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Old 12-26-2008, 07:15 PM   #1
Mike-T93
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Homemade Power Attenuator

I'm going to build a power attenuator footswitch for a Crate V18 2x12, and I've drawn up a wiring diagram with my horrible paint skills. The 8ohm resistor in parallel with the speakers should cut the current in half and the 4ohm resistor in series in the parallel circuit should correct the load to 8ohms and cut the voltage in half, with each stage reducing the power output by a factor of four and the volume by about six decibels. When the DPDT switch is not thrown it should bypass. When the DPDT switch is thrown the first stage of the attenuator will be in the circuit, reducing the power by 1/4. The first 3PDT switch, the battery, and the first LED will also be in the circuit. When the first 3PDT switch is thrown it should reduce power by 1/4 again to 1/16 and activate the second LED. When the second 3PDT switch is thrown it should reduce power by 1/4 again, to 1/64 and activate the third LED. I want to run it from the amp output by a 10' 1/4" male-1/4" male speaker cable and back to the speakers along another 10' cable. It should allow me to reduce my output volume by 6, 12, or 18 decibels with a stomp of the foot. Any comments on whether this will work or not, design improvements, etc?



Last edited by Mike-T93 : 12-28-2008 at 03:37 AM. Reason: Modifying specifications.
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Old 12-27-2008, 05:39 PM   #2
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So nobody has any sort of comment to make?
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Old 12-28-2008, 02:54 AM   #3
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i have nothing to add but i will be looking at this thread to see what happens because im going to need one soon...good luck
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Old 12-28-2008, 03:14 AM   #4
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Well, I probably won't be making it for a few months because it's going to cost about 140 bucks and I need new pickups for my guitar before anything else. I was just planning it out right now and trying to get some feedback because I'm bored and have way too much time on winter break. I guess it's good to have a lot of design time to get expert advice. I'm going to take a better hand-drawn diagram to a local amp shop and see what they think. The design seems too simple to not work. It should work more conveniently than a Hotplate since it's a footswitch, and for less than half the price. The main thing that concerns me is securing the cables so if someone trips over one it doesn't come out; I don't want a fried output transformer on my new amp. I'll be sure to post updates when I start building, probably in May, April if I get lucky.

Last edited by Mike-T93 : 12-28-2008 at 03:33 AM.
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Old 12-28-2008, 03:19 AM   #5
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I could build one of those right now from spare parts. WHat is the $140 coming from?
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Old 12-28-2008, 03:28 AM   #6
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From what I can find online, although I just found some cheaper parts in the past couple hours, didn't change my old estimate. I don't have these spare parts lying around. The aluminum enclosure is $13.90, jacks are $1.80 each, DPDT footswitch is $6, 3PDT footswitches are $9 each, 8Ω 12.5W resistors are about $2.75 each, 4Ω 25W resistors are $3.25 each, 10-foot 12 gauge speaker cables are $10 each, red LEDs are $2 each, and 6V camera batteries are about $3.80 a piece, so I guess with cheaper parts it'll be about $90 before shipping and tax, $110-120 after unless I find everything locally.
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Old 12-28-2008, 03:29 AM   #7
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^^ cause you cant use 1/4 watt resistors on something like this. just looking up a 30W 4ohm resistor on mouser real quick and it looks to cost around 7 to 8 bucks. so thats like $45-50 worth of resistors, plus 3 switches that are around $10 each. toss in a case, nice wire, and some LEDs and ive got no problem seeing this costing over $100.

as for the way you said it should work, it seems like it should work logically but i didnt look at the circuit close enough to go through it.
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Old 12-28-2008, 03:44 AM   #8
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I'm wondering if the third stage is necessary. Removing it would reduce the cost by a significant amount. The output with the amp completely dimed would be about 280milliwatts, which would be split between two 12" speakers and probably sound pretty poor. Is that low enough to plug in to a sound card for direct recording of the power amp output or for plugging straight into a PA system?
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Old 12-28-2008, 04:48 AM   #9
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Quote:
Originally Posted by Mike-T93
From what I can find online, although I just found some cheaper parts in the past couple hours, didn't change my old estimate. I don't have these spare parts lying around. The aluminum enclosure is $13.90, jacks are $1.80 each, DPDT footswitch is $6, 3PDT footswitches are $9 each, 8Ω 12.5W resistors are about $2.75 each, 4Ω 25W resistors are $3.25 each, 10-foot 12 gauge speaker cables are $10 each, red LEDs are $2 each, and 6V camera batteries are about $3.80 a piece, so I guess with cheaper parts it'll be about $90 before shipping and tax, $110-120 after unless I find everything locally.


If you are going to pay 2$ for an LED thats crazy. Either way, you don't need this in stomp box form, and you don't need the LEDs. Replace your stomp switches with simple DPDT on/on switches. I also have no idea where you are getting your decibel cut values from. I will tell you though that this will turn the amp from 18wats to 9watts to 4.5 watts to 2.25 wats. You could get rid of the last stage because ~5 watts dimed is just a loud room level. Also, who ever said that you need the 30 watt resistors is wrong, the most any of the resistors will see is 9 watts and that is just those in the first stage, and you don't need 12 guage speake wire, that is just over kill.

So now it all looks like this:
Box 14
4 resistors 12
Jacks 4
2 DPDT flip switches 8
18g wire, two colours for fun, 8

Total 46 bucks.

PS put a couple air holes in there to let the heat out.

Edit: if you are dead set on spending that much on an attenuator, look into an airbrake clone, you can DIY for about 140 and its a much better design
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Old 12-28-2008, 04:57 AM   #10
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First, how are you getting those numbers? Cutting both the voltage and current in half reduces the power to 1/4 of what it was. Secondly, wouldn't a stompbox would be more convenient for switching from clean to overdrive settings in order to maintain a reasonable volume? Lastly, your cost assessment omits the cost of LEDs, batteries, and 3PDT switches required to activate them. As far as decibel cuts go, it takes a halving of power to drop the volume by 3dB, so quartering power cuts by 6dB. Please explain why I'm wrong if I'm wrong.
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Old 12-28-2008, 05:06 AM   #11
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I cut those out because they aren't neccessary, and your not going to stop box an attenuator like you would a booster pedal. So I left alot of things out to keep costs down. If you dont want to keep the costs down, like I said, check out an air brake clone DIY, its a fool proof design that works with more Ohm ratings, and handles more power, I will edit my stuff with my numbers.

Edit, ok my numbers where off. I was thinking of doing something different, I didn't notice that the speaker was in series with the 4 and parralel with the 8. Or something like that, the diagram is a bit messy and im tired. Either way, you have alot of extra stuff in there.

You should read this, it will prove interesting. http://www.ax84.com/faq.html#spkr-eff-2

I think I got everything, again im tired.

Last edited by PainIsPower : 12-28-2008 at 05:17 AM.
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Old 12-28-2008, 05:52 AM   #12
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Thanks for the suggestions, I'll consider making it built in to the amp so it can sit by the spring reverb or something on the bottom. I think I might want three stages still if it will allow line output to recording or a PA, anyone know is <280mW is low enough?

Last edited by Mike-T93 : 12-28-2008 at 05:58 AM.
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Old 12-28-2008, 06:43 AM   #13
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For the cost of having 3 stages, you're approaching the price of already made attenuators.

A Weber Mass Lite is around $150, and will sound better than your L-Pad attenuator.

The foot-switches are nice, but attenuation is not something one regularly changes on the fly. Stepping up your attenuation drastically alters your tone, so you'd have to adjust your amp either way.

For my Firefly (1watt tube amp), I built an attenuator into it. A simple L-Pad like you're intending but I used a rotary switch for 4 different levels of attenuation. Course I could get away with 1/2 watt resistors, so it was cheap to add.
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Old 12-28-2008, 07:30 AM   #14
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As suggested, a cleaner diagram. Simple two-stage circuit with the first switch acting as a bypass. Designed for building into a box that will be secured to the floor of the amp cabinet. Should allow 18W (full), 4.5W, and 1.125W operation, or 0, -6, and -12dB. So, $14 for the box, $12 for the resistors, $4 for the jacks, $4.50 for the switches, and $4 for a 3-foot speaker cable (Musician's Friend 16-gauge any good? Monster wants 13 bucks for a similar cable). Total $38.50 before shipping and tax, probably a bit over $50 after. Already have soldering iron, solder, wires, etc, so should be a cheap and easy build. Not quite sure when I'll do it, but I'll update when I do, it'll definitely be sooner than I was originally planning.

Last edited by Mike-T93 : 12-28-2008 at 07:34 AM.
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Old 12-28-2008, 08:39 AM   #15
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Quote:
Originally Posted by Mike-T93
Well, I probably won't be making it for a few months because it's going to cost about 140 bucks and I need new pickups for my guitar before anything else

Don't do this until you have a good amp.
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Old 12-28-2008, 10:29 AM   #16
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Quote:
Originally Posted by PainIsPower
Also, who ever said that you need the 30 watt resistors is wrong, the most any of the resistors will see is 9 watts and that is just those in the first stage, and you don't need 12 guage speake wire, that is just over kill.

i didnt say you need 30W resistors, i just looked up the price of them because i didnt see anything around 20W. however, the resistors will see more than 9W. the amp is rated at 18W RMS, which is not the same thing as being rated 18W at full. so if you crank the amp its closer to 25W or 26W. half that is around 13, in which case i would use 15W resistors at minimum. i just picked a random high number to search for. i also looked at power resistors because they are usually within closer tolerance, which i would want in an attenuator. call me crazy, but if im building something like this, i want my resistors to cover any power the amp can put out as well as have very close tolerance resistors.

back to my point, the resistors in the first stage will see more than 9W.
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Old 12-28-2008, 02:14 PM   #17
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Don't do this until you have a good amp.

Don't even start this... Just let it go.
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Old 12-28-2008, 02:55 PM   #18
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I have to agree with what painispower said. 2 dollars for Leds is absurd. They are practically free.

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what is wrong with his amp?
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Old 12-28-2008, 03:00 PM   #19
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Parrotting, probably. It seems you can't do anything these days without having a JCM800 or something.
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Old 12-28-2008, 03:07 PM   #20
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Was that supposed to be criticizing? Im a little tired...
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