yet another reactive dummy load thread

[J M Fahey]

Never saw them, but from what I read here:
1) price and cost are absolutely unrelated.
They are not selling a bunch of parts in a bag but a product which "does something".
If well done and liked, thatīs all the end user needs.

2) in principle anybody can ask whatever he wants for his product.
If perceived as being "worth it", people will buy it.
What brings prices down is competition, once 2 or 3 offer the same or equivalent, prices will start going down, if they want to sell.

3) just a wild guess here, completely unsubstantiated by any statistics: if people expect the magic word "reactive" , a nice mid sized visible inductor will cover that, without actually lying, how much it actually does or not is arguable.
I suspect the actual magic is done in the digital domain ... and seems to be "good" magic.

FWIW about same concept as those "tube powered" multi effects pedalboards and simulators, with some orange (or blue ) glow behind/under some glass bottle, conveniently visible through a window

4) so they also offer/promise a real/passive reactive load, with a few pounds copper/iron?
Why not? It will be quite more expensive, less flexible and probably not sell much, if at all, but will improve brand perceived quality.

FWIW French car makers Renault and Citroen and Italian FIAT are perceived as good quality, average price "consumer/Family" cars, adequate but nothing exciting ... so eventually they made Formula 1 cars and had them race , with good results.

Absolute "waste of money", obviously not meant to be sold anywhere, at any price ... but hugely improved brand image, and thatīs an understatement.

[darkfenriz]

Would there be an interest in an 'AC Electronic Load' product that:

1. worked with amps up to, say, 100W
2. simulated different loads, including 4/8/16R resisitive, speaker-reactive loads of several 'flavours'
3. dumped the excess power in a convenient sink
4. re-amplified the audio signal in a, say, 0%-30% adjustable range of original signal, for connection to scope or meter or external speaker?

[Gregg]

There are all kinds of similar devices on the market. If you think you can come up with something new I'll have to disappoint you.
The only thing you can do is to achieve a better price by offering it directly thus saving from dealer's, taxes etc. but you'll need to do a very good marketing which as already mentioned is not so simple.

[Mick Bailey]

It does surprise me how inductors are used in commercial dummy loads - why they're there in the first place and why they're so small. Maybe just enough to match the resonant peak of a particular speaker? Who would notice whether they're there or not, if the load was otherwise well-designed? Seems to me on of the key elements that none of the reactive loads do is to create the back-EMF that a proper motor does. They seem to me to use inductors to shape the frequency response in a small-signal type of model, the sort of thing that looks good in PSpice, but ignoring the interaction between the OT and the speaker as a motor.

[Chuck H]

Well the Weber units use speaker motors. Though I've never examined one so I don't know if they're just static or if they move. I've wondered if it might be possible to un cone a speaker and use something like a silicone blob in it's stead. Another idea I had was using an electric motor on a spring load.

One problem with big, air core inductors in the principal load is that at high attenuation levels they have a large emf. My attenuator uses a static/active load and the first incarnation squealed like a piggy within six feet of the unit using single coil pickups. I solved for that by using two inductors of half value wired in series/out of phase. Stacked one on the other but with a copper plate in between that is the same diameter as the coils to minimize mutual inductance and preserve most of the original intended value. So now there are two out of phase emf's, that cancel each other in the radiant area, but the coils don't actually "see" each other much. It worked first try and I was very pleased with myself Though I can't say how much of the original intended inductance value was preserved because I didn't know how to test for it at the time.

[Mick Bailey]

Ah, yes - I'd forgotten about the Weber. I don't know how much movement it has compared to a regular speaker. Maybe someone here has taken one apart to have a look.

Another thing that stuck me was how an attenuator of any kind behaves with an open-back speaker. So at higher volume you get the interaction from air movement at the back of the speaker. With an attenuator you lose this. Maybe another reason along with equal-loudness curves why an attenuated cranked amp doesn't sound the same as the unattenuated amp - all controls being set the same in both cases.

I wonder how the perception changes in reverse? Say you profiled an attenuated amp with a Kemper, then played that profile at high volume through a regular cab.

[J M Fahey]

It does surprise me how inductors are used in commercial dummy loads - why they're there in the first place and why they're so small. Maybe just enough to match the resonant peak of a particular speaker?

The proper ones are large and heavy and definitely benefit from some kind of magnetic core, because they must resonate around 90Hz which is not a mean task.

Only slightly saving grace is that they can use relatively thin wire, being lossy is not a problem and in fact is actually needed, while an inductor designed to work in, say, a crossover, with an 8 ohm speaker should not have more than 1 ohm DCR to avoid excessive losses, in a reactive load it can have, say, 6 or 7 ohms and also coverbthe resistive part without need of an external resistor.
Of course, it will dissipate full power in that case

And yes, it will kick back same as a real speaker.

Only honest one I know for sure is Randall Aikenīs excellent one, and probably the large heavy Marshall one, all others are suspect until proven good.

As of the Weber unit, to each his own but I donīt see it as a good idea.
Although a real speaker motor, itīs lacking the most important part, the (cone) load which makes it meaningless.

Think of it as revving up full throttle your car engine ... while in neutral.
Donīt know how the (mechanically) unloaded voice coil does not smash against the back plate or shoot forward and kill somebody in front of it, he he.

Might believe it more if it had a tiny cone, say a voice coil diameter cardboard disk, and the whole shebang were inside an oil filled can, no kidding.
Oil would also help dissipate heat.

[Mick Bailey]

An oil filled reactive load? Mojo by the gallon. Different grades for different damping characteristics. You can hear the difference. Custom lube for your sound. Kinetic liquid. Tiny expensive bottles of additives that do nothing. I can see a strong market for all of that.

[Chuck H]

The Aiken load is the one I'm using in my active/resistive attenuator and it sounds real good, to me anyway. Even at low settings and without any "bright" switches or other tone shaping. But be aware that to use parts as spec'd you're going to find the smaller inductor is an air core and will howl at your single coil pickups unless you do as I described above.

[Gnobuddy]

Well the Weber units use speaker motors. Though I've never examined one so I don't know if they're just static or if they move. I've wondered if it might be possible to un cone a speaker and use something like a silicone blob in it's stead.

Keep in mind, all the low frequency behaviour of a speaker load (the big impedance peak around the fundamental resonance frequency) is caused by the back EMF generated by the voice coil moving in the gap. If you stop the coil from moving, it generates no back EMF, and no longer behaves anything at all like a loudspeaker! Instead, you just have a very big, very heavy, very expensive little inductor, usually less than a mH.

Like many guitar players, I've experimented a little with DIY attenuators for (valve) guitar amps. The best results I got were using an 8 ohm power resistor to load the guitar amp, dividing down the voltage across it, feeding that through a Danelectro Fish-n-Chips 7-band graphic EQ pedal, and running that into my multi-track recorder, or a solid-state flat frequency response amp and speaker. No reactive load anywhere in sight, but the EQ pedal lets me have a fair bit of control over the frequency response.

On the "dummy speaker as reactive load" idea, considering that most guitar speakers have a fundamental resonance around 100 Hz, and the big impedance peak only spans a few tens of hertz on either side of that, I am sceptical about the importance of this peak to good guitar sound. The effect of that bass resonance only covers a very small fraction of the guitars range. Just a few semitones on each side of the 3-rd fret "G" on the low E string. And these are the low frequencies that turn to mud in a band mix with drums and bass, the frequencies that a good mix soundman or mix engineer will high-pass out of the guitar signal.

I think that little voice coil inductance is a lot more likely to make an audible difference, as it boosts a wide range of treble frequencies when fed from a high-impedance valve amplifier. Considering the small size and relatively fine wire used in a speaker voice coil, I can't imagine why you'd need a huge hulking whopper of an inductor to do the same job in a reactive load. It takes only 3.5 amps (RMS) to push 100 watts into an 8 ohm load. A 24-gauge wire can handle that! And a more typical power level - 15 watts, say - requires less than 1.5 amps. 27 gauge wire can handle that! ( https://www.powerstream.com/Wire_Size.htm )

-Gnobuddy

[Malcolm Irving]

The resonant frequency of a loudspeaker quoted by the manufacturer is the 'free air resonance'. Once a speaker is loaded in a cabinet that resonant frequency changes. (It always increases.) A resonant freq. of 75Hz might increase to somewhere around 150Hz in a closed back cab. Would it not be better (and easier) to simulate this higher resonant frequency when building a reactive dummy load?

[J M Fahey]

Good idea, but boxes are never *that* small.
The holy Grail is a Marshall 4 x 12" , which depending on speakers used resonates between 100/120 Hz.
And open backed cabinets (think Twin Reverb), resonate at practically free air resonance frequency, since they are untuned open cabinets, think about 80/90 Hz and you wonīt be far.

EDIT: from the Fractal Audio guys, who thankfully measure things in an attempt to simulate them later:

I measured the LF resonant peak of my circa 1982 Marshall 4x12 260 Watt cabinet with G12-65 speakers. Some people were interested in the LF peak on another thread, so I thought I'd measure my cab and post it.

The resonant peak of my cab is 115Hz.

[Gnobuddy]

Once a speaker is loaded in a cabinet that resonant frequency changes. (It always increases.)

Very true. The resonant frequency goes as the square root of the stiffness of the "spring". So it would take four times as much stiffness (i.e. a very small box indeed) to double the resonance frequency.

For a small 2-watt valve guitar amp I'm building, I tried a couple of atypical speakers. One was a cheap 8" full-range speaker designed for background music / paging / public address. It has a tiny, cheap ceramic magnet, but still achieves a fairly high (91 dB/W@1m) efficiency, by having an extremely thin, light, paper cone. The light cone and stiff (corrugated paper) surround combine to create a fundamental free-air resonant frequency of 132 Hz. The small cone area and stiff suspension mean that frequency doesn't change much when you put the speaker into a reasonably sized cabinet.

The thin paper cone also breaks up in the midrange, even at low power, and gives the speaker a very "vocal" quality to its sound. Quite nice for clean-tone lead guitar work in a band context, but the lack of deep bass is very noticeable when playing solo at home. For my tastes, at least, 132 Hz is too high to be acceptable for good all-around guitar use.

I am still a little confused by people wanting to re-create a big, high Q, bass resonant peak in their guitar dummy load. Why would a musician want a few big fat boomy notes that sound very different from anything else on the guitar neck? As a guitarist myself, this is the sort of thing we normally avoid in a guitar, or guitar amp. In fact guitarists usually highly prize guitars that have an even response across all the frets, with no notes that are either "deader" or "more alive" than the other notes around them.

I have also read a lot of discussions about the benefits of a small amount of negative feedback around the output stage of a (pentode output) guitar amp, mainly because it lowers the amplifiers output impedance, and helps to control that unwanted woofy peak around 100 Hz. Once again, most of us seem to agree that big, woofy, and uncontrolled bass is not a good thing for making music.

So why do people want to put back that woofy peak via their dummy load?

-Gnobuddy

[Malcolm Irving]

....
I have also read a lot of discussions about the benefits of a small amount of negative feedback around the output stage of a (pentode output) guitar amp, mainly because it lowers the amplifiers output impedance, and helps to control that unwanted woofy peak around 100 Hz. Once again, most of us seem to agree that big, woofy, and uncontrolled bass is not a good thing for making music.
...

True. Another factor is the use of open-backed cabinets, which give low cut due to phase cancellation of the sound from front and back. For me, amps without negative feedback (AC30 style) go well with an open back combo (or cab.), the extra bass response near resonance being balanced out by the low cut of the cab. I don't know how 'well tuned' this effect is, but I suspect the typical open-back low cut and speaker resonance boost are 'in the same ballpark'.

[Gregg]

It takes only 3.5 amps (RMS) to push 100 watts into an 8 ohm load. A 24-gauge wire can handle that!

That's when you have a pure sine wave but when the amp is clipping it's a square wave so it would be wise to assume at least 50% more than that.
The wire size affect also the DCR which affects the Q factor but I'm not sure it's that important.