attenuator airbrake
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Re: attenuator airbrake
Hmm, side question: could this be used to make speaker mismatch safer? For example, my cab is currently 8 ohms, but I'd prefer to run the amp at 16 ohms.
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Re: attenuator airbrake
FWIW, I updated the impedance plot in my post on the previous page to show the "0" position where the attenuator is off. The impedances shown are what the amp "sees". The mismatch is pretty significant, but it is a "one-size-fits-all" solution, rather than a specific impedance. Changing the 25R shunt resistor will not correct the mismatch much. The attenuator is better suited for use at 8 or 16R.JMPGuitars wrote: ↑Thu 04/30/20 8:38 pmHmm, side question: could this be used to make speaker mismatch safer? For example, my cab is currently 8 ohms, but I'd prefer to run the amp at 16 ohms.
You could use it to do a mismatch. Here is a quote from an earlier version of the Dr. Z Airbrake manual:
"Helpful Hints:
When using the Airbrake with older vintage amps, mismatching the load will be a safer way to operate. This will eliminate any overheating of the output transformer. For example, a Vox AC30 or vintage 100 watt Marshall should be run at 8 ohms through the Z Airbrake with a 16 ohm speaker load.
A useful technique for live applications is to use the [Z Airbrake] in parallel with another speaker cab. If your amp is equipped with dual speaker out jacks, connect the [Z Airbrake] as you would normally to one speaker cab. Connect the other cab directly to your amp. This will soften the attenuation effect and allow more control in live settings."
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Re: attenuator airbrake
A lot of people generally say it's okay to mismatch as long as the cabinet is higher than the amp (within 2:1). Do you agree with that? That thought fits in line with the helpful hint. What about going the other way though? I would think having an 8 ohm load with amp set at 16 would at the very least reduce the amount of attenuation? Possibly cause other problems given the variation in impedance depending on the airbrake's settings?colossal wrote: ↑Fri 05/01/20 7:13 am"Helpful Hints:
When using the Airbrake with older vintage amps, mismatching the load will be a safer way to operate. This will eliminate any overheating of the output transformer. For example, a Vox AC30 or vintage 100 watt Marshall should be run at 8 ohms through the Z Airbrake with a 16 ohm speaker load."
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Re: attenuator airbrake
An 8ohm speaker plugged into the 16ohm tap halves the reflected impedance back to the power tubes. So with an 18W amp running an 8k primary, with an 8ohm speaker on the 16ohm secondary tap, the power tubes now "see" a 4k primary. This rotates the loadline in favor of more power; not necessarily a good thing, depending on where the amp is biased under "normal" as-designed conditions and load. But speakers are inductive/reactive loads so their impedance is not constant and rises significantly at the resonant peak (usually in the 75-100Hz region). This plot is very similar to a V30:JMPGuitars wrote: ↑Fri 05/01/20 7:55 amA lot of people generally say it's okay to mismatch as long as the cabinet is higher than the amp (within 2:1). Do you agree with that? That thought fits in line with the helpful hint. What about going the other way though? I would think having an 8 ohm load with amp set at 16 would at the very least reduce the amount of attenuation? Possibly cause other problems given the variation in impedance depending on the airbrake's settings?
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Re: attenuator airbrake
Here's what mine looks like. I was going to engrave it with a rotary tool, but that looked terrible. So I used a paint marker instead.
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Re: attenuator airbrake
Very nice! What enclosure did you use? Is that a Hammond? I like the black powder coat.
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Re: attenuator airbrake
Thanks! Yes, it's a Hammond. Any enclosure they offer, you can usually find a powder coated version.
Here's the exact parts list I setup on Mouser: That includes everything I used except all the 18AWG wire.
Thanks,
Josh
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Re: attenuator airbrake
Here's the demo of mine: https://youtu.be/3oSZVg2STXw
Most of the demo is okay, but when I turned it down too low, my camera automatically increased the sound level. Oops.
Most of the demo is okay, but when I turned it down too low, my camera automatically increased the sound level. Oops.
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Re: attenuator airbrake
Good work! Despite the camera increasing the volume, you can hear the strings once you are down into bedroom level, so you know it is definitely is doing its thing. How did you find the transparency/tone retention at higher levels of attenuation?
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Re: attenuator airbrake
That's a good point.
I haven't played enough with it to form a solid opinion on the tone retention. There's maybe a drop in trebles, but not bad at all. And that's with the EQ on the amp flat, so it seemed like it would be easy to compensate for. What I really want to do is compare it to my MiniMass and see which one I like better. My first, and biased opinion, is that I like this shiny new one better.
In the short time I have played with it, it sounds pretty transparent to me otherwise. The perception of the change in tone is hard to measure accurately. So I will focus on which unit I think sounds better, and update this eventually.
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Re: attenuator airbrake
Good work Josh. Thanks for the report back. Looking forward to seeing if you still like it after the honeymoon period.
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Re: attenuator airbrake
I shared the doc I made: files/JMPGuitars_Break_Wind_Attenuator.pdf
It's also in the Miscellaneous Stuff page: viewtopic.php?f=25&t=24414&p=235766#p235766
I simplified it. The other layouts I've seen weren't very logical.
It's also in the Miscellaneous Stuff page: viewtopic.php?f=25&t=24414&p=235766#p235766
I simplified it. The other layouts I've seen weren't very logical.
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Re: attenuator airbrake
OK, great! I want to read up on this thread! I want an economical power soak and dummy load! And I'm on a budget, so a DIY attenuator, sounds just what I was hoping for!
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Re: attenuator airbrake
Great info.
\not sure if I missed in in the threads but is this for a 16 ohm load and how much attenuation will it give.
Jim
\not sure if I missed in in the threads but is this for a 16 ohm load and how much attenuation will it give.
Jim
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Re: attenuator airbrake
I posted a demo above. When the attenuation is increased, you can hear the guitar strings in the video. That's pretty significant.
16 ohm is supposedly ideal, but it also works with 8 and 4 ohm loads.
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Re: attenuator airbrake
As Josh said, the attenuator will "work" for 4 and 8 ohm loads, but is best suited for 16. See the first page of this thread for a plot of impedance vs. attenuation.
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Re: attenuator airbrake
Can I use a different value Rheostat ?
The 150 ohm 50 watt is hard to come by at the moment in the UK as they all seem to be on back order until the end of the year.
Jim
The 150 ohm 50 watt is hard to come by at the moment in the UK as they all seem to be on back order until the end of the year.
Jim
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Re: attenuator airbrake
You can. Lower values will limit attenuation. Higher values will increase it at the expense of increasing impedance.
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Re: attenuator airbrake
Limiting attenuation isn't a bad thing, since there will be less tone sucking that way. BUT there's limited options in that price range. I looked at alternatives, and they get pretty expensive. The RJS125E for example is 125 ohms, and $80. Some other variations were hundreds of dollars.
Personally I got a couple of the RJS150Es on eBay for $40 a piece. There is a seller in the UK selling the 150 also, but not as cheap. The 125 is available on there too, but I dunno about in the UK.
Thanks,
Josh
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Re: attenuator airbrake
Thanks Josh
Just found the one on ebay for $77.00.
A lot of money in these difficult times .
I will have to do some thinking .
The pure resistor attenuator is a cheaper option, But.....
Jim
Just found the one on ebay for $77.00.
A lot of money in these difficult times .
I will have to do some thinking .
The pure resistor attenuator is a cheaper option, But.....
Jim
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