Golden Age Bookshelf Speakers – The Cream of the Crop

The introduction of “big” amplifiers (60 watts instead of 3 watts) at reasonable prices propelled the Golden Age of Audio. These amplifiers allowed a major change in speaker design and improvement in sound. Bookshelf speakers could now go deeper in the bass and higher in the treble than the refrigerator-sized speakers that preceded them. These “small” speakers used designs that traded off sensitivity for size and frequency response. They were very HiFi and great for stereo reproduction in the home.

Acoustic Research and Its Offspring

The AR-3a was, without doubt, the standard of the industry. Introduced in 1969, is was the most refined version of Edgar Villchur’s innovation of acoustic suspension speaker design. Extremely well reviewed and wildly popular, it was the model to beat for all other manufacturers. To beat it with another acoustic suspension speaker was tough because of the limits of Hoffman’s Iron Law. This law held that with three variables (size, low-frequency response, and efficiency), once you have chosen two, the third variable is determined. So, to beat the AR-3a, the speaker designer needed to compromise one of these variables. Ohm tried with a more extended treble while keep the three variables the same. Henry Kloss took his company, Advent, in the direction of deeper bass at lower efficiency with great success.

Enter A.N. Thiele – And Marty Gersten

The Electro-Voice Interface Alpha One demonstrated that it was possible to break the Iron Law by using the back of the woofer as well as the front. A. N. Thiele had explained how to do it and have control of the low frequency response shape. With a little bit of equalization, this system with a smaller box was able to get more deep bass output from an 8” vented system than the AR’s 12” sealed system. Marty Gersten, Ohm’s founder, also used an 8” woofer when he developed the Ohm H. This was Ohm’s first Optimally-Vented system designed using Thiele’s procedure. He had kept the box size and efficiency from the AR-3s and the Ohm B+s to gain very audibly deeper bass on the few recordings that had content that low. Saint-Saëns’s Organ Symphony and the cannons in the Telarc recording of Tchaikovsky’s 1812 Overture were popular examples.

Marty’s next Optimally-Vented design was also an AR-3a/Advent competitor. With the Ohm L, he used an 8” woofer designed to keep the Large Advent’s frequency response and efficiency, while going for as small a box as possible. While the Ohm H, Advent and AR-3a had the performance of refrigerator-sized speakers (except efficiency), the Ohm L had the performance of these “full-sized” bookshelf speakers in a much smaller box. The Ohm L’s smaller woofer had wider dispersion in its higher frequencies than the bigger woofers used in the Advent and AR. The on-axis response was similar for all three, but the Ohm had more energy output off-axis in the voice range, and people liked it. The Ohm L design (helped by a low selling price) proved a winning combination and went on to become Ohm’s most popular Optimally-Vented system. The Ohm L was replaced with the Ohm L2 which incorporated stacked tweeters in mirror imaged pairs for better horizontal dispersion and our patented Sub-Bass Activator (SBA) for higher output on the dynamics of CDs.

Repairs and upgrades:

We have the parts to repair Ohm Ls (although we need the woofer chassis back for recycling). The foam surrounds between the cone and chassis are dying of old age. When we rebuild the original woofers we automatically add SBAs to improve the sound. There is some confusion about which tweeter is which: The original Ohm Ls had their Low Tweeters reproduce the 1700-17,000 Hz range but was located at the top of the cabinet. The Low Tweeter has the Ohm logo in the perforated screen protecting its dome. The High Tweeter supplements the output above 4 kHz and was located lower on the cabinet next to the vent. It had the Ohm logo on the outer trim ring. With the Ohm L2s we used the same tweeters but put the high tweeter on top, lined the tweeters up and shipped speakers as mirror-imaged pairs.

Bargain hunters rejoice! It is possible to find Ohm Ls locally on Craigslist and get great sound for a great price.

Enjoy!

John

John Strohbeen Author

John Strohbeen was the President and Chief Engineer of Ohm Acoustics from 1978-2023.