Simple Attenuators - Design And Testing

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

  1. tmingle

    tmingle Active Member

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    I'm really interested in building 1 to use with the 16 ohm tap to 16 ohm speaker for my 40C. This amp sounds great as is but I can't crank it in a small room.
    Is there a final drawing posted?
    Thanks John.
     
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  2. JohnH

    JohnH Well-Known Member

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    Hi tmingle, thanks for your interest

    The schematic at post #42 (previous page) is the current/final version for using an 8 Ohm tap and 8 Ohm speaker. If you just want a 16 Ohm to 16 Ohm version, just double all values. May need to then adjust to the nearest standard values depending where you buy them. Power ratings would be the same. I'm happy to run any variation through my spreadsheet.

    But the 8 ohm (amp tap) version I built is also sounding good with 16 Ohm and 2x16 Ohm speakers with the DSl401. It gives the option of using an extension cab with twin output jacks. Ill post how Ive wired that with an extra resistor over the weekend.
     
    Last edited: Jun 21, 2018
  3. 1199RS

    1199RS New Member

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    Have put a good few hours on the attenuator today and I am totally sold on it tried it back to back with my hot plate and it definitely sounded markedly clearer to my ears. Also with the hot plate I get v4 & v5 redplating slightly as I play with master above 6 this doesn't happen with the homebrew.
    This has been a real issue for my jubilee clone as I used a plexi power transformer which puts 500vdc on plates but with the homebrew was able to crank the master and lead master... totally different animal with some volume.
    I think I will add another stage to drop some more volume on a switch for late night playing as I saw 96db on the max attenuation setting which is a bit loud for others in the house.
    I would say the case got reasonably hot but not so I could not hold it. Maybe around 50 deg C not sure what that is in Fahrenheit. And that was after constant use for 2 hours approx. Actually my power transformer gets much hotter and I constantly check it as I had one go up in smoke not too long ago.
     
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  4. JohnH

    JohnH Well-Known Member

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    That's great feedback, thanks, very encouraging!

    Ive been blasting away on mine too, but with my 40-50W amp I cant get it more than warm. Even with my compact build, there's so much mass of case and aluminium-clad resistors that any heat gain takes quite a while to build up, and depends on an average power over at least 10-15 minutes rather than peak values.

    Hopefully this w/e I can run some tests to measure what I'm hearing.
     
  5. Micky

    Micky Well-Known Member VIP Member

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    It would be great to post a final 16-ohm design...
     
  6. tmingle

    tmingle Active Member

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    Thanks John.
    I was pretty sure that doubling the values would get me there.
    I have a bin full various power resistors I have been scrounging from old equipment & a lot of the values look familiar.
     
  7. JohnH

    JohnH Well-Known Member

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    Sure. It seems that power resistors often come in a different series of values than smaller ones. Im buying from a Chinese ebay retailer for the ones Ive used, and their range also includes exact x2 values too. But, in a standard range more usual elsewhere, 10, and 15 exist, but 20 and 30 do not, nearest being 18, 22, 27, 33. Lets see what you have in your spares box. Its all not super critical but picking the best combinstions from whats available helps to keep the attenuation steps and impedances as consistent as possible.
     
  8. JohnH

    JohnH Well-Known Member

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    Here are schematics for four variants of the three-stage attenuator:

    Design A is the basic 8 Ohm version, with twin output jacks for either a single 8 Ohm or 2 x 16 Ohm speakers. The resistor values are as per the previous diagram. Power dissipations and attenuation levels are shown for an amp input of 50W.

    Design B is as A, plus the extra resistor between the ground wires of the two outputs, aimed at balancing the output and tone if you use it with a 16 Ohm cab. This works by adding to the output impedance when Output 2 is used, which sounds a bit brighter with a 16 Ohm cab. If Output 1 or both outputs are used, this is bypassed. The effect is achieved using the ring connections on 'stereo' TRS jacks. Design B is the one I have built currently.

    Design C is a simple 16 Ohm to 16 Ohm version, all values are as Design A x2 and so it should perform the same.

    Design D is also for 16 to 16 ohms but uses slightly different values, based on the more common series of standard resistors. The db increments are a bit smaller, typically 6db instead of 7db.


    General Notes

    1. Power dissipations are listed above based on 50W amp input. I’d suggest a factor of at least x3 on these for selecting components and use case-mounted resistors for those taking significant power.

    2. The two switches used together can set up four attenuation settings. Eg, for Designs A and B running into 8 Ohms, a 50W amp can be brought down to 10W, 2W, 0.6W or 0.12W. If more reduction is wanted, then a further -7db switched stage can be added in the middle, based on values for R3 and R4. This would bring power down by another factor of 1/5th, to 0.03W (or 0.06W for a 100W amp).

    3. There’s intentionally no ‘Off = 0db’ switch shown. My view is that if you don’t want to attenuate, then don’t use an attenuator! Having the first stage fixed, and taking most of the power, takes pressure off later stages so there is less current being switched and less power running in later resistors. Also, the amp always gets some load and there is no nasty transient moment when the switches either fully shunt or disconnect the amp. But if an ‘Off’ switch is wanted, I’d suggest a full bypass switch of the whole circuit, like a loop switch or a ‘True-bypass’ switch in a stomp box. But this switch would need to carry the whole current (eg, around 3.5A for a 100w into 8 Ohms), and should only be switched with the amp off or on standby.

    If anyone else tries these designs, it would be great to hear about them!
     
    Last edited: Jun 23, 2018
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  9. JohnH

    JohnH Well-Known Member

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    I realized I cant do recordings until I get my main PC fixed. So instead I did some high and low attenuation listening tests on the DSL401 and VM. I set a clean preamp signal, max attenuation and master at about 8, tried some chords and arpeggios. Then I switched down to min attenuation and reduced the master to the same room volume. I got the same overall tonal balance but could clearly hear the cleaner tone / more headroom due to absence of the missing power amp harmonics. It happened with both amps. I guess that's the whole idea.
     
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  10. JohnH

    JohnH Well-Known Member

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    Id like to ask for a bit of help from anyone building the attenuator shown above, either now or in the future. What amps have you tried it with and what is your impression of the tone?, eg too bright, thin, muddy, or just right? And if it makes a difference, at what settings?

    I’ve worked it out to suit my amps and speakers (a VM2266C and a DSL401 with 1 or 2 x V30’s). I’m happy with it now but different rigs respond differently to various attenuator designs, plus there’s personal taste.

    It would be possible to adjust this design further, and it’s also possible to introduce a control to change output resistance without affecting attenuation levels. This affects the extent to which bass and treble is lifted, changing the tonal balance. Any interest?
     
  11. JohnH

    JohnH Well-Known Member

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    Searching around to learn more, I found this link that may explain the red-plating with the Hotplate, and why this doesn't happen with a resistive attenuator. See posts by 'Club and country':

    http://marshallroadhouse.com/viewtopic.php?f=2&t=2825&sid=6434bb5e971bb62827d124a8c58e6166
     
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  12. 1199RS

    1199RS New Member

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    Thanks for that john. I had found that article a while back when the problem manifested. I went through a lot of changing of capacitors in my amp which didnt cure the red plating before searching for issues with the hot plate. Only thing that helped was running at 4 ohms into 8ohm hotplate and not getting above 5 or 6 on pre M.V.
    Using your design my master is almost never off 10, and no issues.
    Other than heat it does get hot now after an hour or so of playing its around 60 or 70 deg c almost too hot to hold your hand on.
     
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  13. tmingle

    tmingle Active Member

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    I found a 50 ohm rheostat in my pile of junk to use as the 2nd stage of John's 16 ohm version. Do I even need the 3rd stage? I also am going to add a line out to use with impulses & will probably just order the remainder of the items from Mouser. IMG_1286.JPG IMG_1287.JPG
     
  14. JohnH

    JohnH Well-Known Member

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    Hi tmingle. Nice idea about the reostat, but Its maybe not so easy. The key to the consistent tone of this design is the consistent impedances seen by amp and speaker, and using this could throw them off. But if you could let me consider further I might figure something. In which case:

    is it just a two-lug unit or is it three lugs like a pot?
    Power or current rating: is that a 1Amp rating? Lets say you are running at 10w after stage 1 into 16 ohm, then there is about 0.8Amp running, a bit too close for ir all to go through that piece?
     
  15. tmingle

    tmingle Active Member

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  16. tmingle

    tmingle Active Member

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    It's 3 lugs. I was afraid of the same thing, so I ordered the resistors in my previous post. Since they were cheap, I went with all 100W resistors. My 40C starts to sound better above about 2-3 on the volumes. I have a small room & 2-3 is too loud. It really starts to cook above 4, which is way too loud. If I read the schematic correctly, the 40C MV is not the same as the 401.
     
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  17. JohnH

    JohnH Well-Known Member

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    ok, thats great. so you can anyways build the all-switched version as an option. But Im interested to figure out how a pot can be incorporated, so Ill see what I can do.

    Yes I think the 40C has a normal MV while the 401 had a PPIMV. There's also that thing about the C19 bright cap that folk around here seem to snip or reduce on a 40C.
     
  18. tmingle

    tmingle Active Member

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    I removed it and then put it back in while I installed my bias pots a few months back.. I don't use L2 much on the 40C.
     
  19. JohnH

    JohnH Well-Known Member

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    I had a go at designing the 50 Ohm pot into the 16 Ohm attenuator circuit, at Stage 3. Stages 1 and 2 are unchanged.

    There are three other resistors around the pot, aimed at controlling impedances and also current flow through the pot. With Stage 2 off and Stage 3 at minimun, with 50W input, the greatest current through the pot is calculated at 0 73 Amp. Thats within a 1A rating, but it would be expected to heat up.

    In general, I was going for ouput impedance in the range 2 to 2 5 x nominal ohms with these designs. This one peaks at about 46 Ohms, just under 3x nominal Should be Ok, but not tested yet.


    With this arrangement, the pot is giving a 10 db range, providing from 3 to 13 db of attenuation when Stage 3 is engaged. Using this and the other stages together, you have -7db, and then any setting needed in the range from -10 to -27db.

    Note that the switch for Stage 3 now needs two poles instead of 1.

    I think this could be particularly useful for an NMV amp, or for those who just have to twiddle a knob. But for me, I think I prefer the switched-only versions, since they are simpler and more electrically robust.
     
    Last edited: Jul 9, 2018
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  20. JohnH

    JohnH Well-Known Member

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    Its a good idea. I reckon just take your line-out directly from the speaker, via a 10k pot if you want its own control. That way you get all the electrical interactions of resonance and inductance from the speaker. Its almost like a full (expensive) inductive load-box, with the advantage of using the real speaker instead of an electrical model.
     
    Last edited: Jul 9, 2018

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