Advice please? 10 and 6 dB attenuators.

Discussion in 'Hi-Fi Stereo Systems & Separates' started by Paul7777x, Aug 21, 2018.

  1. Paul7777x

    Paul7777x
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    Can any one point me in the direction of a diagram that’ll show me the right way to solder resisters into a phono cable for attentuation please? I’ve lost mine from years ago.

    I’d be obliged if anyone would work out the values necessary for 10dB and 6dB too please. :smashin:

    Most obliged for any help gents. Cheers.
     
  2. larkone

    larkone
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  3. Deleted member 781788

    Deleted member 781788
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    Assuming the phono cable is connecting to a line-level source:
    - as above post, R1 = R2 = 25K (or so) for 6dB
    - R1 = 33K , R2 = 17K (or so) for 10dB
    the above are from top of my head, but practical enough.
    Remember, the cable would not function the same if direction is reversed, so mark it.
     
  4. Paul7777x

    Paul7777x
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    Cheers gentlemen. :thumbsup:
     
  5. dannnielll

    dannnielll
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    That one is easy...
    .. not good on diagrams but you should be able to follow my text... . If soldering is beyond you ,it will also work with an electric cable connector strip...
    1. Sarcrafice a phono cable by cutting it in half... There are normally two interior wires , covered in white and red insulation and an exterior braid. On better cables there will be an individual braid around each of the two leads red and white...
    2. Extract a bit of the braid.. this will always be the return . And remove a bit of the insulation on say the white.
    3. Join two resistors in series. . We will call the middle B and the other end of the bigger value on A and the remaining end C. .. values of resistors will be given later.
    Connect the white to point A . .. this end goes into the source output .. connect the other white lead to point B and the remainder of that lead goes on to the input terminal of next device.
    Connect the braid to point C.
    Then repeat the sequence for the red lead.....
    Now size of resistance??. . This really depends on what you are driving..
    The statement requesting 3db 6 dB 10 dB is usually incorrect for a simple reason..
    The dB scale was invented to simply calculate signal power into matched loads and it's use as a voltage scale into unmatched loads is so error strewn that we would spend a two hour class clearing up the mess with our students...
    The best way to look at attenuators is a voltage dividers,and ignore dBs altogether...
    The maths is that let the input signal voltage be Vi .. this is the signal coming from the source..
    Let the signal output be Vo ..that is the attenuated signal to be sent to the next device.
    Call the resistance connecting Vo and the braid R2. (Points B and C)
    Call the resistance connecting Vi to Vo R1 (Points A and B)
    Vo= Vi (R2/(R1+R2))..
    Worked example let both R1 and R2 be the same value then Vo = 0.5Vi... voltage dropped by 50%.
    Let R1 be twice the value of R2 then Vo=0.333Vi.... voltage dropped by 2/3.
    Let R1 be 9 times the value of R2, then Vo = 0.1Vi. .. voltage dropped to 1/10 of the initial value...some people will call this 10dB, others 20dB and both will be incorrect...
    As you can see these are just ratios . So what about actual values.
    Well for this you need to know the output impedence of the driving stage and the input impedence of the next stage. This is because we need to ensure that the driving stage is not overloaded, by being asked to drive more current into the attenuator than it can provide.
    Also we assume that the input impedence of next stage is sufficiently high that the math for the attenuator works.. ..
    Let's take a plausible example.. an input circuit into an amplifier wants a maximum voltage of 0.775v and has an input impedence of 47k. The CD player driving it has a peak output voltage of 2 volts and has an output impedence of 300 ohms.
    We need a volt reduction of 0.775/2 or an "attenuator gain " of 0.388...lets make it 0.35.
    Or even simpler 0.33. .. then we need R1 to be twice R2..
    Now R1 must be large compared to 300 ohm and R2 to be small compared to 47k. ..
    So lets let R1 be 3k and R2 be 6k..
     
  6. BlueWizard

    BlueWizard
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    If you scroll down to the bottom of this link, you will see a Calculator for an Impedance Balanced Attenuator or an L-Pad -

    APICS-LLC - Crossover Design Chart and Inductance vs. Frequency Calculator


    This is for a speaker L-Pad, so it would require, Power Resistors, probably 20w to 50w.

    However, for a Line Level signal small 1/4w resistors or even 1/8th watt resistors would be fine.

    Impedance Balanced L-Pad means that the source (Amp, CD Player, whatever) sees the exact same impedance independent of the amount of attenuation.

    Using that calculator and assuming the Line Level Input is 47k ohms, which is very common, then for -

    R1 in Series with the Load, R2 in Parallel with the load -

    [​IMG]

    -10db -

    R1 = ~32k ohms
    R2 = ~22k ohms'

    -6db -

    R1 = ~23.4k ohms
    R2 = ~47k ohms

    Check the Input Impedance on your Line or Phono Level input by looking in the Owner's Manual, and make the Resistors accordingly. Generally the range is about 10k ohms to 47k ohms though on the rarest occasion you might find 100k ohm inputs. But 47k is probably going to be the most common.

    Use the closest available standard resistor value, and probably 5% or greater tolerance. There are all very common resistors, should be easy to find.

    Rothwell is a UK/EU company that makes Attenuators -

    Rothwell Attenuators

    You should be able to find the RCA-style attenuators at a pretty reasonable price.

    Rothwell -10db Attenuators - £39/pr -

    https://www.amazon.co.uk/Rothwell-Line-Attenuators-10dB-Source/dp/B00WT3MVEY/

    I use some from a USA Company called Harrison Labs. Because I didn't want that extra weight hanging off the back of my amp, I patched the Attenuators In-Line -

    [​IMG]


    "Harrison Labs 6 dB RCA Line Level Audio Attenuator Pair" from www.parts-express.com!

    "Harrison Labs 12 dB RCA Line Level Audio Attenuator Pair" from www.parts-express.com!

    The Harrison Labs are about US$26 per pair, the Rothwell are about UK£40/pr. It is conceivable, you could POST the Harrison from the USA for about the same price.

    US$26/pr = £20.16/pr

    The Harrison were sold on Amazon-UK but are currently unavailable. Not sure if they stopped selling them or if they are simply waiting for new stock to arrive.

    https://www.amazon.co.uk/Harrison-Labs-Line-Level-Attenuator/dp/B0006N41AG

    https://www.amazon.co.uk/Harrison-Labs-Line-Level-Attenuator/dp/B0006N41B0

    The Black RCA Cable on the left is a short 18" cable, the Silver RCA Cable on the right is a standard 1m RCA cable. And I use these very easy to find RCA-F to RCA-F couplers -

    https://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=rca+female+to+rca+female+coupler

    These are in-line with the DVD Player that I am using to bring the level down to that consistent with my Turntable. These are -12db attentuators which for me worked out just about perfect.

    Though no more than FYI, here is the analysis I did to determine what degree of attenuation I needed -

    For What It's Worth - LP vs CD SPL

    Again, the link is just FYI.

    Steve/bluewizard
     
    Last edited: Aug 21, 2018
  7. BlueWizard

    BlueWizard
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