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Comparison of Popular Power Attenuators

The MASS goes from zero power to the speaker to about 85% of the power to the speaker.  It is a smooth control rather than a stepped switch.  If/when you want full volume, then you can remove the power attenuator.  The nicest thing about having a bypass setting is that it's easy to compare no attenuation to slight attenuation -- the three products I tested (Power Brake, Hot Plate, and Power Soak) all exhibited their signature EQ coloration as soon as I switched from Bypass (no attenuation) to slight attenuation.  It's possible to hook up A/B/C switch boxes to do this A/B comparison -- that's how I did this type of testing.


I'm most concerned about what a power attenuator's signature EQ coloration is.  I don't mean at the Line Out jack, as when the product is used as a dummy load, because I concluded that you can always use an EQ after the dummy load (Line Out jack), to compensate (I A/B tested this carefully).  I mean when you truly use the device as a power attenuator and are hearing a guitar speaker that is driven directly by the power attenuator.  The Hot Plate has a treble switch and a bass switch, which I always use.  I wish these attenuators would have even more high-power EQ controls (not just the Line Out EQ controls like the Weber MASS has).


In contrast to my earlier writings, as of 2003 I'm skeptical about power attenuators sounding worse the more you attenuate.  My testing indicates that a power attenuator with bad EQ response sounds the same no matter how many notches you attenuate, and what little opinion people have voiced about speaker smoothing seems to indicate that the notion is unfounded, so that pushing the speaker fairly hard doesn't sound significantly different than barely pushing it -- this view matches London Power's view and the view of a certain online tech (Neil Keen, I believe); it's important to push the power tubes, but it's not important to push the guitar speaker, unless you really want that special effect which is outright speaker distortion.


The Rocktron Juice Extractor is a dummy load (probably resistive) and 3-band full parametric EQ packaged in a single rackspace unit.  Comparing it in A/B/C testing against the Power Brake, Hot Plate, and Power Soak, the Juice Extractor can approximate the EQ response of the other products.  It's not clear if there are still some subtle differences such as dynamic response.  That's all the time I have now for covering the Juice Extractor.  I focus on the most mainstream and important reference products.


I compare the Power Brake, Hot Plate, and Power Soak in a relevant test setup as dummy loads and as power attenuators at the 1/4-watt private playing level with minor excursions into louder, cafe regions (12 dB down with a 15 watt amp).  The greatest differences between power attenuators happen at strong attenuation.  At small attenuation, the load least affects the sound, because most of the power is simply going straight to the speaker.  The most relevant SPL points to compare attenuators (with 15 watt amp) are:


o  12 dB attenuation (cafe level)
o  30 dB attenuation (private daytime practice)

o  Full attenuation (dummy load)



For reference, here are the standard architectures for a conventional 2-stage amp rig, a 2-stage amp rig with power attenuator, and a 3-stage amp rig with dummy load:



2-stage amp rig:



preamp distortion


power-tube saturation

guitar speaker



2-stage amp rig with power attenuator:



preamp distortion


power-tube saturation

power attenuator (can have spk-level eq controls e.g. Hot Plate)

guitar speaker



3-stage amp rig with dummy load:



preamp distortion


power-tube saturation

dummy load


linear power amp

guitar speaker







preamp distortion


power-tube saturation

A/B/C 2-channel switch box

Loop A contains Power Brake

Loop B contains Power Soak

Loop C contains Hot Plate

Guitar speaker





preamp distortion
power-tube saturation
A/B/C 2-channel switch box
Loop A contains Power Brake (use Output jack)
Loop B contains Power Soak (use Connect To Speakers jack)
Loop C contains Hot Plate (use Line Out jack or Speaker Output 1 jack)

Solid-state power amp

Guitar speaker







Power attenuators:

EQ response of Power Brake: ___.  1/4 watt vs. 1/2 power: _____

EQ response of Power Soak: ___. 1/4 watt vs. 1/2 power: _____
EQ response of Hot Plate: eq boost off: ______  eq boost on: ___. 1/4 watt vs. 1/2 power: _____


Dummy loads:

EQ response of Power Brake: ___.  1/4 watt vs. 1/2 power: _____
EQ response of Power Soak: ___. 1/4 watt vs. 1/2 power: _____
EQ response of Hot Plate: eq boost off: ______  eq boost on: ___. 1/4 watt vs. 1/2 power: _____.  Line Out vs. Spk Out: ____




Power attenuators:
EQ response of Power Brake: ___.  1/4 watt (30 dB down) vs. louder, cafe level (12 dB down):

Volume: can't turn extremely low like Hot Plate.  Lowest setting other than "dummy load" is standard daytime private practice level.  Not loud, but way too loud for night; need to rig up a resistor to further attenuate.


Power Brake: Has a pronounced V shape with bass dominant.  Inverse of Power Soak.

Power Soak: mud - flat throughout mid-bass to mid-treble, with rolloff of low-bass and of upper-mid through high-treble.

Hot Plate: moderate V shape with treble dominant especially when the 16dB pot is within the left half of its range.  That's with the EQ boost switches off.  With the Treble boost off, Bass boost on, it *almost* approximates the Power Brake, but the Power Brake still has more bass and less mid-treble.  When both boosts are on, the Hot Plate has almost the bass of the Power Brake, but much more mid-treble than the Power Brake and of course far more treble than the Power Soak.  The Power Soak bass but esp upper bass/lower mids.  The Hot Plate has moderate bass even with the bass boost on, but has little mud and tons of mid-treble. 


Muddy response of a power attenuator makes you compensate by setting EQ2 (before power tube saturation) to a V, thus hurting the distortion voicing within the power tubes. 


The Power Brake has not much treble, but decent high treble.  The Hot Plate with 16dB pot at noon has too much mid-treble when treble boost is on.  The Power Brake has lots of bass but lots of upper bass and lo-mid, too, making its bass a little muddy at extreme attenuation.  The Hot Plate has not so much bass, even with boost, but it's a very nice, tight, clear bass with little low-mid/upper bass.  The Power Brake's mud problem is not as bad as the Power Soak.  The Power Soak has less treble than the Hot Plate, even when the Hot Plate's treble boost is off.  The Power Soak has a nice round response in the frequencies that is does reproduce, easy on the ears, but lacks bass & treble.  It's got great sound throughout entire midrange from low mid to high mid -- smooth, not peaky.  But no bass thump, no treble singing.


Here is the 9-band response-curve estimate (lower bass, mid-bass, etc.).  "HP+" means Hot Plate with treble and bass boost on. I use a scale of relative levels from -3 to +3.

These are relative to the span found within the products - not relative to perfection.  0 means in the middle of the range found in all the products.
These are quick figures and I should draw them then convert to numbers. 


PB  +2 +3 +3  0  0  0  0 +1 +2
PS  -2 -1 +1 +2 +2  0 -2 -2 -3
HP  -1 -1  0 -1 -1 +1 +1  0 -1
HP+ +1 +1 +1 -1 -1 -1 +2 +3 +2


[this paragraph might be less true, see whitenoise tests below]  All power attenuators sound good at 3-9 dB attenuation, but how good, and how high do you have to turn the SPL before it sounds "acceptable", within range of your acceptable tone?  The Power Soak, you have to turn loud before it finally produces acceptable response; it's only good for the first few settings and quickly starts sounding too lower-midrangey.  The Power Brake is very accommodating -- starting from strong, 30dB attenuation, you  only need turn up a few notches, and it starts sounding *great*.  The Power Soak you have to turn way up loud before you recover great Tone.  Thus the Power Brake has a wider usable range of attten, in practice.  A good figure is, what is the greatest usable attenuation for each product?


The Power Brake sounds great down to ___ dB attenuation.

The Power Soak sounds great down to ___ dB attenuation.
The Hot Plate sounds great down to ___ dB attenuation.


The Hot Plate with boosts on: at 12db attten, too loud, very sharp middle-treble.  The Power Brake has very nice intermediate settings.  The Hot Plate 16 dB pot from about 11 am to 2 pm is nice.  The Hot Plate 16db setting with boosts on has emphasis of mid-treble and lower-treble; brittle, sharp, not enough bass, not round like the Power Brake.  The Power Brake is very round and dynamically squashy.  For high-attenuation settings, the Hot Plate is best because it has decent bass and clear treble.  But for slightly louder -- 12 dB -- the Power Brake wins, when up a few notches above its 30dB setting, up where the mid-treble starts to recover.  The bass strength is always there, unlike the Power Soak.


Two critical ratios:  how much of the dummy load is used (or seen by the amp) vs. the actual speaker?  How many watts are the head and speaker?  Ideal for low-wattage great tone is:  low-wattage amp, so the soak is only used for slight attenuation such as 12dB not 30dB, and perhaps a single, inefficient speaker, to control sound dispersion.  How can you use a soak the least, relative to power sent to the speaker, for a given SPL?  Use a low-wattage amp and an inefficient speaker.


For private playing levels, the Hot Plate wins becauase it has enough bass and lots of treble.  For cafe levels (12dB attenuation with a 15 watt amp), the Power Brake wins: it has lots of bass at all settings, and is very musical, with round treble (singing, not attack you brittlely like the Hot Plate can do).  The Power Brake is never brittle and is only dull at the extreme settings of 27 or 30 dB down.



At 12dB attenuation with a 15 watt amp, the true character of each product comes out best:  the Marshall is the V monster, with great bass and great high treble that's not piercing or rigid.  The Power Soak is the lower-mid mud monster.  The Hot Plate is the upper-mid through mid-treble monster.


At 16dB down, the Power Brake sounds more Hendrixy than the Hot Plate which sounds relatively muddish and lower-trebly.  The Power Brake has a strong V eq response, and is round dynamically.






EQ response of Power Brake: strong V shape, with especially strong bass and a good amount of upper treble but not mid-treble or lower-treble.  With infinite attenuation setting, has a lot of treble and little bass.  At the other settings, much bass, not much treble but pretty strong upper-treble.


Power Brake attenuation levels (unmarked): 

Position, dB down:

1 0

2 3

3 6

4 9

5 12

6 15

7 18

8 21

9 24

10 27

11 30

12 infinite (I don't use - alters the tone - more treble, less bass)


Comparing all 3 products as dummy loads at 12db down, it's very clear and immediately obvious that the Power Soak has a midrange mound, the Hot Plate w/ treble/bass boosted (using Spk Out jack) has a slight V curve, and the Power Brake has a strong V response curve.


Generally there is no difference in eq response when moving among attenuation settings.  But the Power Brake has a thin sounding response when set to infinite attenuation.  And the Hot Plate in the 16dB setting, where the variable pot applies, is greatly affected by the pot position.  7 pm is thin and sandy.  noon is full. 5 pm is dull.


Hot Plate (using Line Out jack not Spk Out) (with treble and bass boosted by settting switches to "not boost") - has less upper-bass than Power Soak, and a little more treble.  The Power Brake has the most V shape response, the Hot Plate has slight V shape, and the Power Soak has a mid-mound shape.


EQ response of Power Soak: mid-mound, emphasizing upper bass through middle-midrange.  No treble, little low-bass.
EQ response of Hot Plate: depends on EQ boost settings.  Depends on 16dB pot setting when using Spk Out as dummy load line out.  Does not vary with Line Out pot position when using Line Out jack.  Overall, has average bass, and strong middle-treble.  A lot of lower mid and upper bass but not extreme like Power Soak.






At the last minute I found one of the most interesting tests.


Turn on the preamp distortion and other processing, turn guitar's volume to 0.


Turn all dummy loads to 0 dB attenuation (full power).  Use Spk Out on all units, then feeds EQ then solid-state amp.  A/B/C reveals no difference at all - as expected (all are bypassed).


For each product, the white noise changes as follows, as you increase attenuation:


Power Brake:

0 - standard white noise
3dB - tons of high-treble and reduced lower-mids (drastic change from 0 dB)

30dB - as you attenuate more, the curve becomes more like the 0 (bypassed) whitenoise curve -- treble hiss gradually reduces.


Power Soak:
0 - standard white noise
3dB - treble immediately reduces; opposite of Power Brake!
32dB - as you attenuate, becomes gradually somewhat duller


Hot Plate:

0 - standard white noise

4dB - essentially same as Power Soak:  treble immediately reduces
~30dB - as you attenuate, treble increases slightly.


This is a major surprise.  I assumed that at 3 dB all power attenuators sound the same.


Now connecting as a power attenuator, the same is true though to a less extent: all products sound the same at 0 dB attenuation.  But at 3 or 4 dB, the Power Brake actually sounds significantly more trebly, while the Power Soak and Hot Plate sound slightly duller.  That means that if you want a brighter sound from your speaker cab, you can insert the Power Brake and attenuate a little bit; as soon as you attenuate at all, the Power Brake brightens up the sound.  I used a low-volume input signal but I think the same behavior would show up at high SPL (with a higher wattage amp and greater input signal from the preamp).



With a 3-stage rig with dummy load, how much can you use the 3 EQs to redeem the sound of the Power Soak?  Certainly a lot. I was able to change EQ3 to force the Power Soak into a V-curve.  I think you would need to change both EQ2 and EQ3 to make the Power Soak sound like the Power Brake.  The dummy load loads the power tubes differently and affects the voicing of the power-tube saturation.


By understanding the quirks and interactions of EQs and specific power attenuator products, you can get great sounds at low volumes, with either a power attenuator approach or dummy load approach.  Speaker isolation cabinets and low-wattage tube power amps are still essential tools that contribute important capabilities for reduced SPL.  Sometimes you don't want to hear the guitar speaker directly at all, such as when recording or for late-night playing.  And a 15-watt amp (or less) with a power attenuator makes sense compared to a 100-watt amp into a power attenuator, and might even sound better because you can avoid extreme power attenuation.



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