Air Brake Attenuator

Throughout my musical journey I’ve always searched for different ways to get good sound at apartment-friendly levels. In recent years I’ve started to implement VVR (Variable Voltage Regulator) in my amps that can be used to reduce the voltage of output tubes and reduce the volume that way, but I wanted to build a standalone attenuator to use either in conjunction with VVR or on its own. There are many attenuators on the market. Some of them use big power resistors to convert portion of the power output by the amplifier to heat, while letting the rest of the power through to the speaker. Some use combination of resistors with reactive load, like Weber’s Mass Motor which is basically a guitar speaker without the cone, so it cannot produce sound.

Dr Z Air Brake is one of the attenuators people really like and it’s not too complicated to build, having only resistive load. I started with the schematic from Air Brake Lite and scaled it down to work with small amps that I typically build for myself. Dr Z uses two 100W wirewound resistors that are tapped in few places and is good for amps up to 45W of power. The circuit itself is very simple. In each position, the rotary switch selects different combinations of resistors, one in series and one in parallel with the speaker, effectively forming an different L-pad in every position. In the last position there is no resistance in the circuit and it does not affect the signal at all.

Mods

The first mod came from The Amp Garage Forum and it involves replacing the 5 position rotary switch on the original with a 6 position switch that has one more attenuation level. In addition to tapping the two resistors in one more place, they suggest adding an 8 ohm 20W resistor in the parallel resistance network, making the total impedance closer to 8ohm all attenuation positions. Dr Z uses a fixed 25ohm resistor in parallel with the speaker making the total resistance less ideal in some positions.

It’s not easy to find those big 100W resistors where I live and I have no need for such high power rating, so I used 7 smaller resistors instead rated 9-10W each. That should be good enough for all my amps that are 6W max, but should probable work well up to 10W.

I also added a simple line-out tap that I can use to record the signal direct from the amp and add impulses in the DAW. It’s easy and simple to do, so why not.

Below is the schematic that includes all the mods mentioned above.

Construction

To simplify construction I used one of the plastic enclosures I had in my bin and used point-to-point terminals to suspend all resistors in the air. That should provide enough air flow to keep them cool and prevent them from touching the plastic box. Since we are dealing with very low impedance signal, noise is not a huge concern, so I didn’t even bother to shield the enclosure with the copper foil.

It turned out really nice. I sounds good on all attenuation levels and doesn’t kill too much tone even on the lowest setting (-11db).

Leave A Comment

  • About

    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.