>speaker feedback to the power tubes, (reverse EMF)
> - the signal generated by the voice coil as it "falls" back through the gap.
>plain cones are more "nodal", ribbed cones are smoother responding (in
>general) not that smoother is necessarily better, just different! :-)
Tube amps produce a spectrum of sound, so it's hard to come up with adjectives that suit all possible tube amp tones. Some players like a dry warm breakup tone with little treble or presence. If you boost bass and cut treble before the power tubes, or boost treble and cut bass, the tonality shifts from crusty to liquidy. The word "clank" implies low-treble resonance, which isn't necessarily present for all different sounds.
The physical sound is the difference between amplifying and eq'ing the signal at the speaker terminals and the signal coming out of the speaker or microphone. The physical sound is especially lacking when you put a resistor-inductor load across the output of the amp, and tap that signal. When people toss out the speaker and just use power tubes and a load, something is missing -- it sounds fake, electronic. It's not just the lack of room reverberation. The speaker adds something other than eq.
I haven't read enough about guitar speakers to describe clearly what I have in mind. Little has been written.
I've tried various approaches to getting rid of the speaker, but I was forced to firmly conclude that the speaker is centrally involved in tube amp tone. Now I'm completely committed to using directly-driven speakers, to get their clank and physical tone. I discourage buyers and manufacturers avoid power attenuators and inductive loads and speaker simulators (low-pass filters). Instead of guitarists spending time trying to get tube amp tone from gear that attempts to substitute for speakers, and instead of manufacturers pumping resources into such gear, everyone should work towards low-power amps, speakers that sound engaged at low power levels, speaker isolation cabinets, and other no-nonsense approaches that involve power tubes directly driving guitar speakers.
The complex dynamic speaker effects do not exist in the signal across a load, and they only exist a little in the signal across the terminals of a hard-driven speaker. These dynamic effects only come to full life when listening to the air that is pushed by the speaker.
Speaker complexity theory -- When you apply a composite signal to a speaker, the low frequencies may move the cone a certain amount, while the higher frequencies riding on the low frequencies move the cone in small steps back and forth. The cone system itself, because it has mass (or weight), will continue to move by itself and actually works against the signal commanding it to move by generating a voltage in the voice coil that counteracts the signal from the amp telling the voice coil to move. This is called motional impedance. As you can imagine, this is a very complex effect and is not very friendly to mathematical analysis. This is why the selection of speakers, by necessity, is very subjective. This motional impedance also plays a big part in how a speaker responds to a particular amplifier output circuit or enclosure. Many speaker emulators on the market today use inductors, capacitors, and resistors in an effort to model a speakers voice coil, but they stop short of emulating the motional impedance. Others use opamp filters as well as other shaping networks in an effort to put peaks or dips in the response curve. The only way you could truly emulate a speaker would be to have some kind of electromechanical device in the signal path that would generate the 'back EMF' and in effect, the unpredictable motional impedance. Several methods could be used to do this. You could build a dummy load/voice coil emulator [like my proposal of a tiny actual guitar speaker and a tiny mic, in a pedal - Michael] and use a small portion of the signal to drive two 4 or 6" speakers mounted face to face. The low powered signal would drive one of the speakers, while the other one acts as a microphone to pick up the acoustic signal of the driven speaker. This 'microphone' signal would then be amplified and mixed in with signals from other parts of the circuit, and you'd then have a fairly complex signal at the output. Another method would be to remove about half of the laminations from an iron core choke and use that in the design of the dummy load/voice coil emulator circuit. The laminations would vibrate and emulate motional impedance. Of course, the laminations would need to be fairly small and lightweight so they would vibrate sufficiently to affect the inductance of the choke. The foregoing description of emulating motional impedance assumes you have a power amp to drive the emulator circuit. Since your question was about using a preamp output for DI, the only choice you really have is to use filters or EQ to emulate the single tone sweep response, short of using a complex algorithm employing a Digital Signal Processor IC. As far as plots on the Internet, I have seen plots for HIFI drivers, but as you have discovered, I haven't found any for musical instrument speakers. I characterized and studied every vintage loudspeaker I could get my hands on while doing the research for our speakers, and plan to add a page to our website to display those plots and their T-S Parameter info.
Weber Vintage Sound Technology - Manufacturers of vintage style loudspeakers. Looks like a full-featured site for speaker freqs. There have been many positive comments about the sound of field coil magnet speakers. Ironically, these were the first speakers available in the early years of speaker development since small permanent magnets with high energy were not available at that time. The field coil acted as a choke filter for the power supply in the radio or amplifier in addition to providing magnetic energy for the speaker. With that in mind, we developed what we call our Emag loudspeaker. It is a 12" speaker with a 1 1/2" voice coil and a controllable field coil magnet. For the first time in the history of speaker development, the user has control of one of the most important parameters of the loudspeaker -- the magnetic circuit. The Emag is supplied with a control unit which allows adjustment of the strength of the magnet in addition to adjusting for compression or expansion of the magnetic flux in response to the speaker input signal. In other words, the Emag speaker can be adjusted to match your amps output circuit for damping, sensitivity, and breakup. The control unit has an auxiliary output which allows several Emag speakers to be ganged together.
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