Simple Attenuators - Design And Testing

[AtomicRob]

With that enclosure you'd also have the two sides where you could mount at least a couple resistors on each side to make more room on the bottom. It's also a good idea if possible to spread out the hottest resistors in different parts of the enclosure even if it means using more wire between - the neatest visual layout isn't necessarily best for thermals.

 

[jfromel]

With that enclosure you'd also have the two sides where you could mount at least a couple resistors on each side to make more room on the bottom. It's also a good idea if possible to spread out the hottest resistors in different parts of the enclosure even if it means using more wire between - the neatest visual layout isn't necessarily best for thermals.

Mounting on the side would free up a lot of room but with the side vents it may only be possible to fit one or two. I recommend countersunk screws! If it were my build I would put all the HOT resistors (R1's) directly in the path of the fan and spread out the other stages between the switches and jacks and perhaps add some venting holes to the bottom if you can.

 

[adrian078]

You should be able to mount the coil on a nylon standoff of some sort from the bottom over the resistors.

All so I do not like those 100w resistors. They are not comparable to size of the name brand at 100w and the bolt holes should be opposite like the 50w so when you bolt them down the thermal paste gets spread out evenly. I haven't tried them so these are just my observations.

Ideally and why a Hammond box is better is you mount the resistors to the top and the top acts as a source for heat management. On the bottom you lose all that and are now relying on the fan.

Its a good looking box and I considered it too but decided on the Hammond.

If they cooked, would they be likely to take out the output tranny of the amp, or the coil or the other resistors?

Its a good looking box and I considered it too but decided on the Hammond.

I'm starting to think it may be a better idea to use the Hammond.

I've used the Chinese 100W resistors. I think they are ok. In an M2 design R1 gets about half the amp power,

I was surprised by the size when I received them after looking at some of the other designs here. I guess if it were likely to be an issue I could upgrade the resistors that take the most power. R1 and R2?

 

[adrian078]

If it were my build I would put all the HOT resistors (R1's) directly in the path of the fan and spread out the other stages

That's what I was thinking with the current layout and why I put those resistors where they are. I was going to make the bottom look like swiss cheese with vent holes.

I like the idea of mounting resistors to the side too.

I recommend countersunk screws!

so you'd have the screw head on the outside of the chassis, and the nuts on the inside?

 

[jfromel]

That's what I was thinking with the current layout and why I put those resistors where they are. I was going to make the bottom look like swiss cheese with vent holes.

I like the idea of mounting resistors to the side too.




so you'd have the screw head on the outside of the chassis, and the nuts on the inside?

Yep, screw on outside. My thought is if that box is a tight fit and screw heads are popping out it could present a challenge unless they were countersunk. They also look better IMHO.

 

[spudmaster7]

Hey guys, have finally found some time to finish up my build of a 100W 16ohm version and have just tested the resistance across the output jack to the amplifier (with a 16ohm cabinet connected to the speaker output). Will add some photos shortly.

I'm reading 10ohm with first switch in -3.5dB position and all other switches off. With the first switch on -7dB it jumps to about 22ohm and then up to maximum of about 23.5ohm with the other switches on.

With the first switch on -3.5dB and the second switch on the reading only goes up to about 11.7ohm, so it seems just that first switch is raising the resistance quite a lot.

The only difference between mine and the M4 is that I've used a 50R resistor for R2A instead of 47R, and I omitted the resonance circuit.

Can anyone advise whether this seems ok, or if I've stuffed something up somewhere? John's advice at the start of the thread indicated that the resistance values should be somewhere between 14-20ohm, so I'm not far off but want to be reasonably sure before I plug an amp in!

 

[JohnH]

Hi @spudmaster7
Thanks for posting those, and I think there is something not right there.

You could post photos of the wiring, though it's often hard to see issues from a photo, but it might reveal something.

Also, post a full table of resistance settings. It sounds like you're measuring the right thing. 16 Ohm cab plugged into Attenuator main 16 Ohm output, and measuring across the jack that would go to the amplifier. You have 4 switches so that's 16 readings.

Identify which switch is which eg
1 = stage 1: -3.5/-7 dB
2 = stage 2: -14dB
3 = stage 3: -7db
4 = stage 4: -3.5dB

Confirm if that's how yours is?

Then, post your full table of ohms readings:
for switches 1 2 3 4,

on on on on (quietest, most attenuation)
on on on off.... 0hms=?
on on off on.... etc
on on off off...
on off on on
on off on off
on off off on
on off off off
off on on on
off on on off
off on off on
off on off off
off off on on
off off on off
off off off on
off off off off (all stages loudest, just stage 1 is at -3.5dB)

The table above is like binary numbers.

When posting readings, subtract the ohms of your meter leads when touching the probes together.

If you can be as methodical as that, then I can compare against the theoretical values and hopefully help figure out what is happening.

 

[spudmaster7]

Hey John, thanks for the reply. Yes mine has the same layout as you mentioned with switch 1 being the 3.5/7db cut.

My readings are as follows. First number is resistance in Ohms, 0000 is all switches off/first switch in -3.5dB. 1111 is all switches on/first switch in -7dB:
9.7 0000
12.1 0001
11.0 0010
11.5 0011
11.4 0100
11.7 0101
11.6 0110
11.5 0111
21.8 1000
23.8 1001
22.9 1010
23.3 1011
23.2 1100
23.3 1101
23.2 1110
23.2 1111

Hi @spudmaster7
Thanks for posting those, and I think there is something not right there.

You could post photos of the wiring, though it's often hard to see issues from a photo, but it might reveal something.

Also, post a full table of resistance settings. It sounds like you're measuring the right thing. 16 Ohm cab plugged into Attenuator main 16 Ohm output, and measuring across the jack that would go to the amplifier. You have 4 switches so that's 16 readings.

Identify which switch is which eg
1 = stage 1: -3.5/-7 dB
2 = stage 2: -14dB
3 = stage 3: -7db
4 = stage 4: -3.5dB

Confirm if that's how yours is?

Then, post your full table of ohms readings:
for switches 1 2 3 4,

on on on on (quietest, most attenuation)
on on on off.... 0hms=?
on on off on.... etc
on on off off...
on off on on
on off on off
on off off on
on off off off
off on on on
off on on off
off on off on
off on off off
off off on on
off off on off
off off off on
off off off off (all stages loudest, just stage 1 is at -3.5dB)

The table above is like binary numbers.

When posting readings, subtract the ohms of your meter leads when touching the probes together.

If you can be as methodical as that, then I can compare against the theoretical values and hopefully help figure out what is happening.

 

[JohnH]

Thanks, yes it is hard to investigate from the photo!

But from those readings, the issue looks to be in Stage 1.

Take that first reading 9.7, with switches 0000 (ie net -3.5dB). With a typical 16 Ohm speaker with its own DCR of about 14, the reading noted should be about 17 ohm. The measurement of 9,7 is pretty close to as if R2A is shorted out in that setting, and R1B is grounded at one end instead of one end disconnected, which could all be due to issues with switch wiring in that first switch

So, next check: set the switches to 0000 and this time unplug the speaker.

Measure across R2A (it should be about 11, in combination with R2C and R2B and the coil). Measure across R1A (it should be 56, since R1B would be disconnected at one end)

 

[spudmaster7]

Thanks, yes it is hard to investigate from the photo!

But from those readings, the issue looks to be in Stage 1.

Take that first reading 9.7, with switches 0000 (ie net -3.5dB). With a typical 16 Ohm speaker with its own DCR of about 14, the reading noted should be about 17 ohm. The measurement of 9,7 is pretty close to as if R2A is shorted out in that setting, and R1B is grounded at one end instead of one end disconnected, which could all be due to issues with switch wiring in that first switch

So, next check: set the switches to 0000 and this time unplug the speaker.

Measure across R2A (it should be about 11, in combination with R2C and R2B and the coil). Measure across R1A (it should be 56, since R1B would be disconnected at one end)

Hi John, I checked over the wiring around that first switch and realised I’d connected one of the poles of the switch around the wrong way! My schematic was drawn incorrectly so half the switch was set to -3.5dB while the other half was set to -7dB

I’ve fixed that up and done new measurements with a 16ohm speaker connected:

0000 17ohm
0001 19.6
0010 18.4
0011 18.9
0100 18.7
0101 18.8
0110 18.8
0111 18.8
1000 15.9
1001 16.9
1010 16.4
1011 16.7
1100 16.6
1101 16.6
1110 16.6
1111 16.6

This seems to fall within your recommendations in the original post so hopefully should be good to test out with an amp

 

[JohnH]

Yes they look good, and safe to try! I'm glad we figured that out.

So ok to connect it up for a test, starting at fairly low volume on the amp and check sound comes through and the attenuation steps are each a steady increase as expected. (If there's no sound at all, don't turn up further!)

BTW the intent for the best tone is that Stage 1 is usually set at -7dB, and you only use it at -3.5dB to get that first 0000 -3.5dB setting. But it's safe to use the switches any way you prefer.

 

[AtomicRob]

I did a little modification to my M2. I wanted a footswitch controlled solo boost but I wasn't sure whether 3.5db or 7db would be enough, and I wasn't sure how much attenuation I would need overall. So simply switching one stage in/out wouldn't work - I want to switch between any two levels that can be set independently.

Using toggle switches for each stage doesn't really work in this case - but rotary switches work really well. With three stages we have 8 levels of attenuation. So I designed a circuit that takes input from two 8-position switches and uses it to drive three relays that replace the toggle switches. Basically it converts the rotary switches to binary, uses a 2-1 data selector toggled by the footswitch to select which control is active, and that controls the relay driver. It's all off-the-shelf CMOS logic chips, no microcontroller needed. I considered a few other options but this seemed like the best combination of simple to use and easy to build.

So that's it. No change to the attenuator circuit or wiring except the toggle switches are replaced with relays. The footswitch only carries logic-level (5VDC) signals. LEDs indicate the selected channel. PCB is pretty small ~ 146mm x 37mm, no SMD or exotic parts. I could probably have made it slightly smaller with SMDs but most of the space is the switches, relays, and terminal blocks so it wouldn't shrink much. Uses 9VDC power supply. Pics below, with my temporary paper faceplate LOL, haven't finished the real one yet.

The default state (NC position on the relays) is -31.5db. While switching without signal there is no pop or noise at all. If you switch while playing it gets slightly softer (or slightly louder occasionally) momentarily because depending on the settings there will be either one, two, or three relays switching which takes 5-10ms and during that time R3, R5, and/or R7 is disconnected.

I don't know what interest others might have in this. I ordered the PCB from Oshpark so I have two extra. If a lot of people are interested, it is much cheaper to buy boards in quantity. But then someone (not me!) has to coordinate all that. Or if you want to etch it yourself it's not too bad - 10 mil traces and spacing, dual sided. I can share more info and files. Maybe in a separate thread to keep from spamming this thread too much.

 

[DirtySteve]

I just want to say that the M2 is the best thing that ever happened to my Lil' Devil! (my high gain "plexi" rebuild amp) I wasn't playing this amp for a long time because I thought something was wrong with it and just I put it aside when I bought my SC20, but after finally getting off my ass to do some troubleshooting it turned out to just be a bad power tube, the one thing I never suspected. Now it's breathing fire again and the M2 allows me to turn my MV up to 6 or 7 (the sweet spot) and still be at tv volume. It's been a blast! I've never been able to turn this amp up enough to get the goods out of it in the 10 or 11 years I've owned it because I can't play loud volume for several reasons. With the M2 it sounds better than it ever has thanks to being able to get that MV up and I'm loving every minute of it!

I talked about adding another stage, but now I don't need to, it works great with this amp.

Just wanted to say thanks to John and "the Guy" that built mine. I couldn't be happier, it's the best money I ever spent!!!

 

[stickyfinger]

I did a little modification to my M2. I wanted a footswitch controlled solo boost but I wasn't sure whether 3.5db or 7db would be enough, and I wasn't sure how much attenuation I would need overall. So simply switching one stage in/out wouldn't work - I want to switch between any two levels that can be set independently.

Using toggle switches for each stage doesn't really work in this case - but rotary switches work really well. With three stages we have 8 levels of attenuation. So I designed a circuit that takes input from two 8-position switches and uses it to drive three relays that replace the toggle switches. Basically it converts the rotary switches to binary, uses a 2-1 data selector toggled by the footswitch to select which control is active, and that controls the relay driver. It's all off-the-shelf CMOS logic chips, no microcontroller needed. I considered a few other options but this seemed like the best combination of simple to use and easy to build.

So that's it. No change to the attenuator circuit or wiring except the toggle switches are replaced with relays. The footswitch only carries logic-level (5VDC) signals. LEDs indicate the selected channel. PCB is pretty small ~ 146mm x 37mm, no SMD or exotic parts. I could probably have made it slightly smaller with SMDs but most of the space is the switches, relays, and terminal blocks so it wouldn't shrink much. Uses 9VDC power supply. Pics below, with my temporary paper faceplate LOL, haven't finished the real one yet.

The default state (NC position on the relays) is -31.5db. While switching without signal there is no pop or noise at all. If you switch while playing it gets slightly softer (or slightly louder occasionally) momentarily because depending on the settings there will be either one, two, or three relays switching which takes 5-10ms and during that time R3, R5, and/or R7 is disconnected.

I don't know what interest others might have in this. I ordered the PCB from Oshpark so I have two extra. If a lot of people are interested, it is much cheaper to buy boards in quantity. But then someone (not me!) has to coordinate all that. Or if you want to etch it yourself it's not too bad - 10 mil traces and spacing, dual sided. I can share more info and files. Maybe in a separate thread to keep from spamming this thread too much.

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Very cool! Id be interested in seeing a gut shot of the completed M2 and curious about the 8 position rotary you went with and if suitable spec wise for a standard M2 build.