Simple Attenuators - Design And Testing

[halcyon]

Interesting that nailing the values for R2A and R2B is so important. Seeing that Mouser has the exact values desired and delivers super-fast makes using them a lot easier. Granted that puts the build cost near $100 instead of $70, but $100 is still quite a bargain for an attenuator that works so well!

Thanks for that. Good reminder, I've ordered from Mouser before. I'll have a look at their pricing. It may be worth it for that one value as I need to order some other parts as well for other projects.

 

[halcyon]

OK did the deed. Mouser is too pricey for my taste, all from ebay. Here's the breakdown:

R1: 15ohm 200W, from 2x 30ohm 100W in parallel
R2A: 22ohm 100W
R2B: 18ohm 100W, from 2x 9ohm 50W in series
R3,6: 15 ohm 50W
R4: 10 ohm 50W
R5: 4.7ohm 50W, actual 4.5ohm from 2x 9ohm 25W in parallel
R7: 33ohm 50W
R8: 5.6ohm 50W, actual 5.5 ohm from 2x11 ohm 25W in parallel

 

[matttornado]

Hi JohnH

As you know, I biult your 100 watt version back on page 28. How would i add a simple tone control? Same as a guitar?

It would be added at the attenuator's speaker output.

 

[Osman Qureshi]

Attached are photos of my M2 build. Works as advertised. I heard the clean channel on my DSL20 break up for the first time in my apartment. There’s not much to say about the awesome M2 design, so I’ll comment to some of my additions.

The line out is a 2.2KΩ 1W resistor and 10KΩ log (audio) 1/4W potentiometer in series. There is a pre- and post-attenuation switch. Switched to pre, it works quite well on my 32Ω headphones. The line-out is wired such that it won’t work in bypass mode. The perfboard is there for the future addition of a passive cab sim. The three resistors and the 1uF cap on the perfboard are there to switch to a lower mic-level signal and to block noisy DC voltage from seeping in from my old DI box. The objective of building the line out was to record on my Mac using GarageBand’s cab sims. Yes, the DSL20 has an emulated out, but I wanted an unemulated signal that I could use with GarageBand’s several cab sim options. I haven’t spent much time on the line out yet but will update on how well it works for recording later.

The switch in the back is there for a totally clean bypass, with the attenuator completely isolated. I have not noticed any difference in volume and tone between the isolation switch tuned on and off.

The ventilation holes on the top are not large but with the holes on the side and the larger ones on the bottom, the unit remains cool (I went with form over function vis-à-vis ventilation). With the DSL20 in 20W mode, fully cranked, the unit barely even gets warm. I put all the resistors, except R3, R5 and R7, on the “ceiling” of the box; the only warmth seems to come from R1 (don’t forget the heat sink paste like I did at first. Just for kicks, I briefly tested it on a 100W modelling amp, fully cranked, and it got hot fairly quickly.

The lettering on the box uses water slide decals printed on a laser printer, then slid on to the Hammond box, and then finally 10 light coats of clear automobile lacquer sprayed on with sufficient drying time between each coat. The last step is key to making the lettering permanent.

Some observations: The attenuation from Stage 4 is very slight—at least with a 20W amp—so much so that I thought that I hadn’t connected it correctly given my poor soldering skills, so I wonder if it is even needed. Overall, it’s a fairly easy project for someone who doesn’t normally build things and is a novice at soldering, especially if you don’t include the perfboard in your build. Also, I used 18 AWG wire, but given the size of the box (Hammond model 1550E) and the addition of the perfboard and line out switches and pot, I probably should have used more flexible 20 AWG wire or a bigger box. But it works really well so far with a 20W amp. Thanks JohnH and everyone else involved in the design and testing.

 

[JohnH]

Hi JohnH

As you know, I biult your 100 watt version back on page 28. How would i add a simple tone control? Same as a guitar?

It would be added at the attenuator's speaker output.

Hi Matt, that was a nice chance to look back at your build, with that big-a$$ resistor soaking up your 100W Plexi! How's it worked out for you after having it for a while? Did you do a fan in the end? Also interested if you built the line out that you were noting there.

On tone control, what sort of tone change would you be seeking? is it a treble cut as on a guitar? A cap and resistor in series, across the output might do it and I could run my sim to make a suggestion. If it had a pot, it would have to be very high power rating. A simpler version would be to get an on-off-on toggle to give you two settings and off. Also, yours is a 16 Ohm build but are you using a 16 Ohm cab? or an 8Ohm?

 

[JohnH]

hi @Osman Qureshi , that looks great, very nice job! thanks for the very helpful write up and pictures too.

I think that box is exactly the one I used in post 1. Its a good solid unit, and Ill build my next one in a bigger version of it. I think you were right to make use of 18 gage wire though, despite its stiffness.

Your heat observations match others and its interesting to compare with Matts above. The temperatures seem to take off once you get past 50W, but below that it just gets warm.

id like to know more about how the line out is working. The values for you keep most of the available signal in case you need it, but it could be reduced with different values.

 

[Gene Ballzz]

@Osman Qureshi
First, Excellent job on a gordeous build. So much better looking than any I've produced! Am I correct in assuming that your "Stage 4" is wired as a -3.15db stage that can be added to any combination of the other stages, as well as being able to be used as a stand alone, resistive only stage?

Next, It sure is nice being able to finally open up that CLASSIC GAIN channel of the DSL20, isn't it? Before these attenuators, it definitely was not my "go to" channel, but now it's my default, fully cranked and the ULTRA GAIN is just used for a bit of fun, now and then! And yeah, even after a couple hours of full volume, 20 watt mode use, my mounted attenuator only gets barely warm. My other electrically identical attenuator does however, get mighty toasty with my JTM30! Although that particular unit is my "updated" first build with a few less and smaller vent holes than later units! I'm planning on opening up the existing vent holes and drilling a few more to see if it helps. I'll have a closer to accurate wattage measurement of that JTM30 in the next day or so. I'm really betting its in excess of 40 watts!

Again, Nice Job! Enjoy!
Gene

 

[Gene Ballzz]

@JohnH
Well, I've confirmed that my "little" JTM30 is in excess of 36 watts, dead nuts clean and approaching 53 watts when fully blazing, and I often use it that way! That likely explains my heating of my attenuator!
Some Pondering To Do Here?
Gene

 

[JohnH]

hi @Gene Ballzz
It's a cookin' little amp, if not smokin'!

That spec you just measured on the JTM is exactly what they say about the 50W Vintage Modern which is my main squeeze. I've looped that at maximum settings and tried to heat up the attenuator but I just can't get my current one hotter than a warm Australian summers day. Maybe Im not getting that power. But the case is really thick and heavy and drilled extensively. From photos it's the same case as Osman used above.

But, how hot is too hot? I reckon if you can touch R1 for 1 second then its under 100C, ie less than boiling point.

 

[matttornado]

Hi Matt, that was a nice chance to look back at your build, with that big-a$$ resistor soaking up your 100W Plexi! How's it worked out for you after having it for a while? Did you do a fan in the end? Also interested if you built the line out that you were noting there.

On tone control, what sort of tone change would you be seeking? is it a treble cut as on a guitar? A cap and resistor in series, across the output might do it and I could run my sim to make a suggestion. If it had a pot, it would have to be very high power rating. A simpler version would be to get an on-off-on toggle to give you two settings and off. Also, yours is a 16 Ohm build but are you using a 16 Ohm cab? or an 8Ohm?

 

[matttornado]

Hi JohnH.

The attenuator been working great! I added a fan with an on/off toggle switch along with a power supply.

I did add the line out like the diagram I posted but the ground must be to the speaker ground, not the chassis ground like the diagram shows (If I remember correctly). Yes I'm looking to cut some treble as I turn the attenuator volume up.

Thanks!

 

[JohnH]

Hi JohnH.

The attenuator been working great! I added a fan with an on/off toggle switch along with a power supply.

I did add the line out like the diagram I posted but the ground must be to the speaker ground, not the chassis ground like the diagram shows (If I remember correctly). Yes I'm looking to cut some treble as I turn the attenuator volume up.

Thanks!

Hi Matt

I did some runs. My SPICE model is set up for 8 ohms, so this plot is for 8 Ohms:



This relates to having a capacitor and a resistor in series, from hot to cold across the output jack. For this 8 ohm assessment, the cap is 10uF. The top line is the full tone, and you can see the usual rise in treble. With a 10 ohm resistor plus the cap, we get the red line, which exactly flattens the rise. Other resistors are plotted from 6 (lowest curve) to 22 (highest)

For your 16 Ohm build, the same curves will happen with half the cap and twice the resistor, ie 5 or 4.7uF cap and resistor from 12 to 44. (or 47 or 50, whichever you can find)

It assumes that the reactive stage 1 is always on

The caps need to be non-polarized, either electrolytic or polypropylene, rated for at least 50 Volts to go with your 100W amp. the resistors should be 25W or more.

it may be hard to know how to relate the change that you want to how these curves look, so a bit of testing may be needed with different values.

 

[Osman Qureshi]

hi @Osman Qureshi , that looks great, very nice job! thanks for the very helpful write up and pictures too.

I think that box is exactly the one I used in post 1. Its a good solid unit, and Ill build my next one in a bigger version of it. I think you were right to make use of 18 gage wire though, despite its stiffness.

Your heat observations match others and its interesting to compare with Matts above. The temperatures seem to take off once you get past 50W, but below that it just gets warm.

id like to know more about how the line out is working. The values for you keep most of the available signal in case you need it, but it could be reduced with different values.

Hi John,

I finally got the chance to record using the my M2 build's line out connected to a DSL20HR. I didn't connect the line out directly to my MacBook because I didn't have a Y-splitter that splits the headphone and the mic connection of the headset socket. Instead, I connected the post-attenuation line out to my iRig Pro, which did the A/D conversion and then fed the digitized signal into my Mac's USB port. On GarageBand I used the Amp Designer function that lets you pick a combination of amp, cab, and recording mic. I picked the clean Transparent Preamp, and then tested out various cab and mic combinations. It all worked as expected. With GarageBand's cab and mic sims, the sound was good for at-home recording purposes. There's also an option in GarageBand to not pick a cab and hear the direct signal, which I tried, and as expected, it was terrible. I also compared the emulated out from the DSL20 to the various cab sims in GarangeBand, and I think the DSL's cab sim is closest to GarageBand's Vintage British 4x12 option with the Ribbon 121 mic. GarageBand also let's you virtually move the mic around with reference to the speaker, but I just went with the default position.

I also tried all of the above with my M2's pre-attenuation line out; other than more volume, I really didn't notice any difference (but my hearing is not the best and I'm not a tone connoisseur at all). The best use of the pre-attenuation line out would be for it to go through a passive cab sim to play with headphones (it has more than sufficient volume for headphones).

The mic-level I put into my M2 build with the three resistors and the cap was pretty useless in the end. I think I'll get rid of it and use the switch and perfboard for the passive cab sim that you're developing.

Happy to answer any other questions and run any tests.

Thanks.

 

[Osman Qureshi]

@Osman Qureshi
First, Excellent job on a gordeous build. So much better looking than any I've produced! Am I correct in assuming that your "Stage 4" is wired as a -3.15db stage that can be added to any combination of the other stages, as well as being able to be used as a stand alone, resistive only stage?

Next, It sure is nice being able to finally open up that CLASSIC GAIN channel of the DSL20, isn't it? Before these attenuators, it definitely was not my "go to" channel, but now it's my default, fully cranked and the ULTRA GAIN is just used for a bit of fun, now and then! And yeah, even after a couple hours of full volume, 20 watt mode use, my mounted attenuator only gets barely warm. My other electrically identical attenuator does however, get mighty toasty with my JTM30! Although that particular unit is my "updated" first build with a few less and smaller vent holes than later units! I'm planning on opening up the existing vent holes and drilling a few more to see if it helps. I'll have a closer to accurate wattage measurement of that JTM30 in the next day or so. I'm really betting its in excess of 40 watts!

Again, Nice Job! Enjoy!
Gene

Thanks Gene, I spent some time making it look pretty and labeling it to make using it self-explanatory for others, like my kids. On your question regarding Stage 4, my build is to the exact specs laid out on page one of this thread, so I suppose my Stage 4 can be added to any combination of the other stages (?), although I haven't tried it yet. (My only add-ons to the standard 8Ω M2 design are the two bypass switches, one at the amp-in socket and the other at the speaker-out socket. I also added a line out that's switchable between amp-in and speaker-out sockets, and a perfboard for a future cab sim circuit). Regards, Osman

 

[matttornado]

Hi Matt

I did some runs. My SPICE model is set up for 8 ohms, so this plot is for 8 Ohms:

View attachment 86075

This relates to having a capacitor and a resistor in series, from hot to cold across the output jack. For this 8 ohm assessment, the cap is 10uF. The top line is the full tone, and you can see the usual rise in treble. With a 10 ohm resistor plus the cap, we get the red line, which exactly flattens the rise. Other resistors are plotted from 6 (lowest curve) to 22 (highest)

For your 16 Ohm build, the same curves will happen with half the cap and twice the resistor, ie 5 or 4.7uF cap and resistor from 12 to 44. (or 47 or 50, whichever you can find)

It assumes that the reactive stage 1 is always on

The caps need to be non-polarized, either electrolytic or polypropylene, rated for at least 50 Volts to go with your 100W amp. the resistors should be 25W or more.

it may be hard to know how to relate the change that you want to how these curves look, so a bit of testing may be needed with different values.

Thanks JohnH! So If I want to add a pot to control treble or tone, would I add a cap & resistor to the the pot? How would that work?

 

[JohnH]

Thanks JohnH! So If I want to add a pot to control treble or tone, would I add a cap & resistor to the the pot? How would that work?

Let's say you wanted to sweep through those tone curves. You'd get a small resistor (like 12 for you )and a high power pot (using 2 lugs) and tbe cap, all in series. Maybe 100 ohm pot. But youd want a switch too to get it all out of circuit.

I think a better way might be a rotary switch with 6 steps. One is no tone reduction, and the other 5 step down in even increments following those curves. A series of resistors in a chain, plus the cap.

 

[Timonew]

I’ve been using my first M2 8 Ohm 50W attenuator a lot with my Tweed Champ style amp so I’ve built another one for it.

I mounted all the Rs on a thick piece of aluminium and then into a aluminium case made from a shelf from Pete’s greenhouse. The connections use solid core wire - this made the soldering much easier. The reactive stage resistors are scaled back for an attenuator that handles 25W.

At home I’m typically using 10.5, 14, 17.5 or 21dB of attenuation even on this 4W amp. It all works perfectly with all the amp settings - eg NFB off and extra preamp treble boost switched in. The loudspeaker is a 12” Eminence GA-SC64

Thank you @JohnH for sharing your design, I’m playing my guitar much more now.

 

[JohnH]

A very nice clean looking build @Timonew I like the side vent slot idea.

 

[Gene Ballzz]

@Timonew
Very nice job. I like the solid wire concept and agree that it's likely a bit easier to manipulate and work with. And yes, these attenuator designs are the greatest thing since sliced bread! And as you've obviously determined, economical enough to build a unit for each amp you own!

And @JohnH
I've determined that when using one of my units with my 50-ish watt JTM30, if I leave the lid open, the hottest resistors do still get fairly hot, but even after an hour or more of pounding at the the amp's fully cranked sweet spot I can still touch them for a while without discomfort. So the unit I build/dedicate for this amp will simply be more openly ventilated and I should be good to go. I'm really hoping to avoid all the hassles and pitfalls of adding a fan! My experience does certainly demonstrate that a cranked up 50 watt amp is the absoulte limit for these units!

Enjoy,
Gene

 

[Timonew]

A very nice clean looking build @Timonew I like the side vent slot idea.

Thanks, and if it gets hot with a more powerful amp I could put the case on its side - with a chimney effect!