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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: Valve dampingPosted by anthony on: 4/17/2017
I would contend that most of the microphonics and feedback in valves is usually caused by the resonances and vibrations of whatever they are plugged into. Glass is very rigid which is only enforced by the shape into which it is molded. Sure, sound pressure waves directly contacting the valve internals can cause the internal valve structure to resonate, I am not disputing this at all, but an amplifier chassis is generally less rigid than the valve, has a much larger surface area for pressure variations to induce vibration and is often a good mechanism by which energy can be stored for later release. My "extreme acrobatics" have been aimed at producing as inert a chassis as I reasonably can so that the largest source of valve vibration feedback (in my opinion of course) is managed to some degree.
Then there is the feedback caused by direct sound onto the valve. The quietest part of the entire room is probably immediately behind the horns. After all, the horns manage their increased sensitivity by narrowing the dispersion of the driver and at all frequencies above the Schroeder Frequency for the given room behind the horn is significantly quieter than in front of the horn especially given the high mass of my horns and their reduced sound transmission through the horn walls. Therefore, the only frequencies that I am worried about at this stage occur in the frequency range below the Schroeder Frequency where room pressure nodes occur, which is where the wavelength of sound is greater than a room dimension and pressure nodes and nulls will occur based on the characteristics of the room. These frequencies are generally below about 200Hz and are nearly impossible to dampen wherever you place the valve in the room.
So the big question becomes where the speakers will be situated within the room for best sound and does that correlate to a generally less or more low frequency pressure at the valves? I have no way to tell, modelling will not work but empirical testing will give an indication (take SPL measurements at the amplifier). Of course not all low frequencies have nodes and nulls at the same locations so the results will be 'average' based rather than for a specific best location for low SPL. The amplifiers can be disassociated from the horn stack, that is not too difficult, but I really doubt that will be necessary and I really do not want to do it. What might end up happening is I find the best positions in the room for the horn stacks and then see what happens when I integrate the bass arrays. That is a bridge to cross at that particular time and if I have problems with the amps sitting in too many pressure nodes then I will have to reassess. But I do not expect any issues from sound wrapping back from the horn stack, because apart from the lowest octave (100Hz - 200Hz) that will probably just not happen to any significant degree.
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