Simple Attenuators - Design And Testing

[EC Strat]

Hi @EC Strat , Some do build these in. Can you remind me how many output sockets are you using, ie just one or two in parallel or are you doing the output 3 as well (makes bypass more tricky)

Also, do you really need a bypass? because with a bypass engaged, you could instead just not use an attenuator. After a few tests you are likely to find that just one or two settings are all you use. Unless your use involves often adjusting between attenuated or not, then maybe you dont need one? A bypass switch is switching the full power and current of the amp, and must never be operated unless the amp is off or on standby, whereas with the other switches they can be operated on the fly.

I’ll be switching from attenuated to non-attenuated. I like to use pedals and the JTM 45 clean mostly often. But I want an attenuator so I can use the amp as intended when I’d I like to

I’m planning on 2 output jacks in parallel

 

[EC Strat]

Been working on this and planning it most of the day today. Here’s the fruits of the labor thus far: I’m going to pre-install the jumpers prior to installing the jacks and pots tomorrow

@JohnH one of the resistors is covering up a heat sink on the bottom Should I drill more? I have 8 up top

Well now it won’t let me attach a pic

 

[EC Strat]

Ah here we go -

 

[JohnH]

Hi @EC Strat

If you can get another hole or two in the base it would be good, so long as it doesn't hit anything.

Here's a good way to do a bypass on M2. Ignore the Out3 wiring in red.



The looping green and blue wires give a double set contacts in parallel when bypassed. The inset picture shows how to wire it on a DPDT switch.

 

[EC Strat]

Hi @EC Strat

If you can get another hole or two in the base it would be good, so long as it doesn't hit anything.

Here's a good way to do a bypass on M2. Ignore the Out3 wiring in red.

View attachment 105014

The looping green and blue wires give a double set contacts in parallel when bypassed. The inset picture shows how to wire it on a DPDT switch.

Thank you for posting the schematic. I’ll print it out

Btw - what would happen if I did say turn on the bypass while the amp was on / engaged? Ka-boom?

 

[ToneWitch]

Thank you for posting the schematic. I’ll print it out

Btw - what would happen if I did say turn on the bypass while the amp was on / engaged? Ka-boom?

Yes, don’t ask me how I know lol

 

[EC Strat]

Yes, don’t ask me how I know lol

Sometimes experience is the best teacher LOLOL

 

[JohnH]

As you go through a full bypass, there could be a nasty transient when the contacts change where the amp is momentarily open-circuit, particularly with toggles, which are break-before-make. Also due to the speaker coil being suddenly connected. This may be bad for the amp. So to use a bypass safely, better to avoid that risk.

This is not really different to any other passive attenuator with bypass. Tone King Ironman for example, tells you to mute the strings, even though they offer it on a footswitch so you might more likely to be in mid riff when you did it!

Another watchit is, while an 8 Ohm M2 can work well with 8 or 4 ohm speakers, or 16 (whether or not you use the Out3 circuit which is just a tone correction), if you do a bypass, then you have to match speakers to amp.

I do have one on my build, and I have never used it except for testing. I could see it'd be useful sometimes though, eg for an attenuator built into an amp.

A suggestion would be to make it distinct from the others, maybe around the back if the main switches are on the front, and/or, a different style of switch.

 

[EC Strat]

As you go through a full bypass, there could be a nasty transient when the contacts change where the amp is momentarily open-circuit, particularly with toggles, which are break-before-make. Also due to the speaker coil being suddenly connected. This may be bad for the amp. So to use a bypass safely, better to avoid that risk.

This is not really different to any other passive attenuator with bypass. Tone King Ironman for example, tells you to mute the strings, even though they offer it on a footswitch so you might more likely to be in mid riff when you did it!

Another watchit is, while an 8 Ohm M2 can work well with 8 or 4 ohm speakers, or 16 (whether or not you use the Out3 circuit which is just a tone correction), if you do a bypass, then you have to match speakers to amp.

I do have one on my build, and I have never used it except for testing. I could see it'd be useful sometimes though, eg for an attenuator built into an amp.

A suggestion would be to make it distinct from the others, maybe around the back if the main switches are on the front, and/or, a different style of switch.

thanks, yes i put it in the back. I'm about halfway through the wiring schematic. I had to redrill a few holes bc the pots / jacks were too low and contacting the resistors. So, it's a bit swiss cheese like but hey the more heat sinks the better i suppose.

VERY tough to get the switches lined up all nice and near. The hand drill really drifts a bit

I'm using 14 AWG wire and bare copper wire for the jumpers.

that 1590D box is really tiny! Had to get creative and redrill some holes on the sides etc for the resistors. If i ever do another one, i'll look for the next size up probably. taking me longer than i thought it would, but i'm not rushing and going slowly, check connections with the multimeter as i go and verfiy where the wires should be running before i get too far down the road. Learned the hard way on that one with some partscaster builds and pickups / pots installs.

Deeply appreciate your mentorship! This has been a fun project and i'm sure it will be rewarding in the end

 

[Gene Ballzz]

@EC Strat ,
Just to clarify some terminology to avoid confusion. The holes drilled in the box are "vents" that aid cooling through convection/air movement not "heat sinks." The "heat sink" is actually the thick aluminum box that the resistors fasten to and it aids cooling through "conduction" of heat by the resistors being securely bolted to and held in contact with it, with thermal heat sink paste/compound/grease in between them. You ARE using "thermal heat sink paste/compound/grease" for assembly, correct?

thermal heat sink paste



Just Checkin'
Gene

 

[EC Strat]

@EC Strat ,
Just to clarify some terminology to avoid confusion. The holes drilled in the box are "vents" that aid cooling through convection/air movement not "heat sinks." The "heat sink" is actually the thick aluminum box that the resistors fasten to and it aids cooling through "conduction" of heat by the resistors being securely bolted to and held in contact with it, with thermal heat sink paste/compound/grease in between them. You ARE using "thermal heat sink paste/compound/grease" for assembly, correct?

thermal heat sink paste



Just Checkin'
Gene

Great catch Gene - had no idea I needed to use it. I’ll pick up a tube at Home Depot and apply it

I should basically apply it to the underside of each resistor?

Thanks again

 

[JohnH]

Yes, and the ideal is the thinnest layer that covers the whole surface. It's good to di a practice or two on a spare surface. Smear it over the resistor and squish it down, moving it side to side. Then pull it up and see how it went.

 

[Jason deBroux]

Yes, don’t ask me how I know lol

How do you know !!?
You know all of us guitar amp/electronics nerds love stories about how to make errr, I mean how things blow up.

 

[tfletchii]

Is there a wiring diagram for wiring the -3.5 stage on its own?
I see it mentioned and drawn in the original M attenuator but I'm a novice and I'm not following things clearly. I have the bypass wired and working well. I've built the M3 version and really enjoying it with my plexi and my BF princeton. The jump to -7db though is a bit much with the princeton and I think being able to use the -3.5 stage independently would be a great addition for me.
Thanks John, the attenuator has made using my amps at there sweet spot on gigs so much easier!

 

[JohnH]

Is there a wiring diagram for wiring the -3.5 stage on its own?
I see it mentioned and drawn in the original M attenuator but I'm a novice and I'm not following things clearly. I have the bypass wired and working well. I've built the M3 version and really enjoying it with my plexi and my BF princeton. The jump to -7db though is a bit much with the princeton and I think being able to use the -3.5 stage independently would be a great addition for me.
Thanks John, the attenuator has made using my amps at there sweet spot on gigs so much easier!

Hi @tfletchii and welcome. Im very happy to hear that your M3 is working well. What specs did you build to, eg power and 8 or 16 Ohm?. Also how many outputs did you have?

There have been a few builders who have wanted something inbetween 0 and -7db, mainly when gigging with medium or lower powered amps where you want to keep most of the volume.

The way I showed in in Post 1 for design M can still apply to M2 or M3, and it works ok but you mayhave to get new larger resistors for the -3.5 stage since it may take most of the power, and it has some watch-its when using different speaker ohms. Plus, its just a resistive stage, not reactive although you get some speaker reactance coming through. But there are two other ways developed since, which are probably better:

1. Easiest, if it works for you, nothing to build!

Set the attenuator to max attenuation with no speaker, and plug it into the amp in parallel with your speaker, using a second output jack on the amp. Set your amp to half the speaker ohms. Now the attenuator takes half the amp power acting as a load box, and the speaker gets half. This is an attenuation of -3db. It should work perfectly with an M3, if you have the right amp outputs,

2. You can add a switch that turns the -7db Stage 1 to -3,5db. Use this when you want this setting and other switches are off. You can also add the other stages in too, but the tone may be very slightly better in that case if you revert to -7db in the first stage if possible. Here's the schematic. Can you follow it?: If its of interest I can provide values to suit your build.

M2 With 70-35 220126

www.marshallforum.com

Its for an M2, but with M3, it would be similar with the extra coil and cap in series with L1, as in the base M3 design.

 

[tfletchii]

Hi @tfletchii and welcome. Im very happy to hear that your M3 is working well. What specs did you build to, eg power and 8 or 16 Ohm?. Also how many outputs did you have?

There have been a few builders who have wanted something inbetween 0 and -7db, mainly when gigging with medium or lower powered amps where you want to keep most of the volume.

The way I showed in in Post 1 for design M can still apply to M2 or M3, and it works ok but you mayhave to get new larger resistors for the -3.5 stage since it may take most of the power, and it has some watch-its when using different speaker ohms. Plus, its just a resistive stage, not reactive although you get some speaker reactance coming through. But there are two other ways developed since, which are probably better:

1. Easiest, if it works for you, nothing to build!

Set the attenuator to max attenuation with no speaker, and plug it into the amp in parallel with your speaker, using a second output jack on the amp. Set your amp to half the speaker ohms. Now the attenuator takes half the amp power acting as a load box, and the speaker gets half. This is an attenuation of -3db. It should work perfectly with an M3, if you have the right amp outputs,

2. You can add a switch that turns the -7db Stage 1 to -3,5db. Use this when you want this setting and other switches are off. You can also add the other stages in too, but the tone may be very slightly better in that case if you revert to -7db in the first stage if possible. Here's the schematic. Can you follow it?: If its of interest I can provide values to suit your build.


Its for an M2, but with M3, it would be similar with the extra coil and cap in series with L1, as in the base M3 design.

Hey John, option 1 won't work for me because I modded my princeton and put a mid knob where the extra speaker jack was located but option two sounds perfect and its what i was hoping since this option keeps the reactive load. I'll give it a go and report back. Thanks!

 

[JohnH]

hi @tfletchii , that's great. Have you got the values for the extra 2 resistors?

 

[tfletchii]

hi @tfletchii , that's great. Have you got the values for the extra 2 resistors?

No, I just sat down to take a good look at it. I would love your suggestions. At first I thought it would be as simple as taking the values from R7 & R8 and increasing the wattage but it doesn't look like thats the case.

 

[JohnH]

Values for the amended Stage 1 are in post 2359 on page 118, presented there as a standalone -3.5db/7db reactive stage, for either 8 or 16 ohm builds.

On page 120 post 2382 you can see the full circuit with i as i posted yesterday. You can see how stage 2 is now with 3 resistors. But if you don't use the stages 2, 3 and 4 unless stage 1 is at 7db (which i think will be better anyway) then you can keep them as they are.

But you could also consider just building the one stage on page 118, which might be a nice simple version for your gigs.

 

[tfletchii]

Values for the amended Stage 1 are in post 2359 on page 118, presented there as a standalone -3.5db/7db reactive stage, for either 8 or 16 ohm builds.

On page 120 post 2382 you can see the full circuit with i as i posted yesterday. You can see how stage 2 is now with 3 resistors. But if you don't use the stages 2, 3 and 4 unless stage 1 is at 7db (which i think will be better anyway) then you can keep them as they are.

But you could also consider just building the one stage on page 118, which might be a nice simple version for your gigs.

Hey John, thanks again for all the help. I'm about to order the needed resistors but should I get 100w for the R1 values? I saw in your post that 50w should be fine but is that still ok if I'm using the attenuator with my plexi