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Good ole John Karlson

Posted by bb2 (A) on July 22, 2013 at 05:00:11

In Reply to: RE: Is the Karlsons for me? posted by bb2 on June 22, 2013 at 14:10:38:

Well, here we go. The K-12's are practically finished: They lack the corner steel reinforcements and edge linings but otherwise they are good to go.

I have to buy some trolley wheels for them as well: These mothers are HEAVY!

Units used:
- Fane Crescendo 12MB bass
- Fane M5 squaker
- Fane ST5020 slotted tweeter
- Crossover home brewed over a modified commercial unit 500Hz/5.3kHz 12dB/octave (treble 24dB to protect the coil) with resistive compensation.

Amplifier (music) Revox S25 2 x 60W
CD Micromega Stage 2
Amplifier/mixer (synthesizer and vocals) Nakamichi Stasis7/Soundcraft FX-8
Synthesizer Korg OASYS 88

Refer to the picture: the M5's were left open to the rear in hope of utilizing radiation from both sides of the cone, amplified and smoothed by the front chamber and the curtains.

Well, the scheme works. The critical midrange is far superior to the same set mounted in a traditional bass-reflex box. Very distinct and precise.

Treble is as always perfect with the famous ST5020 (still some around on eBay, I hear) if you like sharp, crisp high frequency sound that will cut butter. The efficiency of the tweeter with its 2kg magnet is a reasonable match for the acoustically enhanced midrange speaker.

What about phase relations in the midrange when using both sides of the cone?

I use my ear. If it sounds good it IS good (quote Avery Fisher; he should know). Measuring phase is not my favourite passtime as it is close to mission impossible. Too many disturbing factors.

The bass is something else. As anticipated with a K it is massive, precise and very, very powerful. Also, as anticipated, it has a distictive hump around 200-300Hz calling for some equalizer reduction in order not to sound boomy.

But the bass power is incredible! The efficiency is such, that the modest 2 x 60W from the Revox amp is more than adequate for large halls and outdoor use.

The acoustic picture with recorded music is close, precise and present. Due to the midrange properties piano and double bass come through in a natural way. The first test were conducted in a fairly small room which is an arror: These K's perform optimally in large rooms and at high sound levels. They are not exactly bookshelf speakers either.

Whereas music was reproduced without any equalization at all the synth was fed through the FX8 mixer console and equalized a bit, dampening the 2-300 Hz hump.

Wow! This is just what the doctor ordered: The superb transient handling capabilities of a K really come to use here. Piano sounds are clear and crisp like nothing I heard before. For the first time I hear a speaker system that matches the headphones I use for practise.

For vocals using a Neuman KMS-105: Perfect. No less. Well balanced, present and clear. The mike has something to do with it too, of course, but no chain is stronger than its weakest link.

The original K-15 was constructed for use with church organs and capable of reproducing the 64' pipes. I can tell, that the K-12 is not far behind. Frequencies below app. 30Hz are clearly attenuated steeply since the mechanical impedance changes significantly there and the amp sees a higher impedance, thus educing the power - exactly as it should be.

This K is a hybrid, i.e. it does not use a full-range unit as the oroginal construction did - hence my experiment sending midrange into the front chamber - and no particular calculations have been done other than nmechanically matching the speakers to the box, but nevertheless the result is better than expected.

The midrange and treble have a hard time matching the woofer since the bass reproduction is enhanced 3dB (double sound pressure) compared to an infinite baffle. Woofer cone movements are very small for a given sound level and thus we avoid the usual doppler effects from wild cone excursions - and we get a dry, precise bass. Some mild attenuation of the bass renders a better balance expecially when used with musical instruments. Not the worst condition since it takes load off the power amp.

The thick plastic curtains work in many ways as the hard surface disperses the treble and midrange well. Old time movie theatres used the same principle for sound using a huge (typically 4 x 4 meters) 1" sheet of plywood with the woofer placed in a closed box on the rear and the treble unit radiating from the surface center, which was hardened with several layers of varnish, as dispersion medium. Dispersion was nearly 180 degrees at all frequencies and the 4 meter dimension rolled off bass smoothly from 60Hz and downwards - a setting called a dialoque filter in the recording industry: Optimal for speech recognition.

The back radiation from the midrange seems to work too. So far no measurements have been made (lot of work setting it up and little time to do it in) and who cares anyway. I like the noise they make.

On the other hand measuring may render some useful information: I changed several parameters for the boxes èn route so it is hard to tell which make a difference and which do not.

The speakers were not intended for HiFi but they came out not too shabby in that respect anyway.

Was it worth the effort?

Building a K is a lot of work, but hell yes: The result is not 100% perfect but then what is?

The Basher

Edits: 07/22/13 07/22/13

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