Atenuador

Alguien escribió:

Other Weber Attenuators utilize an actual speaker motor to apply a reactive load on the amp. The attenuators are unlike any other on the market, and provide a much more interactive, natural sound when in use. They affect the tone less than the resistor-based models.

Alguien escribió:

These are the motors from our MASS attenuators. Many techs like to use them to quietly test an amp with an authentic load so they can look for instabilities. These are speaker motors, they will blow up just like a speaker. So, don't think they are indestructible. We have 4, 8, and 16 ohm units available in power ratings of 50 and 100 watts. The red and black leads are 18" long. I would say the power ratings are conservative, RMS ratings, but I don't want to give the idea the motors are invincible. Be careful when using these and keep an eye on them if you are cranking an amp into them with heavy distortion.
We prefer to sell these only to amp builders and those who understand their proper use.

Alguien escribió:

There seems to be something wrong with the physics of the idea: there is no power dissipation.

The physics works like this: The impedance of a speaker is influenced by the movement of the coil. As the coil moves, it acts as a microphone. This lowers the 'effective' voltage across the coil and therefore the current. This increases the impedance (=AC Voltage / AC Current).

If the coil can vibrate freely, the impedance is much higher then it normally would have been with a speaker cone. Also, the coil can move much farther out of the magnet as it normally would. I've seen with an audio speaker that I destroyed. (Look how loud it can go!!! Bang... ) I had launched the coil out of range of the magnet and it was damaged on its way back into the magnet.

On the other hand, when you keep the coil stationary, the impedance will be smaller than 4 Ohms. (Closer to the DC resistance.)

The problem is, there is no resistive component in the system that dissipates energy. In a normal speaker, the resistive component is the resistance of air to the movement of the speaker. Energy is dissipated as the sound is emitted and from friction when the air is compressed by the cone. The speaker cone assembly is designed dissipate as little energy as possible.
Therefore the coneless speaker will not behave electrically as a speaker. The complete physical description of a speaker involves a lot more than its DC resistance and its magnetic inductance.

To clarify this: I've observed two junior science students trying to build an electronic actuator from a speaker. (Not a bad idea) In other words, they wanted to move something at a few Hz with a speaker coil. They ended up blowing up the speaker at a power 10 times lower than the maximum for the speaker.
All because the slow sine waves was more like DC and the movement of the speaker cone was impeared (8 ohms AC became 3 ohms DC).

Alguien escribió:

Still my question remains, where is the power dissipation?

I agree that it is out of the question to let the coil vibrate freely. But when you add a mass to it it will still be fundamentally different from a normal speaker cone (I haven't even considered all the resonances in a normal speaker. I've been amazed by the resonance curves of just a simple mass/spring system actuated by a similar coil/magnet.)
A mass does not absorb energy, it stores energy, and gives it back to the system as it makes the speaker work as a microphone. My point is: it is not the same system, both electrically and mechanically.

My only concern is this: The total impedance of the dummy load will be more "reactive" than a standard speaker: If there is insufficient resistance for energy dissipation in the load it will be dissipated in the only place where it can go: back through the transformer to the power tube. This might be a major problem at full volume.

I think that it will probably will work as the coil itself has DC resistance. (A lot of people problably use it too) Anyway, if it was my amp I would add an extra 1 ohm resistor (5 watts) in series with the speaker coil. This would "simulate" energy loss from the cone to the air.

Alguien escribió:

Me: Is the element called "motor" just the magnet of a speaker as it is or
do you modify it in order to get better results?

Weber: It is the voice coil, spider, small magnet, and voice coil wires. It is exactly like a speaker, unmodified.

Alguien escribió:

That schematic is "correct" in the sense that it is the example schem that Ted offered for people buying Mass motors that wanted to build their own attenuators. The series 100 ohm resistor helps isolate the loading effects of the pot/spkr from the Mass load on the amp, which helps maintain a constant load on the amp independent of the pot setting. The disadvantage is that, as drawn, it will knock off at least roughly 20db (for an 8 ohm spkr) even when the volume pot is maxed. (For an 8 ohm spkr: 8 ohm || 100 ohm pot = ~8ohm which with the 100 ohm series resistor gives you a 100/8 divider.)

For the 10-13W amps I build this was way too much attenuation. So I experimented and ended up subbing the 100ohm resistor with a 27 ohm power resistor I had lying around and that worked much better for me. This increased the "volume at max loss" from ~-20db to ~-10db and was a much more useable volume for my purposes. It also decreased the "loading isolation" but I felt the effect was negligible. I keep thinking there must be a better way of "isolating" the volume pot "tap" from the Mass load, maybe with some active electronics or something. Then again, it's a very simple circuit that works well and maybe it's just a matter of tweaking it for your individual needs.

Otherwise, for a low wattage amp a variable L-pad may make more sense. Grab a handful of power resistors and a rotary switch and come up with 2 or 3 volume levels that you find useful. For more "accurate reactive loading" you could use some inductors too, then again it may not be that significant depending on what you are trying to do.