Built an Air Brake over the weekend…

[Raoul Duke]

My PS-2 was altering the tone of my recent Liverpool build too much for my taste and my Mini-Mass was muffling it; even using the built in treble boost settings - so I decided to try the KF solution as documented in the TW Files section here.

I have to admit after playing it on and off for 3 days now whilst comparing it to the aforementioned two - it is the most transparent to the amps true voice. I don’t know how KF figured it out - but it definitely works extremely well with both my TW clones.

Best part is I did it almost completely with “much cheaper than retail” e-bay parts. The only things I got from Mouser were the Lorlin switch and two additional adjustable resistor lugs. Total cost was roughly $72 all in. I was lucky enough to find two Ohmite rheostats of the correct value - one was 50w (for $5) and the other was a 75w (for $9.95); so I guess I’ll use the 75w to build a second one. The large resistors were all “ANOS” in their original packaging with the mounting brackets and of course the rheostats are used - but both checked out fine and cleaned up well. I even managed to find a cheap box that was just the right size.

The moral of the story is: the Air Brake is a great design and if you’re not in a terrible hurry and don’t mind picking through e-bay, you can still put one together with all the features reasonably. Here’s the pics:

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[solderhead]

I don’t know how KF figured it out - but it definitely works extremely well with both my TW clones.

It is indeed a good sounding and simple design, though the design is nothing remarkable. He figured it out by reading an old radio design textbook's chapter on attenuation.

[Raoul Duke]

Interesting. My inexperienced electronics “eye” thinks it seems pretty simple… but that doesn’t mean much, lol.

I’ve tried about a good half-dozen resistive attenuators out there (both well-known and lesser-known) as well as the PS and this simple design seems to be the clearest at high attenuation. I do think that with some experimentation and fiddling around the PS might sound better than my first try with it; but I’m not sure it would sound as true to the amp. Tried the Air Brake with my AC30 and 2204 today and had the same (positive) experience.

I guess none of the other manufacturers read that same book? Or they interpreted it differently?

[solderhead]

I don't know much about what other attenuator designers are doing, as I don't use their products. I just use a common radio attenuator design and I avoid going into deep levels of attenuation.

With that said, I don't see that there is a lot of money to be made by manufacturing designs that were common in the early days of radio and are now in the public domain, with the hope to profit by selling them -- there's no "economic moat" for a manufacturer and very little room for profit in what is a crowded market.

It doesn't surprise me that what's commonly offered in the market today are "improved" designs that add various features to try to justify themselves as higher priced / value-added enhancements to the original designs. The addition of EQ settings that try to adjust for Fletcher-Munson effects,the addition of EQ for recording outputs, the addition of cabinet sims and the like would be examples of why a simple attenuator design has been replaced by something that is more modern and more expensive. People design and sell what the market is willing to bear, with the result that there are many complex and expensive options available now.

[Raoul Duke]

I absolutely agree with you. One of the reasons I was motivated to start learning about amplification and electronics is because of the hyperbolic marketing surrounding everything out there - attenuators being a great example.

I was lucky to have a few friends who let me borrow and test theirs; none of which performed ideally in my opinion. Watching attenuator reviews on YouTube led me to bite the bullet and pay for a PS2 - which is a great piece of gear - but way more than what I needed. It worked better than any of the attenuators I tried, but still not exactly what I was after.

Had I been smart or lucky enough to try an Air Brake design first (knowing what I know now); I’m pretty confident that would have been my solution.

Live and learn I guess…