120W Amp Attenuator/Powerbreak

[Rethfing]

Hey there folks

Is there any good solution for bedroom level playing with 120W high gain amp? Im using Bugera 6260, its a beast, but its sooo loud. I was thinking about building some powerbreak and was suggested a following sollution:
12 or 13 pieces of 100 Ohm resistors with 10W power handling connected in parallel, and then connected in series with 0,47 Ohm 10W resistor.

Im worried that the reduction is too big and I would have to crank the volume way too high than needed actually. Also I dont think that the resistance (with speaker connected) is good for the amp. On the back of the amp I can choose between 16, 8 and 4 Ohms cabinet. My cabinet is 8 Ohms.

Im looking for some cheap and stable solution, the attenuation doesnt need to be big, just so I cant turn the lead volume to 1-2/10.

Any suggestions?

[Seljer]

A typical attenuator can be of the "L-pad" type, which is basically just a voltage divider, with the resistors chosen so the total impedance present at the amp's output remains unchanged.

Code Select Expand



 ------+
       |
      | |
      | |  R1
      | |
       |
       +------+  /|
       |      | / |
      | |    | |  |  8ohm
   R2 | |    | |  | speaker
      | |     | \ |
       |      |  \|
       +------|
       |
 ------+



A 10dB power reduction means a 10x power ratio, which means only 10% of comes out of the amp gets to the speaker. Which would mean 120 watts from your amp would be 12watts and the speaker which more than enough from bedroom playing where you'd should find an acceptable level with in the first quarter turn of the volume knob. Even 1 or 2 watts into an efficient speaker cabinet is loud enough to annoy anyonve in the vicinity who is not inclined towards your playing 

For that same 10x power ratio, you need a 3.16 voltage ratio, because over a constant load, the power is the square of the applied voltage (P = U^2 / R which leads to sqrt(10) = 3.16)


So you have an 8ohm speaker, and lets say you want the total impedance of the attenuator to stay at 8ohms (though you could also dimenion the attenuator so it turned into 16 ohms or 4 ohms if you wished)


In this example you want the resistive divider to have a voltage ratio of 3.16.

The top part of the divider is R1. The bottom part of the divider is R2 in parallel with the speaker: R2||8ohms = ( R2*8 )/( R2+8 )

The equation for the voltage divider is:
1/3.16 = ( R2||8ohms ) / ( R1 + R2||8ohms )

But you already know you want the total impedance of (R1+R2||8ohms) to also be 8ohms. Thats quickly simplifies the divider equation and you can then solve it for the combined value of R2 and the speaker in parallel
1/3.16 = = ( R2||8ohms ) / 8ohms
R2||8ohms =  8/3.16 = 2.53ohms

From that you can get that the value of R1 which is whatever is missing to get up to 8ohms total -> R1 = 8-2.53 = 5.47ohms

Which leaves us to find the value of R2
2.53ohms =  ( R2*8 )/( R2+8 )

2.53ohms * ( R2+8 ) =  ( R2*8 )
2.53 * 8 = R2 * ( 8-2.53 )
R2 = 2.53*8/( 8-2.53 ) = 3.7ohms

Whats left is to chose the power ratings of the resistors. If want to be sure you can run the amp at its maximum 120W into an 8ohms load, P = I^2 * R which leads to I=sqrt( 120/8 ) = 3.87amperes
The power used up by the top resistor P = I^2 * R = 3.87*3.87*5.47 = 82watts
And for the bottom resistor, you can subtract the 12watts that on speaker, the 82watts on the upper resistor and you're left with 120-12-82 = 26watts

So for a -10dB attenuator with an 8ohm input and output impedance you need an 5.47ohm that can handle at least 80watts resistor and a 3.7ohm resistor that can handle at least 26watts. You also probably need to mount them on a hefty heatsink. You could de-rate the power ratings, but that'd mean you would have to be very careful with the volume knob to not accidentally overload it and burn up the resistors and then kill well as your amp's output transformer and/or power tubes

If you google for "L-pad attenuator" you can find calculator applets that simplify this process so you could quickly find some values that would work for you, you should probably aim for an attenuation between -6db and -10dB. From the speakers point of view, the resistive attenuator will look like the amp has an increased output impedance, which means the speakers own sonic quirks will be more prominent but thats the drawback from a design this simple (and tube amps already have relatively high output impedance compared to solid state amps so it's no that much of an issue). You just have to accept it if you don't want to cause an earthquake  

[R.G.]

Get a smaller horse.

A 120W amplifier in a bedroom is like keeping a Ferrari in the back yard when what you need is a child's wagon.

Power brakes and attenuators are not particularly good ideas for the amps, either. You get a lot of wear and tear on the amp. I don't know the insides of that amp, but if it has an effects loop, you can take the preamp sound out of the effects loop and into a much smaller power amplifier to feed speakers. A 1W to 10W amplifier can be deafening in a bedroom, so a quite-small amp can do what you need there.

[Rethfing]

it does need to be power amplifier? I have Noisy Cricket MKIV that I have built a while ago, but I suppose it has to be power amp construction, right?
And even if I can do that - I still need the cabinet to be connected to tube amp right? So I need 2 cabinets to do that slaving?

[jimbeaux]

Don't know if it will tame your Bugera enough to play in your bedroom.

http://www.guitarplayer.com/article/power-attenuator-project/930

[Rethfing]

Yeah I came across that too, I might try to do it, but I'd like to hear what R.G. says first, his method seem more "natural" to me.

[R.G.]

I'm completely unfamiliar with the Bugera. But if it has a pair of effects loop jacks, and a pluggable speaker connection, that would be ideal.

You would plug your hypothetical external power amplifier input into the effects send, nothing into the effects receive, and unplug the speakers on your amp and plug them into the external small power amp. Notice that you still have to worry about not leaving NO speakers plugged into the Bugera if it's a tube amp, and you may have to plug some kind of load, even a 100 ohm power resistor on a plug, into it to keep it from having "no speakers" tube amp problems. This can damage your amp if you remove all speakers from a tube amp and leave only an open circuit. But even a modest power resistor is complete protection.

If your amplifier does not have its speakers plugged into the chassis, or in a separate speaker cab, you may have to do more cable work to make this easy.

As I said, the real answer is that a 120W amplifier doesn't need to be in a bedroom.

[slacker]

I only had a quick look but it seems to have a master volume so isn't R.G's suggestion pretty much the same as just turning it down? Either way you're losing whatever magic the power amp adds by being turned up.

Thinking about it I guess a smaller power amp turned up more might sound better at very low volumes than a high Watter barely turned on.

[bluebunny]

it does need to be power amplifier? I have Noisy Cricket MKIV that I have built a while ago, but I suppose it has to be power amp construction, right?

Is there a definition for "power amp"?  Hmmm...

Just plug the Noisy Cricket into your speaker cab.  Ignore the Bugera.  Season to taste with overdrive/distortion/etc. pedals in front.

[Hemmel]

As I said, the real answer is that a 120W amplifier doesn't need to be in a bedroom.

Some people (like my brother) argue that they can only afford one amp and the only one they have is higher than 50W. Since he wanted to practice at home, he asked me to box an attenuator for him. He's pretty happy about it !
Although I admit I totally copied it from a certain Karl with a C.

[Rethfing]

@R.G.
Bugera 6260 is a head amplifier with effects loop (copy of Peavey 5150). I might try that idea, only need to get my hands on some high power resistor (but I guess it doesnt matter since I wont be using Bugeras power amp at all).

@Slacker
Even with the master volume this amp just sounds too loud if you want good distorted tone

@Bluebunny
I've been playing this way all the time, got Fulltone OCD copy, SansAmp GT-2, but Noisy Cricket tone is very dull and I just fell in love with tube amps clean sound

I guess I just stick with that and wait for my band to rehearse so I can fully use Bugera...

[bluebunny]

. . . but Noisy Cricket tone is very dull . . .

Shame.  Dare I suggest a Tiny Giant?  Quite a bit louder, but clean and works nicely at low volumes.  Should work well with the pedals you mention.

[anchovie]

Splice a 1meg volume pot into the middle of a patch cable - you can mount the pot in a little metal enclosure if you like. Plug this into the effects loop. Set the amp master volume to the point at which it becomes too loud.

The pot in the loop now becomes your "bedroom master volume", giving you a full pot's worth of travel in the range of "silent" to "just too loud".

Being a 5150 clone, all of the distortion is coming from the string of cascaded 12AX7s in the preamp. You'd have to dime the master volume to get any sort of power amp distortion added on top, so the dummy load/attenuator route is of no benefit here.

The quietened amp might still not sound as good as when it's set loud, but as a major piece of the sonic chain is the air being physically moved by the speakers then you simply can't put that punch back in. If you need to feel like your ears are being pummelled when practising at home, I guess you'll need a POD and a pair of headphones!

[DougH]

From what I remember of the 6260, it is a "preamp-distorter". In other words, it gets its sound from the preamp, the power amp is just basically there for amplification, more or less. In that case, there's no advantage to using an attenuator over just using the master volume in the amp to turn it down. If it doesn't sound good with the master volume turned down to the level you need, it won't sound good with an attenuator turning it down to the level you need- it will just work the power amp a lot harder and wear out power tubes quicker.

Attenuators are used for attenuating amps that develop their overall tone by driving the output stage for distortion. These are usually vintage amps with comparatively simple preamps. They don't get their "sound" without cranking them really loud. Most modern production channel switching amps, like the 6260, are designed to develop their tone through the preamp, with channel switching, EQ-ing, fx-loop and etc which is more complex than a typical "vintage" amplifier. The master volume is meant to really be used as a master volume, delivering a reasonably consistent tone over a wide range of overall volume. That said, the tone of any amp/speaker combination will suffer when turned down below a particular threshold due to the way our ears work. (Google "fletcher-munson curves".) In that case, an attenuator or even smaller amp may not help you any more than the master volume would. You just don't have enough db's for your ears to hear it right.

So either your "bedroom volume" requirement is too restrictive for a guitar amp and speaker- in which case you might want to investigate some headphone solution like a POD or something...

Or, like R.G. mentioned, you need a smaller amp.

[Rethfing]

From what I remember of the 6260, it is a "preamp-distorter". In other words, it gets its sound from the preamp, the power amp is just basically there for amplification, more or less. In that case, there's no advantage to using an attenuator over just using the master volume in the amp to turn it down. If it doesn't sound good with the master volume turned down to the level you need, it won't sound good with an attenuator turning it down to the level you need- it will just work the power amp a lot harder and wear out power tubes quicker.

I may be wrong, but I have to disagree. This amp develops its tone along with turning master volume up. It sounds very hissy and muddy when its really quiet, but starting to gain power when you turn it up. I know, that with 12" speaker you need to deliver enough power to move some air, but I've used some smaller amps with the same speaker and it sounded better on lower volumes. I guess the master volume and the preamp work together to provide desired tone.
As I said I might be wrong and the tonal differences may be present just because of subjective hearing perception and difference in volume.

I tried using Pocket POD but its not my thing I will continue using Bugera on low master volume level, or just use Noisy Cricket as I did before. Bugera will wait for better times then

[Seljer]

^what your mentioning is probably more due to the response of your ears and how the amp approaches it https://en.wikipedia.org/wiki/Equal-loudness_contour

In a big amp the EQ components are chosen so it sounds good at high volume on stages, a small little amp will be designed so it sounds good at it's lower volume level