Dr. Z - Air Brake

[Greg]

.... Second, the folks who generated that first layout remarked that the tone changes at "bedroom" level were unacceptable.

madditch, do you find this to be so with your Trainwreck approved Airbrake?

This is definitely the case. The bedroom settings sound bad.
For reasonable levels of attenuation, the AirBrake sounds great to me... and I know a lot of people prefer it over THD and some of the other major brands.

Thanks to those who posted pics and schematics.

[timersk]

Hi,
I have a Z Air Brake that I like but I would like to have the line output like the THD Hotplate and the Altair has. I would like to find a safe mod that would allow me to do this with the Z Air Brake.

By the way, the Marshall power brake sucks. This is why I got a Z air brake. It sounds much better without changing the tone of my amp, for sure.... I like the THD but there is some colouration I don't like. I had some ground loop problems using the THD with the line out and was not able to correct them, even with their tech support.

Tim

[MrSparkle666]

I compiled this for somebody on another forum, but I thought I would post it here too in case anyone is interested:

http://www.speedyshare.com/130845309.html

It's a bunch of detailed gutshots I took of my (pre Dr. Z) Trainwreck Airbrake thrown into a zip folder with some of the documents from this thread. The link won't last forever, so grab it while you can.

[SPeter]

I compiled this for somebody on another forum, but I thought I would post it here too in case anyone is interested:

http://www.speedyshare.com/130845309.html

It's a bunch of detailed gutshots I took of my (pre Dr. Z) Trainwreck Airbrake thrown into a zip folder with some of the documents from this thread. The link won't last forever, so grab it while you can.

Thanks!

[DougH]

You can get the schematics/layouts/etc for this at the Amp Garage site: http://ampgarage.com/forum/index.php (You may have to register to get access to the files.)

It is a resistive Lpad type of attenuator that works for up to 100W amps at 4/8/16 ohm loads. The loads are a compromise, obviously, but I've used it with my 100 watter at 16 ohms and my 10 watter at 8 ohm and it sounds very good on both. There's a very subtle high freq rolloff but otherwise it is very close to the bypassed amp sound and it stays pretty consistent at all attenuation levels (except at the very lowest).

One of the best parts about it for me was it only cost me $35 to build it ( ). I already had the jacks and knobs and got the switch from Mouser. I was able to get the remaining parts surplus, which saved me a lot of money.

It's a very simple build and a very useful tool. I highly recommend this to anyone interested in this sort of thing. There's a lot of good info at Amp Garage about it including a writeup about it by Ken Fischer which is interesting.

[SPeter]

Thanks!

[culturejam]

Excellent.

[DougH]

Looks like modman merged the threads, thanks! I didn't see the original thread or I would have just used it.

Here's a few comments re. earlier comments in the original thread-

The pdf schem someone posted (attenuator1_a1) is wrong, according to what they reversed on the amp garage. The input is connected to one of the switch poles. The throws for that pole are connected to the hot side of the 25 ohm resistor. But beware, even the schematic posted at amp garage is wrong for a similar reason. It's buried in one of the threads. I used the layout instead, it was created first, from the actual unit.

Yes, since this works for 4/8/16 ohm impedances, the load it presents to the amp will be a compromise to some extent. However, resistive and reactive impedance are two separate things. Reactive loads in speakers will have many multiples of its "rated" impedance at certain frequencies. Look at a speaker impedance curve for example. The average impedance of a reactive load over the useful frequency range is higher than the "rated" load. So you can't make a direct comparison between the two but you can say that is probably safer to run an amp with a higher resistive load than it is to run with a higher reactive load.

But in the end, this one sounds pretty good. There is a subtle high freq rolloff (which I also hear in my MASS-based attenuator) but otherwise it is very transparent. Even the "bedroom level" sounds good to me, unless you turn the rheostat down very low. There will be sound degradation at that point, and Fletcher-Munson comes into play anyway. But I use the higher rheostat settings of the "bedroom level" for my 100 watter and it really sounds good. I have used it both with the 16 ohm 4x12 with the 100 watter and with the 8 ohm 1x12 with my 10 watter just dialed down a click or two and it sounds pretty "transparent" for both of them.

I've been reading up on attenuators a lot lately, and I believe part of the reason that some people have problems with them has more to do with the speakers than the attenuator. If you are using low wattage speakers a la greenbacks or blues, part of the sound you get when the amp is dimed is from the speaker changing in some way dynamically and coloring the sound that it doesn't do when it's not pushed. Using an attenuator in this instance will not replicate what the speaker does, so it will sound "different" even though the amp is turned up. Higher-wattage speakers that produce a consistent sound no matter what volume the amp is will work better, since the way they color the sound is unrelated to the volume of the amp. My 4x12 has 50W speakers and it works pretty well with this attenuator.

Attenuators are not perfect, but they can be a real useful tool.

[culturejam]

I've been reading up on attenuators a lot lately, and I believe part of the reason that some people have problems with them has more to do with the speakers than the attenuator. If you are using low wattage speakers a la greenbacks or blues, part of the sound you get when the amp is dimed is from the speaker changing in some way dynamically and coloring the sound that it doesn't do when it's not pushed. Using an attenuator in this instance will not replicate what the speaker does, so it will sound "different" even though the amp is turned up. Higher-wattage speakers that produce a consistent sound no matter what volume the amp is will work better, since the way they color the sound is unrelated to the volume of the amp. My 4x12 has 50W speakers and it works pretty well with this attenuator.

Very interesting, Doug. I hadn't thought about the speaker's wattage as being a factor.

[DougH]

Yeah, it may not be the wattage so much as the way the speaker behaves "differently" when it is being pushed hard. If it's a higher wattage, there's less chance it will be "pushed hard".

[Greg]

I mainly use a 22 watter with 1 x Alnico Blue and 1 x G30H.
I've tried a Weber Mini Mass, a Dr Z .. and I now have an Ultimate Attenuator.

The Weber and Dr Z both sounded pretty muddy at lower volume settings, but the UA doesn't suffer from this at all.
It really sounds like the amp cranked, but at low volume.
Everyone who's tried it, and has tried other attenuators in the past, has been amazed at how good it sounds right down to very low volume.

[Ripdivot]

I have a 24 ohm 200 watt resistor in my shop. Would this be ok to use for the resistor that is wired between the hot and gnd (the non tapped resistor)? I'm thinking it is plenty close but thought I would ask anyway.

[Ripdivot]

Hey Greg_G, how about some gut shots of your ultimate attenuator

[Greg]

Hey Greg_G, how about some gut shots of your ultimate attenuator

Yeah, no problem..
I'll do 'em soon as I have a spare minute.

[DougH]

From what I've read, the UA is a re-amping device with a resistive load and a built-in SS power amp. The lower impedance drive of the power amp to your speakers may help the sound.

You can do this kind of thing pretty easily by putting a line-out on a load box and running the line to whatever amp you want for "re-amplification". Or you could use a chip amp for a built-in amplifier. I've done re-amping with a 25W MASS-based attenuator I built. i gave it a switch to make the MASS the load and then implemented a line-out. I ran the line into my SS amp and it sounded real good. I also did stuff with wet/dry and wet/wet setups with it that sounded real good as well.

[Attila]

Hi Guys,

Not wanting to rain on anyone's parade, and gees Dr. Z most certainly know his stuff ....however I think that attenuators are a lot complicated than stringing a few resistors, L-pads etc together. Tone loss/sacrifice as a result of the induced attenuation is inevitable. It therefore makes sense to use of smarter circuitry to achieve the best end result as your treble frequencies will suffer, Someone also correctly points out that one must consider the amp and the speaker in your calculations .....

Some folks I know here in South Africa have done a great job doing just this have a look at their website ( before you say it...I am not punting their product) http://www.tubetamer.co.za/

enjoy ...

Attila

[dai h.]

some people do find the simpler attenuators satisfactory, so I don't think that more complicated and sophisticated necessarily means better. I used to think that but after observing user exeriences I'm not convinced that the simple ones are poorer or unusable. This is not to say the Tube Tamer product is going to be poor or anything like that but I don't think technical sophistication is a guarantee of superior sound.

[DougH]

Yeah, on the contrary IME I've found that people tend to overcomplicate attenuation. I built an attenuator with a MASS motor that's supposed to be a more accurate reactive load blah, blah and I've built this purely resistive L-pad Airbrake and 1) the differences are fairly subtle, 2) I actually prefer the Airbrake as it sounds more consistent over the range of attenuation. As far as "tone degradation" etc is concerned, I'm not hearing it. There are slight variances in the sound, mainly a subtle high-freq loss, but that's easily restored by adding some presence (that's what that control is for). In any case, it only cost me $35 to find out and I'm very satisfied.

[Ed G.]

Seems like the high-frequency loss should be able to be corrected using a bypass capacitor arrangement.

I'm getting ready to build one. I find that my Weber MiniMASS thins out the bass of my amp too much and at moderate attenuation levels, it actually distorts the bass. I think the speaker motor is actually distorting at those bass frequencies.

[rosetta]

hi guys, I'm decided to build this but I can't found 588-RJS150E RJS150E Ohmite Power Rheostats 50watts 150OHMS on stock is on the bill of components, so I wonder If it can be replace for this one Rheostats 50watt 125ohm 750V Std Shaft which is the same but 25ohm less, so the questions are; do I need to use a sitch with a larger ohm value instead? it will work well? is there any issue with temperature?

can it be used with 4, 8, and 16 ohm with no further problems, isn't it? By the way I want to use it with a 45watt amp I'm building.