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What I meant was that most which claim to be 'reactive' in fact are still mostly resistive even though they have a reactive element, and don't actually have the same sort of impedance curve as a real speaker. Oversimplification comes back to bite me .
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I've built / installed variable HT into several amps.
As a general rule with most forms of attenuation, they seem to work more naturally at lower levels of attenuation (of course there may well be Fletcher Munson effects working here as well).
My experiences with power scaling is that when you turn down the HT, you need to also reduce the drive to the power valves / power amp, otherwise you can experience a harsh octave doubled type distortion as the power valves are driven harder with respect to the HT as the power is turned down.
Essentially I think the "problem" is that the end user expects to have a single knob on the amp that will make the amp sound the same at all volumes. This is unrealistic. A good practical solution is having a range of controls that allow the use to adjust the sound at different power levels.
Actually a well-designed resistive attenuator can sound very good - the Dr.Z Airbrake is a good example. They work especially well with no-negative-feedback amps, in fact.
I actually think the bigger problem is that at high levels of attenuation, the parallel resistance across the speaker is quite a low value, and this will heavily damp the speaker in comparison to it being connected directly to the amp, which is what makes the sound 'flat'. It may need a three-way variation with a series resistance to the amp, a parallel load and then another resistance in series to the speaker to eliminate the damping effect, but without custom-made resistive elements that can't be done with a single rotary control. It could be done with switching instead of a continuous control though.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
It's moderately invasive, but it depends on how it's installed. In some cases you can use all existing holes to mount new controls, so no new holes are needed in the control panels. Other installations need 2 new holes so they are ore of a commitment. You may also need heatsinks and fans on larger amps. Internally, it's all reversible.
The RBX kit is an auxiliary raw bias supply for those amps which have too high an impedance in their own bias supply, i.e. they can't supply enough bias current. The Power Scaling tracking bias regulator draws more current than some amps' bias supplies can provide, usually because they tap off the HT primary via a high value resistor to drop the voltage.
That's what I do, and advise my customers to do. It's always best to use it to add some power amp colouration, not as a kind of rough volume control. @jpfamps makes a good point about user expectations, and it's a reason why it's not more prevalent IMO. One knob systems are limited and can sound poor at times, whereas two knob systems require a little finesse to set up. I'm lucky in that my customers are self selecting (and few!)
There are two issues to consider with attenuators.
1) The attenutor / speaker combination, should "look" like a speaker to the amplifier. With a properly designed reactive load this is relatively easy to achieve.
2) The amp / attenuator combination should "look" like an amplifier to the speaker. This, is much more problematic, and most (if any) attenuators don't achieve this.
What the speaker "sees" will vary very much with the amount of attenuation, eg ICBM says above the very low parallel resistance with the speaker causes the speaker to be damped more than if it were connected directly to the amp.
I wonder if a stereo L-Pad could be adapted to try this out...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I'm sure you could design a load that emulates a speakers impedance curve using resistors, inductors and capacitors. You just need to create a peak at the resonant frequency and then an increase in impedance at the high end.
I have some free time later today so I'll have a go and test it using LT SPICE
It's a shame they stopped making it, but apparently it's because the transformer was too expensive - which also most likely explains why no-one else has taken up the concept. If you added something which produced the resonant peak to that, it should be pretty accurate.
Whether that would make it sound better or not, I don't know - the remarkable thing is that a well-designed purely resistive one can sound really good as well, and in theory it shouldn't...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Could there be any benefit in using Power Scaling in combination with a reactive load for the sound/feel, or just to reduce wear on valves?