Simple Attenuators - Design And Testing

Discussion in 'The Workbench' started by JohnH, Oct 21, 2017.

  1. JohnH

    JohnH Well-Known Member

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    Summary: July 2019

    This thread started as an exploration of what can be done with a few resistors to make a simple attenuator for valve amps, and it then develops into multistage resistive and then reactive designs. The latest designs work much better than I’d expected at the start. But they are still simple and fairly inexpensive to build. A number of others have also built them with good results, and posted in this thread. This is a summary is to present the current attenuator design, with build photos and testing. Plus I’ll explain how it works and how it's different to other attenuators. The rest of the thread has more info and discussion, with several builds presented and discussed.

    Background

    This is about passive attenuators, which are wired between the amp output and the speakers. Their basic function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to be run at higher power, allowing the glorious tone of a good valve output stage to develop, but without excessive volume in the room. In doing this, the attenuator must present a load to the amp that is similar enough to a speaker to at least maintain safely of the amp.

    But in addition to safety and an overall reduction in volume we want the attenuator to also maintain the tone of the amp as volume is reduced, with a consistent response in terms of highs, mids and lows, plus dynamics and feel, and to maintain that performance from a small attenuation level, right down to sub-bedroom level, even with a powerful amp. This is where the simplest designs can prove to be inferior, and the best commercial designs get very expensive indeed.

    With a lot of feedback and ideas from others on this thread and on other forums, and considerable testing by several builders, I think we have a design that achieves this. That’s a big claim, which I wouldn’t make unless I believed it. For about $100, anyone with workshop skills and the ability to follow a circuit schematic can build this.

    An important point:

    Anyone who builds this does so at their own risk, and takes responsibility for working out their own wiring for their own private, non-commercial use, and completing it safely.

    Attenuator M

    This is the latest design, which myself and others have built since January 2019.

    The design needs to be matched to the output tap of the amp, eg 8 Ohm or 16 Ohms. So component values for both are given, which basically differ by a factor of two:


    There are a number of attenuation stages, engaged or bypassed by switches.

    Stage 1 is the key reactive stage and includes two inductor coils. This stage on its own, will reduce power by a factor of 5, which is -7db, reducing a 50W amp to 10W. The coils design is configured so that the impedance presented to the amp is similar to that of a real speaker (values based on various Celestions), particularly how impedance rises with frequency.

    After Stage 1, several more stages are provided. These can be mixed and matched, but the design shown is based around additional -3.5db, -7db and -14db stages. By combining these switches in combination, and with Stage 1, reductions of up to -31.5db can be achieved in small, equal steps of -3.5db, at which point a 50W amp, at full power, is playing quietly at about 35mW.

    At the output, one or two speakers can be used. The most even attenuation steps and consistent tone is if an 8 Ohm attenuator is used with an 8 Ohm speaker (or two x 16 Ohm), or 16 with 16. But both versions are safe to use with either 8 or 16 Ohm speakers. In the red box, some additional parts are engaged to a third output socket, which will make a slight tweak to the tone for use when a 16 Ohm speaker is used with an 8 Ohm attenuator. These add a couple of db to correct the high treble, whereas if a 16 Ohm speaker is used with 8 Ohm attenuator with the standard output, there is a bit more mids (not much difference though – its optional).

    Another optional aspect is the bypass switching. As shown, full bypass can be achieved, and also the -3.5 stage can be run on its own, as a small resistive reduction. This gives the widest range of volume settings. But for many users (including myself), full bypass and -3.5db may not be needed, and switching can be simplified if the -7db stage is always engaged and all further switched steps are below that.

    Component values and power ratings

    The table below shows the maximum expected power being dissipated by each resistor. The component ratings need to have a good margin above this. I Suggest a factor of at least 3 for case-mounted aluminium resistors, bolted (and using thermal grease) to a substantial metal chassis or heatsink , and a factor of 5 or more for air-cooled resistors. These values also fit in with the recommended spec in the schematic diagram above.

    Wire for hookup and also the winding of air-cored inductors should be 18 or 19 gage for 50W attenuators, and this is also OK for a 100W one, if built to the 16 Ohms values. For switches, I suggest at least 5A rating (at 125V ac) for a 50W 8 Ohm build. Get the best jacks you can find.


    Cooling

    If running amps > 30W at high power, then the unit will heat up as it absorbs power and dissipates it. A good size die-cast aluminium case is best. Once components are positioned, then a substantial number of additional large vent holes should be drilled, in the top and in the base, with feet to raise up the base. This will help to promote good convective flow of air out through the top, replaced by cool air at the base. The best colour for cooling is black, or another dark colour, which promotes radiative cooling.

    Build

    My current build is in a case approximately 170 x 120 x 55mm, thick aluminium.:



    A 'watch-it': don't mount the air-cored inductors using normal steel bolts, since this can significant change their inductance. Use nylon, or stainless steel bolts, or another method such as zip-ties. (That being said, if you want to experiment, put an M3 or M4 steel bolt through inductor L2, which in theory should increase treble slightly!)

    Performance

    In the schematic above, there is a graph at lower left that shows a calculated frequency response at each attenuation level from 0 to -31.5 db. These are made using a spreadsheet to calculate the signal at each stage of the circuit, as a series of voltage dividers, using complex number theory to keep track of magnitudes and phase angles. The speaker was represented, for analysis, by an equivalent load model, by Aiken, (http://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator) adjusted to match the measured performance of a G12M 4x12 cab. The plots are based on small signals, with the amp output impedance assumed to be 20 Ohms, for an 8 Ohm tap, based on measurements of my VM2266C. These calcs were used to adjust the values in the design.

    Here are some sound clips

    Attenuator M: Max attenuation to non-attenuated:
    https://vocaroo.com/i/s0QOpqTD3WAo

    Attenuator M: Normalised:
    https://vocaroo.com/i/s1GJvYK04i00

    It’s a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in 3.5db steps. Then, the second file, which is based on the same recording, has each stage normalised for volume so you can hear any differences in the tone.

    The VM2266c was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off the speaker with a Rode M1 into a neutrally set mixer.


    The plots are taken from the sound sample posted above. The lower set of data are the basic plots, from full volume down to -31db (db scale is arbitrary, but relative db's are right).

    The upper plots are intended to show the differences between responses. I took the -7db recording as the base case, so this is shown as a flat line. The others are the various other settings, with the -7db trace subtracted. The ideal for these traces is therefore also a flat line. And for all the traces below -7db down to -31db, this is what is happening, there is virtually no further tonal change at all as you attenuate down as far as you want. It measures as consistent.
    The -3.5db and full-volume traces show some wiggles relative to -7db. The peaks are consistent though. I think we are seeing extra resonances and distortion generated in the speaker itself at this high volume, and no attenuator can capture those. The -3.5 trace (resistive) shows a very slight treble fall-off, hard to hear in practice though.

    How it works

    The tone of a guitar speaker in a valve amp depends on low damping due to a high effective amp output impedance. This allows the natural speaker inductance and resonance to develop a rise in treble, and a bass resonance. So the output impedance of the attenuator needs to be consistent and representative of a real amp, which is quite high. Most simple attenuators do not get this right, and it is rarely discussed. Based on this, a good resistive attenuator can be designed, as shown through the first few pages of the thread, which follow.

    At high volume, the amp reacts to changing impedance of the speaker. This is what the inductor coils do in the attenuator. This design matches impedance of a real speaker from low mids up to high treble. It doesn’t show the amp the bass resonance, which is not necessary since it is developed at the speaker.
     
    Last edited: Aug 23, 2019 at 8:12 AM
  2. tmingle

    tmingle Active Member

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    Here is the best attenuator comparison Ive seen so far.



    Does your attenuator work better IYO than the MV? Did you try it with the 401?
     
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  3. JohnH

    JohnH Well-Known Member

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    This was originally the first post, on 22/10/2017, moved here to allow my July 2019 summary in post 1




    A Fixed Attenuator


    This is about a simple fixed-level attenuator that I've been using with my Vintage Modern VM2266c. I built this mainly for low volume practice at home, in order to be able to get the volume up a bit but without disturbing others too much. Also, there's a lot written about attenuating a valve amp and how it does or does not change the tone, and I wanted to understand this better myself. The following is still in a testing form.



    Schematic


    I've tried a few recipes, but the current one that I've been using these past few months has three fixed resistances, combined to in theory take -7db off the input power, which is a x0.2 power reduction, so 40W becomes 8W. Each resistance is made up of several 5W power resistors. With 40W from the amp and an 8Ohm load, each resistor is dissipating between 2W and 4W.


    [​IMG]



    The idea of the three resistances is to not only match the Ohms as seen by the amp, but also to control the impedance as seen by the speaker. This gives the speaker a bit of damping but not too much, as when directly connected to a tube amp. This is different to an SS amp which has super-low output impedance, which creates high damping and suppresses some of the speakers natural response.



    Construction


    It's currently built on tag-board, inside an aluminium box 120mmx90mm. Plastic jacks make sure the box is not connected at all.


    [​IMG]


    [​IMG]



    Sounds in use


    I like the sound of my VM2266c best at about 5 on the dial, at which point it is way too loud for home, but this box makes it feasible. At much lower settings, the attenuator squashes the residual hiss and hum and lets me get up to 2 or 3 for a reasonably quiet practice. At anywhere up to half-way volume, the resistors are not noticeably warming up at all.


    Although, based on the nominal 5W ratings of the resistor, it could theoretically absorb the full amp output, as currently built it's not really set up for that. The resistors would need to be 10W types to give a margin, and it needs to get some better ventilation happening too. So I don't use it for rehearsal or gigs, and there really is no need with this amp since it doesn't need to be cranked to 10 to sound great.



    Tests


    Here are some tests from the VM2266c:



    I miced up the amp with a Rode M1 and recorded via a neutral mixer into Audacity. I put a strum from a bridge Hb into a loop pedal, then recorded at Volume 5.5. I used LDR mode with Body at 6 and Detail at 8. Then with no change of settings and the loop still running, I closed down the amp, inserted the attenuator, switched on and re-recorded.



    Normalizing both traces showed that Audacity was finding a -9.7db difference in the attenuated, rather than -7db as calculated.



    This is the sound: There are three of the full strum, then three of the attenuated strum, to show what this approx -10db attenuation sounds like. Then there are three of each, full then attenuated but normalised to equal total volume. With those you can listen for tonal differences:


    https://soundclick.com/share.cfm?id=13646224

    This is the first part of the waveform, full and then attenuated and normalised:


    [​IMG]

    They are very similar but you can see a few differences.

    Then I exported the frequency spectrums into a spreadsheet to compare them, by subtracting one from another. This showed that, after adjusting for overall level, the attenuated sounds have a slight boost in the range up to about 1500hz and a slight dip above that. The differences are about 1 to 2 db. Its not much but you can just hear it. It's not enough to really change the character of the tone or turn a good sound bad, but it is a small noticeable difference, within range of EQ if needed.


    This is the two spectrum plots. the green line is the difference in db, showing how the attenuated sounds has a db or so more mids and less treble:


    [​IMG]

    --------------------------------------------------------------------------------------------------------------


    That is another great vid from JS. I could hardly hear any difference in the normalsed tones right down to -30 db reduction. But his attenuator is more sophisticated than what Im testing.

    Im using my fixed attenuator in conjunction with the MV on my VM2266c. For quiet playing, it sounds much better at volume 3 and using the attenuator, than at volume 1 without. At very low volume settings, my VM combo sounds a bit thinner than it might, and the residual hiss is more noticeable. The attenuator improves this significantly.

    To be honest, I havn't yet tried it with the DSL401. I find this amp to sound great all the way to zero volume. But I do want to test it just for the sake of science. It has a very different power amp circuit, with a different ppimv design and no negative feedback. Im thinking tbat could mean a higher amp output impedance, less damping. I want to see what that does to the slight tonal shift that I found with the VM. I also want to test with an SS amp (Crate PowerBlock), just for info even though I'd not use it in practice.
     
    Last edited: Jul 13, 2019
  4. SG~GUY

    SG~GUY Well-Known Member

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    -ARACOM- best ive ever heard for a MARSHALL or Marshall platform
    -MESA- -(Trans Atlantic)- whatever ALCHEMY,.. or Black Magic deal they have with satan that sacrifices power tubes,... Id gladly pay to have it in a stand alone unit right next to the big bag of power tubes thst need to be sacrificed,.. Its like a PERPETUAL MOTION MACHINE- the more attenuation that you use the better it sounds

    ---*(here's the sad part of the story, the very important missing ingredient with any such device,... they cant account for a pushed speaker,.. or a speakers character,.. or the sound a speaker makes with significant wattage)*---
     
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  5. JohnH

    JohnH Well-Known Member

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    Yep, there's no replacing the tone of a loud speaker except with a loud loudspeaker. But when it has to be quiet, id still rather have a bigger amp with all its features and full sized speakers than a small boxy-sounding one. My hope for an attenuator box like this it a one step change to bring a gig-sized amp down to the power of a 5W home amp, without losing too much and no need to buy the small amp.
     
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  6. SG~GUY

    SG~GUY Well-Known Member

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    -im right there with you John, i think we both come from a time when a 50w amp was/is considered small. All else being equal I'll take the 100 watter every time. I dont understand the "lunch box" thing, they're full of features, they're cute & -- "sound" -- good, I guess,... ya know,... First it was our dogs,... Now,...its our amps.... We're next John,...... THERE GONNA CUT OUR BALLS OFF TOO.....
     
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  7. tmingle

    tmingle Active Member

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    I tried several of the lunchbox amps (Blackstar HT-5, VHT Special 6 Ultra, Peavey MH20) and found the DSL40C to be better at low volume than any of them.
     
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  8. SG~GUY

    SG~GUY Well-Known Member

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    -but does it have balls?-
     
  9. SG~GUY

    SG~GUY Well-Known Member

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    -the acoustic part sounds really good John, ive listened to it a few times, is it original?

    -also some random things on attenuation-
    -i have a EH 12AX7 that one day went south, amp went almost silent, i turned it up and it sounded really good but still quiet, ended up turning every nob on the amp up to 10.... Completely BAD ASS!!! but its a time bomb and i took it out and put it in a marked box, i pull it out every now and then for 15 minutes of the sound in my head

    -always use a variac @ 80-90v-

    -Mostly with a 2203KK in V-1. But works in every slot just not as well. Tried it on a JJ100 as well, same effect, same sound-( a heavy metal violin crying violently as her metal soul melts to liquid )-it scares the hell out of me thou, afraid. its gonna turn my amps to liquid lava!!!! And it seems i lost it in my move to the west coast, but i havent really looked for it either-(What could it be?)-

    -theres a guy on TGP who has a socket adapter/s for this obscure power tube that reduces output to whisper levels, a friend has this set up and it sounds kinda similar to my -bad tube-
     
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  10. BowerR64

    BowerR64 Well-Known Member

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    What about an Iso cab? is there a comparison of the 3? no attenuation, attenuation and then a miked Iso cab?
     
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  11. JohnH

    JohnH Well-Known Member

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    Thanks SG. I think what's happening is that the link to my small attenuator sample is putting up my whole play list, which is all various tests. If so, the one that comes up next is me testing a Digitech phaser - its electric but recorded without amp, and the riff is based on Crowded House 'Take the weather with you'

    A variac is interesting - I wonder if it would affect any solid-state parts of the amp that were looking for certain supply voltages.
     
  12. Micky

    Micky Well-Known Member VIP Member

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    It will try...

    A variac will lower the input mains voltage, which will in turn lower the output voltage of the PT.
    If a PT has a tap for 12 or 5V to drive a solid state circuit (say, for switching or other control circuitry) it will lower that feed voltage as well. Sometimes there is a regulator circuit that will try to keep the voltage stable at a certain voltage, but many times there is not, and there will be a point where the SS stuff will become unstable.
    It depends on how much you lower the voltage where that unstable point will be.
     
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  13. SG~GUY

    SG~GUY Well-Known Member

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    -ive found when --ON-- indicator light goes dim or out that in 3 to 5 secounds the caps will drain & the amp fades away to silence-

    -Marshall's -(especially JMP's)- could go the lowest in the high 70's to 80 volts area, usually keep it at 85-90 volts...

    -odd thing,... when lowering the voltage, the amps get very quiet, loses sensitivity to other electrical devices that create oscillation.... Exposes's witch doctor "power conditioners" show what they really are.. Uselesss placebo's.... Variacs are Useless against "DIRTY POWER" -(like 99% of power scrubbers unless you have big $$$ on something like is used in big raised floor server rooms)- i have "dirty power" from a 12v DC TRICKLE CHARGER...... had to put an off/on swith on it....
     
  14. JohnH

    JohnH Well-Known Member

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    I tried the attenuator box with my DSL401. I was quite interesting. This has an internal 16ohm V30, and I usually run it with another V30 in an extension cab, for an 8ohm load.

    My attenuator, current version, is calculated for an 8ohm load. But using it with a 16ohm speaker makes only about an ohm of difference to what the amp sees, so its safe to try. So I ran it with one 16ohm speaker, via the attenuator, to the 8ohm amp tap.

    It worked fine, but with a more significant tonal change than before (which was the 8ohm 2x12 pair on the VM). More loss of highs, and also a few db off the low bass. Not a bad sound, but not consistent.

    Then I hooked up the second speaker, to restore an 8ohm load. Highs and bass were restored, with just the slightest treble roll-off, as with the VM before, equivalent to about 2 steps on the treble plot.

    Then I dug out some specs and worked some numbers. All of this was predictable, and probably well known but it’s a learning process for me. I think one of the key parameters in a simple resistive attenuator, after power and impedance seen by the amp, is the impedance as seen by the speaker. This has two effects. At treble frequencies, it is part of an LR filter together with speaker inductance and dc resistance. Higher output Z of the attenuator reduces treble roll off in proportion to low frequencies. At bass frequencies, a higher attenuator output impedance reduces damping of the cone, allowing a bit more bass to develop. In the tests above, due to these effects, there was a greater tonal affect on the higher impedance 16ohm single speaker than the 8ohm pair.

    The current box lets the speaker see an output impedance of about 5.5ohms. The next build will raise that slightly to about 7ohms for slightly more treble consistency. This one will be with case-mounted high power resistors (100w each in a ‘Pi' arrangement as before, 20, 8, 20) which I’ve ordered. It will be with a box drilled for ventilation and should be an interesting test and hopefully sound good.
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Are you finding the drop in wattage to the speaker worth the loss in frequencies/tone?
     
  16. JohnH

    JohnH Well-Known Member

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    Yes indeed, particularly for the VM when playing quietly. It allows the volume to be raised smoothly to about 3, where the mv is sounding good and the tone is smooth. It also squashes out the residual hum and hiss, which is not at all high but its nice to knock it down. Tone change with the 8ohm pair is very small and is well compensated with 2 steos up on ths treble knob.

    The dsl401 Ive only tried briefly for the testing above, but I think that this amp may deal better than the VM with very low volume.
     
  17. JohnH

    JohnH Well-Known Member

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    I’ve had my simple attenuator running with my VM2266C, as a test version for a few months. So now I’ve rebuilt it in a more permanent form with better robustness and cooling.

    This one goes for a single reduction of -9db, which is a 1/8th factor on power, bringing a 40W amp down to a home-friendly 5W. There are three resistors, all nominally rated at 100W, two 10Ohms in parallel to make 5 Ohms, and a 4 Ohm, forming a voltage divider. With this and an 8 Ohm load, the amp sees 8 Ohms and the speaker sees about 3 Ohms. The resistors dissipate 9 or 13W each max.

    For a -6db version, all resistors would be 8ohms.

    [​IMG]


    It's all in a compact case of about 120mm x 90mm, so although it has plenty of electrical power rating, its useable capacity is controlled by cooling. There are several features to promote this:

    1. Power resistors bolted with thermal grease to the underside of the top plate

    2. Aluminium case for high conductivity and thermal capacity

    3 Top plate drilled for internal ventilation, with holes in base for make-up air inlet

    4. Base raised on feet to allow ventilation underneath

    5. Outer surface black to increase heat loss by black-body radiation

    [​IMG]

    [​IMG]

    I've tested it and it sounds fine, nice and clear. I can play at 4 or 5 in a small room quite comfortably and fully crank if I want to. At these levels, the presence control is quite effective so there are ways to tweak the tone if needed.

    But for me, half the interest in trying this is play through it and the other half is to test it and learn about how its working.

    I plan to run some tone samples to see how tone is changed. On cooling, so far I cant detect a noticeable temperature rise, but it would take sustained thrashing at high volume to heat it up. Another test I might try is to take it off the amp and run some dc through it to make it dissipate a known power. I can measure case temperature with an IR thermometer.
     
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  18. Micky

    Micky Well-Known Member VIP Member

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    I'd like to see how hot you can get it.
    300W of resistance is probably overkill...
     
  19. JohnH

    JohnH Well-Known Member

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    I agree, but you can get them for a dollar each from china!. Also, I suspect that that nominal rating will only apply if they are totally cooled to remain at ambient temp.
     
  20. Micky

    Micky Well-Known Member VIP Member

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    I use a couple of those mounted to a heatsink for a dummy load when testing the output of an amp with a scope. Even pushing a hundred watts continuous they barely get warm...
     
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