Simple Attenuators - Design And Testing

[MarkS2]

Thanks for sharing this and being so helpful to everyone! Fantastic work.

Does anyone have any reservations or caution about doing a simple bypass switch like the top of the following?

 

[JohnH]

Thanks for the quick reply @JohnH this is for R7 on the M2 16 ohm build, i hope iom reading that correct as to what i need, also is there a range reuqired for the switches in terms of amps volts etc.

Im looking at cliff uk for the jacks as im based in uk, which do you think seems most suitable, not sure of the differences of S1 S2 S4, thanks again

Oh yes, R7! 75 Ohms for a 16 Ohm M2, and 5% tolerance is fine, even if 5% off it makes less than 0.1db or 0.1Ohm difference overall.

I had a look at the Cliff specs, which i wasn't familiar with in detail. Actually they are all good in terms of that they will work fine in this design. The key thing about all of them is they are plastic bodied, so the case in not connected to the circuit, also they spec a rating of 5Amps and are high quality.

But, ideally, we want panel mounted, so solder lugs with a hole for the wire, instead of smaller PCB lugs. So I think that rules out S1 types which look to be small bodied and just for PCB (but if that's what you had you could use them). Not sure what the difference is with S4, maybe the nut style? still OK.

i don't think we need gold contacts

S2 looks fine. For your 16 Ohm M2 build, you only strictly need the simplest with two contacts. But I highly recommend getting the stereo type. The added spring contact grips the plug better even though you wont use it. (it does get used in the 8 ohm version). Many jacks also come with switched contacts, which connect across the switch and disconnect when you plug in. Again, all fine to use but make sure you use the lugs that are connected to the plug when inserted. (In the foot-switched version that Don is building, we do use the switch contacts)

On switches, most use a full-sized toggles either SPDT or DPDT in which case you can just use one side, or bridge the two sides in parallel to get more redundancy. Most switches are specced for mains switching with current ratings using 125V or 250V. Our voltages are much less, so we can use the 125V ratings. Minimum current rating should be 5A.

 

[JohnH]

Thanks for sharing this and being so helpful to everyone! Fantastic work.

Does anyone have any reservations or caution about doing a simple bypass switch like the top of the following?

Hi Mark and welcome to our thread. Yes you can have a bypass, but not as you show, which shunts the top of the circuit leaving the R3, R5 and R7 resistors still in circuit.

Bypass circuits need two poles to disconnect at both ends. I see yours starts the bypass after the first stage, so you would still get the first -7db reactive stage in circuit. That's all fine (with a two-pole bypass switch) if that's what you are wanting. May I ask what you want to use the bypass for? i have a bypass on my build (design M on post 1), but I took it off the ongoing current base design M2, since it makes it simpler and less subject to errors in use.

 

[MarkS2]

Hi Mark and welcome to our thread. Yes you can have a bypass, but not as you show, which shunts the top of the circuit leaving the R3, R5 and R7 resistors still in circuit.

Sorry for the false start and waste of time. This is what I get for trying to bang out a quick question with a distracting toddler on hand.

What I meant was to connect the left side to the input tip. i.e. a single pole switch connecting/disconnecting from input tip to output tip.

May I ask what you want to use the bypass for?

I'm simply trying to be able to restore full volume without unplugging and plugging cables. Would the direct input-to-output tip be the best bet for this? If not can you tell me a bit more about what you had in mind? Or alternatively why I don't really need it? EDIT: I took a closer look at the original M circuit and yes, I see what you mean. I'm simply going to bypass from input tip to output tip with a DPDT.

Thanks again!

 

[Bearzooka]

Hello Marshall Forum! Quick question: On the M2, instead of using 3 separate switches could you use a rotary switch instead? Thanks!

 

[JohnH]

Sorry for the false start and waste of time. This is what I get for trying to bang out a quick question with a distracting toddler on hand.

What I meant was to connect the left side to the input tip. i.e. a single pole switch connecting/disconnecting from input tip to output tip.



I'm simply trying to be able to restore full volume without unplugging and plugging cables. Would the direct input-to-output tip be the best bet for this? If not can you tell me a bit more about what you had in mind? Or alternatively why I don't really need it? EDIT: I took a closer look at the original M circuit and yes, I see what you mean. I'm simply going to bypass from input tip to output tip with a DPDT.

Thanks again!

OK yes, like this:


Things to watch out for:

Not operating the bypass while the amp is on, since the full power is being switched, so you'd have thte amp off or on standby
Also, M2 is able to be used for different cab Ohms, provided the attenuator ohms matches the amp. But in bypass mode, a match of amp and speaker is needed and also the Out3 if you have it, is not in the bypass loop as shown, so you need a load in Out 1 or 2

Bypass could be useful if your power levels are such that you often want to go from attenuated to not attenuated, or if you are building it into a head or combo as a fixed unit. But usually, a given set up such as a home studio, will always need the attenuator, or always not need it in which case you dont need it then.

 

[JohnH]

Hello Marshall Forum! Quick question: On the M2, instead of using 3 separate switches could you use a rotary switch instead? Thanks!

in principal yes, but getting a robust rotary with enough current capacity and enough poles and switch positions can be difficult and expensive. The three toggles are cheap, simple to wire up and reliable and together they give you 8 even steps.

 

[Bearzooka]

in principal yes, but getting a robust rotary with enough current capacity and enough poles and switch positions can be difficult and expensive. The three toggles are cheap, simple to wire up and reliable and together they give you 8 even steps.

Thanks for your reply JohnH. What kind of switches would you recommend (Volts, Amps, Watts)?

 

[JohnH]

Thanks for your reply JohnH. What kind of switches would you recommend (Volts, Amps, Watts)?

Actually we had this earlier today!

On switches, most use a full-sized toggles either SPDT or DPDT in which case you can just use one side, or bridge the two sides in parallel to get more redundancy. Most switches are specced for mains switching with current ratings using 125V or 250V. Our voltages are much less, so we can use the 125V ratings. Minimum current rating should be 5A.

 

[Bearzooka]

Much obliged.

 

[Graham G]

Hi JohnH, I've read quite a lot of the thread & although i'm capable of building on of these I have no circuit reading or technical knowledge at all, I would need a paint by numbers or detailed to pics to build, is anything like this available in the thread ?, or is it not for dim wits .
Just in case it can be done by a dim wit, I have a HF Modded ORI50 (8 ohm speaker) which I only use for gigs, the problem is I don't know how much attenuation I will need, so I suppose a Variable knob would work best, as I guide my ORI20 & DSL20 are not loud enough so I suspect half way between the 20 & the 50.
If this is not possible i'd just like to say that people like yourself, that give up so much knowledge & time for free is generous to the extreme.

 

[JohnH]

Hi Graham, thanks for your interest. I've actually made a point of not posting a pictorial paint-by-numbers guide. The reason is, it's really important to be able to relate the diagram to the build and have a sense of how its all connected, and to be able to review and check it, and test it with a multimeter. It's almost impossible to trouble shoot over the internet, and the only person who can verify a build is the one who built it. Also, all builds come out a bit different due to their parts and the needs of their owner.

It's actually a simple circuit with robust parts. If you can figure out how to wire it, and are proficient with a soldering iron and drill, then im happy to discuss. But reading the diagram is the entry exam !

 

[MarkS2]

OK yes, like this:
View attachment 93963

Things to watch out for:

Not operating the bypass while the amp is on, since the full power is being switched, so you'd have thte amp off or on standby
Also, M2 is able to be used for different cab Ohms, provided the attenuator ohms matches the amp. But in bypass mode, a match of amp and speaker is needed and also the Out3 if you have it, is not in the bypass loop as shown, so you need a load in Out 1 or 2

Good info thanks. I'm omitting out2 and out3. Just a single output for 8ohms only.

Bypass could be useful if your power levels are such that you often want to go from attenuated to not attenuated

Exactly. I'm going to be trying this on a 15W that's just on the border of too loud.

 

[Gene Ballzz]

@donwagar & @JohnH
So cool to see another one done. Don's usage experience mirrors and confirms my own regarding amps with multiple wattage settings, especially concerning the use of this attenuator design. The amp will nearly always sound better at it's full wattage/power setting and attenuated to suit, as opposed to a lower wattage/power setting with less attenuation to achieve similar sound pressure levels. This is not always the case with other attenuators!

I am however, going to pick a few nits here.

> I'm not familiar enough with that style/brand/model of jacks, but if they are not fully isolated from the chassis/frame/enclosure, they should be. Particularly in the case where you will be sending AC voltage through a wire, to a pedal that gets stepped on by a human who is solidly connected to ground/earth through that human's guitar/cable etc! This in addition to a few other anomalies that have popped up throughout this thread with non-isolated jacks. If the jacks actually facilitate isolation, "never mind!"
> Never been a real fan of push on/blade connectors, especially in something that may get bounced around in a gig-bag, cable box etc.​

Other than those couple nits, looks great and really glad you've discovered how well it actually works!
Nice Job!
Gene

 

[Graham G]

Hi Graham, thanks for your interest. I've actually made a point of not posting a pictorial paint-by-numbers guide. The reason is, it's really important to be able to relate the diagram to the build and have a sense of how its all connected, and to be able to review and check it, and test it with a multimeter. It's almost impossible to trouble shoot over the internet, and the only person who can verify a build is the one who built it. Also, all builds come out a bit different due to their parts and the needs of their owner.

It's actually a simple circuit with robust parts. If you can figure out how to wire it, and are proficient with a soldering iron and drill, then im happy to discuss. But reading the diagram is the entry exam !

Thanks John, so if I only need an 8ohm out, which is the simplest diagram/version to get to grips with, on the plus side I do have a decent iron/solder & a multi meter & I have a lot of experience building hi-fi amps including PCB stuffing, so maybe I should start to understand in a small way how what i'm doing

 

[donwagar]

@donwagar & @JohnH

> I'm not familiar enough with that style/brand/model of jacks, but if they are not fully isolated from the chassis/frame/enclosure, they should be. Particularly in the case where you will be sending AC voltage through a wire, to a pedal that gets stepped on by a human who is solidly connected to ground/earth through that human's guitar/cable etc! This in addition to a few other anomalies that have popped up throughout this thread with non-isolated jacks. If the jacks actually facilitate isolation, "never mind!"
Gene​

​

Thanks Gene.

I am a pack rat (I prefer that to 'hoarder' or 'junk collector'). Those jacks are, I think, these:

https://www.markertek.com/product/s...gqabVIx9myr2XmDcnFvHCwhQiSuea8WhoCDk0QAvD_BwE

They are stereo switching from a Mackie circuit board I picked up years ago (had 50 of them on the board, I think I paid $10 for it), so I've used them anywhere I need a jack. I don't think they are isolated, it's not a big deal to switch them if you think it's warranted.

 

[MarkS2]

Any issues or concerns about switching the 2-4 stages on and off while powered up?

 

[Gene Ballzz]

Two important suggestions to all!

A> It is my considered opinion that we should all firmly and truly embrace the title of this thread: "Simple Attenuators - Design And Testing!" While many bells, whistles and/or additions are certainly possible and almost endless, the simplest form and usage of this design performs so flawlessly at the task that many folks have been dreaming of since the first guitarist playing gloriously into a cranked amplifier was asked/told to "TURN THAT DAMNED THING DOWN" that some of those additions may end up compromising (or at the very least "complicating") the whole build, simplicity, durability and longevity of the finished product! The plain, unadorned unit does exactly what it was initially intended to do: "Allow the use of an amplifier, cranked up into it's glorious (yet usually way to loud) sweet spot, and then have that overall volume tamed to reasonable levels "WITHOUT" compromising tone and tactile response/feel. There is a lot to be said for "Keeping It Stupidly Simple!"
B> While I realize this is @JohnH 's thread, as an avid fan, contributor and participant I humbly beg anyone who modifies anyof John's drawings, please, PLEASE, PLEASE clearly notate on that drawing that it has been modified/edited from the original! Who knows what kind of achiving, condensing, etc may happen to this thread in the future and it would be sad to see erroneous and sometimes incorrect diagrams to be saved and passed on!​

I've said it before and I'll say it again: "I've built multiples of the M-Lite and have found them to be the greatest and most liberating pieces of gear I've ever owned, in my +50 years of "Screechin' & Squawkin'" with guitars through amplifiers!"

Thanx @JohnH & All,
Gene

 

[donwagar]

Any issues or concerns about switching the 2-4 stages on and off while powered up?

Someone that actually knows what they are talking about will probably chime in. But I don't think so, I do that when I'm setting my volume, also I'm building a -3.5dB footswitch to cut volume on the fly at gigs.

 

[JohnH]

Thanks @Gene Ballzz and Hi @donwagar

Gene makes a good point about the jack insulation, that I should have picked up when you showed me your jacks before and i apologize. I agree that yours are not isolated, since the metal threaded part connects to the plug barrel and its also what is contacting the case. This puts the attenuator case connected to the 'cold' wire of your cable from the amp. This is not usually a good ground, and may be just one end of the OT output winding. Im not sure if this will really cause a problem, but Ive been advising to use the plastic bodied Cliff jacks which keep the case not involved in the circuit.

I had a look at pictures online of Weber Mass attenuators, which clearly use non-isolated open jacks, but he may have an isolating washer/bush each side of them. Gene, if you still have your Weber, maybe you could take a look? eg On the front panel, is there a white isolating washer under the head of the jack nut?

But, even non-isolated, it still works, as you have proven. Your jacks loo fairly tigt next to the switches, and lugs are at the back, so changing them may be tricky?

But for your footswitch jack, i think it should have isolation (since the footswitch box is more likely to meet something else on the floor). You can use the Cliff type with 6 lugs that I have a small photo of on the footswitched diagram.

To isolate a non-isolated jack, it needs a plastic washer both sides and something around the threads as it passes through the chassis.