Simple Attenuators - Design And Testing

[Gene Ballzz]

Here is one of my M-Lite versions installed in my DSL20CR:

 

[JohnH]

@JohnH ,
Has it ever been determined whether or not the 8 ohm M2 is as friendly to a 4 ohm load as the 16 ohm version is to an 8 ohm load?
Just Curious?
Gene

Yes indeed, it should be much the same. In theory a tad more high treble but unlikely to be noticed or easily adjusted for at the amp.

So an 8 Ohm M2 with all the Outs, is good for 16, 2x16, 8, 2x8 and 4 Ohm cabs.

 

[JohnH]

Hi @Lancer X , dB numbers are ratios expressed on a log scsle. So anytime you halve the power, that's -3db. 1/4 power is -6db. 1/10th power is -10db. Our first stage is -7db so that's 1/5th power. etc

At 2W from the amp, there's really not much power. So I think any sensible mounting will be fine.

For your building into the amp chassis, which I haven't tried, I'd watch out for placement of parts particularly the coil. Maybe test before committing. The coil may produce a tendency to induce signals into any input wiring nearby, particularly on its axis. And mount it with a spacer to the chassis, using non-ferrous bolts or other means such a zip ties.

 

[Lancer X]

@Gene Ballzz , I love your attenuator peeking out of the bottom back vent hole. That's a handy match up.

Hi @Lancer X , dB numbers are ratios expressed on a log scsle. So anytime you halve the power, that's -3db. 1/4 power is -6db. 1/10th power is -10db. Our first stage is -7db so that's 1/5th power. etc

Ahh, gotcha. As a scientist, you'd think I would have understood that unit better... That's why this new hobby is fun - learning!

At 2W from the amp, there's really not much power. So I think any sensible mounting will be fine.

For your building into the amp chassis, which I haven't tried, I'd watch out for placement of parts particularly the coil. Maybe test before committing. The coil may produce a tendency to induce signals into any input wiring nearby, particularly on its axis. And mount it with a spacer to the chassis, using non-ferrous bolts of other means such a zip ties.

Sorry, I misspoke! I intend to mount the M2 in its own metal enclosure inside the amp cabinet - outside of and separate from the amp chassis.

I have a lot of free space on the transformer side of the cab (furthest from the input section). Hopefully the metal enclosure will help with shielding too. Will try to use a box small enough that I can reposition the mount if needed.

Just to make sure I understood you correctly, ideally I want to point the torroid axis of rotation away from the input section? In other words, align the axis of the coil in the same orientation as the speaker cone axis?

Thanks again!! Time to place a Mouser order!

 

[JohnH]

Yes sounds OK! What does @Gene Ballzz think on these placement issues?

 

[Lancer X]

Hi @Lancer X , welcome to our world!

Yes you can scale down the power rating of the resistors. For a nominally 50W amp, R1 is spec'ed at 100W rating, ie 2x the amp power. It takes about 30% of the power of the amp, which gives is a factor of x3 for a case-mounted resistor. You might chose to try the ceramic block resistors, for which Id give a bigger factor. say 5. 2W x 30% x 5 = 3W. So you could go 5W or 10W.

Sorry, @JohnH - seeing now that this info was in your page 1 master post. Thanks for your patience.

 

[Lancer X]

@Lancer X ,
Next> I strongly recommend that you build the full blown, full wattage version of the M2. Once you plug your "full sized" 5E3 into the M2, your Weber MiniMass will collect dust!

A Hammond 1590D will work well, but @JohnH prefers the slightly larger 1550G. These are the fairly thick die cast boxes, not the thin aluminum sheet metal units! I implore you to build it as a well ventilated tank and you will never have a problem!
Just Sayin'
Gene

You know what - you've convinced me.

A 1590D will fit in my Deluxe Micro cabinet, so I'm gonna just build the 50W M2, with bypass I think. (Out of curiosity, I priced out resistors for a 25W model, and it was only $8 less.)

If I build the Bassman Micro as a piggyback head as planned, I can stuff the M2 in the speaker cab.

 

[Barnsley Boy]

I'm rarely speechless, but .......................

 

[Barnsley Boy]

................................

 

[Barnsley Boy]

...... come on man, get a grip on yourself!

It's all up and running and working correctly, and I have to say it is awesome. I have knocked my JCM 900 down from the beast that it was, to a purring kitten (with large teeth and claws). Thank-you @JohnH , @Gene Ballzz and all of the various contributors along the way - this is something very, very special.

Yes indeed, it should be much the same. In theory a tad more high treble but unlikely to be noticed or easily adjusted for at the amp.

So an 8 Ohm M2 with all the Outs, is good for 16, 2x16, 8, 2x8 and 4 Ohm cabs.

Now this has also got my interest too. Effectively John, what you are saying is my 8ohm M2 is also useable with my Peavey Classic VT and VTX (both of which run a 4ohm speaker load? If that is the case, happy days - I can just replicate the build with probably a few cosmetic tweaks and a good dose of "lessons learnt" from my first one. One thing though, the VTX is 1 65 watt amp, is there enough redundancy in the power resistors to accommodate this? or would I need to build a 100 watt 8ohm variant?

 

[Barnsley Boy]

Completed build just posted on @Gene Ballzz 's Completed Attenuators thread if anyone is interested.

​

 

[JohnH]

Hi @Barnsley Boy , great news on getting that going!

Here's a maths puzzle, for anyone to try: If BB is running his 50W amp, at the max attenuation of -31.5db, plus the added -28db stage, how many Watts are reaching the speaker?

With all the different cab options that the 8Ohm M2 can do, just note that it has to always run from an 8Ohm tap on the amp. If that leaves out some amp/cab combinations that you need, a front end can be added to convert.

 

[BlueX]

Here's a maths puzzle, for anyone to try: If BB is running his 50W amp, at the max attenuation of -31.5db, plus the added -28db stage, how many Watts are reaching the speaker?

-3 dB is half the power (1/2), -6 dB one quarter (1/4 = 1/2 * 1/2), etc.
(-31,5) + (-28) = -59,5, or almost -60 dB. That equals -3 * 20, which would be (1/2)^20 = 9,5 E-7 = 0,00000095
50W * 9,5 E-7 = 4,8 E-5 W = 0,000048 W = 0,048 mW = 48 microW

 

[Barnsley Boy]

48 micro-watts, that's not very rock and roll at all!!!
Jimi would be spinning in his grave if he knew!

 

[JohnH]

Pretty close! The full logarithmic formula goes:

power ratio = P1/P2 = 10^(decibels/10)

or, decibels = 10.log(P1/P2)

In this case
P1/P2 = 10 ^(-59.5/10) = 0.00000112

power is therefore 50W x 0.00000112 W = 56 microW

I expect you're finding a setting to use quite a few dB's louder, but if you engage this full attenuation, can you still hear it?

 

[Lancer X]

@JohnH , @Gene Ballzz , and crew:

Here's a link to an interesting post about placement and orientation of a pair of air core induction coils versus its effect on measured inductance. Likewise, they discuss placement close to 15mm thick aluminum panels, which seems to have a major effect on inductance.

Now of course you guys have been building in ~2mm thick Al enclosures all along, and you're happy with the curves that your attenuator designs are generating, so this is largely academic. Thought it might be helpful for future designs though, since you're probably not getting the exact inductance value that you planned for the circuit.

Cheers!

 

[Lancer X]

@Gene Ballzz , I see that you're placing your attenuator flat in the bottom of your speaker cabinet. I assume this means you've never experienced any noise associated with the field generated by the inductance coil?

I want to place my M2 box in my amp cab. @JohnH was concerned about EMI from the inductor, and was asking your advice about placement.

Per my quick research, it seems that the strongest fields are generated in the direction of the axis coil (of course, since this is how electromagnets work). If you aren't suffering any noise from your coils aimed up towards your amp chassis, then I should be in good shape for pretty much any well-placed location.

My chassis doesn't have a slot in the bottom like yours (see below), so I may need to stand my box up on end. Was concerned about blasting my input section with noise.

 

[Gene Ballzz]

@Lancer X ,
Kudos on being so actively participatory as a new member! Great info on the coil proximity/orientation testing!

I've been using the 19 gauge from Madisound and @JohnH determined it would be fine for a 50 watt unit. All my units so far have been M-Lite and 16 ohm, requiring two coils. I believe Parts-Express has a fairly wide selection of Dayton, Jantzen and other brands, although none are bobbin wound, which I would prefer. I've since decided that cutting appropriate sized discs out of Lexan (or similar), wood or corian, with a hole saw makes for nice mounting.

As for location, I've moved my units around, before mounting in both my combos (5E3 & DSL20) and found no audibly discernible artifacts, anywhere. In the 5E3 Deluxe (similar cabinet to yours) I simply mounted it in the bottom (feet for stand offs, of course) and while I can't see the switches or labeling, I know what's what and I just reach down in to switch it! What is that amp, anyway? Cool color for the Tweed!

Just Attenuatin'
Gene

 

[JohnH]

@JohnH , @Gene Ballzz , and crew:

Here's a link to an interesting post about placement and orientation of a pair of air core induction coils versus its effect on measured inductance. Likewise, they discuss placement close to 15mm thick aluminum panels, which seems to have a major effect on inductance.

Now of course you guys have been building in ~2mm thick Al enclosures all along, and you're happy with the curves that your attenuator designs are generating, so this is largely academic. Thought it might be helpful for future designs though, since you're probably not getting the exact inductance value that you planned for the circuit.

Cheers!

Thanks for posting that. Yes it is interesting. I think I've seen something from that study before, though not that page.

It led me to try some tests of my own, using parts nearer to what we are using. I have a spare 0.4mH coil, and a lid from a Hammond box. In air it measures 0.40 mH using my meter. When it's placed down flat onto the lid, it read 0.40 or 0.39, like it's in between. That's as close as I can read it with the meter that I have. It shows that it's not a big effect though in this case. Lifting a few mm returned the full value. My coil is on a bobbin, so it has a spacer built in. Others are wound without a bobbin. I think a few mm spacer is worth having.

The Web page is using 3.3mH and 15mm, so as seen, there's a lot more effect then.

But what makes a huge difference is any steel bolt. My coil was about 25mm thick. A small 10mm bolt on-axis raised its value from 0.40mH to 0.44mH. I've tested bolts right through that can add 50% or more to coil inductance.

 

[Lancer X]

Thanks, guys! I ended up ordering a Dayton Audio coil from Parts Express. Will try to create a little spacer to place it centrally in the box.

Kinda makes sense that you aren’t getting any exterior EMI effects if the field is that interactive with the aluminum box.

@Lancer X ,
What is that amp, anyway? Cool color for the Tweed!

The pic is my Deluxe Micro - built in a Blackface Vibro Champ cab.

That tweed is actually Tolex! I wanted grey fabric tweed but couldn’t find it. The pattern is really indistinguishable from lacquered fabric from more than a foot or two away.

I wanted to make a crossover look between Blackface and Tweed cabinets. It’ll prob be done today - still fiddling with resistors to get the B+ voltages just right. I’ll post a glamour shot when it’s done.