I got tired of listening the Boom bench pretty quickly, and took a saw to it and cut it in half!
Pulled out the amplifier and turned the speakers back in a vertical position. Connected to the Audioromy FU13 the sound picture instantly found its place again. I guess I have found out the hard way why speakers are always vertical. I know I have read It so you don´t have to pester me with I told you so´s.
When the speakers are vertical the high frequencies which carry the positioning information, are diffracted in a horizontal pattern. That why!
And this is the absolutely last time I will listen to a cheap transistor amp. They Are Boooring.
I have brought the Boom Bench home for some comparative listening. Actually its an uneven match between a Single ended Class A tube amp powering Magnepan SMGa with subwoofers and a 4 inch full range speaker in a TQWT cabinet powered by a lowly transistor amplifier pulled from from an old pair of computer speakers.
But it is the recording material and the speakers that make the most impact on music presentation. I remember the TQWT speakers as very clear and slightly forward speakers with beautiful rendition of female vocals in jazz when they where ordinary standing speakers powered by the MiniBlok. In this incarnation the wonderful sound stage is somewhat unfocused, probably due to diffraction in the vertical plane. The sound is flatter fatter and more suitable for electronic music. I attribute the changed sound to the cheap amp. It could be remedied with a good T-amp. While the imaging probably is an effect of the geometry. Although I would change the amp first to check.
Anyway, I am losing interest in the project and will probably going to sell it of for cost price, or get the saw and revert them to ordinary speakers.
But they are beautiful to look at, and would fit well in a livingroom with the television on top. The massive walls (30mm) are still slightly vibrating, but I doubt it will have any effect on hifi equipment stacked on top.
After much surfing I decided on Fred Nachbaur´s Miniblok SET amplifier for my first foray into amplifier building. Se Fred´s website for a detailed description of how it works and what it is good for.
I decided to build the prototype on a wooden box from IKEA. that way I could keep a clean and safe exterior, and I could turn the lid over so it was just as stable to work on upside down. I ordered most components from Parts Express and Parts connexion. Found power transformers in some cheap local battery chargers, and ordered 13EM7 tubes and Hammond 125 CSE output transformers from Norwegian supplier Motron. Then it was just a matter of soldering everything together.
This is the finished circuit. I have not cut wires and component leads down because I was planning to transplant the design into a double mono chassis if I was pleased with the sound. I did a couple of stupid things here, like running heater negative on the ground bus, and getting the wires to the output transformers right in among the 235VAC, but actually it settled down after 5 or 10 hours of burn in and behaved well with no audible hum on the 89db/w fullrange BiB horns I built to play them on.
Here is a view of the first serious listening session.
This story continues with the 13EM7 dual mono
Update 21.october 2011: Here is the frequency response measurement of the 13EM7 playing through the Tangband W4 TQWT. Measurement is done through a Panasonic WM61 into Mac Pro with FuzzMeasure. the signal is sent to the amp through a M-Audio Fasttrack. The Tangband has Fs of 80Hz, the output transformers of the 13EM7 amp go down to 60Hz.