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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    Hi @TonyK , very happy if you'd like to post about it on other forums, and put a link to them here so we can follow.

    On the last clips I posted, from -7db downwards there was no change in tone that I could see or hear, but from bypassed then to -3.5 and then -7db ( ie the first reactive setting), there was some relative ups and downs in response plots, but no general trend.

    The consistency of the tone comes from the circuit arrangement and also the specific component values. It looks like we have hit on a sweet-spot that works well with a range of amps. But, to arrive there, there's some luck and judgement involved because I don't actually have a lot of data on which its all based. I only have two amps!

    The key info relating to the amps is the small-signal output impedance, and its measured using a consistent, non-distorted input tone, feeding a resistive load, and recording or measuring output level as that load is changed. Its actually pretty simple to do with minimal kit. If anyone is interested to run such a test, I'd be grateful and can discuss further how to do it using whatever equipment is at hand.
     
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  2. assaf110

    assaf110 Well-Known Member

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    Ok, finished assembling a new brother to the family.
    Got delayed cause of life getting in the middle..
    Pics will come soon.
    I made a couple of readings connected to a 16ohm cab (measured 14.3),
    I’m not sure I got it right .
    I measure at the amp in socket. It is a 16ohm version.
    Completely bypassed ( sw1 sw4 bypass mode) - 15.5 ohm.
    Sw4 on ( 3.5 stage) - 24.5 ohm
    When sw1 is on (reactive stage connected) it goes to around 30ohm,
    Sw1-3 on 37.3 ohm
    All switches 36.8 ohm.
    I will go over the connections, I think I have some error in my wirings - does the measurements make sense?
     
  3. JohnH

    JohnH Well-Known Member

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    ok thanks...it doesn't seem quite right! Ill think of some suggestions....
     
  4. assaf110

    assaf110 Well-Known Member

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  5. JohnH

    JohnH Well-Known Member

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    The metalwork looks nice. How did you make the labels?

    Those resistances measured from the input socket are too high, assuming that cab is plugged into the speaker out.

    There's a quick check on full bypass: There should be infinite resistance from ground to hot input or to hot output. Also zero or very low ohms between hot input and hot output.

    Im suspecting a problem with L2 and R1 being not connected. Not sure though, there may be other things. Good luck!
     
    Last edited: Jul 14, 2019
  6. JohnH

    JohnH Well-Known Member

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    BTW, I've edited the first post on the thread to make a summary of the main aspects of the current design. I couldn't add as much as I wanted since I've found there's a limit on post length of 10000 characters. I hope it may be useful though.
     
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  7. JohnH

    JohnH Well-Known Member

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    Another easy test, use a 1.5V battery and touch it across the input, to make sure you get a credible thump in the speaker, same as direct when fully bypassed, and reducing as you engage the first few attenuation steps. Buit I'm still suspicious of L2 and R1!

    With all stages fully off (no attenuation) and bypassed, no speaker connected, you should measure about 31Ohm from the top of L2, through R1, to ground.

    Now just switch Sw2 to attenuated, and it should put R3 in parallel for that same reading, and it should be just over 15 Ohm.

    How about do those tests, unless you've found the problem already?
     
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  8. assaf110

    assaf110 Well-Known Member

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    thanks, will check, hopefully tomorrow...
    The labels made with a simple p-touch label maker.
    The retro one is
    DYMO Office Mate II Embossing Label Maker.
     
  9. assaf110

    assaf110 Well-Known Member

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    Well, Bedroom Blaster is in the house!
    I checked the wiring this morning, starting with stage1, I rewired the L2 R1 to be on the safe side, but the connection was solid to begin with.
    I then re measured the input resistance, it must have been the late hour and being so tired, but everything looks as it should now :rolleyes:
    Made the battery check which turned up fine.
    To be on the safe side, I first connected it to the Origin head, as I rather melt that transformer and not the SV :)
    Then made an unscientific test comparing to a L Pad passive attenuator & Jhs volume box in the loop.
    First impressions -It’s awesome! :dude:
    First time I was able to hear the baby plexi in 20w mode like that.
    It has more gain & harmonics compared to the volume box in the loop - as expected.
    No real influence on tone ( was trying mainly in the 14db attenuation ) if anything it seemed slightly brighter when attenuated.
    I don’t have the proper “technology” to make a recording (or the playing skill :)). I will mess around with the attenuator some more and try to form a more detailed opinion later on.
    As always, thank you JohnH for the amazing support and sharing. It is a very enjoyable & effective project!
     
  10. JohnH

    JohnH Well-Known Member

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    Great news! And congrats for following it through. Yours is the first build I know of running from an SV20.
     
  11. TonyK

    TonyK Member

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    Well, sorry for the delay in finally getting back to the earlier question of how the master volume and drive channels worked out on the Hot Rod Deville. My first recording attempt gave a bizarre result: the MV was at 12, dimed of course, and the drive was incremented in graduations from 2 up to 12, but the recording showed very little difference in sound/tone between each. Anyway, I scrapped that and recorded a very abbreviated version of the classic Alright Now onto my Boss looper, then played it back with the drive at 2, then 4, 6, 8, 10, and 12, each time recording once through with Drive engaged, and then again with More Drive engaged (so two versions for each level of Drive). The attenuator was set at -17.5db, which was as loud as I could manage (pushing around 100db in SPL). Obviously it would have been better if I could have started with no attenuation, but, reality prevailed! I figured that this is NOT a Marshall, but for classic rock tones, perhaps... Finally, for the clearer purposes of this thread, I left the amp totally dimed on More Drive and MV, and increased the attenuation in 3.5db increments. Guitar was my ES-335 (I've never owned a LP).

    Enjoy - and sorry for the sloppiness!

    Oops, warning -- even though the riff is very abbreviated, this is a long recording as there's so much going on!

     
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  12. TonyK

    TonyK Member

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    And just as an interesting comparison, I recorded a similar very simple riff through my BassBreaker 15 amp using -7db of attenuation on the medium overdrive structure. The MV is about 8 out of 10, and the gain is about 5.5. This amp is often said to be a Marshallesque attempt from Fender (not really true, but definitely not standard Fender fare either. So this is the very short sample again using my ES-335.



    Would love to hear thoughts - but in my humble opinion, the gain tone is much more interesting than that of the HRDv :)
     
    Last edited by a moderator: Aug 16, 2019
  13. JohnH

    JohnH Well-Known Member

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    Thanks, Im not seeing a link in the last post though.
     
  14. TonyK

    TonyK Member

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    It's really bizarre. I started a thread on TDPRI and my first two audio samples worked perfectly well with links to SoundCloudm and the others I posted did the same as what's happening here. If I edit my post the link is there as expected but disappears as soon as I save the post. The actual links are live. I guess I'll look for an alternative to SoundCloud and repost but if anyone has some ideas please let me know.

    Ohm the TDPRI link is: https://www.tdpri.com/threads/how-does-hot-rod-deville-410-really-sound.964930/#post-9219404

    And just to try the music link again, it's here:
     
  15. TonyK

    TonyK Member

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    Last edited: Jul 23, 2019
  16. JohnH

    JohnH Well-Known Member

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    Thanks Tony, they are sounding good (just from my phone so far, since I'm travelling)
     
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  17. TonyK

    TonyK Member

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    I think I've figured out what's going wrong, so here's another attempt at just posting the easier to access SoundCloud version of the BassBreaker 15

     
  18. tmingle

    tmingle Active Member

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    BUMPITY
     
  19. kaludjerko

    kaludjerko New Member

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    Maybe this was mentioned before, but I have a question:

    How critical are exact values for L1 and L2 inductors?
    For inductor L2=0.5mH, can I use L2' = 0.47mH or L2' = 0.56mH, as substitute, without adverse effects on the tone?
    These two values I can obtain easily, but 0.5mH not.
    What would be better option of these two values?

    Thanks to JohnH for design and TonyK that posted this topic on TDPRI where i got this info.
     
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  20. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    you put in a bad url...
     
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