Simple Attenuators - Design And Testing

[chocol8]

So, I got to drinking...I mean thinking about building a couple of these and a thought crossed my mind. Why not replace sections 2-4 with an L-Pad?

An L-Pad would show stage 1 a steady 4, 8, or 16 ohm load while giving you a continuously variable attenuation from -7 db to max, and a much simplified BOM and build with 3 switches and 6 resistors replaced by the single L-Pad.

Parts Express has various sized 8 and 16 ohm L-Pads, plus 8 ohm stereo L-Pads that could be wired in parallel for a 4 ohm load. Two of those in parallel either mechanically connected or manually synced could be used for 2 ohms if you are adventurous.

What am I missing?

 

[Gene Ballzz]

So, I got to drinking...I mean thinking about building a couple of these and a thought crossed my mind. Why not replace sections 2-4 with an L-Pad?

An L-Pad would show stage 1 a steady 4, 8, or 16 ohm load while giving you a continuously variable attenuation from -7 db to max, and a much simplified BOM and build with 3 switches and 6 resistors replaced by the single L-Pad.

Parts Express has various sized 8 and 16 ohm L-Pads, plus 8 ohm stereo L-Pads that could be wired in parallel for a 4 ohm load. Two of those in parallel either mechanically connected or manually synced could be used for 2 ohms if you are adventurous.

What am I missing?

The balance/ratio of series and parallel resistance in each stage, as well as overall is kinda crucial for tone and dynamics/feel retention. This is the main basis of this design, in addition to the reactive first stage! An L-Pad may accomplish the goal of "continuously variable" volume reduction, but…….? Should you read through the whole thread, you'll see that a few have tried L-Pads and a couple other methods of making the attenuation "variable" with mixed and not necessarily stellar results! Until you actually build and/or use one, you won't truly realize how small the -3.5db steps actually are.
Simply Sayin'
Gene

 

[chocol8]

Won't the L-Pad keep the series and parallel resistance ratios intact? I thought that was what they were designed to do. I guess I will have to look into it more.

 

[What?]

Until you actually build and/or use one, you won't truly realize how small the -3.5db steps actually are

Supposedly, a change of 3db up or down is just perceptible to the average person. I think that is different if you are focused in on sound, such as when mixing audio. I can reliably hear 1db volume changes when mixing audio. But 1db is just barely perceptible while being really focused in on balancing track levels. I can't reliably hear less than that. Some people can reliably hear 0.5db changes when mixing audio. Any way, point is, 3db of attenuation is a small amount, and it is just noticeable. And 3db is also the change in output when halving the power of an amp, i.e., 100 watt to 50 watt, 50 watt to 25 watt, etc.

 

[chocol8]

Won't the L-Pad keep the series and parallel resistance ratios intact? I thought that was what they were designed to do. I guess I will have to look into it more.

To answer my own question, I see a comment from John that the issue is the output impedance to the speakers is not held steady by the L-Pad. So, the amp would react the same and the line out would be OK, but speaker damping would be off.

 

[JohnH]

Hi @chocol8 yes thats one one the key things that make our design work better. Consistent input and output imdedance to show the speaker consistent damping and the amp a consistent load. Lpads only control what the speaker sees. Hence attenuators based on Lpads tend yo get dull at high attenuation.

 

[chocol8]

Supposedly, a change of 3db up or down is just perceptible to the average person. I think that is different if you are focused in on sound, such as when mixing audio. I can reliably hear 1db volume changes when mixing audio. But 1db is just barely perceptible while being really focused in on balancing track levels. I can't reliably hear less than that. Some people can reliably hear 0.5db changes when mixing audio. Any way, point is, 3db of attenuation is a small amount, and it is just noticeable. And 3db is also the change in output when halving the power of an amp, i.e., 100 watt to 50 watt, 50 watt to 25 watt, etc.

I have conducted and participated in a number of speaker and audio tests. In an A/B test, I have yet to run into a person with normal hearing who could not perceive a change of 0.5 db and many can perceive .3 db when the entire signal changes. They don't always recognize it is louder, but they hear a difference, and often say things like "fuller". That is a sample of size of hundreds of people, some audiophiles, some just random people we plucked out of a mall.

In a mix, where you are making EQ changes, or just changing one instrument, those numbers go up, but usually not to 3 db in the middle of the hearing spectrum. I have seen that number quoted a bunch of times online, but my experience matches yours, most people can hear a change of 1 db or less if they are listening for it. 3-3.5 db is an easily noticeable solo-boost.

I don't need a smaller increment, but I thought the L-Pad would simplify construction and use, but I guess not.

 

[Gene Ballzz]

I have conducted and participated in a number of speaker and audio tests. In an A/B test, I have yet to run into a person with normal hearing who could not perceive a change of 0.5 db and many can perceive .3 db when the entire signal changes. They don't always recognize it is louder, but they hear a difference, and often say things like "fuller". That is a sample of size of hundreds of people, some audiophiles, some just random people we plucked out of a mall.

In a mix, where you are making EQ changes, or just changing one instrument, those numbers go up, but usually not to 3 db in the middle of the hearing spectrum. I have seen that number quoted a bunch of times online, but my experience matches yours, most people can hear a change of 1 db or less if they are listening for it. 3-3.5 db is an easily noticeable solo-boost.

I don't need a smaller increment, but I thought the L-Pad would simplify construction and use, but I guess not.

Once you get used to the switches, they become second nature. Typically you get to a level where all you need is 3.5db more or 3.5db less! Any finer tuning can be esaily accomplished at your picking hand or the guitar's/amp's volume knob. When given an in between situation, I tend to opt for that "tiny bit" louder and enjoy the extra dynamics afforded by lightening up my picking and strumming attack! These units really liberate your playing techniques, letting you use amp levels that you only used to dream of! Just imagine a 1987 or other 50 watt amp dialed in to it's sweetest tonal spot, and then brought down to a volume that won't get you thrown out of the venue!
Yeah Baby!
Gene

 

[chocol8]

Well, I ordered parts for a 2 ohm and a 8 ohm M2/4. I am building them with the -3.5/7db Stage 1 and Stage 2 from the M4 design, but I am not including bass resonance, line out, or the output 3 and 4.

Edit: No foot switch and no bypass switch either.

 

[JohnH]

The way I think about our steps, is that each one scales down the amp power, to a lower version of itself with no tonal chsnge. It's kind of like on a Marshsll studio, you have a 20w and a 6w option, a difference of 6 dB.

So with our 3.5dB steps, a 50W amp becomes any of 50, 22, 10, 4.4, 2, 0.9, 0.4, 0.18, 0.08, or 0.035 W.

And apart from the windows aren't rattling, the guitar isn't feedbacking, the speaker isnt distorting and your pants aren't flapping, they all sound the same!

 

[chocol8]

I am starting to map the layout and I have a question about mounting inductors. For the 2 ohm build, I am planning on using 2x 0.44 mh coils in parallel to reduce the current through each one. Since they will have the same signal going through them, would it be OK to stack them? Can the centers be lined up but with a gap, or should I mount them at a 90 degree angle and as far apart as practical?

 

[JohnH]

Hi @chocol8

I would think that stacking two inductors would cause a mutual inductance between them that will increase their values. So they would need to be separated. I think the theory would say that 90 degrees between axes would be ideal, but parallel and separated should be ok, but I don't know what the gap would need to be. Id guess a couple of cm's would do.

But really, I cant see any advantage in that approach rather than just buying a single inductor wound with a larger wire. It'll still not be huge because its a low value for the 2 Ohms version, What can you get with 16 gage or thicker and 0.22mH?

Other things with the coil is, no ferrous bolts on the coil axis and put a non-metal spacer between coil and case

 

[chocol8]

With in stock and quick shipping as a priority I found 0.22 in 18 gauge wire or much larger and more expensive 16 gauge copper foil.

The 18 gauge wire options I found are both rated for 300w RMS so are probably safe, and I may go that route for plan B. I should have plenty of room to put the coils offset by 90 degrees and 2" apart so I will probably just do that for a first run.

 

[JohnH]

You could also use 0.25mH which is actually close enough, but can also unwind it a little down to 0.22. I do that with an inductance meter, but it can also be worked out in terms of what length of wire to remove (I can help). But I also believe that suppliers can do this for you. eg Madisound:

https://www.madisoundspeakerstore.com/sidewinder-16-awg/sidewinder-0.25-mh-air-core-inductor-16-awg/

 

[chocol8]

Thanks, I will save that option for a plan B.

 

[parsnip]

Hey @JohnH and co. I just got my 4 ohm M2 through testing. HOLY COW. This thing is amazing. Running my silvertone 1482 dimed, it sounds great, and no longer risks neighbor relations. Attenuating 15 watts, it never got warm. I tried it with my 60 watt blues deville 410, and it tamed it wonderfully. Dimed, it definitely got warm verging on hot. (But I don't need to crank that amp and then play it at bedroom volumes; Unlike the silvertone, its sweet spot isn't 11. I was just curious.)

I am excited to play out and about with folks and to see whether and to what extent I'll want to be able to fine tune the attenuating (related to discussion above). My hunch is it'll be just fine as is.

Once I get the enclosure enclosed and labeled, I'll post on the "completed" thread, with pics and more info on the build. In the meantime, I just wanted to say thanks for the awesome invention, the generosity, and the great, and patient advice. Over the course of figuring this out I realized (with some embarrassment) how many times you've had to answer the same questions! So thanks! I promise to pay it forward.

 

[JohnH]

hi @parsnip that's great news and congrats getting it together.
We'll look forward to the pics and further reports in due course.

On 'hottness' , a practical and simple test is that if you can touch the hottest place (usually R1 or nearby) for a few seconds, then it's likely not more than about 60C, which is all fine.

 

[SmutjeHannes]

Well, I ordered parts for a 2 ohm and a 8 ohm M2/4. I am building them with the -3.5/7db Stage 1 and Stage 2 from the M4 design, but I am not including bass resonance, line out, or the output 3 and 4.

Edit: No foot switch and no bypass switch either.

Ha! Someone who's also building a 2 Ohm version. Would you be keen in sharing your plans and parts? =)

 

[chocol8]

Ha! Someone who's also building a 2 Ohm version. Would you be keen in sharing your plans and parts? =)

Sure, I will post what I am using this weekend when it all (hopefully) arrives. However, I would definitely suggest you order the Madisound inductor John linked above instead of following my dual .44 inductor route.

I got most of the resistors from Digikey so they are a bit more expensive but faster shipping than the Chinese ones. For R1a and R1b I ended up going with 4x 3.9 ohm 25 w resistors instead of 2x 50 watt equivalents. Most of the rest follow the 2 ohm M2 values John posted. Again, I will post all values here for JohnH, Gene and others to comment on and for you and others to copy or not copy. I did get 50 watt resistors for R2 a through c, but I later realized that these can all be 25 watt for my build. I will explain why later so you can make a more informed choice for your build.

I will also post my layout and cooling. I am going to be hitting it with over 35 watts most of the time so I am not going to even try to rely on passive convective cooling with lots of case holes in the top and bottom like many I see posted. My plan is to use a front to back cooling flow powered by small exhaust blowers and one or two small intake openings. I have a few to try out to get the right blend of airflow and noise level. I think it will run significantly cooler with no spilled drink attracting holes on top this way. You also won't have to worry about some idiot (often the bass player or someone's future ex-wife) setting something on top and blocking airflow.

 

[JohnH]

Hi @chocol8

Just trying to remember - for your 2 Ohm build, and since your were thinking of including the -3.5/-7 dB front end, did we have resistor values for the front end? The post #1 covers 2 Ohm builds but only for a -7dB front stage. But in principle - values scale down in proportion but sometimes that then leaves a value which needs to be shifted to hit an available value.