Simple Attenuators - Design And Testing

[Dogs of Doom]

Still can't post a link. It's on Amazon.

Hammond 1590F Aluminum Diecast Case​

I posted the link earlier...

https://www.amazon.com/bud-industries-cn-5708-aluminum-enclosure/dp/B005T78D8K

but, you should be able to post the link now.

that seems to be a different one though...

https://www.amazon.com/Hammond-1590F-Aluminum-Diecast-Case/dp/B0002BBRCW

 

[JohnH]

Hi @Dretot

100W attenuator, 16 Ohm, with -3.5dB and -7dB attenuation

Ok, cautiously, I think this will do it. I think it needs another check through though:



Its based on all 50W resistors, in series pairs. So for this 16 Ohm version, all values are pairs per the 8 Ohm ones.

Ive shown each resistor as a pair meeting at the middle. This should be a solder joint, with a bit of wire through. Ive shown them meeting tip to tip, but you might overlap the tips and crimp the ends a little left and right so the lug holes line up but the resistor bodies are still in line

Its all reasonably to scale based on real parts eg Arcol. I allowed 60mm for the fan and also the coil (eg 1.8mH 16 awg from Madisound.com). There shoud be enough case depth for the resistors on the top (= base of the case) and fan and coil below, fixed down on the lid which becomes the base.

Feet are needed below to raise the base

Plenty of holes needed, obviously drill out for the fan, and a few more say 10mm in the base, and lots in the top, and/or, at the side walls near the top.

See @Gene Ballzz 's builds for inspiration!

cheers
John

 

[Dretot]

John... Wow! That is absolutely amazing! You went above and beyond, You laid everything out so clearly and with nice instructions. Thank you! now I'm really excited.

I'm going to order all my parts and get started on this. I'll post pics as I progress.

Again, I can't thank you enough, really special what you did here.

Andreas

 

[AtomicRob]

Basicaly, to do it right at 100W, we need 5 x 100W resistors per the diagram. Its actually more compact to use 2x 50W for each, due to smaller tabs.

I was thinking about this on my 100W build and my conclusion was that while 2x50w is more compact, and electrically equivalent, the 2x50W resistors are probably less efficient at dissipating heat than a 100W resistor and will run hotter at the same power level. The 100W part has more than 3x the "footprint" of the 50W part - 3100mm^2 vs 920mm^2. My case isn't ventilated so it's mostly cooled by conduction. "In theory" I would expect the 2x50W parts to be quite a bit hotter (1.5x?) but I didn't have the parts or patience to actually test it out, I only bought the 100W parts...

 

[JohnH]

Hi @AtomicRob My estimate would be that many smaller parts are often actually easier to cool, in the same way that conversely, larger animals are more likely to keep warm and suffer less from being too cold than small ones.

But taking say, Arcol resistor products, their design of high and low power ranges are presumably based on similar criteria.

For our units, with a good distribution of parts within the chassis I think it's mostly about the surface area of the case and its ventilation , than the design of separate parts.

 

[Dretot]

John, a couple of questions regarding thermal paste. First of all, How much do I need for my 10 resistors? Is 10 grams enough?

Also, how thick should I apply it and then how tight do I bolt down the resistors? Just enough for a little squeeze out or tight to the box?

Thanks,
andreas

 

[JohnH]

hi @Dretot one small tube is enough. Just the minimum amount so when squeezed down it spreads out and covers the area. It's easy yo practice the amount on a scrap surface. Try, then pull off. Ideal is the least amount for full coverage that you can check when you pull off your test. Then just do up the bolts normally firmly.

 

[Dretot]

John, another question for you. In your instructions you wrote, "you might overlap the tips and crimp the ends a little left and right so the lug holes line up but the resistor bodies are still in line". (see my drawing, resistors on right)

Can I just offset the whole row of resistors by a couple mm and connect them like this? (resistors on left). The resistor bodies won't be perfectly in line, but it would be easier this way.

I apologize if this is a silly question. I want to get this right.

thanks,
andreas

 

[JohnH]

hi @Dretot Sure that's fine, whatever is easier for your build. Just a bit trickier for marking out the drilling for the holes maybe?

The main thing is If you can get the holes in the solder lugs to line up, you can put a scrap of wire through and solder the joint, then trim the wire. I think that's more positive than just a solder bridge.

I had a couple more looks to check the drawing, and I think it's ok. The jacks I drew look more like the plastic Cliff jacks, but I'm sure you can interpret them to Switchcraft if that's what you are using.

Also, for graphic reasons, I drew the switch lugs facing up instead of to the side, so they can be seen on the diagram. Whether when it's mounted, the -3.5dB and -7dB are up or down depends on your switch. Most toggles when you flick down, the middle pole lugs connect to the upper lugs. But I have some that are opposite. If this matters, we can work it out.

An opportunity: If you build this and then find you'd like the option of more attenuation, there's probably room to add it. But you can also build a separate box with just resistor and switch stages and put that after this one, linked by a short speaker-cable patch.

 

[AtomicRob]

The main thing is If you can get the holes in the solder lugs to line up, you can put a scrap of wire through and solder the joint, then trim the wire. I think that's more positive than just a solder bridge.

This is important advice - you want to use wire between them, even if just a short bit, because these resistors will move as they heat up and cool down, and experience vibration, and get knocked around like all gear does sometimes. A solid soldered connection between two fixed components that experience temperature cycling, vibration, and shock is sort of a test machine for solder joint failure analysis!

 

[JohnH]

Would it be even better to make them more offset and have a short wire length between lugs? A neat way, for the two resistors of a pair could be, noting that they have a lug on each side: For the 4 holes that mount two resistors, put all the holes in a straight line. So easy to mark out and drill. Then inside, the resistor bodies are therefore offset at an angle with a gap between lugs, to be connected by a short wire .

 

[AtomicRob]

Would it be even better to make them more offset and have a short wire length between lugs?

Yep, probably easier to solder too. I felt like a real beginner trying to solder those Arcol lugs at first! I'm using a 90-watt Edsyn soldering station, and finally I gave up and put on the largest tip I have - a 1/4" chisel tip that looks like it's for stained glass or something. That did the trick.

 

[Dretot]

Hi John, Would it be beneficial to spread the resistors out for heat dissipation? I would lose space for future expansion, but I could mount resistors on the side walls if I ever decide to add another stage.

 

[JohnH]

Yes the wider spacing would be good for heat, so long as you have enough space and access for the jacks and switch on the side wall.

 

[Gregmiata]

Does anyone on here build these for others? I'd love one but not sure I have the capability to build one myself.

 

[Gene Ballzz]

Does anyone on here build these for others? I'd love one but not sure I have the capability to build one myself.

Yes, we "know a guy!" I sent you a PM/started a Conversation!

And to the forum!

Cheers,
Gene

 

[StratoMarshall]

Many thanks to JohnH for this sweet design and to Gene Ballzz for "knowing a guy"!
I was put in touch with the "guy" and had him build me the JohnH M2.
This thing is amazing! Does exactly what everyone says and had me playing late into the night on a work night too!
The "guy" did an AMAZING job on the build and quality of work, great attention to detail.
I'm thrilled with the ability to attenuate at any of the settings with NO change in tonality and it allows me to run the SV20H into its sweet spot easily at lower room volume!
Anyone needing an attenuator absolutely could not do better without spending over a thousand bucks (I've even heard a couple of those that did not do as well)!
My compliments to "the guy"!

 

[matttornado]

Hello Again. I watch this thread from time time after building mine a few years ago when the thread started. I have a Line Out on mine and that works great and was easy to implement. But what If I want a line out Return like an effects loop? How would we bring the LIne Out back in?

Being able to add an effects loop would be the icing on the cake for me. As of now, I use the line out to feed effects then to a second amp.

 

[JohnH]

hi @matttornado , thanks for staying with us!

Sadly Line 'IN' or Return is not something that can really be included in a passive attenuator. All the commercial units that do it are based on active re-amping, where you can deal with line level signals then feed that to an amp. The signals that would be coming in from the effects are far to high impedance and small signal to mix with the low impedance speaker-level signals in a passive attenuator.

 

[Gene Ballzz]

Many thanks to JohnH for this sweet design and to Gene Ballzz for "knowing a guy"!
I was put in touch with the "guy" and had him build me the JohnH M2.
This thing is amazing! Does exactly what everyone says and had me playing late into the night on a work night too!
The "guy" did an AMAZING job on the build and quality of work, great attention to detail.
I'm thrilled with the ability to attenuate at any of the settings with NO change in tonality and it allows me to run the SV20H into its sweet spot easily at lower room volume!
Anyone needing an attenuator absolutely could not do better without spending over a thousand bucks (I've even heard a couple of those that did not do as well)!
My compliments to "the guy"!

So nice to hear of another happy @JohnH M2 user! Truly one of the most useful and "liberating" pieces of gear that any tube amp user could own!

Thanks!
Gene