High Efficiency Speakers
Copyright 2010-2016 © Troels Gravesen


Pleased to present my first construction dedicated for corner/near-wall placement. Based on the fact that most people have to place speakers near walls or close to corners due to simple lack of space or spouse having specific ideas on living room decor. And also based on the fact that most people enjoy music while doing other things, they hear music while others (few) listen to music (audiophiles). Now, I'm also sure the latter group do not necessarily enjoy music more than the former group. Quality is highly subjective.
Setting up these speakers in various rooms in our house was a rewarding exercise in how the rooms played their different roles in shaping the overall sound. Moving the speakers a little back and forward with regard to front wall and side walls impacts the sound and experimentation is imperative in getting the sound you like the best.
Making the cabinets was fun. They are so easy to make, just like the very first speakers I made in the Seventies. What I couldn't do back then was making a very simple series crossover, delivering a flat response with point of crossover exactly where I wanted it. Only really good drivers and proper measuring gear and simulation software can create such results from so little. Only 4 (four) components in the crossover.
High impedance (min 7 Ohms) makes this speaker very suitable for tube lovers.

OBL-15 Open Baffle Loudspeaker

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Well, the OBL-11 was sold a few years ago (space issue) and I'd forgotten what a 15" on an open baffle does. I knew it was good, but that good? Frankly, it beats any other construction on these pages regardless of price. None of my other speakers even comes close to this. Listening to Niels Henning Ørsted Pedersen on This Is All I Ask, track four, "Just In Time", and you'll know what I mean. The articulation in lower registers is phenomenal and overall dynamics scary. This is close to what an upright bass should sound like. Even the best 10-inch bass in a bass reflex box will never be able to deliver such detail and transient response. Never! Price to pay is size..


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There are two main features of the new TQWT-mkIII and DTQWT-mkIII constructions: New JA8008 HMQ driver and stepped baffle allowing a true LR2 filter to be implemented. The JA8008-HMQ driver features a high mechanical Q (low damping) and a better match to the magnificent Eminence bass driver(s), thus enhanced transient response and low-level detail. The stepped baffle allows true LR2 filter, which has become my favorite from the constructions made over the last few years. To properly implement LR2 filters we need to acoustically align the drivers, thus the stepped baffles.


OBL-11 Open Baffle Loudspeaker


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This is my homage to Mr. Gilbert Briggs, founder of Wharfedale loudspeakers and writer of numerous books on hifi. My first book on loudspeaker building was Briggs' "Loudspeakers", edition 5, 1958, reprint 1970, and I still have it on the shelf. Much of what Briggs wrote back in the Fifties and Sixties is still valid today, thus highly recommended.
In particular one construction caught my attention, the SFB, Sand Filled Baffle, and maybe it was due to the simplicity of cabinet construction for a young man with limited funds, or maybe I just liked the physical appearance of the baffle. The latter prevails to this very day.

3-way tapered quarter-wave tube design

Replaced by DTQWT mkIII

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To my ears, this is the best speaker I've ever made - and the biggest! 150 liter brutto volume is what you have to be prepared for should you decide to follow. From the front it seems fairly modest, 28 cm wide and 105 cm high, but depth is 50 cm and it will require some 20-25 cm minimum to the rear wall to breathe. Overall sensitivity is 95 dB/2.8 volts and having a minimum impedance of 6 ohms, it can be driven by low-wattage SET amps. I do not recommend 2 watts SET; these amps need 98-100 dB minimum no matter what manufacturers may claim. I have no trouble driving the DTQWTs from my 8 wpc 300B SET amp. Deep powerful bass - and that very special 300B midrange. With a proper 300B SET amp you can play these speakers for hours and hours without listening fatigue, a clear sign of low distortion and proper balancing of drivers.

In many ways life gets easier when we make big speakers. We may have more and bigger drivers and distortion may all of a sudden be reduced significantly because none of the drivers have to move much to produce high sound levels and we don't have to rely on drivers being capable of large cone excursions either.
We may all have experienced a 6" driver being driven to high SPLs and doing its best to pump air in order to produce deep bass notes - and at the same time trying to produce a smooth upper midrange/lower treble, the 640-2560 Hz range. It's not ideal but a compromise most of us have experienced. As long as we don't push a "six + one inch" too hard, things go well.

2-way tapered quarter-wave tube design

Replaced by TQWT mkIII

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Designing your own driver is a major challenge. Having worked with hundreds of different drivers, you eventually come to a point where you would like to design your own driver, hopefully fulfilling a wide range of desired features. Some people may think that cost is a major constraint here, but it is not, unless you want to use a lot of chrome, copper and fancy metal coatings; things that do not necessarily enhance sonic performance. Metal for the motor system, magnets, chassis, voice coils, cones and surrounds basically cost very little. The problem is to get the parts you want - and in the quantities you want. If you order 10,000 units, no problem. Smaller quantities are trouble. Obviously we're not ordering thousands of units here. That would be financial suicide. The driver presented here is not designed to be a commodity product, rather a carefully designed high-end product aimed at people enjoying low-wattage, high-class amplification, but it can obviously be connected to any high-quality system. Be prepared it will reveal possible deficiencies in the system driving the speakers. Be also prepared that it will urge you to use the best crossover components available like those seen below.


Supravox DTQWT experiments

A study into rather nerdy high-efficiency speakers using extended range drivers, allowing point of crossover to be taken above 7-8 kHz, thus a single driver covering the entire range of basic notes plus most harmonics.
Nerdy? Well, this is not a speaker for everyone - I think. It's a speaker for those who want to explore the world of low-wattage, single-ended triodes - and not least - the delicate presentation of the very best recordings from either CD or vinyl. It's a discriminating speaker and if you have less than the best source material, this speaker will tell you. Those addicted to heavy metal or contemporary type of music having a constant "noise" level over the entire frequency band, please look elsewhere. I like music having breaks, music with low level passages, etc. How can we build up expectation and excitement if we don't have breaks? Anyway....

Driven by the best of SET amps it can bring you close to the music like only good fullrange drivers or electrostatic speakers can do - but there's a price to pay: The paper cone of the 215RTF64 is extremely thin and careful damping behind the driver is necessary to avoid rear reflections and there's a limit to loud we can play before the membrane starts breaking up and smears details. T

The 215RTF64 is a large 8" driver having a membrane area of 265 cm^2 and efficiency is around 96 dB. In commercial terms this means 98-99 dB from the usual overrating of products. In the treble range it actually does deliver some 100 dB/2.8V, but this won't help if the upper bass and lower mid does not follow - and it doesn't.

OB9, 2009 Open Baffle Experiments

The recipe:

- 1 x JA8008/TW034 kit (= a pair)
- 2 x Eminence Deltalite II 2515 bass drivers
- 1 x electronic crossover, 24 dB L/R with variable point of crossover
- 2 x stereo power amps, anything from 20 watts and up will do. Small SET amps can easily run the mid/tweeter section.
- A decent clone of the beautiful vintage Wharfedale SFB speaker.
- Liniarise Deltalite II impedance from 15 uF + 10 ohms across terminals.
- Add baffle step loss compensation to the bass drivers from 8.2 mH bypassed by 4.7 ohms.
- Hook it all up, set point of crossover between 80 Hz and 160 Hz to your liking - and you're flying!

OB7, 2007 Open Baffle Experiments

I'm always ready for surprises when it comes to speakers. The sensitivity of this panel is phenomenal, reaching a healthy 100 dB/2.8 volts in all of the midband. Actually the JBL tweeter is working hard to make up to the two other drivers, but despite the apparent sloped response towards the top octave, treble seems to be well balanced from having no attenuation on the tweeter.
Reaching ear-shredding levels, I measure no more than 5-6 volts peak RMS on speaker terminals, accounting for some 5-6 watts. This means that even flea-powered amps can be in here and I haven't been able to drive the MAE1 amp to any noticeable distortion levels.

Obviously there's no boxy sound from this panel. The bass comes out clear and punchy and listening to acoustic bass is most enjoyable. It's also obvious that the bass is not as deep as had the Goodmans been placed in an e.g. Onken vented box. So, how about the mid running so high? Well, I hear treble crisp and clean, but dispersion - or lack of the same - in upper mid and lower treble is very noticeable. In that sense it resembles electrostatic speakers. Move your head a little and the soundstage changes a lot. There's a hot spot, no doubt about it. However, due to the dipole status, this is more tolerable than expected. A dipole tweeter might be interesting to try, but where do we find a suitable planar at this level of sensitivity?