16 Ohm High Efficiency Speaker
Perfect For OTL Amps
Low Cost- Easy to build
The problem with creating a top quality 16 ohm speaker for OTL use has always been size and cost. Since there are virtually no 16 ohm drivers on the market, we have to double up and that doubles the cost. Then there is the size problem as cabinets also must increase in size. Multiple driver speakers are more difficult to design because all of the drivers must coordinate acoustically.
The primary motivation for creating this speaker was to create an ideal match for the new Mini Beast amp. It certainly does that and more. It works perfectly well with all Transcendent Sound OTL amps and allows them to reach their full potential.
This is a two-way design, incorporating planar tweeters with a sort of bass reflex low frequency section. The tweeters are dipolar and are mounted in a open baffle arrangement. This really opens up the sound.
The cabinet has an internal volume of 3 cubic feet with internal dimensions of 12 inches by 12 inches by 36 inches. Using 3/4 inch thick partical board, that yields external dimensions of 13.5 inches by 13.5 inches by 37.5 inches for the cabinet. Very little floor space is required.
The port is simply a 3 3/8 inch square hole cut in the front panel six inches off the bottom. The bottom end was very boomy. Closing the port killed the bottom end. These drivers need to be ported. Also, the port was projecting several resonate frequencies that colored the sound. The symmetrical shape of the cabinet certainly didn't help to dampen standing waves.
The solution was to evenly pack the cabinet with 2 pounds of standard fiberfill that you can buy in the craft stores. It comes in five pound boxes. That solved the problem. No more resonances, and good solid bass down to 50 Hz. If I had it to do over again, I would have lined the cabinet with lightweight vinyl damping sheet. You want the box to be sonically inert so you aren't listening to cabinet vibrations. A couple of cross braces would not hurt either.
Whatever you want to call it, it works. The midrange is clean and the bass is solid. These are fast drivers. Moving mass is only 10.7 grams yielding fast transients.
The tweeters are a planar type mounted in an open baffle. They are quick and clean. The open baffle measures 9.5 inches wide and 11 inches tall, fashioned into an L shaped bracket with the bottom plate screwed into the cabinet. No calculations were made to determine the dimensions. It is important to mount the front surface of the baffle 1 inch behind the woofer panel. This better time aligns the drivers.
The crossover point is set to 1000 Hz for both. The woofers use a two pole 12 dB/octave section and the tweeters use a one pole. These tweeters have a rising high frequency output. They are too bright. A small inductor was placed in series to tame that down.
The new Mini Beast will drive these to very, very loud levels. You can really rock out. The higher powered OTL's will pin your ears back. This is not a near field speaker. Because of the multi-driver arrangement, you have to get back about 12 feet or more to allow the sound field to develop. It will work exceptionally well in large rooms.
The reason this speaker work so well with low powered OTL's is because of its impedance curve, shown below.
Notice that the impedance is 13 ohms at 200 Hz and then rises to 35 ohms at 100 Hz and then peaks out at 44 ohms at 85 Hz. This is the power band. Since the impedance is so high, very little current is needed, hence the two watt amp can easily drive it. Due to the resonance peak, speaker efficiency skyrockets, yielding a flat acoustic response. Nominally, the sensitivity is 95 dB for one watt. The rising impedance above 5 Khz is caused by the tweeter inductor, further unloading the amp.
The crossover network is shown below:
Parts List:
4 tweeters: GRS PT6816-8
4 woofers: Dayton Audio PM220-8
2- 0.3 mH inductors
2- 3.0 mH inductors
2- 12 mfd, 250V capacitors
2- 8.2 mfd, 250V capacitors
2- 0.33 mfd, 250V capacitors
2- pairs binding posts
4 pounds of fiberfill
3- half sheets (4' by 4') of 3/4 inch particle board
Five hours of break-in will make a world of difference. You will be amazed what a 2 watt OTL can do with these speakers.
