Simple Attenuators - Design And Testing

[matttornado]

Thanks. That's what I kind of thought but I wanted to asked just to be sure.

Oh one more question for now if you don't mind?

I'm sorry if this question is a repeat. I'm building the 16 ohm "amp tap" version. The diagram states I can use either a single 16 ohm cab or two 16 ohm cabs that equal 8 ohms or a single 8 ohm cab, correct? If the amp is set at 16 ohms and I'm running an 8 ohm load, isn't that a mis - match in the wrong direction? I understand an amp can be set on 8 ohms going into a 16 ohm cab but not the other way around.

Does the circuit compensate for that mis-match?

 

[JohnH]

Thanks. That's what I kind of thought but I wanted to asked just to be sure.

Oh one more question for now if you don't mind?

I'm sorry if this question is a repeat. I'm building the 16 ohm "amp tap" version. The diagram states I can use either a single 16 ohm cab or two 16 ohm cabs that equal 8 ohms or a single 8 ohm cab, correct? If the amp is set at 16 ohms and I'm running an 8 ohm load, isn't that a mis - match in the wrong direction? I understand an amp can be set on 8 ohms going into a 16 ohm cab but not the other way around.

Does the circuit compensate for that mis-match?

No problem, thanks for asking. I appreciate that you need to be sure.

Its all OK. The short answer is that most of the power is being absorbed in the attenuator and only a small part gets to the speaker. So if that speaker has a different Ohms, only a small part of that difference reaches back to the amp.

The long answer is, every possible setting has been calculated, taking into account the reactive and resistive properties of the speaker and the attenuator. Here are the results for each setting, with impedance calculated at a mid frequency of 440hz. The upper table is with a 16 Ohm speaker and the lower is with 8. It stays close enough to 16 Ohms. The yellow column is for an 8 ohm cab into a 16 Ohm attenuator and 16 amp tap. It varies from about 15 to 17 ohms. This table assumes step 1 is -3.5db, per the full circuit. When with a 16 ohm cab, this first setting is 19 Ohms to the amp, still close enough to 16.



The other things you can see here are the steps of attenuation and the consistency of the response as you work the switches. With the 8 ohm cab, there is a db or 2 more high treble and bass, which you may or may not notice.

 

[matttornado]

Wow! Thank you so much, JohnH! I appreciate you taking the time to answer and explain it.
Should be starting the build next week sometime. Almost ready.

 

[Gene Ballzz]

Red is the signal seen by a speaker directly connected to an amp
The upper black and green lines are the signal at the amp feeding into the attenuator
The lower blue and black lines are the signal after the attenuator, using attenuation values of -7db and -14db.

@JohnH

I am confused by the above quoted portion of your post #529? Of particular confusion is the phrase: "The upper black and green lines are the signal at the amp feeding into the attenuator" While I understand the general "jist" of it, I don't understand the difference of what the green line depicts, as opposed to the upper black line?

Please Illuminate?
Gene

 

[JohnH]

Hi @Gene Ballzz
All those plots come from a Spice simulation, which is a modelling software. Its very powerful and user friendly with regard to what it can calculate, but the version I use has a few constraints on how it can plot, hence a bit of ambiguity creeping in! With these circuits, once I've figured out a general arrangement, I like to write my own spreadsheet from first principles, so then I get total control.

Anyway, those lines: The plot overall is representing what you would expect if you fed a sweep frequency through the amp, and put an oscilloscope or signal trace on the speaker, or on the output of the amp. The signal at the speaker is what the speaker reacts to and what we hear. The signal at the amp output shows what the amp is seeing and what it reacts to. Plots are provided at full volume and at two attenuated levels.

The red trace for full volume, is obviously what the amp outputs and this equals what the speaker sees, so one trace, and it represents the benchmark for comparison.

The lower two traces are the speaker signal at two attenuated levels. You can see that the volume is lower, but the general shape is close to that of the red trace.

Now the actual answer!: The two upper black/green traces are the signal coming out of the amp at the two attenuated levels that are included in these particular runs. So it is high volume, but not necessarily the same shape as that of the red trace. The shape of these curves shows what the amp feels, and the ideal is that they are the same shape as the red.

So, with the all resistor version, although we get basically the right response at the speaker, avoiding the wet-blanket effect of most resistive designs, the amp is only seeing a mostly flat response as it interacts with the attenuator. The green line is with the lower attenuation, and you can see a bit of the rise and bass bump coming through but not much.

Then go to design M and the new M2 which is similar, you can see the upper green and black rising with high treble to match red. This pulls more out of the amp at the higher frequencies, making the amp react more, particularly if you play harder where the power stage then compresses more, which dynamically changes the response until the note decays. I think that is the main thing that our design does, and the benefit of a reactive system (thanks for being the first to really notice it earlier this year!). This effect is working well to match the upper curves from around 150-200 hz up.

The with the speculative M3 design, the same is added to the bass response. I think this is much less important to tone, and much more expensive to capture! I'm still thinking about how to confirm this.

 

[Gene Ballzz]

@JohnH
Thanks for that clarification, always nice to be speakin' the same language, so to speak!

With that said, I think the graphs of the response show the most "pleasing" curve with the "M" version. Looking at it and thinking about it, it acts almost like a "loudness" switch on a stero. It allows for a slight accentuation of highs and lows at lower volumes. I see this as a beneficial byproduct of that particular design iteration!

I've been using mine, in anticipation of subsequent builds, I've been trying to decide about what (if anything) to change or do differently and decided no change was needed except for changing one of my two -7.0db stages to a -14.0db. I'm simply amazed at how natural and consistent all my amps sound through this attenuator at a widely varied range of volume levels!

I'm just now working on an updated, lower profile layout for at least one of my next builds. By the time I'm done, every one of my amps will have one of these (or similar, yet trimmed down version) installed in it. I may even install into a couple speaker cabinets!

Thanks Again John!
Gene

 

[JohnH]

Hi @Gene Ballzz ,

I use my -14db stage most of the time, to get my 40/50W amps down to home volume.

I wonder, if you are looking to build a refined, slim-like build, you'd be interested in trying the M2 design? It's the same performance as M, but with just one coil, so cheaper and more compact. If so, we could work on fine-tuning the values for your use, using components from your preferred sources. Nothing wrong with the basic M though, if you prefer.

 

[Gene Ballzz]

Hi @JohnH ,

Yeah, with my DSL20 I toggle between -10.5db & -14db and with the Tweed Deluxe between -7db & -10db.

I guess I'd just need to know the values (wattage and ohms) of R1, R2A, R2B & L1, for a 16ohm build.

Thank You Sir!
Gene

 

[JohnH]

Hi @JohnH ,

Yeah, with my DSL20 I toggle between -10.5db & -14db and with the Tweed Deluxe between -7db & -10db.

I guess I'd just need to know the values (wattage and ohms) of R1, R2A, R2B & L1, for a 16ohm build.

Thank You Sir!
Gene

Hi Gene,

I reckon, for a 16 Ohm build, Design M2 and up to a nominally 50W amp:

R1: 30 Ohm, 100W
R2A: 47 Ohm, 50W
R2B: 39 Ohm, 25W
L1 1.8mH (eg Madisound 19AWG, or 18AWG if from elsewhere)

They all allow for a factor of x2 on power plus and extra x1.5 to allow for overdriving

R1 is as before for Design M. 27 Ohm is also an alternative for it (loses about 0.5db)

All other values as Design M, and can be 25W if you are skipping the '-3.5 db on its own' option.

(Note to others: if an 8 Ohm build, Its looking like
R1 = 15
R2A = 22
R2B = 18
L1 = 0.9mH)

 

[matttornado]

Finally got some parts in and some work done!

Working on the enclosure now. This will drop right in with four bolts through the bottom.
Inductors will be mounted on the side.

R1 is a 250 watt 30 ohm! Hopefully that'll handle a cranked 100 watt metal panel Super Lead?

I have never seen a resistor that big in any attenuator LOL

More pics to come. Hopefully I'll be doing some testing this weekend.

 

[JohnH]

Nice! That sure is one BIg Mutha of a resistor!

The trick will be getting the heat out of it, so lots of ventilation needed. Is there thermal grease between resistors and base?

It'll be very interesting to see how hot you can get it with that Plexi. Definately there's plenty of electrical power rating in those parts. And they can work at high temperatures too, more than you can touch. It'll take quite a long time if cranking to approach it's max temperature. On mine, the main resistors are direct to the thick aluminium case, and it takes about 30 minutes to get to max, which isn't very hot with my VM combo.

A guide to temperatures, if you can touch it for 3 sec, it's not too hot. With these simple conservatively -specced components, there's nothing that can suddenly blow.

 

[matttornado]

Yes I use thermal grease for each resistor. Here at my work, we use the 130 deg. F (54.4 Deg.C) as a max temp. for being able to touch something. I thinks it is an industry standard?

Anyway, Yeah I can't wait to see how hot that big old 30 ohm gets. Hopefully I can finish wiring it up today or tonight and start taking some preliminary measurements before firing it up. Then, I'll have to finish drilling some holes in the enclosure.

I will be honest though- I will be very nervous trying it for the first time.

 

[matttornado]

When I add a line out, does the ground have to be isolated like the rest of the circuit (plastic jacks) or can I use chassis ground?

The volume pot for the line out is metal and will be chassis grounded. I was thinking of using a standard metal jack for the line out too.

 

[JohnH]

That's looking really good.

Down here in Australia, we call 54 degrees C a 'warm summer day'.

Im not sure what standards apply in industry, Im not in that field myself. But, Id say something that is inside a box would not need to apply such a consumer standard for the safety of touching it. There are plenty of much hotter things inside many appliances. I got an IR thermometer for some projects I run with engineering students, and its a great tool and not too expensive. Maybe you have access to one?

On the line out...hmmm...not sure. So far the chassis is not connected at all, but if its connected to your output ground, then it could be connected also to the amp tranny depending on how you wire the line out. A couple of other guys have used a line-out with this circuit, perhaps we could get advice as to how it went.

You'll have a pot, plus I assume a pad resistor in line with it? I think I would do a pad resistor at both ends of the pot, so there is no direct connection from pot, through the circuit, to the amp. I reckon get the attenuator going first, then experiment with caution.

As for the first test, its ok to be nervous. but I feel reassured once ive checked the wiring (again), if I can measure a reasonable resistance, with speaker plugged in the amp input. That way I know the amp cant get hurt.

 

[tmingle]

I used metal jacks & an aluminum case for mine. I have had no issues.

 

[dudu]

@JohnH, many thanks for putting this together! I have a couple of questions:

1. For a 100W amp using the 16 Ohm tap, I presume I need to double the power rating of each component yet again, is that correct?
2. Is the impedance curve (seen by the amp) in the graph only modelled in Spice or also measured, e.g. with the Dayton Audio DATS?

 

[matttornado]

I used metal jacks & an aluminum case for mine. I have had no issues.

Thanks tmingle! Is your attenuator circuit like the one mentioned in this thread w/ isolated ground with plastic jacks?
Did you just use a metal jack for the line out?

 

[matttornado]

I finished wiring it up and took some preliminary resistance measurements with a 16 ohm resistor load prior to firing it up.
Here are my measurements:

I think I thought of all of the possible combinations?

Next I gave it a test run. IT SOUNDS AWESOME!!!! Even with the first stage dropping the power down by ~80%, it is still very loud, probably too loud for a gig. It must be more than 20 watts. I think my 100 watt amp is putting over 150 watts into the attenuator.
After playing for about an hour with maximum attenuation, ALL of the resistors got very hot meaning that I could touch them but only for a second.

I still need to add the line out and waiting to confirm how it should be grounded. Also, I'm thinking about adding a fan using the Marshall Powerbreak schematic. The fan is tapped right off of the input and rectified. Looks pretty simple.

Not sure if I need to do that or not yet. I need to figure out what too hot is and if it is needed?
I'd like to add some ventilation holes to the case as well but I don't feel like taking it apart now that it works so well! LOL

 

[matttornado]

Here is the Line out I will be using. I got this from the MetroAmp forum many years ago but never tried it.

 

[JohnH]

Hi @matttornado , that's great news and an awesome looking build! The resistance readings are totally looking right on the money given normal tolerances. I'm really glad that its working so well and I appreciate how carefully you have built it and I thank you for trusting the design.

On the temperatures, you are cranking a 100W Plexi and I reckon that attenuator is just cruising nicely. If you can touch the resistors for a second, then it would mean they are considerably less than 100C, eg, as an example of 100C, you cant touch a metal pan of boiling water without pain?

When it gets a lid, vents are needed and air in through the base and out through the top is best. but I also think that self-powered fan idea could be a very good one to explore too, for a 100W build. Need to experiment to set it up right. I think it could be set so it only starts turning if the power gets up above say 30-50 W.

The line-out circuit looks right, and you might like to try it either at the input or the output. At the output, obviously its then on an attenuated signal but it'll pick up some more of the characteristics of your speaker, particularly the low bass resonance. But, we also know from @tmingle 's build that it works at the input too. At the input, you then have the option of not using a speaker at all, just set it all to max attenuation and use it as a load box. At -31.5db, the amp won't see any difference.