Post by tubestack on Jun 6, 2010 9:44:24 GMT -7
I know this question has been discussed in other threads, however I haven't come across any conclusive responses, and have not found any definitive answers from the good dudes at Dr Z Amps. Please excuse me if answers have been provided and I've missed them.
Does the performance of the Air Brake vary if you're using it with cabs of different ohms, and if so, in what ways?
According to Randall Aiken at aikenamps.com, the dB reduction at each position on the Air Brake changes, depending on the ohms of the cab being used. Aiken states that the Air Brake operates as advertised only when used with an 8 ohm cab. When used with a 16 ohm cab, the dB reduction becomes half of that in each position (ie. -3 dB becomes -1.5), and when used with a 4 ohm cab, the dB reduction is doubled (ie. -3 dB becomes -6dB).
This can be found in the Q&A section of his site:
Aiken also provides a chart, over at TGP, showing what he says are the corresponding dB/ohms relations, when using the Air Brake with 4, 8, and 16 ohm cabs:
The reason I'm asking this is, I own a switchable 4/16 ohm 4x12 that I've been using with the Air Brake, as well as an 8 ohm Z Best cab. I would like to know if the Air Brake is behaving differently when used with each cab and ohm setting, and if so, why.
Also, I've found large tonal differences between running the amp & cab at 4 ohms and running them at 16 ohms, when using the Air Brake. (I prefer it at 4 ohms, although the Air Brake seems to become less effective...), and would like to understand what's going on.
And finally, my experiences have actually been contrary to Aiken's statements - I've found the dB reduction to apparently be less when using the 4x12 at 4 ohms, rather than doubled. So what, then, is he going on about?
Any help sorting all this out would be greatly appreciated - thank you!
Does the performance of the Air Brake vary if you're using it with cabs of different ohms, and if so, in what ways?
According to Randall Aiken at aikenamps.com, the dB reduction at each position on the Air Brake changes, depending on the ohms of the cab being used. Aiken states that the Air Brake operates as advertised only when used with an 8 ohm cab. When used with a 16 ohm cab, the dB reduction becomes half of that in each position (ie. -3 dB becomes -1.5), and when used with a 4 ohm cab, the dB reduction is doubled (ie. -3 dB becomes -6dB).
This can be found in the Q&A section of his site:
Q: I have a Dr. Z AirBrake attenuator and am ordering an Ultimate Attenuator. Are these safe to use with my Sabre?
A: The Airbrake presents a widely varying load to the amp, depending on the impedance of the cabinet connected to it and the attenuation setting (contrary to their marketing claims, there is no such thing as a "one-size-fits-all" attenuator that properly matches all impedances - this is in reality an 8 ohm unit being marketed as "universal", because it presents a "close enough" load in most cases, although it doesn't really match that well even at 8 ohms for higher attenuation levels - up there it is closer to a 16 ohm load no matter what impedance cabinet you have connected).
For example, at -3dB attenuation, the impedance is 11 ohms for a 16 ohm cab, 8 ohms for an 8 ohm cab, and around 6 ohms for a 4 ohm cab. At -12dB, the impedance is 15 ohms for a 16 ohm cab, 14 ohms for an 8 ohm cab, and 13 ohms for a 4 ohm cab. The more you attenuate, the more this type of attenuator approaches a 25 ohm load.
Here is a link to the impedance load you can expect to see as a function of the speaker load and attenuation setting. Also note that the attenuation steps are only accurate for an 8 ohm speaker load. If the load is 16 ohms, the attenuation steps are 1/2, i.e. at -12dB you are really only -6dB down, and at 4 ohms they are twice, i.e. at -12dB you are really -24dB down... [snip]
... Of the two, the AirBrake is the safer unit to use in my opinion (and it sounds good, too), but I think both of them should be better designed to correctly match impedances.
A: The Airbrake presents a widely varying load to the amp, depending on the impedance of the cabinet connected to it and the attenuation setting (contrary to their marketing claims, there is no such thing as a "one-size-fits-all" attenuator that properly matches all impedances - this is in reality an 8 ohm unit being marketed as "universal", because it presents a "close enough" load in most cases, although it doesn't really match that well even at 8 ohms for higher attenuation levels - up there it is closer to a 16 ohm load no matter what impedance cabinet you have connected).
For example, at -3dB attenuation, the impedance is 11 ohms for a 16 ohm cab, 8 ohms for an 8 ohm cab, and around 6 ohms for a 4 ohm cab. At -12dB, the impedance is 15 ohms for a 16 ohm cab, 14 ohms for an 8 ohm cab, and 13 ohms for a 4 ohm cab. The more you attenuate, the more this type of attenuator approaches a 25 ohm load.
Here is a link to the impedance load you can expect to see as a function of the speaker load and attenuation setting. Also note that the attenuation steps are only accurate for an 8 ohm speaker load. If the load is 16 ohms, the attenuation steps are 1/2, i.e. at -12dB you are really only -6dB down, and at 4 ohms they are twice, i.e. at -12dB you are really -24dB down... [snip]
... Of the two, the AirBrake is the safer unit to use in my opinion (and it sounds good, too), but I think both of them should be better designed to correctly match impedances.
Aiken also provides a chart, over at TGP, showing what he says are the corresponding dB/ohms relations, when using the Air Brake with 4, 8, and 16 ohm cabs:
The reason I'm asking this is, I own a switchable 4/16 ohm 4x12 that I've been using with the Air Brake, as well as an 8 ohm Z Best cab. I would like to know if the Air Brake is behaving differently when used with each cab and ohm setting, and if so, why.
Also, I've found large tonal differences between running the amp & cab at 4 ohms and running them at 16 ohms, when using the Air Brake. (I prefer it at 4 ohms, although the Air Brake seems to become less effective...), and would like to understand what's going on.
And finally, my experiences have actually been contrary to Aiken's statements - I've found the dB reduction to apparently be less when using the 4x12 at 4 ohms, rather than doubled. So what, then, is he going on about?
Any help sorting all this out would be greatly appreciated - thank you!