Airbrake impedance question

[rogb]

Hi guys, I have decided to make an Airbrake more for fun and using HUGE resistors than need as I have PPIMV on my Fender and Marshall types and a Dumbleator on my D-mble.

I run at 16R actually 15R - all 1x 12 combos of G12-65s - sometimes I hook up a 1x12 extension of 16R so it gives 8R in parallel.

Now I have ordered all this stuff from Mouser USA I find posts not recommending the Airbrake except for 8R cabs.

Has anybody run 2 output jacks from the Brake in parallel like in an amp to give 1/2 R impedance?

If someone could run through what they do with the Brake I would really appreciate it.
Thanks Rog

[vibratoking]

The airbrake will work with 4 and 16 Ohm loads. The airbrake design only accomplishes an approximate match for any load that you attach. Don't think that the matching is spot-on when you use an 8 Ohm load...there are mismatches depending on the attenation setting. The same is true for 4 and 16 Ohm loads. I wouldn't get hung up on it. The component values in the airbrake can be tweaked for better matching if you want, especially if you remove the rheostat. There is no magic in the airbrake, in fact, quite the opposite. It is far from an elegant design, but it works within limited parameters and it is easy to implement. I don't the sound of the airbrake, but many do.

[rogb]

The airbrake will work with 4 and 16 Ohm loads. The airbrake design only accomplishes an approximate match for any load that you attach. Don't think that the matching is spot-on when you use an 8 Ohm load...there are mismatches depending on the attenation setting. The same is true for 4 and 16 Ohm loads. I wouldn't get hung up on it. The component values in the airbrake can be tweaked for better matching if you want, especially if you remove the rheostat. There is no magic in the airbrake, in fact, quite the opposite. It is far from an elegant design, but it works within limited parameters and it is easy to implement. I don't the sound of the airbrake, but many do.

Thanks for that VK. As usual I wasn't paying attention and ordered the 8R 25w for the rheostatless version. It does say quite clearly 8 Ohm I guess I should have looked for a 16 or 15R - oh well too late now....
Could I adjust the sliders to get nearer to ~15R as the schematic in the airbrake section does to 8R.

[Doug H]

Don't agonize over impedance matching for this. The airbrake is a resistive load, speakers are inductive. What this means is that a speaker's impedance varies a lot depending on the frequency. Look at some speaker curves and you will see this. The actual impedance of an airbrake is sort of an average of what an amp sees with a typical inductive load, which is why it does not match the typical speaker rating, which is much lower.

Ken knew what he was doing. He looked at speaker curves and came up with a reasonable "resistive" average that worked with 4, 8, or 16 ohm loads. I built an airbrake a few years ago and have never had an issue with it or the amps I've used with it. It works great and sounds great too. It has a slight treble rolloff which can easily be compensated for with the treble or presence control on the amp.

[rogb]

Don't agonize over impedance matching for this. The airbrake is a resistive load, speakers are inductive. What this means is that a speaker's impedance varies a lot depending on the frequency. Look at some speaker curves and you will see this. The actual impedance of an airbrake is sort of an average of what an amp sees with a typical inductive load, which is why it does not match the typical speaker rating, which is much lower.

Ken knew what he was doing. He looked at speaker curves and came up with a reasonable "resistive" average that worked with 4, 8, or 16 ohm loads. I built an airbrake a few years ago and have never had an issue with it or the amps I've used with it. It works great and sounds great too. It has a slight treble rolloff which can easily be compensated for with the treble or presence control on the amp.

Thanks for that, a good explanation. I looked at some impedance curves and can see what you mean. THe impedance is particularly high at high frequency, nearly 100 Ohms yet a nominal impedance can be 4 Ohms.

I will tweak it as close as I can to my 16 Ohm cabs, using my parallel resistor calculator and this.

[rogb]

Thanks for the advice on this - I have been using it with no problems to amp OT. The resistors don't get too warm though.
Also 1,2,3 seem to reduce the dB OK but 4 and 5 seem to have no effect. I have checked the wiring and it seems fine.
Any members with similar results?

[mididoc]

Don't agonize over impedance matching for this. The airbrake is a resistive load, speakers are inductive. What this means is that a speaker's impedance varies a lot depending on the frequency. Look at some speaker curves and you will see this. The actual impedance of an airbrake is sort of an average of what an amp sees with a typical inductive load, which is why it does not match the typical speaker rating, which is much lower.

Ken knew what he was doing. He looked at speaker curves and came up with a reasonable "resistive" average that worked with 4, 8, or 16 ohm loads. I built an airbrake a few years ago and have never had an issue with it or the amps I've used with it. It works great and sounds great too. It has a slight treble rolloff which can easily be compensated for with the treble or presence control on the amp.

Thanks for that, a good explanation. I looked at some impedance curves and can see what you mean. THe impedance is particularly high at high frequency, nearly 100 Ohms yet a nominal impedance can be 4 Ohms.

I will tweak it as close as I can to my 16 Ohm cabs, using my parallel resistor calculator and this.

Hi rogb,

your calculator is great.
shunt is the resistor going to masse? right.
my english is not so good.

did you succed setting up this shunt resistor,
matching the series resistor,
to keep a constant Z like 8 Ohm for example?

does the shunt not get too hot?

in the airbrake stock calculator there is a field on the very right side saying "power", always beeing 0
what is this for?

thanks for some information, please.

cheers