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New to the forum!!! OSA/api gain pot replacement?

Discussion in 'Pro Audio Equipment' started by oldguitars, Aug 6, 2007.

  1. oldguitars

    oldguitars Active Member

    Hi!
    Great forum here. I am new to the forum. I figured you guys could help. I have some old OSA (old school audio) api clones and i want to replace the pots. Currently they have 10k, 1w, linear taper. The gain slope is weird with linear. Brent averill uses 25k log/audio. some people say to use 22k rev log.

    I am not sure which to use, what impact they will have and where to get them. I would looooove some help from the wisdom here. Thanks!!!!!!

    note: These are old OSA's with actual 2520's. nice and crappy paint too! but they sound good... also they have pad, mic/line and polarity
     
  2. AudioGaff

    AudioGaff Well-Known Member

    I'd suggest you call to OSA and get the info from the source.
     
  3. oldguitars

    oldguitars Active Member

    i can't they don't seem to exist anymore. Website is down. Dale has done this before, when he moved from PA to AZ
     
  4. AudioGaff

    AudioGaff Well-Known Member

    Contact Atlas Pro Audio, OSA US Distributer. They should be able to help. http://www.atlasproaudio.com
     
  5. AudioGaff

    AudioGaff Well-Known Member

    IIRC, the first batch of OSA mic pre's I saw for sale were API clones with the API 2520 so I would expect the pot to be the same as for an API 512.
     
  6. oldguitars

    oldguitars Active Member

    That sounds right. I will contact atlas. I still can't find what the 512has or a good place to get them... Thanks!
     
  7. Boswell

    Boswell Moderator Distinguished Member

    The original API circuits had the variable gain trims connected as part of the lower end of the feedback resistor pair. The gain is thus 1 + (Rf/Rv) where Rf is the fixed feedback resistor and Rv is the sum of a small fixed resistor (to limit the gain) and the trim pot. From the equation, you can see that none of the standard linear, log or antilog law pots will give a constant number of decibels of gain per radian of rotation. However, the anitlog could well give the best result, but because it's wired as a rheostat and not as a potentiometer, a reverse-wired log pot is equivalent. If you use linear, you will get a steep gain increase near the top end of the travel.

    I am puzzled by your saying that the pots in your circuit are 10K and that you have been told to use 22K. This does imply that these are not connected as in the original API circuit, but are conventional faders on the output. If this is the case, then log law is what you want. The value is not critical, so 10K to 25K should be acceptable depending on what is connected to the wiper.
     
  8. oldguitars

    oldguitars Active Member

    That is awesome info, Boswell. Thanks! I will see what i can dig up at Mouser or digikey.
     

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