Emails from Andy Kotsasis

[Guest gkentsmith]

This is a rather long piece, lifted from Vintage Assylum posts (thanks Google) and is quite informative. Enjoy, everyone.

From Vintage Asylum:

Posted by Ken Perkins ( A ) on January 20, 2005 at 09:08:59:

Over the last two days, I've had the incredible fortune to have had several email exchanges with Andy Kotsatos of Boston Acoustics. He contacted me after seeing a post here regarding the manufacturer of th Advent drivers (they built them in-house, BTW). Andy worked with Henry Kloss at Advent and apparently for a while at KLH was well. Over the conversations, I was given a world of great information and history of the Advent, the drivers and the design, and with Andy's permission wanted to post it here for all to know. I'd like to thank him for taking his time to put up with my questions and for freely giving out this most excellent information surrounding this speaker's lineage. Some of the detail I've never seen posted anywhere before. I've also invited him to join us here if he likes for further discussion.

Enjoy!

Some tidbits and highlights, taken directly from the emails:

re: the original masonite frame woofer:

"The woofer cone in the original Advent, whose diameter we refused to define in our brochures, was the same size as commonly used in 10-inch baskets. However, we had to use a 12 inch basket because Henry wanted a deep cone for stiffness, and it was too stiff to fit in a standard 10-inch frame! The masonite ring was used to take up the space around the circumference of the surround. Because of the excursion capability, the baffle had to be recessed and the grille panel spaced off of it. It made for an unusual looking woofer!"

re: the second generation all steel frame woofer:

"In 1974 I went to Japan on business, and on the long flight back I sketched out a woofer frame that would step down to allow the excursion and eliminate the ring, and look better, I might add. I also sketched a new tweeter frame that would eliminate a number of parts, reduce the moving mass by shortening the voice coil former, and also look more finished. I showed the drawings to Henry when I returned to the office, and he was surprised and pleased, and gave me the go-ahead to have engineering drawings started. The new woofer frame was stronger than the original one, and had a number of other neat things about it. It's amazing what you can do on a 20 hour flight! My primary objective was to move the speaker baffle forward in the cabinet, but in the process of solving that issue, I was also able to reduce cost, improve appearance and make stronger parts. I also wanted to make the woofer basket be backward compatible, so it would fit in the original speaker if need be, which is why it has the same bolt hole circle.

The new frames were tooled and first samples completed around the time Henry was leaving Advent. So some time in 1976 I started work on the New Advent, which was the first system I designed from the ground up. The woofer was derivative of the original. It used the same moving parts, but I flipped the surround around so that it bulged out. The tweeter used the same cone but a different coil, and it was one of the first tweeters in the world to successfully use ferrofluid."

re: the "fried egg" tweeter:

"The original tweeter was Henry's innovation. It was a new twist on the KLH-6 tweeter, the first phenolic ring tweeter that was widely imitated around the world. The edge suspension on those tweeters was a dispensed elastomer, that was always problematic in production. Henry wanted to eliminate it by substituting the outer "donut" as the suspension. It was an ingenious solution, given the problem. Henry was always a big proponent of cones over domes because he felt that the former could generate more energy

at low frequencies (for a tweeter) and that a dome would simply become a ring radiator at very high frequencies whereas the cone would be stiffer and not break up in the same way. He is right that domes, no matter what the material is, become ring radiators, but I disagree with his conclusion. At BA I chose to go with 1-inch domes because they are the best compromise (everything short of having the live performers in your room is a compromise--the trick is to choose the best compromise)."

re: the performance of the box/woofer combination:

"The Fc of the both the original and New Advents is 43 Hz with a Q of about 0.7. The Smaller Advent also has an Fc of 43 Hz, with about the same Qtc. That was the double entendre in the name: it was the same bandwidth as the original in exactly half the enclosure volume and 3 dB lower efficiency. The Advent/1 was a replacement for the Smaller Advent, and it was more efficient but had a higher Fc.

As for design, without going into great detail, the low frequency limit of a speaker can be accurately expressed by the following relationship: Efficiency is proportional to Fc cubed times Volume of the enclosure divided by Qtc. No matter how clever the designer, he can't overcome that relationship. The designer has control over for parameters to manipulate where he wants to be: volume of the enclosure, strength of the woofer motor, moving mass and area of the diaphragm."

re: voicing of the Advent lineage:

"As for the voicing of the systems, I did all that. I did the voicing of the original Advent, the Smaller Advent, the Advent/2, the Advent/3, the New Advent, the Advent/1 and the Advent 400 radio. I also designed a really neat car speaker, the Advent EQ1, that was being evaluated at GM headquarters when I left in 1978. If Advent hadn't self-destructed, that first branded car system in the Cadillac might have been Advent instead of Bose."

re: the myth that the Boston Acoustics A150 was supposed to be a three way Advent:

"Finally, there is no truth whatsoever that I was working on the A150 while I was at Advent. When I left Advent on November 30, 1978, I had no idea what I was going to do. Credit my late partner with talking me into forming Boston Acoustics in February 1979. Everything here was designed here. I brought nothing with me from Advent except the experience I had gained there. The A150 was the sixth speaker to be designed here."

That's it for now and stay tuned. Again, my sincere thanks to Andy for sharing this information with me!

Well, here's my second installment of "Little Known Facts About Well Known Advents" from my emails with Andy Kotsatos of Boston Acoustics. I was waiting for a reply to my most recent email to post even more information from Andy, but he's either been busy or gotten tired of answering my questions.

Regardless, I was waiting for something definitive regarding the doping treatment for KLH's cloth roll surround woofers so maybe he'll find time to respond to my last round of questions. Here's my latest email from him from last week...

re: QC at Advent and why the tweeters are colored:

"Ken,

With regards to QC, there were two major episodes. The first occurred the first month of production. The tweeters didn't sound up to snuff, so I brought it to Henry's attention. I remember it was Columbus Day 1969 that Henry looked into the problem, and he discovered that the tweeter cones, which we made at Advent, were too porous and leaking air so that the efficiency of the tweeter was too low. His remedy was to paint the cones with a sealant. The most expedient sealant at hand was some bulb die that one of the employees had on his desk. It was red bulb die, and thus was born the red/pinkish Advent tweeter cones! It was a great solution because the color made it easy for the person painting the cone to be sure to cover the entire surface.

The second episode occurred a year or two later. I started getting

complaints from some customers that their two speakers didn't sound the same. After making sure that they compared the speakers in mono, side by side, I had a few customers return their speakers to the factory. Sure enough, they were right, they sounded different. When I investigated (I did the detective work this time), I found what was causing the differences and put in process control steps in manufacturing to make certain that important parameters didn't drift in production. Thereafter, every Monday we took a dozen woofers and a dozen tweeters at random from production and measured them to make sure they were consistent. It was amazing that if you controlled the proper specifications of the critical parts, every driver would be identical to the reference drivers.

A year or so later I had Engineering design a test rig that enabled us to measure the response of every production speaker against the reference. By dividing the read-out into six different bands (the bottom end, the piston band, and the top end of the woofer, and likewise for the tweeter), the test also served as a diagnostic tool for the tester. If a speaker failed, the tester could tell by which band or bands it failed in how to repair the speaker so it could pass. I'm sure that Advent, which was making a finished speaker a minute by then, was the first company in the world to test its finished product to such a close tolerance that any two would constitute a matched pair.

When the New Advent was reviewed around the world, many of the reviews in foreign countries included frequency response curves that were run on the review samples. I was really pleased that they all looked like they had been taken of the same speaker! That's how good our production testing was.

At Boston Acoustics, we went even further, and from the very beginning we tested to an even closer tolerance. At Advent the overall tolerance for matching to the reference was +/- 2dB. At BA every speaker is tested to within +/- 1dB of the reference. That means that someone who has bought our $150/pair speakers gets the same unit to unit matching as someone who buys our most expensive systems. I don't know of any other speaker manufacturer in the world who tests the response of every speaker they make, let alone

tests to such a close tolerance.

Well, that's it for this installment. I have to get ready for the Playoff games this afternoon. Go Pats!

Andy"

[Guest Ken Perkins]

Yep, I just got lucky I guess. I nearly passed out the day I got the first email from him and he actually replied back when I responded to it. At first I thought it was a joke until I checked out his email address and found out it was for real. He's a really nice fellow.

Now if I could just find my Ouiga board, I'd channel Kloss himself from the grave =^)

[Guest gkentsmith]

Hi Ken,

I'm glad you saw this. I assumed you wouldn't mind my sharing this with the rest of the forum. Please feel free to share any detail with us that you may not have chosen to place on Vintage Assylum.

You are one very lucky guy to have gotten that. I am hopeful that I'll be able to get a brief (phone) interview with Tom Holman; his present company's site says they'll schedule interviews on advance request.

Thanks again for the golden nuggets!

Kent

[Guest Ken Perkins]

Don't mind you posting this at all. As a matter of fact, I thought I had already posted it myself on this site back then but I can't find anything in the archives so I'm glad you found it. As I recall, that was about the extent of our conversation other than some chat about loudspeaker design and just general stuff. I tried to keep the exchange brief so as to not be a pest.

A talk with Holman would be nice as I'd love to hear his thoughts on his phono stage designs. It would also be cool to talk to Vilchur as well for more than obvious reasons! It's great to know that so many of these guys are still around and it'd be great to pick their brains.