Reactive Load For Amplifiers Teaches Lessons About Inductors

The sound produced by any given electric guitar is shaped not just by the instrument itself but by the amplifiers chosen to make that sound audible. Plenty of musicians swear by the warm sound of amplifiers with vacuum tube circuits, but they do have some limitations. [Collin] wanted to build a reactive load for using tube amps without generating a huge quantity of sound, and it resulted in an interesting project that also taught him a lot about inductors.

The reactive load is essentially a dummy load for the amplifier that replaces a speaker with something that won’t produce sound. Passive loads typically use resistor banks but since this one is active, it needs a very large inductor to handle the amount of current being produced by the amplifier. [Colin] has also built a headphone output into this load which allows it to output a much smaller quantity of sound to a headset while retaining the sound and feel of the amplifier tubes, and it additionally includes a widely-used tone control circuit as well.

There’s a lot going on in the design of the circuitry for this amplifier load, including a lot of research into low-frequency inductors that can handle a significant amount of current. [Collin] eventually ended up winding his own, but the path he took to it was long and winding. There’s a lot of other circuit theory discussed as well especially with regards to the Baxandall EQ that he built into it as well. And, if you’d like to learn more about tube amplifiers in general, take a look at this piece which notes one of the best stereo amps ever produced.

26 thoughts on “Reactive Load For Amplifiers Teaches Lessons About Inductors

  1. And then there are gyrators: Would save a lot of pain in making monster inductors.
    Though a gyrator to handle that kind of power brings its own headaches too, since it would require a heatsink.

    1. People love distortion. And total RMS output wattage. Most of the audioheads who claim to want to eradicate distortion and listen to things at a reasonable volume are just lying. If your set can reproduce music at close to concert levels without hard clipping and fuzz, it’s a good set.

      1. I used to work in telecomms and a colleague told me that back back in the dawn of the era of HiFi one of their colleagues was waxing lyrical about distortion-free perfectly linear amps and whatnot, so eventually they offered to build him one (baseband audio being not much of a challenge compared to the HF analogue transmission gear they were working with).

        He was incredulous, but they built it and used their lab test gear to positively demonstrate that it had an absolutely flat linear response across the full range (and then some) – and it sounded awful, and was never spoken of again!

        1. Bob Carver, purveyor of audiophile gear popular in the 1970s and 1980s was maker of “magnetic” power amplifiers. Snooty audiophiles looked down their noses at Carver’s amplifiers. There are apocryphal stories of Carver’s standing challenge that he could make his equipment sound exactly like any arbitrary fancy audiophile device. Supposedly Caver would be presented with a piece of gear at a conference, go back to his hotel room and after adding some analog components to his own device, emerge in the morning having met the challenge. At least to the limit of golden ears’ ability to discern any difference.

    2. Yes, you are quite correct. Predominantly 2nd harmonic frequency. A tube amp can, and often are built to lower distortion standards by employing much negative feedback. Measurements show near SS figures yet listening tests show confusing results as the listener often prefers the “warmer” sound of the tube amplifier not so rigorously designed. Of course the type of speaker load presented makes nearly the same amount of difference. For even greater comparison look into voltage vs current drive as related to this discussion.

  2. When “audiophiles” are wiling to spend up to $300 on special AC wall sockets with exotic metal contacts in order to supposedly get a better sound in spite of the fact that the wall socket is on the other side of the power transformer and filters I tend to not pay much attention to whatever they think is a better sound.

    Google “audiophile wall sockets”, and then especially check out this link …

    1. My favourite was the equally expensive custom-machined volume knobs that were claimed to greatly improve the sound.

      Honestly if I were devoid of ethics I could make a fortune in the audiophile world.

      1. There’s nothing unethical in parting an audiophool from his money. You’re not running a monopoly, your customer has the freedom of choice and will usually be happy with his purchase. Not much different from selling luxury cars or expensive timepieces.

    2. I generally just play my vinyl with my body as the conduits between the mains outside my house. You would likely not believe how crisp the highs are when your body is convulsing at the same frequency. If you don’t do the same you don’t deserve to be called an audiophile. C’mere let me grab my oxygen-free 2 gauge jumper cables, I’ll show you…

      1. Why hasn’t anyone tried to error .. market (yeah yeah that’s it, ‘market’) DC powered amps to them?

        “No hum in this power!”

        “This is DC so there’s no skin effect to worry about”

        “Zero frequency power doesn’t generate harmonics”

        “Non reversing current, so no copper grain hysteresis to worry about”

        .. or whatever other buzzword driven nonsense sounds enticing to those with money burning holes in their pockets.

        Not to mention the possibility of recurring sales of hideously marked up custom battery packs

        1. Oh, they have explored and exploited that path a while ago. The Audio Note( brand ) “On-Gaku” system was a preamp/ power amp that was 5 rather large, heavy components at a MSRP of $350,000.00. There was an optional DC power supply that used batteries that charged when it was off. It added 2 more boxes and added $38,000 to the price. Tesla batteries don’t sound so overpriced now, huh?

          1. I feel any cheap car radio sound pretty decent, I wondered if is it because of beatiful power supply (Aka car battery). Maybe industry doesnt try because they predate all is super high end thousand dollar products.

    3. Well maybe it’s more like someone’s willing to sell some? Hi-fi accesories could be something to laugh at, but I can’t see how that makes amps or speakers not worthy of attention.

    1. Many but not all amp power attenuators are resistive in nature. They use arrays of resistors to soak up the power. There was one some years ago that actually looked like a toaster inside.

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