Simple Attenuators - Design And Testing

[JohnH]

Hi @Yamariv

its really a good idea if you work out the wiring yourself on a sketch, taking into account the size of the parts and the case that you have. Then you will have thought it through and have something to work off.

Every layout built so far has needed to be a bit different and all builds that I know of have been a success. I haven't wanted to post a wiring layout because then it becomes possible to try to build these without thinking it through and hence mistakes could be made. Interpreting the schematic into a wiring layout is the entry exam for using the design!

When working out a physical layout related to the schematic, in this case, it doesn't matter how the wiring goes so long as the correct lugs are connected. eg A to B to C is the same as B to A to C or A, B and C all to one point etc. (not true in an amp but perfectly ok here)

On the schematic, the switched stages are shown -7, -14 then -3.5, for a small electrical reason. You can follow that in terms of wire connections but physically, you probably want to place the switches 14, 7, 3.5 or 3.5, 7, 14

Think about how to place the jacks and switches in your case, so that it operates conveniently in your rig.

Try to minimise lengths of wire runs where possible

Think about the order of wiring and how you will work on it and open and close the case

The case should not be connected to anything.

When its built, test it with a resistance setting of a multimeter before using an amp

 

[Maurizio Minguzzi]

Good morning thanks for the work you are doing and it looks great. I made the m-4 version with two inductors my idea is to add a control for the treble by inserting a capacitor with fc 1000 hz on r2. for more crystal clear sound. There are other changes to the scheme to achieve this

 

[Yamariv]

Hi @Yamariv

its really a good idea if you work out the wiring yourself on a sketch, taking into account the size of the parts and the case that you have. Then you will have thought it through and have something to work off.

Every layout built so far has needed to be a bit different and all builds that I know of have been a success. I haven't wanted to post a wiring layout because then it becomes possible to try to build these without thinking it through and hence mistakes could be made. Interpreting the schematic into a wiring layout is the entry exam for using the design!

When working out a physical layout related to the schematic, in this case, it doesn't matter how the wiring goes so long as the correct lugs are connected. eg A to B to C is the same as B to A to C or A, B and C all to one point etc. (not true in an amp but perfectly ok here)

On the schematic, the switched stages are shown -7, -14 then -3.5, for a small electrical reason. You can follow that in terms of wire connections but physically, you probably want to place the switches 14, 7, 3.5 or 3.5, 7, 14

Think about how to place the jacks and switches in your case, so that it operates conveniently in your rig.

Try to minimise lengths of wire runs where possible

Think about the order of wiring and how you will work on it and open and close the case

The case should not be connected to anything.

When its built, test it with a resistance setting of a multimeter before using an amp

Thanks John, makes perfect sense about the layout when you explain it that way! I was reviewing the schematic after your reply and it's looking much simpler than I remember, I should be fine! Fingers crossed!

Quick (possibly dumb) question.. if my inductors have a plastic bobbin as a surround, is there any issue with me using a metal bolt to mount it if the inductor itself is not touching any part of the chassis? Seems obvious to me that a metal mounting nut would be ok but just wanted to make sure that I wasn't missing anything as I haven't worked with inductors before (and I don't want to buy a bunch of nylon bolts and nuts if I don't have to.. )

 

[JohnH]

hi @Maurizio Minguzzi , that's a great solid looking build!

That cap idea should work to add further treble - you'll need a bipolar capacitor and youd work it out so 1/2pi.f.C = R2 if you want to start to roll in at f = 1000 hz. Using nearest standard values, I make that 15uF for the 8 Ohm build and 8.2uF for the 16 Ohm version.

Quite a few attenuators have this to try to compensate for their inherent tone issues, which we don't have in this design.

But, my concern with doing that is that it will throw off the input impedance as seen my the amp, reducing it at high frequency. So be careful, probably OK but some amps might not like it. I have heard that switching in such a cap on some other designs can stress the tubes

I tested it in a Spice run, using an 8 Ohm version of Design M (I think M2 will be similar, also 16 Ohm ones will show similar curves).



The upper lines represent what the amp sees and the lower ones are at the speaker, in this case at -14db. Switching in the cap raises the response from the lowest black line up to the blue line, so you get a few db more treble. The amp then sees the green response instead of the upper black one (or red with full volume no attenuator). What's good is that although the green line drops, it doesn't dip lower than a the red/black ones at mid frequency, so it looks safe to try with care.

 

[JohnH]

Hi @Yamariv . Even if a steel nut and bolt is not contacting anything, it can still affect the inductance, raising its value. Its because steel is much more permeable to magnetic flux than is air. That's why transformers and larger inductors have iron cores.

I tested this will mine and was surprised to find a significant affect on my coil inductances even with just a small steel bolt.

As an alternative, nylon or stainless steel bolts are ok (stainless is generally non permeable), or zip ties (see Maurizio's above), or, I just used an adhesive and put my coils on the base.

 

[Yamariv]

Hi @Yamariv . Even if a steel nut and bolt is not contacting anything, it can still affect the inductance, raising its value. Its because steel is much more permeable to magnetic flux than is air. That's why transformers and larger inductors have iron cores.

I tested this will mine and was surprised to find a significant affect on my coil inductances even with just a small steel bolt.

As an alternative, nylon or stainless steel bolts are ok (stainless is generally non permeable), or zip ties (see Maurizio's above), or, I just used an adhesive and put my coils on the base.

Oh wow.. I'm glad I asked then! I thought it was a dumb question So now to decide from all those options, I might just do some silicone adhesive right to the case.. Thanks again John, gonna start building soon

 

[Maurizio Minguzzi]

Thanks JohnH for the very accurate answer. Now I'm thinking of building the M3 version with two coils and a capacitor, these attenuators work really well, have a good day Maurizio

 

[Mcentee2]

Hi @Mcentee2

Good questions, and there's good news:

Provided that at least the first -7db stage is on, it should work fine with a four ohm load on the attenuator, and using the 8 Ohm amp tap. You checked the resistance seen by the amp and it still stays close to the 8 ohms that the amp expects. The attenuation levels stay close to the same but with about another -1to -2 db attenuation through the mids compared to bass and high treble. You may not be able to hear this difference, but if you wanted to compensate, a small reduction in amp resonance and presence should fix it.

For using different amp taps, weber uses just a parallel resistor, but if you do that you partly loose the benefits of the reactive input. I worked out an add-on comprising another coil and two resistors which you can put in series with the input to convert to 16 ohms or in parallel to convert to 4 ohms. But in either of these cases, all attenuations drop another -3 db.

The diagram below has values to take a base 8 Ohm M2 and use it with 16, 8 or 4 ohm taps, or to start with a 4 Ohm M2 and use 8, 4 or 2 ohm amp taps (useful for certain old Fenders)

Attenuator M2v 200420

• JohnH
• Sep 20, 2020

I knocked up that new pre stage over the weekend, and it works brilliantly, even with my slightly off resistors.

Can this stage be used just by itself as a -3db drop with no ill effects as I can see the R12 and inductor arrangement is slightly different to the main -7db stage?

I know my resistor lower ratings are ok.

 

[JohnH]

Yes i think that's ok to use it on its own with a speaker. It takes off -3db and the impedance changes x2 or x1/2 depending how its connected.

 

[Mcentee2]

Yes i think that's ok to use it on its own with a speaker. It takes off -3db and the impedance changes x2 or x1/2 depending how its connected.

Apologies though, I probably didn't think it through fully before asking the question!!

I suppose on its own it is an 8r load, it is only when in parallel/series with a simple 8r M2 does it do it's useful 4r/16r thing.

On its own, it still a useful thing to have, a 3db 8r stage, but I was probably asking the "wrong question" , doh!!

My actual unit doesn't match your calculated R12/R13 values exactly, so when I use just it and my 2x8r speakers (4r), I get 2.8r, which is a bit low to just use on its own as I don't have any 2r amp outputs.

 

[crashcarlson]

I feel kind of dumb here, but I'm hoping this is a simple issue that someone else has run into and knows a way around it.

There's a few places where I need to solder two wires to a single lug of a resistor. For example, on the M2 design, L1/R1/R2B all need to connect, either one lug of R2B or one lug of R1 will have two wires connected to it. But with the 18 gauge wire I'm using, there's no way I can fit both wires through the lug - it's just too small.

Do I have some oddball resistors? Are you drilling those solder lugs a little wider before installing them? Are you putting one wire through the lug and soldering it, and then soldering the other wire directly to that wire, instead of going through the lug (I'm sure that would work but I'd prefer to have a physical connection to the lug if possible)?

Thanks!

 

[JohnH]

hi @crashcarlson , personally, I feel ok about my joints if I have good tinned ends on my wires and a good amount of contact, even if the wires aren't all through holes. Then melt with a little more solder to flow smoothly. I want my solder joints to look like the liquid-metal bad guys in Terminator 2!

 

[crashcarlson]

hi @crashcarlson , personally, I feel ok about my joints if I have good tinned ends on my wires and a good amount of contact, even if the wires aren't all through holes. Then melt with a little more solder to flow smoothly. I want my solder joints to look like the liquid-metal bad guys in Terminator 2!

Sounds good - they are SUPER close to fitting. In fact, the inductor is 19 ga. instead of my 18 ga. wire, and I was able to fit those both into one lug. I might try getting a small circular file and seeing if I can very slightly increase the diameter. But if that fails I'll just glob them all together with more solder and ensure it's all connected with a multimeter. Thanks for the feedback!

 

[crashcarlson]

@JohnH - I just wanted to say THANK YOU for all the time and effort you put forward on this project. I was able to get it all wrapped up and tested out today and I am VERY happy with it!

My scenario might be a little different than most. I have a Blackstar HT Club 50 Mk II 50 watt amp. I don't need to crank it to get OD, since it has it's own OD channel...but I don't really use that anyway. I use the clean channel and a few nice dirt pedals. The amp does have a power scale (either 5W or 50W), as well as a master volume. I found that my pedals sound MUCH better with the added headroom on the 50W mode, but even with the master way down it was still pretty loud. With the attenuator hooked up, even with just the first -7dB stage, I can get some AWESOME sounds without getting way too loud for the room!

Now that I know what I'm doing I think I'm going to make another one but make it a little cleaner and maybe do some decals instead of just writing on it.

I did have one question - if I did want to add an overall "bypass everything" switch so I could run it with no attenuation, would I just need another 5A/125V switch? I'm thinking I would then wire the signal (Tip) from the input to the COM on the switch, and then one side directly to the output jack, and the other side to the R2A/L1 components right? If I go that route are there any "gotchas" (like, don't switch to fully bypassed while the amp is on, etc.)?

Thanks again for everything - this design you've done here is really awesome!

Attenuator in action

• crashcarlson
• Oct 8, 2020

Inside attenuator

• crashcarlson
• Oct 8, 2020

Bottom of attenuator

• crashcarlson
• Oct 8, 2020

Back of attenuator

• crashcarlson
• Oct 8, 2020

Front of attenuator

• crashcarlson
• Oct 8, 2020

Top of attenuator

• crashcarlson
• Oct 8, 2020

 

[JohnH]

Hi @crashcarlson , I really like that build! Good use of space in the case. I like the hand graphics too, which remind me of my old Crate PowerBlock amp.


Bypass:

You can have it if you want it. It should be a two pole switch. One common to the input, one to the output. In bypass mode, one set of outer lugs are connected together, totally disconnecting the attenuator parts from input or output. The other pair of outer lugs direct the signal into and out of the attenuator. Its similar to a basic true-bypass wiring on a stompbox.

I set the current M2 base design without that, to keep it simple, save a switch and save two sets of contacts in the signal path. I figure there's rarely a reason to keep switching from bypass to attenuated, other than just to try it.

But, my own one is Design M. For that one, I show a bypass switch that happens not right to the output but to just before the final stage which is -3.5db. This allows the -3.5 stage to work on its own, so every 3.5db increment from 0 to -31.5db is available. See post No.1. I just wanted to try it, and it works and sounds fine. But I haven't found it to really be any use to me! But if wanted, it could be done either way on Design M2.

But the watchits with either of those bypass options:

Without the bypass, its not sensitive to switching on the fly. But, if you switched to bypass during playing, unknown transients might get back to the amp.

Also, without bypass, its very tolerant of different cab Ohms , provided you use the right amp tap as designed. But if you use bypass, or that extra -3.5db only setting above, then cab and amp Ohms need to match.

 

[crashcarlson]

Without the bypass, its not sensitive to switching on the fly. But, if you switched to bypass during playing, unknown transients might get back to the amp.

When you say "during playing" do you mean actually switching it while you're playing guitar, or just having the amp on? For example, if I put the amp in standby would it be safe to switch to bypass, or would the only real safe way to do it be to switch between bypass when the amp is completely off?

 

[Paul Buxton]

Hi everyone, just joined the forum after finding this awesome thread!
There is so much information here it is mind blowing.

Like a lot of people when the pandemic started, I decided to pick up an instrument, after a few weeks playing a guitar I had kicking around in the garage (a birthday present from my teenage years), I decided I would like to build my own guitar from a kit. Which I did. That naturally led me to look at amps as well, and it wasn't too long before I ordered a tube amp kit and built that too
But it turns out that the amp was a little too good and I couldn't crank it up enough to get some nice distortion without incuring the wrath of my girlfriend, so I then started to look at attenuators. I built an lpad based one, which lets me crank the amp up, but when using it with my neck p'ups it really sounds like I am under-water! So the search for better attenuation led me here!

Skimming through the thread I think I will be looking at the M2 design. Could anyone confirm that using the 8Ohm version with a single 8Ohm speaker is fine? I note the two outputs on the design and just want to check that it is not intended to run 16Ohms in parallel which would be equivelant to an 8 (I think trying to dredge up whatever electronics I can remember)?

Given the volume of information here would it be useful to put it somewhere more structured? I would be happy to setup a github repository to collate it...

Thanks!

Paul

 

[JohnH]

When you say "during playing" do you mean actually switching it while you're playing guitar, or just having the amp on? For example, if I put the amp in standby would it be safe to switch to bypass, or would the only real safe way to do it be to switch between bypass when the amp is completely off?

With the amp powered but on standby its totally safe to switch anything or reconfigure your speakers no problem. That's what I do if I want to unplug/remove or add the attenuator, or use its bypass switch, also if I'm going to the -3.5 setting that I have.

What I would definitely not do is hit a chord and go to or from bypass.

Between the lower attenuated settings eg at least -7db and more, it should be ok to switch on the fly to find the level that you want but I prefer to just mute the strings or amp volume while I do that switch change.

 

[JohnH]

hi @Paul Buxton , welcome! you have found the right attenuator!

The 8 ohm M2 design is intended for using the 8 Ohm tap from your amp with either a single 8 ohm speaker cab, or two of 16 Ohms, hence its two parallel output sockets. Its also safe to use it with 8 ohm amp and a single 16 Ohm speaker, and if you build the optional red coloured parts in the diagram with the third output, it just corrects the tone for that arrangement.

What amp do you have?

Yes there's a lot of info and most of it is a journey rather than a destination. I try to keep all the current key info in post 1, though its limited to 10000 characters so it gets squeezed

 

[Yamariv]

Hi @Yamariv . Even if a steel nut and bolt is not contacting anything, it can still affect the inductance, raising its value. Its because steel is much more permeable to magnetic flux than is air. That's why transformers and larger inductors have iron cores.

I tested this will mine and was surprised to find a significant affect on my coil inductances even with just a small steel bolt.

As an alternative, nylon or stainless steel bolts are ok (stainless is generally non permeable), or zip ties (see Maurizio's above), or, I just used an adhesive and put my coils on the base.

Got my chassis's drilled out and started painting them today..just about to make up some decals and clear coat tomorrow

One quick question about the M2 16ohm Version I'm building.. Are there any drawbacks to running my amp's 16 ohm output through the Attenuator into an 8 ohm cab? Others have said it's safe to do, are there any other drawbacks to doing that, most of my cabs are 8 ohm.. I'm now thinking the 8 ohm version is what I should have built but I have all the parts for the 16 so..