Simple Attenuators - Design And Testing

[JohnH]

Will using a 16 AWG at 0.9 for the coil work? I see Madisound has them. Would that be too large for most enclosures?

That would be fine, obviously its bigger and heavier than the 19 gage though. id use that if it needed to run with 100W amps, otherwise, Id be happy with their 0.85mH 19 gage.

Are you building the 8 Ohm version?

 

[mike_lawyer]

That would be fine, obviously its bigger and heavier than the 19 gage though. id use that if it needed to run with 100W amps, otherwise, Id be happy with their 0.85mH 19 gage.

Are you building the 8 Ohm version?

Yes, building the 8 ohm version. I have the case and most of the resistors. Just need to get the jacks, switches, and air core resistor.

 

[TheOtherEric]

@TonyK if your premise is that your amp sounds different at speaking volume versus screaming-loud, I’d encourage you to test that assumption before spending time and money building stuff. Back around page 69 or 70 I posted some spectrums and sound clips played thru a looper (recorded with SM57) at REALLY loud then max attenuated speaking-volume (without touching any amp controls). They were indistinguishable. Led me to tentatively conclude that the whole “pushing air” argument is bogus. You should try it too.

 

[Gene Ballzz]

@TonyK if your premise is that your amp sounds different at speaking volume versus screaming-loud, I’d encourage you to test that assumption before spending time and money building stuff. Back around page 69 or 70 I posted some spectrums and sound clips played thru a looper (recorded with SM57) at REALLY loud then max attenuated speaking-volume (without touching any amp controls). They were indistinguishable. Led me to tentatively conclude that the whole “pushing air” argument is bogus. You should try it too.

While I often play at near bedroom volumes through either one of my main amps, using a 16Ω, 25 watt/75hZ G12M greenback in their open back combo cabs, I've found them to sound much bigger and better through the exact same speakers in my 4x12 1960B Marshall cabinet from '72. I'm not sure if its because of it being closed back or simply because of 4 speakers. Its not actually any louder, just more full sounding! Of course, I've been playing through this particular cabinet since 1974, it's my favorite, so I'm kinda partial to and very familiar with how it sounds!
Just My Take On It,
Gene

 

[acromarmot]

Schematics software

I've just been monkeying around with some software for drawing schematics. My diagrams to date have just been hand-carved out of MS Word, so I've been looking for something a bit more mission-specific. I've tried Tinycad, which is a free open-source download and works quite well, and also Scheme-it which is an online app based at Digi-key (you have to sign up to save diagrams).

So here's some versions of the Attenuator M2 diagram. No design changes here, just playing with graphics:

This is from Tinycad:

View attachment 84960

Here it is from Digi-key Scheme-it:

View attachment 84961

And here it is tricked-out with some colours and added info, using Word:

View attachment 84967

Tinycad was easy to use, and you can work offline, but I like the clarity of the Scheme-it linework and its also user friendly.

Hi @JohnH,

You may wanna look into KiCad. It's a free electronic design suite, from schematic drawing, some simulation tools to pcb design.
It might be a big over the top just for the attenuator drawings but is a great help if projects expand and somehow end up with a pcb or alike.

Cheers and thanks for the great efforts and work!

 

[Timonew]

Many thanks to JohnH for the design.

I’ve enjoyed reading the thread - lots of good tips throughout - thanks to Gene and LordoftheLivingRoom.


I’ve kept my M2 8 Ohm build as simple as possible with plenty of space in my recycled bits case (cheers Pete). I bought DPDT switches with large contacts to make the soldering easier.


The inductor came from qtasystems.co.uk and after an email confirmation and PayPal payment it arrived well before the resistors from Mouser!


After checking the input resistance with the speaker plugged in was around 8 Ohms in all switch positions I tried the attenuator today with my 18 W combo (ampmaker) and it works! First impressions are it’s excellent and I’m going to be able to enjoy using the amp much more now.

 

[JohnH]

hi @Timonew , thanks for posting. That looks really well thought out with a very clean layout. I like the heatsink behind R1, which is where the greatest heat would be, plus it looks badass!

 

[Gene Ballzz]

@Timonew
Yessiree! Very nice and tidy indeed!
Great Work!
Gene

 

[TonyK]

Hi @JohnH, sorry for the delayed response, we've been struggling with some immediate effects of the changing climate here - floods unlike any for about 40-60 years! Luckily our house didn't get flooded but most of our property is only just beginning to dry out

Anyway, thanks very much for your thoughts. So if I understand you correctly, the 50W version of M1 should be adequate to simply disconnect the speaker from the attenuator box if I then use a Line Out to go somewhere else? That is very encouraging. As things stand, this would be with my 60W Deville. But I'm beginning to think that after 20 odd years of enjoying that amp, it's time either for me to give up guitar as the sound and feel no longer make me feel "good", or perhaps it's time to simply get an amp that is significantly "better", perhaps a Deluxe Reverb or more expensive boutique flavour of such, or I've even been toying with the idea of a Vintage Modern 50. But in any case, it sounds as though the solution you are beginning to outline would be just fine with these kind of alternatives.

So a quick question. Running through the attenuator at maximum attenuation without a speaker ... what actually would pass through a Line Out? Would it essentially be the same sonic characteristics as the amp at real volume (with appropriate IRs of course), or would be more likely a watered-down version of such?

Good luck with your cab-sim project and thanks a bunch!

 

[TonyK]

@TonyK if your premise is that your amp sounds different at speaking volume versus screaming-loud, I’d encourage you to test that assumption before spending time and money building stuff. Back around page 69 or 70 I posted some spectrums and sound clips played thru a looper (recorded with SM57) at REALLY loud then max attenuated speaking-volume (without touching any amp controls). They were indistinguishable. Led me to tentatively conclude that the whole “pushing air” argument is bogus. You should try it too.

Hi Eric, quite some time ago I actually recorded some clips using a looper while simply going through the different attenuation levels. When I normalised the recordings they were almost indistinguishable. So I think that's what you're getting at.

I guess what I'm saying is what many of us already feel, is that once the volume has dropped significantly, the feeling/depth/fullness/pleasure/[insert any other subjective adjectives] becomes different. And in my case, it's less pleasurable so I try to practice with a fairly loud volume, typically with SPL in the 80-90db range. That's what I was alluding to, but it is possible that you are referring to something different again.

As @Gene Ballzz says, there is something about certain speaker cabs that makes us "feel" like we're hearing something "better". In his case a lovely 4x12, but in my case generally the 4x10 of my Deville, or an oversized 1x12 cab loaded with an ET65. But neither feels very fulfilling at very low volume even though the actual tone perceived is very similar to the unattenuated sound.

It was my understanding, and perhaps I'm wrong, that one of the pluses of using amps such as the Yamaha THR, or powered monitors like the Genelecs, is that these very small speakers are somehow designed to be far more pleasing without the SPL to reinforce the sound. That's what I was considering. But as I mentioned in my response to @JohnH, the reasons for my misgivings could indeed be something quite apart from all of this. with a tiny bit more detail, although I've been playing for well over 40 years, I have a neurological disorder that has significantly cramped my playing technique and finesse. So I adapt by looking for different songs to play, focussing on tone (obviously very technique-based) rather than fast or flashy type leads and melodies. Recently I decided to learn John Mayer's "Slow Dancing" for this reason, and no matter which amp I use in my collection, I am very dissatisfied with the tone that I'm getting (particularly Deville 4x10 with a G&L Legacy). So either it's me, or perhaps a "sweeter" amp might help, and perhaps with lower volumes as per my original question/idea!

Sorry to digress

 

[JohnH]

hi @TonyK , thanks for your comments above. I hope your place drys out!

So, attenuator at max attenuation, no speaker and line-out, you take the line out from the amp out = attenuator input, and dial down the line-out pot to a level to suit your connected device's input .

The amp is seeing a pretty good electrical facsimile of a real speaker, and hence passing this on to the line-out. There's a couple of provisos:

Real speakers create a very local resonant peak in the bass at between 80 and 110 hz, for open backed and closed back cabs respectively. When we use our M or M2 attenuators with a speaker, the speaker creates this itself at the attenuator output. But these designs don't create this peak at the amp, and so its obviously not there when running with no speaker. This is not a problem at all. This peak is only relevant if you play, in a closed cab, around low A and a note or two either side. With open back, it would only affect low E, maybe up to G. Without the peak, you theoretically get a more even bass response, and feel free to add a bit of bass somewhere in the amp or downstream if you wish.

But if you are not needing to run silent, you can keep the cab in and take a signal from there after the attenuator. It is probably already at a good level with little more reduction needed. I think a switch for line-out pre or post attenuator would be good.

From about 150hz and above, our design is a very close electrical match to a real cab, its as close as any $1000 active unit that you can buy, and better than some, because it closely tracks the response of a real measured cab. What it misses is the acoustic response of the speaker, and any room and mic effects, which is where an IR box would come in. Something like a Mooer radar looks like a good small fun option for this.

My cab sim ideas have gotten as far as making some raw recordings loaded by the attenuator only, twisting them with an EQ curve in software so their response matches a miced recording, and working out a network of components that will reproduce that in real time. I haven't tested the real version yet but from these sims, it sounds like the results will be very clear and direct sounding, great for tracking or PA, maybe add a little reverb to it. There are commercial all analogue cabsim boxes that do this too, such as a Hughs and Kettner Redbox.

Vintage Modern

If you think that you might like one of these, then you definitely need one! The combo is a heavy mutha though, but excellent in every way.

 

[Gene Ballzz]

@JohnH
Do I recall, somewhere earlier in this thread, that you may have mentioned that the two choke M & M-lite designs do a slightly better job of mimicking and/or tracking that "local resonant peak" that you're speaking of? And would that mean that the "two choke" desigm might lend itself better to using as a load box, with line out functionality? All I kow is that my 16Ω, M-lite units sound absolutely fabulous in every sitution I've tried/used them in, with multiple amplifiers and speaker configurations. While I can't speak for the 8Ω units, M-2 and/or multi impedance designs, if they are even close to my M-lite units, they would compete quite well and even surpass the performance of most, if not all of the currently, commercially available offerings at and above the $500 price range! I would think that the multi input impedance/multi output impedance units couls easily sell in the $600 to $800 range, especially if combined with a selectable pre/post attenuator line out. The fact that they require no power supply is a huge plus!
Just Curious?
Gene

 

[JohnH]

hi Gene

The M and M lite versions that you and Tony have done have virtually identical performance to the M2 (if anything, M2 may actually be a tad better). None of them try to make a bass rssonant circuit, leaving that to be done by the real speaker.

There is the M3 design that has that bass circuit, but the benefit looks to be minimal, at the cost of a lot of extra $ to do it right.

 

[Gene Ballzz]

Thanks for that clarification @JohnH ! So maybe still plugging an actual speaker into the attenuator and maybe putting that speaker into an isolation box or out in another room (even at bedroom volumes) may be beneficial? The isolation box could be much simpler than one intended to be used for miking purposes. Simply using the speaker for it's resonant peak response? Of course, that extra "speaker in an isolation box" would likely cost more and be much more "messy & cumbersome" than the extra "bass circuit" you mentioned!
Thanks John,
Gene

 

[JohnH]

The bass circuit is just two extra parts added to an M2. For an 8 Ohm base, its a 180uF cap (or two at 90uF in parallel), and a 9mH iron core inductor. At 16 Ohms, they become 90uF and 18mH, which is probably less $. The big Kahuna there is the cap which needs to be bipolar with high ripple current spec. The best are polypropylene. Those parts can add up to another $100. Id like to try it to test it. Note its not feasible on an M or M-lite, since it would then need two sets of caps and extra coil instead of one

If its use is primarily as a load box, then it may be of value to add those. But without actually trying it, Ive tried to find any tonal problem with not having it, and I just cant find it, even when really trying to provoke it with deep loud notes from Drop D up.

 

[Gene Ballzz]

@JohnH

Well, with the first consideration that all of our ears are subject to some version/degree of the anomalies of the Fletcher munson Curve. Once we get volumes down to the "TRUE" bedroom levels of babies sleeping in the next room, all bets for realistically full range sound and tonal/dynamic response are off! Coming really close at those levels is quite an accomplishment. Now, with these attenuators, just bringing the volume up by 3 - 7 or 10db from those "bedroom" levels can be a very "recordable" sound with plenty of natural dynamics at a volume that may wake the babies in the next room, but not likely elicit calls to the cops!

Now, if I recall correctly, the whole impetus of this somewqhat long thread was to generally tame (not eliminate/mute) the output wattage/volume of a fairly cranked tube amp, without losing it's tone and dynamic response. And it accomplishes this specific task nearly flawlessly, except at "babies in the next room" volumes! I kinda think that moving into the realm of being a silent load box/direct box for recording and/or headphone playing is a bit contrary to the whole original point. Its not that those functions and features are not really cool things, but controlling the speaker volume of a great amp while retaining its sound and character seems to be the most important point here.

This attenuator design is one of the greatest tools that a true tube amp aficionado could ever dream of. I can achieve all the same harmonic overtone feedback and response at speaking volume as I can at "I can't hear a word your saying" volumes! This is a massive accomplishment and evolution in the universe of playing electric guitar! The liberation of being able to play through a 30-35 watt, JTM45 (or similar), cranked to its sweet glory spot, with all its inherent character, in a situation and at a volume that calls for the volume level of a Tweed Champ is simply mind boggling!

Sure, adding on the bells and whistles can be fantastic, but I'm just urging us to not lose sight of the VERY REAL benefits provided by this design for real gigging players. These JohnH units are by far the best and most useful pieces of gear I've ever owned in my +50 years of Screechin' & Squawkin' with a guitar!

Another benefit I derive is being able to bring my full performce rig/setup into my personal, semi lower volume practice space and set it all up for performance levels and simply knock the volume/sound prssure level down as required. This affords me the luxury of rehearsing all of the intended feedbacks, squeals and howls required to achieve my style of said Screechin' & Squawkin'! They don't call me "The Ballzz" without reason!

There It Is, And There Ya Have It!
Gene

 

[Osman Qureshi]

hi @TonyK , thanks for your comments above. I hope your place drys out!

So, attenuator at max attenuation, no speaker and line-out, you take the line out from the amp out = attenuator input, and dial down the line-out pot to a level to suit your connected device's input .

The amp is seeing a pretty good electrical facsimile of a real speaker, and hence passing this on to the line-out. There's a couple of provisos:

Real speakers create a very local resonant peak in the bass at between 80 and 110 hz, for open backed and closed back cabs respectively. When we use our M or M2 attenuators with a speaker, the speaker creates this itself at the attenuator output. But these designs don't create this peak at the amp, and so its obviously not there when running with no speaker. This is not a problem at all. This peak is only relevant if you play, in a closed cab, around low A and a note or two either side. With open back, it would only affect low E, maybe up to G. Without the peak, you theoretically get a more even bass response, and feel free to add a bit of bass somewhere in the amp or downstream if you wish.

But if you are not needing to run silent, you can keep the cab in and take a signal from there after the attenuator. It is probably already at a good level with little more reduction needed. I think a switch for line-out pre or post attenuator would be good.

From about 150hz and above, our design is a very close electrical match to a real cab, its as close as any $1000 active unit that you can buy, and better than some, because it closely tracks the response of a real measured cab. What it misses is the acoustic response of the speaker, and any room and mic effects, which is where an IR box would come in. Something like a Mooer radar looks like a good small fun option for this.

My cab sim ideas have gotten as far as making some raw recordings loaded by the attenuator only, twisting them with an EQ curve in software so their response matches a miced recording, and working out a network of components that will reproduce that in real time. I haven't tested the real version yet but from these sims, it sounds like the results will be very clear and direct sounding, great for tracking or PA, maybe add a little reverb to it. There are commercial all analogue cabsim boxes that do this too, such as a Hughs and Kettner Redbox.

Vintage Modern

If you think that you might like one of these, then you definitely need one! The combo is a heavy mutha though, but excellent in every way.

Hi John, the parts have arrived finally and I’m about to start building the M2 including a line out with a pre- and post-attenuation switch. But before I start, I have three probably stupid questions. Can you please clarify for me (i) that the pre line out would be a parallel connection at the attenuator’s input jack, (ii) the post line out would be a parallel connection at that attenuator’s output jacks, and (iii) with full attenuation engaged both the pre and post line outs can safely go through a 2.2k ohm 1 watt resistor with a 10k ohm 0.25 watt 2.4cm log potentiometer in series? (By the way, I’m also going to leave space to a perfboard inside the attenuator and space for a couple of switches on the attenuator’s face to accommodate your future passive cab sim design.) Thanks in advance.

 

[dougie tamson]

The last items I need to build John's attenuator are the inductors, Instead of buying I made my own
First I 3d printed a bobbin, I used '3d Builder' feee app from Microsoft, the whole thing is made up of a series of cylinders of various sizes, added to and subtracted from each other. I included 4 off-set screw holes to mount it.
The bobbins are made of ABS (same stuff Lego bricks are made of).

I then wound the 1.8mH coil with the following spec:
1mm wire, 278 turns, internal diameter is 15mm, 40mm OD, 20mm bobbin height.

I'll wind the 0.9mH one next on the empty bobbin, this will be the same spec but 215 turns.

Pic of empty bobbin and a completed wound inductor attached.

Doug

 

[JohnH]

Hi John, the parts have arrived finally and I’m about to start building the M2 including a line out with a pre- and post-attenuation switch. But before I start, I have three probably stupid questions. Can you please clarify for me (i) that the pre line out would be a parallel connection at the attenuator’s input jack, (ii) the post line out would be a parallel connection at that attenuator’s output jacks, and (iii) with full attenuation engaged both the pre and post line outs can safely go through a 2.2k ohm 1 watt resistor with a 10k ohm 0.25 watt 2.4cm log potentiometer in series? (By the way, I’m also going to leave space to a perfboard inside the attenuator and space for a couple of switches on the attenuator’s face to accommodate your future passive cab sim design.) Thanks in advance.

Great that all sounds fine. That line-out recipe is intended to be very versatile in terms of giving whatever signal level you need, from a wide range of amp powers. There may be times when you'll have to set it very low to suit your next device without overloading it, but also if you put it after the attenuator, with speaker attached, you should get a good signal even at max attenuation, if you turn the knob up high. Once you have tested it with how you want to use it, you might want a different resistor for the front end connection, but this is good to start.

On the component powers, it depends. But lets take a worst case, say you are driving an amp at 100W, and if this was a 16 ohm attenuator. (is it 8 or 16?). The signal voltage at the amp output = attenuator input would be (100 x 16)^0.5 = 40V. The power taken into your line-out circuit of 2.2k+10k would be 40^2/12200 = 0.13W. Of this, 10/12.2 x 0.13 = 0.11W goes into the pot. So there's still a good factor of more than 2 against its 0.25W spec.

Good luck with the build!

 

[JohnH]

The last items I need to build John's attenuator are the inductors, Instead of buying I made my own
First I 3d printed a bobbin, I used '3d Builder' feee app from Microsoft, the whole thing is made up of a series of cylinders of various sizes, added to and subtracted from each other. I included 4 off-set screw holes to mount it.
The bobbins are made of ABS (same stuff Lego bricks are made of).

I then wound the 1.8mH coil with the following spec:
1mm wire, 278 turns, internal diameter is 15mm, 40mm OD, 20mm bobbin height.

I'll wind the 0.9mH one next on the empty bobbin, this will be the same spec but 215 turns.

Pic of empty bobbin and a completed wound inductor attached.

Doug

View attachment 85337

Hi dougie, thanks for posting that, its awesome work!

Are you able to measure your inductance or is it based on a calculation?

i like the standoffs for the screws. Ive been thinking we should generally space the inductors off the case (eddy currents, more on that to follow soon) and this does that.