yet another reactive dummy load thread

[Zozobra]

There was a discussion on building reactive dummy loads on another forum I frequent and a common issue that folks run into it trying to source suitable air cored inductors that can handle a 100W amp and have wiggle room spare.
In that thread someone posted gut shots of the TwoNotes torpedo, which is a popular unit featuring built in digital speaker sims for direct/FoH use and there is a distinct lack of huge air cored inductors!



To my eye, all the dummy load parts live in the left hand, with the main load resistors being bolted to the large heatsink, which is fan cooled for good measure. I can only see the one, small, inductor on the PCB to the right of the heatsink, which is surprising as I would have expected to see one for the treble and resonance as per the Aiken design. The rest is the DSP stuff for the cab sim and whatnot and an SMPS PSU.

Anyone got any ideas what's going on here? I know it's not an especially hi-res picture to work from but it looks quite different to what I was expecting!

[darkfenriz]

Either it is not as reactive as it's advertised to or it uses a rather sophisticated switching circuitry to act as the load.
It is doable with a pair of switching mosfets converting and dumping the power into resistors, not a very easy task though.

P.S. I can also imagine a power opamp (like TDA7294) in gyrator simulated inductor application...

[Zozobra]

Interesting about the possible use of silicon here. I've often wondered if it could be done but I guess it's harder to protect against failure modes that could kill the amp being loaded.

[Enzo]

I would venture it is the design rather than the silicon. Look under the hood of your car, there are silicon rectifiers in your alternator, terrible conditions, yet they last forever usually. Industrial applications like railroad locomotives use silicon, and they run millions of miles in terrible conditions. SIlicon is reliable and robust if used in well designed circuits. Trying to squeeze 100 watts from a TDA7293 is not good design.

[Chuck H]

It looks to me like it's probably a Ho/Ultimate type attenuator with additional cabinet simulation for the mostly resistive attenuated signal. That is, the amp signal is partly resistively attenuated and then sent to a lower wattage reactive circuit for final attenuation. Then the now line level signal gets additional tonal touchup just like a preamp and is re amped with a SS power amp. So there IS reactive circuitry and it IS made to sound more like a guitar cabinet with active circuitry, but indeed it's mostly a resistor the amp "see's" and reacts to as a load. Just a guess looking into the chassis though.

[Gregg]

For the money requested for this device they could have done better job on the reactive load I guess. The parts are not so expensive and bviously space is not an issue either.
I can see an inductor on that PCB but for better approximation of a guitar speaker/cabinet there should be at least one more. In the Palmer PDI-03 the signal goes to a resistor (4 in parallel) and to an air core inductor so it's not that reactive either.

[bob p]

Parts count and parts price doesn't necessarily have anything to do with retail price. Trying to justify the price asked by looking at what's inside doesn't always make sense. To a manufacturer the perfect business model disconnects retail price from the manufacturing cost.

[Enzo]

MArketing is everything. There is a case in a restaurant. A nice upscale restaurant, comes up with a delicious new dessert, they put it on the menu for $3. No one orders it, or almost no one. Just as an experiment, they put the price up to $8, and it flies out of the kitchen. Perceptions sell things.

[darkfenriz]

Guys, please remember we're all just speculating based on an incomplete photo

[Gregg]

MArketing is everything.

You're absolutely right!
One can have a perfect product but fail miserably selling it if the marketing sucks .
You can have an average product but make good money if you're good at marketing it.
That means the popularity of a product may not have much to do with the quality.

[Zozobra]

FWIW, the unit is well received and the clips/demos of it I've heard have all been pretty impressive. They're bringing out a unit shortly that dispenses with the DSP stuff and is just the dummy load with a line out and an optional analogue speaker sim. I'd be interested to see if that has a conventional inductor arrangement.

[Gregg]

FWIW, the unit is well received and the clips/demos of it I've heard have all been pretty impressive.

I didn't mean to say anything negative about this one. There are products that are good and well marketed. I just gave an example of two real life situations.

They're bringing out a unit shortly that dispenses with the DSP stuff and is just the dummy load with a line out and an optional analogue speaker sim.

Did you mean the Captor series?
I guess the Reload has more iron in it

[g1]

I'm still not understanding where the expectations are coming from?
Why the idea there should be more air-core inductors?
Is there something in the marketing that implies this? The word 'reactive' leaves a lot of leeway for interpretation.

[Gregg]

The idea maybe is from Aiken's article on reactive loads and from other manufacturers' designs. Maybe that can be simplified but when people hear "reactive" they expect iron and copper.

[g1]

The idea maybe is from Aiken's article on reactive loads and from other manufacturers' designs. Maybe that can be simplified but when people hear "reactive" they expect iron and copper.

Thanks. To paraphrase the Bard, "methinks they doth expect too much?"
Kind of like guitar players critiquing errors in performances that most of the audience will never hear, I doubt these guys are designing for designers. Most guys who buy this stuff will never look inside.
Sorry if I'm going off on an irrelevant tangent, just thought I would point out the expectations aspect.