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  1. #1
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    Analog attenuation

    There is a lot of information about the necessity to match the source and amplifier using analog attenuation.
    Since everything is a bit technical for me (e.g. http://wiki.slimdevices.com/index.cg...ectToPowerAmp), can someone please help me with this issue:
    I have the Transporter as an only source and the amplifier is the XdA from NHT (part of the Xd system).
    It has an Input Sensitivity of 1.5V RMS (http://nhthifi.com/2006/products/s/xda.html).
    To get the best performance should I:
    1) Use the attenuation inside the Transporter and use it's 'digital' volume.
    2) Buy a passive preamp.
    3) Buy a regular preamp.

  2. #2
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    Quote Originally Posted by zork17 View Post
    1) Use the attenuation inside the Transporter and use it's 'digital' volume.
    You should be able to use the balanced XLR cables without any analog attenuation at all if your listening volume control is > 1/4 of the volume bar on the transporter. If you can't get above 1/4 volume on the transporter, you will need a passive resistor pack between them.

    I tested my transporter with an Atma-Sphere amp a while ago..

    I powered everything off and connected XLR cables.

    I turned on the transporter and turned the volume control all the way down.

    I powered on the Atma-Sphere amp's tube warmers. Then I turned the amp section to on.

    I then started playing a track that would start out quiet, seemed ok, so I started turning up the volume when the VU meters were bouncing.

    We hit a low listening level around 1/2 volume, and a normal listening level around 3/4 volume.

    It was a perfect match.

  3. #3
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    Quote Originally Posted by SuperQ View Post
    You should be able to use the balanced XLR cables without any analog attenuation at all if your listening volume control is > 1/4 of the volume bar on the transporter. If you can't get above 1/4 volume on the transporter, you will need a passive resistor pack between them.
    I didn't understand why I must use a balanced XLR cable and not the RCA one?

    If I need a passive resistor pack, why shouldn't I use the attenuation inside the Transporter?

  4. #4
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    Quote Originally Posted by zork17 View Post
    I didn't understand why I must use a balanced XLR cable and not the RCA one?

    If I need a passive resistor pack, why shouldn't I use the attenuation inside the Transporter?
    XLR is a balanced connection, it provides for much better audio quality over RCA. You could use a coat hangers to connect XLR and get better audio quality than $1000 RCA cables. (exageration, but it's mostly true)

    XLR provides an electricly balanced signal which cancels out noise. Any reasonable XLR cable will do.

    The resistor adjustment inside the transporter is for RCA only. But if you have an amplifier with XLR, there is no reason to use RCA.

    A pair of these will work fine:
    http://www.sweetwater.com/store/detail/CMK010/

  5. #5
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    Its relatively simple to build a set of attenuator ....

    take
    1 locking type phono plug and one socket
    take one resistor of 10k and one of 6k.

    solder the 10k from centre pin to centre pin and the 6k fron plug centre pin to ground...this wil give you about 8db if my memory is correct..use the best resistor you can find ...

    total cost should be about $40 for a pair...

    this will allow you to run the source at max volume and have a decent ammount of volume knob to play with ..


    I assume thats one solution to your problem ...if not I've miss read the question!

  6. #6
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    Quote Originally Posted by zanash View Post
    Its relatively simple to build a set of attenuator ....
    .... But if you're going to use the RCA output, then the Transporter's built in attenuation makes more sense, and comes free with it.

    I don't own one, but if I did I would choose the RCA connection and internal analog attenuation setting that allows 100/100 to be a comfortable level, rather than balanced cables with 75/100 being the max comfortable level. The latter is a mismatch, in that a maximum level noise burst would be 12.5dB louder than what you'd like to hear or have your speakers play.

    There is nothing magical about >1/4 on the volume bar either. The sound degradation is linear all the way from 99/100 to 1/100, as far as SNR goes anyway. You want the digital volume as close to 100/100 as possible for critical listening, and ideally spot-on, which will require some sort of variable (stepped) analog attenuation, even with the internal RCA attenuation, because your critical listening level changes according to time of day etc.

    Rather than preamp, though, you might try searching 'passive attenuator'. Endler makes a balanced stepped attenuator, but you might also look to pro audio retailers for xlr equipment. I noticed this one for $100 at sweetwater: http://www.sweetwater.com/store/detail/ATTY/

  7. #7
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    Someone else wrote:
    Balanced outputs were developed for use in professional studios where long cable lengths and lots equipment (which = lots of wires) provided lots of opportunities for electromagnetic interference (hum, noise) to develop.
    Since I'm going to have only one pair of interconnects from the Transporter to the amplifier and they will be 1/2 a meter, I don't think balanced cables are such a big issue for me.

    Which brings me back to my original question:
    The Transporter has built in attenuation of: -30, -20, -10, 0 dB.
    If I want the best sound, what is better (and why?):
    A) Use one of the built in attenuations in the Transporter.
    or
    B) Use a passive pre-amp (like Creek's OBH-22 http://www.creekaudio.com/main_produ...?prolook=obh22) to control the volume and let the Transporter's volume be always 100.

  8. #8
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    The attenuation provided by jumper settings in the Transporter is there to protect your amp and speakers if you should choose 100 on the regular volume control. It is not intended as a "volume" control, since it isn't variable.

    For volume control you have three choices:
    1. Using the volume control in the Transporter (could be bad because you turn down the music volume but not the noise).
    2. Using passive volume control (variable attenuator aka "passive preamp").
    3. Using a normal active preamp either with a power amplifier or as part of an integrated amplifier.

    2 or 3 are considered the way forward because when you turn the music volume down you are turning down the noise too (at least, the noise from the source). As for whether 2 or 3 is better, "it depends".

    (Some might say even 1 "depends" and might be acceptable, since 2 and 3 introduce their own problems - but it isn't recommended by Slim Devices.)

    Regards, Darren
    Last edited by darrenyeats; 2007-07-05 at 08:19.

  9. #9
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    Quote Originally Posted by darrenyeats View Post
    For volume control you have three choices:
    1. Using the volume control in the Transporter (could bad because you turn down the music volume but not the noise).
    2. Using passive volume control (variable attenuator aka "passive preamp").
    3. Using a normal active preamp either with a power amplifier or as part of an integrated amplifier.

    2 or 3 are considered the way forward because when you turn the music volume down you are turning down the noise too (at least, the noise from the source). As for whether 2 or 3 is better, "it depends".

    (Some might say even 1 "depends" and might be acceptable, since 2 and 3 introduce their own problems - but it isn't recommended by Slim Devices.)
    Please don't misrepresent what Slim Devices have to say about option 1. What they DO say is that you should not use the built-in volume control to correct a mismatch between Transporter output level and amplifier input sensitivity. I have never seen any suggestion that, having established a match (possibly using passive attenuation), you shouldn't use the built-in volume control for day-to-day adjustment. Indeed, if that wasn't recommended, it wouldn't be there as an option.

    You are correct when you say that using the built-in volume control reduces only the signal and not the noise. But since the Transporter's noise is down around -130dB to start with, this is a theoretical issue, and has no practical relevance.

    In my experience, option 3 is the one which should be avoided. Active preamps are the least transparent electronic devices in most HiFi systems, and eliminating them usually yields large sound quality gains. (I acknowledge that there are megabuck preamps around which do not audibly degrade the signal, and that some people might actually like the sonic signature of their chosen preamp. I'm talking here about striving for transparency on a finite budget).
    Transporter -> ATC SCM100A

  10. #10
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    Quote Originally Posted by cliveb View Post
    having established a match (possibly using passive attenuation)
    What is the difference between attenuation inside the Transporter and an external one like the passive preamp I asked about above?

    How do I match between the Transporter output level and amplifier input sensitivity? My amplifier has 1.5 vrms.

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