Redesigning the Airbrake

[murky69nz]

Hi all

Have built a standard Airbrake and love it – to my ears it sounds much better and open than my hotplate – and it’s cheaper and lighter to boot!

Thank you to everyone here for your help!

Now, building the AB has got me thinking and the creative juices flowing: what I want to do now is build an AB that meets my specific (quirky) needs a little better.

I’ve googled to despair, and trawled various forums but can’t find an answer: not wanting to hijack any of the other AB threads here, I thought I’d start a new one……

What I want to do is:

1) replace the standard adjustable series resistor, rheostat and 6 way switch with a single rheostat – such that I can get continuous attenuation with just one knob.

2) Up spec the single rheostat to a higher resistance so that I can get greater maximum attenuation

3) Add a second identical rheostat and install a relay that switches between the two so I can set one for less attenuation/solo boost.

Now this is where I’d appreciate some help:

A) What up specd rheostats should I be going for? Obviously they need a maximum resistance of greater than 175ohms – but I can’t find an calculator that tells me how high I should go (they all seem to work as a function of cabinet resistance and desired attenuation to give you required fixed resistor values – I need one that kind of works the other way – input series resistor value, this is the attenuation you’ll get – or alternatively a table of some sort)

B)Associated with specing the rheostat – what value shunt resistor do I need? Again can’t seem to find a calculator that tells me the required shunt value as a function of cab resistance, required attenuation and max rheostat value.

C) Is there an all in one L Pad out there that I could use as an alternative to the rheostat/shunt? I’d only be using the attenuator with the following amps: DC30 (30w?), Ceriatone Overtone (50w), Vox AC30 and Trainwreck Express – no 100w Plexis here…..yet.....

Appreciate any advice…..

[ampdoc1]

I think that's going to be very hard. Those resistors are 100W for a reason. If you try to replace one with a variable resistor, it's going to have to be a monster.

adoc

[Cliff Schecht]

I used a 1.25A, 25 Ohm potentiometer in my AirBrake and it stays quite cool. Just realize that as you increase the amount of attenuation you'll also have to increase the power handling capability of that power potentiometer. Just doubling 25 Ohms to 50 Ohms also doubles the power handling requirements to about 80W at max attenuation. I don't know of anybody that makes an 80W power potentiometer and even if you do find one, you're going to be paying $50 minimum for that one part.

The better thing to do IMO is to find two values that you like and just switch between those. Another option is to put two or more of the power pots in series although with this, you're going to pay out the ass not only in cost but in real estate, those suckers are big!

[Structo]

Isn't one of the problems with doing that is that you are not maintaining a matching impedance that the amp sees?
I realize that changes dynamically with frequencies but wouldn't be harmful to the OT if that is not observed somewhat?

[murky69nz]

Thanks for the responses guys.

Structo: there’s been quite a bit of discussion on boards about the impedence mismatch. The standard airbrake doesn’t present a consistent ohmage – it varies at different levels of attenuation (there’s a graph around that shows this). Apparently anything up to a 2:1 mismatch is acceptable, and I understand the standard airbrake’s range is within this.

The recommended AB Rheostat is 50w, 150ohms, 700v with a maximum of .575amps tolerated. This current rating seems low, but am I right in that because it’s being used as a voltage divider, at maximum attenuation the lions share of power is being burnt off in the parallel shunt, rather than the Rheostat – hence why you can get away with 50w, .575A?

So – how about:

1) Ditching the series tap resister and upping the rheostat to one of the following options
(a) 75w, 200ohms, .612A
(b) 75w, 300ohms, .5A
(c) 75w, 400ohms, .433A
(d) 75w, 500ohms, .388A
(e) 100w, 200ohms, .707A
(f) 100w, 300ohms, .575A (Ohmite part RKS300E)
(g) 100w, 400ohms, .5A
(h) 100w, 500ohms, .447A
(i) 100w, 200ohms, .707A
(j) 150w, 250ohms, .775A
(k) 150w, 350ohms, .655A
(l) 150w, 500ohms, .548A (Ohmite part RLS500E)

2) Upping the parallel shunt to a higher wattage (max available seems to be 225w in 25ohms: Ohmite part L225J25RE)
Problem is, I still can’t find a calculator that lets me work out attenuation with the above

Alternatively will a 100w, 16ohm L pad do the same job? http://www.parts-express.com/pe/showdet ... er=260-261

Remember, I'm only using 50w amps max.....

[MasterDisaster]

R1=pararell resistor or shunt
R2=series resistor
R3=rheostat
Zo=speaker load
Zi=input impedance

The Airbrake attenuator is basically a Pi Attenuator with the speaker replacing the last shunt. R1 sets the maximum impedance of the circuit and R2-R3 sets the maximum attenuation.
With a 25R R1 the spread is quite severe and can be as much as 3,5-25 Ohm at maximum attenuation with Zo at 4 Ohm.

To add attenuation you need to increase R2-R3 and there the rheostat comes into play. Using a 500R R3 will get you a maximum attenuation of about -30db which equals about 0,03W with a 50W input.
At maximum attenuation R1 will take most of the power so take that into consideration.
The impedance spread will lessen with higher speaker loads and will also relieve R2 at lower attenuation.
Actual physical measurements will vary especially as amps do not produce full power when mismatched.

Anyway, it's a lot of math and I might have made a mistake among the algebra so if you feel that I have missed out on something then please fill in.

Some added info:

A 50W amp will output about 3,5A into a 4 Ohm load.
Current through R2 will be 0,89A but Zi will be around 13 Ohm.
With a 16 Ohm speaker load the current will drop to 0,68A
If you have your amp set at 4 Ohm we are looking at a 3-1 mismatch and the amp will most likely be far from producing it's rated power.
Increasing Zo and decreasing R1 will make life easier for the rheostat and will decrease the mismatch.
If somebody has made any actual physical measurements then please join in.

The L-Pad you are referring to is usually used in a HIFI speaker crossover and not designed to handle continuous power from a blazing guitar amp.
Maximum attenuation is normally -16db so you will still be far away from any bedroom level and I doubt that it will survive even a 25W amp for long without a heat sink.

Mike