Simple Attenuators - Design And Testing

[jfromel]

Update on the SMD version. The first prototypes are solid for a 30 watt amp. The derating went above 50% with anything more. Photo at 30w for 60 minutes. It was running fine but pretty warm.

The goal with V2 is to get it to handle 60 watts.

Added a fan that at full speed is 5000rpm at 15.9db, moved the hot resistors in line with the fan, sinks top and bottom and radiator style sink to the chassis. Moved jacks to the bottom of the board for better air flow, added mounting for the coil and finally a light show. Boards and parts should be here next week.

 

[stickyfinger]

Update on the SMD version. The first prototypes are solid for a 30 watt amp. The derating went above 50% with anything more. Photo at 30w for 60 minutes. It was running fine but pretty warm. View attachment 150247View attachment 150248View attachment 150249

The goal with V2 is to get it to handle 60 watts.

Added a fan that at full speed is 5000rpm at 15.9db, moved the hot resistors in line with the fan, sinks top and bottom and radiator style sink to the chassis. Moved jacks to the bottom of the board for better air flow, added mounting for the coil and finally a light show. Boards and parts should be here next week.View attachment 150250View attachment 150250

I'm interested in what other commercial electronics would use this type PCB resistor heat management? I had never heard of it until you mentioned it. Just has me thinking.

 

[jfromel]

I'm interested in what other commercial electronics would use this type PCB resistor heat management? I had never heard of it until you mentioned it. Just has me thinking.

Apple uses the chassis as a sink on many of their products (often with a heat pipe), Dell on laptops. Not sure who else. I did a lot of research on best practices and didn't find much. The manufactures don't give tips on mounting in their data sheets so there has been a lot of things I did on this design I have never seen before because I was making a best guess on how these resistors would work. I came up with this after reviewing with some other engineers in my son's robot club. If it keeps cool with the stick on sinks I will be really happy. It won't need as many sinks as are in the 3d model because only the first stage gets warm. Everything downstream runs cool.

 

[dtier]

hi @dtier

Thanks that's a good question.

On most other passive attenuators (actually all that I know of), the tone shaping controls are mostly targeted at making a work-around to deal with inherent tone changes at different levels, on account of the way that they work. The classic common one is the tendency of some to get dull at lower volume, and so a treble bleed cap is offered. But this messes with impedance that the amp sees, and although it is useful, the sound changes.

My preference has been to target the designs to keep absolute control of tone at all levels and to optimise each component to keep this very close to within less than a dB so far as is possible. Then with a neutral and transparent attenuator, other controls on the amp or EQ can work as needed, and as they do at higher volume.

But I can understand how options are helpful, and a player will want to adjust for the best sound for a given use.

The kind of tweaks on the M2 and M4 that keep tone consistent can be adjusted to make changes.

When setting for different speakers, eg using an 8 ohm cab with a 16Ohm M2, you can put a resistor across the output to lower output impedance, which trims high treble and bass to better match an 8ohm cab. Thus can also be used with a 16ohm cab, as a switch, to reduce treble etc.

At the front, the reactive response can be bypassed by shunting the L1 coil. This then defaults to a resistive design. Still good and basically the same as early designs on this thread. Similar or different frequency response depending on the amp, but less lively and reactive. It might be good in some cases. Also at the front, changing the ratio of R2A and R2B can slightly raise or lower the high presence.

Easily analysed but not tested.

Thanks! Great information. Your knowledge, patience, and devotion to this thread is always appreciated.

 

[jfromel]

V2 of the SMD prototype solved many issues with V1, and found new minor issues. Added vertical fan, improved heat dissipation on PCB, added light show. The fan and PCB changes were dramatic (reduction of 20c on average). The light show is currently in failure status. I want to make the thing glow ultraviolet through the top as it gets more juice, I need to work on regulating the fan and un-regulating the LED array to make that happen.

End panels are FR4 PCB. There is no circuit in them, all the copper is for aesthetic, just holes and silk screen. I think I am done with ordering custom faceplates that cost a fortune, these are beautiful.

This has been the most fun I have had on a project in a long time and learned so much about thermal management. Many thanks to JohnH for his help.

One of my goals with this project was to make it DIY for about the same cost as using die cast enclosures and big ass resistors with less time and drilling involved. I will post all the files for the PCB's, schematic, BOM, 3d printed parts in STL, 3MF with slicing instructions, when all the kinks are worked out.

The V2 proto cost me $175 including shipping, and I have 4 sets of PCB's left over. YMMV

I am hoping to put this into production as a completed unit or DIY kit. The kit will not require any soldering and will take about 30 minutes to assemble with a screwdriver (like IKEA or the JHS Notaklon). I am hoping to keep the kit cost $199 with stock lid and a bit extra for the 3d printed lid ($25), and completed unit at $299.

If there is enough interest I can send one out on tour for trial and feedback.

 

[Gene Ballzz]

View attachment 150485View attachment 150486View attachment 150487V2 of the SMD prototype solved many issues with V1, and found new minor issues. Added vertical fan, improved heat dissipation on PCB, added light show. The fan and PCB changes were dramatic (reduction of 20c on average). The light show is currently in failure status. I want to make the thing glow ultraviolet through the top as it gets more juice, I need to work on regulating the fan and un-regulating the LED array to make that happen.

End panels are FR4 PCB. There is no circuit in them, all the copper is for aesthetic, just holes and silk screen. I think I am done with ordering custom faceplates that cost a fortune, these are beautiful.

This has been the most fun I have had on a project in a long time and learned so much about thermal management. Many thanks to JohnH for his help.

One of my goals with this project was to make it DIY for about the same cost as using die cast enclosures and big ass resistors with less time and drilling involved. I will post all the files for the PCB's, schematic, BOM, 3d printed parts in STL, 3MF with slicing instructions, when all the kinks are worked out.

The V2 proto cost me $175 including shipping, and I have 4 sets of PCB's left over. YMMV

I am hoping to put this into production as a completed unit or DIY kit. The kit will not require any soldering and will take about 30 minutes to assemble with a screwdriver (like IKEA or the JHS Notaklon). I am hoping to keep the kit cost $199 with stock lid and a bit extra for the 3d printed lid ($25), and completed unit at $299.

If there is enough interest I can send one out on tour for trial and feedback.

Beautiful work! And in this one instant, all of my rather crude (comparatively speaking) offerings, at the same price have become cosmetically and generally OBSOLETE! No harm, no foul and kudos to he who can build a better mousetrap!

My offering:



Again, Great Work!
Gene

 

[jfromel]

Beautiful work! And in this one instant, all of my rather crude (comparatively speaking) offerings, at the same price have become cosmetically and generally OBSOLETE! No harm, no foul and kudos to he who can build a better mousetrap!

My offering:

View attachment 150543View attachment 150544View attachment 150545View attachment 150546

Again, Great Work!
Gene

Your build is absolutely amazing in every way. It looks like a box that works and it's built like a tank!! I am also completely against a bypass switch because I don't want to talk to someone who smoked their amp and explain they basically put their car in reverse while going 80 on the freeway. Just different ways to catch the mice, I think they are both better than anything else out there. I just don't have the patience to do point to point work anymore unless it's a really cool custom amp. I'm currently working on a tube amp that is mostly SMD. Less expense, less environmental waste and IMO generally sound better.

 

[Gene Ballzz]

Your build is absolutely amazing in every way. It looks like a box that works and it's built like a tank!! I am also completely against a bypass switch because I don't want to talk to someone who smoked their amp and explain they basically put their car in reverse while going 80 on the freeway. Just different ways to catch the mice, I think they are both better than anything else out there. I just don't have the patience to do point to point work anymore unless it's a really cool custom amp. I'm currently working on a tube amp that is mostly SMD. Less expense, less environmental waste and IMO generally sound better.

@jfromel
Thank you sir for the kind words! Yours looks like a "Million Bucks" while mine just looks all green and wrinkled, by comparison. I too, ALWAYS recommend against a BYPASS switch and will only do one under insistence, like the one pictured!

I'm surprised by your comment about "point to point" as that is what I find to be the most pleasurable part of a build. I still build amps with turret board construction!

With all that said, for me the most tedious and time consuming part is drilling and otherwise preparing the box, prior to powder coating. It would be fantastic to find a source for having the boxes CNC/laser/water jet cut for anything near reasonable small batch prices. And then of course, screen printing after the powdercoat would be nice, cosmetically!

Oh Well, Again, Nice Work,
Gene

 

[stickyfinger]

Apple uses the chassis as a sink on many of their products (often with a heat pipe), Dell on laptops. Not sure who else. I did a lot of research on best practices and didn't find much. The manufactures don't give tips on mounting in their data sheets so there has been a lot of things I did on this design I have never seen before because I was making a best guess on how these resistors would work. I came up with this after reviewing with some other engineers in my son's robot club. If it keeps cool with the stick on sinks I will be really happy. It won't need as many sinks as are in the 3d model because only the first stage gets warm. Everything downstream runs cool.

I'm confused. Apple uses the chassis as a heat sink but your using a PCB. The typical JH attenuator used the aluminum chassis as a heat sink. How are you comparing apples heat management to what your using? Maybe I'm not understanding but I'd like to know in what commercial products would use such resistor/PCB for heat dissipation.

 

[jfromel]

The PCB and chassis both act as a sink in my design. The V2 with the radiator style through hole slots dump a lot of heat before it reaches the chassis. Maximizing a PCB for thermal dissipation can be seen in many consumer electronics. If there is a copper pour top and bottom connected with a bunch of via's that's a sink. If that sink is around a mounting hole and the copper is exposed on the post that acts as a sink as well. The buck converter I'm using to test was built this way. This paper from TI explains the process of using thermal via's and copper pours as a PCB sink. https://www.ti.com/lit/an/sloa120/s...11345&ref_url=https%3A%2F%2Fwww.google.com%2F

 

[Mjh36]

Quick question:

On an 8-ohm unit, can the optional R9 for the lower 4-ohm output #4 be adjusted to make it a 2-ohm output? So an 8-ohm input with 8,8,16,2 as my outputs? Too big of a jump?

 

[JohnH]

hi @Mjh36
I think that double step will be outside of what can be configured properly to keep the tone sounding right using the simple R9 arrangement. Might be possible to come up with a different arrangement of output circuit.

As far a safety goes, it would be possible to put a 2 Ohm load into the 8 Ohm attenuator, with the M2 version ie, without the switchable 3.5/7 Stage 1 that is in M4 . The load that the amp sees would still be ok at about 7 Ohm. The tone could get a bit more high treble and bass with relatively less mids, and overall you lose about another 5dB on power at the loudest attenuated setting.

But stepping back, it seems like you'd be running a 2 Ohm cab or two 4 Ohm cabs, from an 8 Ohm amp? Can you describe more about what it is and how you want to use it?. Maybe there are options... Also what amp and power and is it for gigging, or home studio? etc

Best advice if 2 Ohm is an important use is to build an M2 just for that

 

[Mjh36]

Thanks @JohnH

I figured it didn't seem as easy as swapping resistor values for R9. It was a thought for my new '64 Super Reverb mentioned on the previous page.
The Weber Z-Matcher will arrive next week. This will help it go into my 8-ohm M3 unit. I'll update that later, but it will acheive two things for me:
-Be able to load box it 100% at night.
-Use with my other 8 or 16 ohm speakers. (and attenuate with those)

However, the con is I can't use that setup with the stock 2-ohm, four-speaker combo setup. I'm going to want that option as well, and I did not build my 8-ohm M3 with the lower 4-ohm output #4. I just have the 8-8-16 arrangement.

I was just trying to work smarter not harder and maybe get by without building another one. But if I really want to attenuate within its own 4x10 combo, I know I'll need a 2-ohm unit.

I think I'll convert my first 50w 8-ohm build that I don't use since I built a 100w version. I can just replace I think 9 resistors + the coil with 2-ohm values. They'll probably be the same footprint. And the wattage is just enough for a Super Reverb. Thought of that the other day and that should be an easy change.

 

[JohnH]

If you add one two-pole toggle to the combo speakers, you can add an option to change it from four 8 ohm all in parallel at 2 Ohm, to series-parallel at 8 Ohm (like a normal 4x12).

Then you can go combo (2 Ohm) to Z-matcher to attenuator (8 Ohms) and back to combo speakers (set to 8 Ohms). (Lots of things to get wrong though!)

 

[Mjh36]

That's right I recall you mentioned that about the series-parallel, If I can find a way without any chassis or cabinet mods that would be ideal. I'll give that some thought because that sounds fantastic.

 

[JohnH]

If its an open back, maybe you could make some simple bracket out of folded metal to mount the switch to, and fix it inside the cab area, unseen from outside?

 

[Mjh36]

Thanks @JohnH, I think that'll be a good plan. I can fix something together with some shallow screws I bet with no problem. Just gotta brainstorm it. I assume atleast a normal 5A On-On DPDT mini toggle switch will do?

I had to deep dive the speaker switching. I'm terrible at visualizing switches. I think this is the best way to do it, but correct me if I'm wrong. Essentially a switch from parallel pairs in parallel, to parallel pairs in series. The parallel/series vs. series/parallel definition is kind of confusing because different diagrams called them each differently. This was my thought process:

Super Reverb is wired up all parallel:


What I assume Parallel-Series is with schematic (speakers will be 8 ohms):


Best visualization I could find and what I need to do, working off of the left model:


Final diagram with switch to parallel 2-ohm load:

 

[JohnH]

Final diagram looks good to me. I'd suggest a full-size toggle for easier wiring with 18 gage wire

If its a 45W amp, at 2 ohms it runs about 5 Amps total, plus more if over-driven. This switching splits that into two halves, but I'd try for about an 8 to 10 Amp switch if possible.

 

[BC108]

Great thread and idea. Great knowledge shared, thank you all.

I'm diving now into all info and posts on this thread. I would like to start a M2 for 100w amps and 8ohm cab.

Is there any list updated for all materials and components on mouser, digikey, etc?

 

[jfromel]

Great thread and idea. Great knowledge shared, thank you all.

I'm diving now into all info and posts on this thread. I would like to start a M2 for 100w amps and 8ohm cab.

Is there any list updated for all materials and components on mouser, digikey, etc?

Here is the BOM at Mouser for the resistors I would use for a 100w M2 8 ohm in 4/8/16 out, no bypass. Jacks and switches to taste but get beefy ones for a 100w, lots of enclosure options. Coil is .9mH at least 16ga, Parts Express has a 14ga that would work splendidly. Use 16ga Wire. Go over to the build thread to see how others have done it. Perhaps Gene will chime in. Even if you only need 8 ohms out right now it may make sense to add the two jacks and $15 worth of resistors and build with all the outputs.

	Mfr. #	Manufacturer	Description	Order Qty.	Price (USD)	Ext.: (USD)
1	HSC20015RJ	TE Connectivity	Wirewound Resistors - Chassis Mount HSC200 15R 5%	1	$34.59	$34.59
2	HSC10022RF	TE Connectivity	Wirewound Resistors - Chassis Mount HSC100 22R 1%	1	$18.39	$18.39
3	HSC10018RJ	TE Connectivity	Wirewound Resistors - Chassis Mount HSC100 18R 5%	1	$13.17	$13.17
4	HSA505R6J	TE Connectivity	Wirewound Resistors - Chassis Mount HSA50 5R6 5%	1	$4.53	$4.53
5	RHA05010R00FE02	Vishay	Wirewound Resistors - Chassis Mount 50watt 10ohm 1% AEC-200	2	$6.44	$12.88
6	RHA05075R00FE02	Vishay	Wirewound Resistors - Chassis Mount 50watt 75ohm 1% AEC-200	1	$7.91	$7.91
7	RHA05015R00FE02	Vishay	Wirewound Resistors - Chassis Mount 50watt 15ohm 1% AEC-200	2	$6.44	$12.88
8	RHA05020R00FE02	Vishay	Wirewound Resistors - Chassis Mount 50watt 20ohm 1% AEC-200	1	$6.56	$6.56