DIY attenuator

[lowell]

I am building a very crude attenuator. By crude I mean that the impedance is far from what the OT secondary wants to see. I figure it's an attenuator...and as is often the case.. I've accepted that it will change the sound a bit. My concern however is stressing the OT or power tubes.

I'm basically inserting series resistance before the speaker and not compensating for impedance.

My question is: just how much of a no-no is this?

It is often wise to add a resistor of 470 ohms or so across a speaker jack to mitigate damage from a no-load scenario. Being that this can save the OT, my conclusion, albeit founded on my own opinion, is that it's probably not a huge risk to use an attenuator as such.

With a 16 ohm speaker, my attenuator has 140 ohms before the speaker when set to max reduction. So the impedance the OT secondary sees is 156ohms. Just how much of a concern is this?

I'm taking a very meat and potatoes approach here.

Thanks for the comments.

[Malcolm Irving]

That would seriously stress the amp and is likely to damage it with prolonged use. You just need to put a resistor in series and another in parallel with the speaker to offer the correct impedance to the amp. For example, an 8 ohm resistor in series, with a parallel 16 ohm across the speaker, gives an overall impedance of 16 ohms and a power reduction of 75%. Other combinations are possible to give any power reduction you want.

[eschertron]

Put a 20R resistor in parallel with what you have to get ~18R total resistance and about a -30dB gain for the speaker. Crude enough?

[Chuck H]

It's not hard to build an attenuator. So why build a $h!tty one? I've posted my attenuator design repeatedly here. It's easy to build, relatively inexpensive, keeps a reasonable load on the amp, is variable from about -2dB to -16+dB instead of a fixed level of attenuation and sounds just fine.

EDIT: Oh hell, I'll post it again:

http://music-electronics-forum.com/a...30-8ratten.jpg

For a 16ohm load just change the 10r resistor to 20 and the rheostat value to 50r. This one is for a 20W amp. Double the wattage ratings for a 50W amp. Quadruple for a hundy, etc. Build it in a box with some space and arrange it so heat can dissipate.

[bob p]

That would seriously stress the amp and is likely to damage it with prolonged use. You just need to put a resistor in series and another in parallel with the speaker to offer the correct impedance to the amp. For example, an 8 ohm resistor in series, with a parallel 16 ohm across the speaker, gives an overall impedance of 16 ohms and a power reduction of 75%. Other combinations are possible to give any power reduction you want.

With an 8 ohm speaker, putting an 8 ohm in series with it and a 16 ohm in parallel with those would yield a net load of 8 ohms for the amp, not 16 ohms, right?

If you place an 8 ohm speaker in series with an 8 ohm resistor they provide a 16-ohm load. Put those in parallel with a 16-ohm resistor and you've got an 8-ohm network. That's essentially equivalent to a 4x12 speaker array wired in series-parallel, as if you had used one 8 ohm speaker with 3x 8 ohm resistors. that'll give you -6dB.

I once scored a box of vitreous enamel 15-ohm/15W wirewound resistors on ebay and used them in a series-parallel arrangement with a 16-ohm speaker. -6dB wasn't a lot, but it helped.

[bob p]

As long as we're posting attenuator schematics, consider these. All are easy to build and inexpensive.

The first PDF is a copyrighted work by Gerald Weber; I post it citing the principle of Fair Use for academic study.

The second PDF is a drawing made from a genuine Ken Fisher Airbrake back in the 90s. Out of respect for Ken this drawing was never published until the 3rd attachment came along:

The 3rd attachment is the jpg image -- it is a layout diagram of a Fisher Airbrake that was posted on The Amp Garage as soon as Ken passed away. It seems that peoples' respect for his intellectual property rights died along with him -- as soon as he was in the ground people who had promised not to post these circuits began posting them. Since the layout diagram is now being widely circulated it makes no sense to withhold the schematic any more. The Airbrake schematic makes it a bit more evident how the circuit works.

Weber: attenuator-gerald-weber-simple.pdf

Fisher: attenuator1_a1.pdf

Fisher:

Yet another option would be to use a traditional T-attenuator or a "ladder" attenuator. These circuits are easily found in electronics textbooks. Ladders are more expensive to build but they allow you provide stepped reductions in power while maintaining a constant load for the amp. They're the best option when it comes to maintaining a constant load for the amp, but at extreme settings they tend to change the sound of the amp.

The Airbrake is riskier to use as it doesn't maintain a constant load for the amp. But it works well, it sounds good, and the rheostat allows you to dial down to bedroom levels with good results. (Notice the cap!)

I've had my Airbrake clone for about 20 years now. Highly recommended.attenuator-gerald-weber-simple.pdf

[Malcolm Irving]

With an 8 ohm speaker, putting an 8 ohm in series with it and a 16 ohm in parallel with those would yield a net load of 8 ohms for the amp, not 16 ohms, right?
...

True, but OP was talking about a 16 ohm speaker, so I was suggesting another 16 in parallel with the speaker, giving 8. Then another 8 in series with that combination to bring the total back to 16.

[bob p]

Oops. I missed that he was using a 16R speaker. Thought it was 8R. My bad.

[Zozobra]

Another option is to just build a straight up big dummy load with a line out and then run that to a small (or large!) SS power amp. This is basically what the bad cat unleashed is. The Fryette power station is pretty much the same but with a 6L6 power amp.
I recently bought a Two Notes captor reactive load box for direct recording. It also has a 20dB thru which is okay for taking the volume down on my amps with a decent MV but still too loud for amps with a shitty MV or NMV amps. My plan with those is to use a small class D amp to feed the dummy load line out into so I can monitor with my cab and then dick about with speaker impulses later.

[catalin gramada]

depends how much attenuation you ask. If you want to dime a hundred Marshall amp to bedroom level forget it. For more decent level of attenuation in clubs or studios practice found Airbrake works surprising well till 12-15db arround (that.s very little but can be usefull). You don-t need a rheostat for that. I found it useless as time tone goes to the dump.

[Mick Bailey]

Here's another schematic, this time from Guitar and Bass magazine. The treble bypass makes a big difference.

[Chuck H]

Part of the charm of the Airbrake is reasoned to be the higher than ideal load. I'm not going to analyze it now but I read it's as high as 32 ohms for the "8 ohm" load at some settings. I think the reason a higher than usual resistive load works is that an actual speaker presents a load that is only as rated at certain frequencies and is MUCH higher at all others. The Ultimate/Ho type attenuator is a re-amp device and is also said to sound very good. I used one with my amp at NAMM 2009 and I thought it sounded just fine. The Ultimate/Ho type is said to use a 30 ohm resistive load for all impedance requirements. I've considered trying a higher load for my own design.

The one diagram from G&B magazine looks like it straight up parallels the rated load WITH the rated load. Thereby halving the load AND making it largely resistive. So this would be the opposite affect of a higher resistive load for impedance averaging over the frequency range. The bypass caps work to reach this lowered impedance at any setting at the caps pass frequency. As far as what the amp "sees" this is opposite to what a speakers impedance is like at higher frequencies. I don't like the design electronically, but I can't comment on how it sounds.

[eschertron]

We haven't really discussed the application for this attenuator. A few dB for a gig/practice scenario, or bedroom level? Some units are designed to help the gigging musician find the sweet spot at (marginally) lower volumes, others load down to personal monitor levels, or quiet entirely and provide the line out. These different targets call for different sonic weaponry. So are you hunting for squirrels, or for elephants?

[bob p]

^ This. Application is key.

As much as we all like attenuators, they tend to get over used by most people. They're not a universal solution, so the answer to the question tends to change depending on your goals.

But attenuators are SO popular as the "magic bullet" that they tend to get over-recommended.

For example, go to one of the popular guitar sites and ask what you should do to make your 50W marshall half-stack more quiet for bedroom use and everyone will tell you to buy a HotPlate or some other attenuator. Wrong answer. The first thing they should tell you is that instead of using a 4x12 you need to be using a 1x12, because the increased cone area of the 4x12 couples better with air, and provides more acoustic watts than the 1x12 when fed the same signal. I guess there are a lot of people out there who give a knee-jerk response "get an attenuator" because they aren't up to speed on the physics of matching a cabinet to the impedance of air. If you want it LOUD then a 4x12 is a great recommendation, if you want it quiet then it isn't such a great idea. If you want it quiet, then opt for smaller cone area and lower efficiency in your speaker.

One factor that people have gotten hip to in recent years is that you can make a lower powered amp sound pretty good, tonally like a higher powered amp, with good design. There are lots of great sounding 15-20W amps out there, and for a small room something like an 18-watter or a November makes a lot more sense than a Marshall Major or even a Plexi.

Personally, I don't try to use attenuators to get a screaming 100W amp down to bedroom levels, though I do like them to help me to get a power amp to bark at a lower volume level. As others have said 6-12 dB of cut tends to be the sweet spot. IMO that's where attenuators really shine.

But what about bedroom use? IMO for bedroom level tone, people need to start thinking about stompboxes, not attenuators.

I don't think attenuators are all that great for bedroom levels, though I think the Airbrake is actually pretty good in that respect. But to be fair, there isn't any device that's going to allow you to make a Plexi scream while other people in the house are sleeping. And if you're after power tube distortion, it's pretty hard to get that tone in spades while taking the volume down to bedroom levels with an attenuator. If that's the goal there are better choices.

For bedroom levels my preferred weapon of choice is an amp that uses cascaded preamp gain and a cold bias power amp. With that sort of "Mesa-type" preamp distortion it's pretty easy to get decent tone at whisper quiet levels. With a good cascading preamp/MV setup I can make it scream like Santana at whisper quiet levels. Seriously. My favorite gear for playing a screaming guitar at ultra low volumes is my Mesa MK IV, just because it's got a very flexible 3-channel preamp with graphic EQ and a master volume.

Other options for bedroom volume, or maybe small home studio volume would include an isolation cabinet and re-miking (if you want the tone of speaker distortion) or speaker load emulator with re-amping (if you want to avoid the tone of speaker distortion).

I've used all of these techniques and for me the isolation cabinet and re-amping paradigms involve a lot of overhead that makes them a bit of a nuisance just for practicing. I'd put up with them for recording but for practicing at quiet levels I really like a 3-channel cascaded gain preamp going into an amp with a master volume, and a 1x12 or 1x15 cabinet. I tend to reserve attenuators for amps that are designed without a master that are going to be played LOUD, and get their barking power tube tone at high volume. In that scenario the best that you can hope for from an attenuator is to tame the SPL while you run the amp into the sweet spot, but it'll still be pretty doggone LOUD.

There are lots of good options, and depending on what you want to accomplish, there are different choices to make.

[catalin gramada]

Yup. Bob P covered all scenarios pretty much. If you ask for bedroom levels go cascading and post inverter volume control (that was invented for). If have a Marshall type the amp will not bark , but compression and overtones will be there, Slave into other amp you processed signal if you feel and mix acousticaly. Then you will have dry and wet modulation effects (like reverbs or delays) from two separate acoustical sources, like in real life, if you want "live" in you house