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    50k or 100k Attenuator.. 250k ?

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    mantha3

    Posts : 298
    Join date : 2010-11-10

    50k or 100k Attenuator.. 250k ?

    Post by mantha3 on Tue Mar 22, 2011 9:29 am

    I have a question about how to choose between 50k or 100k or 250k attenuators….

    Question background

    I purchased and built a VTA ST-120 high gain and I got it with the 100k Stepped Attenuator option. I am really happy with this (both the amp and the attenuator). I have the amp on a coffee table in front of my couch. I have a 15 foot run of RCA running from my DVD, CD player, Cable TV Digital converter box etc. I keep the amp on the coffee table so I can just adjust volume without getting up and down.. Lazy, I know! I use this amp for everything now.. Even Cable TV and commercials have the volume levels all over the place.

    I am getting a passive pre-amp. I want this so I can keep the amp on a rack with the CD player, Cable TV Digital converter etc and control the volume with a hand remote. I want the amp close to these pieces so I have short RCA runs to the passive preamp and then short RCA cord runs from the passive preamp to the ST120 amp.

    So, I’m having Mapletree audio build me a passive amp. This will have a 24 step attenuator (100k ), and a BENT motor to turn the attenuator with a hand held remote. Then I’ll have a source select and 4 input sources…

    The question… I have a 100k attenuator that came with the amp and it works perfect. I am having this passive preamp built with a 100k attenuator because the 100k works well for me now… I am having the passive preamp built with 2 parallel outputs (one pair of RCA outputs to the ST120 amp and the other pair to a sub woofer for times when I feel like turning it on). So, I was wondering if having the 2 parallel outputs impacts the choice of the 100k value on the attenuator… Should I be using a different value then the 100k? I was just assuming I should use the 100k as it works well now in my amp. I really don’t know much about the decision factors in when to use 50k or 100k or 250k to spec out the attenuator.

    PS – Bob, if you have any detail on the decision factors that drove the choice of the 100k on the attenuator I’d be interested in hearing and learning.

    As always, I appreciate the input Bob and all

    Andy

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: 50k or 100k Attenuator.. 250k ?

    Post by mantha3 on Tue Mar 22, 2011 11:50 am

    I found this for some help in deciding the k value in the need to decide between 50k or 100k or 250k attenuators-

    When choosing the stepped attenuator value for an in-line level control or "passive preamp", the attenuator value is chosen to match the input impedance of whatever it will be controlling. example: If the amplified monitor speakers or power amplifier you will connect the output of your your passive preamp to has an input impedance of 20K, then order a 20K stepped attenuator for that application.

    Does anyone know the impedance of an ST-120?
    I'll get this also for my speakers.

    Thanks all

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: 50k or 100k Attenuator.. 250k ?

    Post by Bob Latino on Tue Mar 22, 2011 11:59 am

    The input impedance of the VTA ST-120 and the VTA ST-70 is 270,000 ohms (270K). I chose the 100K stepped attenuator because the volume steps seem to work well the standard gain 12AT7 driver board. Many times the value of any attenuator must be done by trail and error. If you want to match the input impedance of the ST-120 then try a 250K attenuator which is fairly close to the ST-120's 270K input impedance.

    Bob

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: 50k or 100k Attenuator.. 250k ?

    Post by mantha3 on Tue Mar 22, 2011 12:18 pm

    Thanks for responding Bob.

    I'll stick with what works for me. I'm real happy with the sound and the volume steps are just right.

    If I paid the $$ to buy a few DACT Attenuators to test trial and error I'd be in the dog house with the wife for sure!

    I'm going to drop a decent chunk of change on this so I just want to research a few things before I go "all in"

    As always, I def. appreciate hearing from you Bob. Thanks for taking the time to respond.

    Andy

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