than new.

The infinite baffle concept is probably the oldest and most intuitively obvious methods for housing a speaker. Because the back wave of the speaker is out of phase with the front wave and would cancel it out if allowed to radiate to the front unaltered thereby sharply reducing bass, the idea of the infinite baffle was to effectively wall off the front and back so that the back wave could never reach the front. In real terms, this meant a very large enclosure. In the 40s and 50s before stereo, many hi fi do it yourselfers (that was when this was a real hobby and any red blooded American male would tinker with anything given half a chance) would mount speakers in a closet door using the closet as the enclosure. The demand of stereo for two systems strategically spaced brought that idea to an end. I think the largest Bozak speakers were designed as infinite baffles. The only maker I'm aware of who offers drivers today specifically designed for infinite baffle installation is Dayton and they can be found in the Parts Express catalogue which you can order free on line. They are competitively priced with other woofers of their type (in the range of $150 each.) Some people use their attic or basement as the back side of the enclosure. Infinite baffles have many advantages but size and convenience are not among them.

[dynaco_dan]

Real nice photos.

[Guest baumgrenze]

Hello Soundminded,

Thanks for your interest and suggestions.

I am a retired synthetic organic chemist whose sum total experience with 'arcs and sparks' is restricted to house wiring, appliance repair, and the successful comletion of a kit version of the EICO ST-70 amplifier in 1967.

I'd love to rebuild these crossovers, but I am reluctant to mess with something that seems to work. The capacitor values I could read on the capacitors in the woofer part of the one circuit do not seem to add up to the value specified by circuit sketch.

There is also the network of what look like large resistors under one of the inductors in the lower left hand corner of the photo. They do not appear in the diagram at all!

I get the impression that these crossovers could be much smaller and neater if they were rebuilt and both the capacitors and the inductors were replaced. BTW, are the large, rectangular capacitors oil-filled? I saw in an online catalog a set of brackets for holding down an oil-filled capacitor similar to the ones used here.

I did a quick check and parts do not seem overly expensive, but then I was not certain just what to price.

Thanks for any further insights.

Baumgrenze

[soundminded]

>Hello Soundminded,

>

>Thanks for your interest and suggestions.

>

>I am a retired synthetic organic chemist whose sum total

>experience with 'arcs and sparks' is restricted to house

>wiring, appliance repair, and the successful comletion of a

>kit version of the EICO ST-70 amplifier in 1967.

>

>I'd love to rebuild these crossovers, but I am reluctant to

>mess with something that seems to work. The capacitor values I

>could read on the capacitors in the woofer part of the one

>circuit do not seem to add up to the value specified by

>circuit sketch.

>

>There is also the network of what look like large resistors

>under one of the inductors in the lower left hand corner of

>the photo. They do not appear in the diagram at all!

>

>I get the impression that these crossovers could be much

>smaller and neater if they were rebuilt and both the

>capacitors and the inductors were replaced. BTW, are the

>large, rectangular capacitors oil-filled? I saw in an online

>catalog a set of brackets for holding down an oil-filled

>capacitor similar to the ones used here.

>

>I did a quick check and parts do not seem overly expensive,

>but then I was not certain just what to price.

>

>Thanks for any further insights.

>

>Baumgrenze

Those do look like oil filled capacitors to me. The problem is that they may have changed value making performance different than intended. (It's possible they may contain PCB-as a chemist I'm sure you know what that is, so be careful if you have to dispose of them or they leak.)

Here's a link to L-pads.

http://www.bcae1.com/lpad.htm

As you can see, there is both a series and a shunt resistance. When they are chosen correctly for the impedence of the load, the combined circuit impedence it and the load present remains unchanged while adjusting it will vary the voltage across the load. The advantage to presenting a constant load is that since the crossover frequencies and Q of the circuit are affected by load impedence, using an L-pad instead of a simple single potentiometer does not cause this change, all that changes is the relative ouput of the drivers, not the crossover frequency or slope. The original ARs regrettably used potentiometers instead. The flattest response where the crossover was optimal was at least theoretcially only obtainable when the setting was exactly the correct series resistance. This site has listings I think for all of them for those who wish to replace the pots with fixed resistors.

Good luck with your restoration.