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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    Edit: August 12 2018: This thread has developed to include a range of resistive and now reactive attenuator designs - I've changed the title accordingly.

    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 nomalised:

    [​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]




    I hope that is interesting to some. I'm finding this useful, and it was a good learning experience to test it.
     
    Last edited: Aug 11, 2018
  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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    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.
     
  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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