Attenuator - what is it?

[Explorer]

Hello,

is an attenuator basically just a volume-pot with higher ratings than ordinary pots, or is there some other stuff in there too?

I see the THD has versions for 4, 8 and 16 ohms, giving the correct load.. While the Dr.Z don´t.. hmm...

[Rob Strand]

An attenuator is anything which drops the leve of signal down.  It can be for low or high levels signal.

I suppose the things you are talking about should be called *speaker* attenuators.  Yes many are some form of high powered volume pot.  The Marshall one with the transformer is works slightly differently.  Quite a few variations on the theme here.

[Explorer]

An attenuator is anything which drops the leve of signal down.  It can be for low or high levels signal.

I suppose the things you are talking about should be called *speaker* attenuators.  Yes many are some form of high powered volume pot.  The Marshall one with the transformer is works slightly differently.  Quite a few variations on the theme here.

Thanks. I´ll go have a read at the link in the archives. Fun fun!

[Mark Hammer]

"Attenuation" is simply that: reduction.  In principle any voltage can be "attenuated" by any voltage dividing circuit, which is usually (but not always) a pair of fixed resistors, a potentiometer whose wiper changes the ratio of two resistances as it moves, or the combination of a fixed and variable resistance that provides a shifting resistance ratio functionally equivalent to a pot (e.g., some limiters or compressors use one variable resistance in conjunction with a fixed resistor to simulate a volume control being turned down and attenuating the signal).

I don't think there is any hard and fast rule, but I find the term "attenuator" often used when either:
a) There are very specific impedance matching requirements between what comes before the attenuation circuit and what comes after.  "Power brake" circuits are in this category since output transformers expect to see a certain load to function properly.
b) The circuit assists in providing identifiable amounts of signal reduction.  This is the thing you often see in circuit collections aimed at audiophiles.  An example might be a stepped "attenuator" between preamp and power amp using precision matched resistors to provide stereo signal reduction of 3db, 6db, 9db, etc.  This accomplishes the same thing as a dual-ganged volume pot, but in this context it is more likely to be referred to as an "attenuator".

I guess from another perspective, with volume controls one is more interested in simply turning things up, whereas with "attenuators" the focus is on how much something has been reduced.  It may seem like a banal half-empty/half-full distinction but it changes the expectations and requirements of the circuit involved.

[petemoore]

Someone was teling me the new Marshall attenuators [been a while I can't remem the ame of the unit] actually has a dummy coil [likea speaker] that loads the output of the amp like a speaker thereby attenuating the signal [like plugging in a cabinet..getting the amp loading without sound] with variable ohms [like a speaker] that swing up and down with every movement of the coils [liek a speaker]...basically a coil that loads, but instead of a cone it is just mounted so that it can move but in restricted fashoin [like a spkr].

[Mark Hammer]

Now there's a topic that takes me back.  I think this one exists from pre-archival times back on Ampage.

Yeah, speakers do not behave as fixed resistors, so using fixed resistor to drop the output does not mimic the way speakers would do it.  There are different ways to perform that mimickry (with differing degrees of success/authenticity one presumes) but the most straightforward way to do it is to use an actual speaker coil.  Naturally, what you want the dummy speaker for is just its moving coil and not its air-moving properties, so some folks tinker with using speakers that have some of the cone excised.  I've been playing with a homebrew reverb experiment, using a small speaker as spring driver.  Of course you want to hear the spring not the speaker, so I painted the cone with white glue to stiffen it up and took a scalpel to it, removing as much of the cone as I could while still retaining the coil-aligning properties of the cone.  You can still hear the speaker AS speaker, but at a drastically decreased volume.  Stick it in the proper box and you can't hear it above the ventilation.

[The Tone God]

From what I remember the Marshall Power Break is an inductive power attenuator which means it uses a transformer of sorts. Seems like a fancy idea to brag about in ads but not really. Its not the only one on the market that works in that manner. Others are resistive attenuators that use big power resistors. To be honest it didn't make much of a difference tonally between the two when I tried them.

I built a power attenuator out of some spare parts I had sitting around in the workshop. Cool item to have. It can switch between 4 and 8 ohm loads to which is nice. I included an inductor that was wounded to the same value as one of my speaker in series with the resistors. Made the attenuator much better sounding,

Andrew

[Rob Strand]

Marshall Power Break

Yes, it uses an autotransformer connection- basically a transformer.

It also has coils which are part of a the reactive load, which makes it look more like a speaker.  For example,

http://www.geocities.com/SunsetStrip/Studio/2987/speaker.html

The main reason the  Power Brake uses a tranformer is because it's hard to make the reactive load adjustable.  The transformer has taps so you can tune the fixed reactive load to the load impedance to your amp.  The Power Brake uses the same transformer for attenuation, the attenuation function could have used resistors IMHO.

From what I can seem, the Dr Z Airbrake doesn't even consider load issues.

[The Tone God]

One thing I notice with almost all commercial power attenuators is that they barely handle 150 watts if that. It makes it hard to use it with more powerful amps or even as a shop tool. The power attenuator I built can handle between 1200 and 1500 watts. :twisted:

Andrew

[fretbuzz2003]

Hi,
Anyone know where I can find a schematic for a power attenuator? I was interested in the THD hotplate but don't really want to spend $280.

Thanks.