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Thread: Attenuators v passive pre to reduce gain

  1. #11
    Join Date: Mar 2017

    Location: Seaford UK

    Posts: 1,861
    I'm Dennis.

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    Quote Originally Posted by philv View Post
    Amplifier -
    Input impedence is 60k
    100mv - input level at 1kHz for rated power output.
    13mv - input level for 0dbw output both channels, with volume control at max.
    8v - maximum input before input overload at 1khz.

    Tilbury -
    input impedance 10k.

    Whatever that means!
    Your results can be achieved with a simple resistor in series with the O/Ps of your source device.
    All O/Ps like to be loaded as little as possible, this reducing the current demands on the O/P cct, and placing a resistor in series with the O/P of your source will raise the effective I/P impedance (Z), that it 'sees', this is a benefit to the source circuitry.

    You can just get a pot and use it as a rheostat, (adjustable series resistor) and approximate the condition you wish to achieve, and when you have done that replace it with a resistor. I would suggest a pot of 100k because it will give you quite a range of attenuation.

    I'm sorry, but I've taken great care in using the qwerty to draw a cct diagram, but it becomes changed and unintelligible when I save it.

  2. #12
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,858
    I'm openingabottleofwine.

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    Adding a simple series resistor will increase the output impedance of the source; whereas a pot used as a voltage divider will maintain the low output impedance as well as providing suitable attenuation.
    Barry

  3. #13
    Join Date: Mar 2017

    Location: Seaford UK

    Posts: 1,861
    I'm Dennis.

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    Yes, but that is of no significance, what the I/P of the amp 'sees' is of no consequence, unlike a speaker O/P where a damping factor would be wrecked. A simple series R is a standard solution in AC signal ccts. (Thevenin and Norton).

    A pot used as a potential divider would have the advantage of great control provided that its value is great enough to ensure that the Z presented to the O/P of the source is high enough, but that would also contravene your implied criticism that the load would perhaps see a high source Z.

  4. #14
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,858
    I'm openingabottleofwine.

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    In audio circuitry the source is treated as a voltage source, so has a low output impedance (< 1000 Ohm) feeding into a large load impedance (> 10,000 Ohm). Otherwise the transfer loss is high: for example, if the source impedance equals the load impedance the transfer loss is 6dB.

    Using a pot as a voltage divider, the load 'sees' that part of the divider to which it is directly connected as being in parallel with the remaining part, which in turn appears in series with the source impedance. So overall, the source impedance is kept low, more or less, regardless of the voltage division ratio of the potentiometer.
    Barry

  5. #15
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 1,757
    I'm Chris.

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    Quote Originally Posted by philv View Post
    Hi,

    possibly have a little too much gain with itargeted amp.

    Now, i am only using it via amazon echo dot analogue out at present.
    But i assume cd and streamer (airport express) will be similar.

    What is best, rothwell attenuators or tilbury passive pre to reduce gain before integrated?

    When just using the echo dot output, this can obviously be reduced by turning down the volume on the echo.
    Is this better than setting at high volume and using attenuators/passive pre?

    Thanks
    Of those two the attenuators is the better choice , as you are avoiding losses associated with contacts, other than RCA plugs and sockets- but they can be removed as well.
    https://pinkfishmedia.net/forum/thre...tortion.68076/

    However if you require variable resistance and no contacts in the signal path at the same time including signal paths associated with input switching, then using NSL32SR3 LDR's
    will give you what you need. There are poor ways of powering LDR's and much better ways, discussed here: https://www.stereo.net.au/forums/top...r-discoveries/

  6. #16
    Join Date: Aug 2012

    Location: Eton wick

    Posts: 1,677
    I'm Philip.

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    Maybe it is better long term to have the amp sensitivity reduced.

  7. #17
    Join Date: Mar 2017

    Location: Seaford UK

    Posts: 1,861
    I'm Dennis.

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    Quote Originally Posted by Barry View Post
    In audio circuitry the source is treated as a voltage source, so has a low output impedance (< 1000 Ohm) feeding into a large load impedance (> 10,000 Ohm). Otherwise the transfer loss is high: for example, if the source impedance equals the load impedance the transfer loss is 6dB.

    Using a pot as a voltage divider, the load 'sees' that part of the divider to which it is directly connected as being in parallel with the remaining part, which in turn appears in series with the source impedance. So overall, the source impedance is kept low, more or less, regardless of the voltage division ratio of the potentiometer.
    Yes, but the tedium when tired of putting it into words.

  8. #18
    Join Date: Sep 2010

    Location: Chiswick, West London, UK

    Posts: 42
    I'm Paul.

    Default Variable atrtenuators

    Some years ago, while experimenting with attenuators, I got these:

    https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1

    in-line attenuators with variable 10/15/20db attenuation. ok for experementing, but eventually I used fixed rothwell attenuators.
    khozmo passive pre/nytech ca252/cxa252 amps, arc 101 speakers
    vinyl: --- lp12/kore/lingo4/ekos2/dv 10x5/gs accession
    digital: --- caiman SEG/allo digione, synology nas, rotel 965 cd

  9. #19
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,858
    I'm openingabottleofwine.

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    Quote Originally Posted by philv View Post
    Amplifier -
    Input impedence is 60k
    100mv - input level at 1kHz for rated power output.
    13mv - input level for 0dbw output both channels, with volume control at max.
    8v - maximum input before input overload at 1khz.

    Tilbury -
    input impedance 10k.

    Whatever that means!
    Assuming the CD player has a typical output of 2V, this is some 13dB above what is required for full output from your amplifier.

    I would suggest you fit a 50K Ohm potentiometer between the source (whatever you use: CDP, Echo Dot etc.) and the power/amplifier. This will give you some control of the volume level.

    The Tilsbury is I believe a stepped potentiometer in conjunction with a switch to select the input source. As such it would be a neat solution to your problem.
    Barry

  10. #20
    Join Date: May 2016

    Location: Much Wenlock

    Posts: 1,522
    I'm Gary.

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    Quote Originally Posted by Barry View Post
    Assuming the CD player has a typical output of 2V, this is some 13dB above what is required for full output from your amplifier.

    I would suggest you fit a 50K Ohm potentiometer between the source (whatever you use: CDP, Echo Dot etc.) and the power/amplifier. This will give you some control of the volume level.

    The Tilsbury is I believe a stepped potentiometer in conjunction with a switch to select the input source. As such it would be a neat solution to your problem.
    Wasn't he the ""Dough Boy"

    Gary
    It is easier to seek forgiveness than to ask permission

    Rules are meant for the obedience of fools and the guidance of wise men

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