Standard output specs of sources and preamps and help with stability theory
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 30th September 2001, 07:38 AM #1 jteef   diyAudio Member   Join Date: Feb 2001 Location: Ogden, UT first is there any standard? What kind of RMS output voltage can I expect to see from CD players? I am asking because the amp I built begins to clip with right around 2 VRMS on the input and I am considering how much gain I should use in the pre amp I am going to build. Also, somebody has told me that reducing the gain of a circuit makes it more prone to oscillation, but in rod elliots high speed op amps article, he claims the opposite. While I believe RE more than somebody from a web board at this point, I would still like to know for sure. I am somewhat new to this so please bear with me. The circuit I am generally referring to is basically a burr brown OPA134 op amp operating as basically an input buffer with a gain of two. This would be directly coupled to a TI THS6012 CFB op amp with a gain of 5.5 to start with. (this is the value obtained from Rod Elliots circuit using a 1k ohm as RF and a 220 ohm for RG. This provides an overall circuit gain of 21dB(11.0) My audio book explains the stability criterion of a single pole amplifier through the use of phase margin. A small phase margin is ideal...nearing 180 degrees would then be more and more unstable. The equations my book lists are: loop gain transfer function: G(s)H(s) = bK/(1+s/w1) where b is the feedback ratio and K is the gain constant, and w1 is the pole frequency. The book is unclear about what exactly K is in the real world. is it just the open loop gain? I didn't see anything in the data sheet about open loop gain. This leads me to believe that I am looking too deeply into something that doesn't matter. in any case, with a constant K and a varying b...a larger b = smaller stability margin, but with a high value of K, both are going to be approximately 90 degrees anyway(see end of last paragraph ). So I guess my question is, what else is affected if RE's statement is true. Again, I do not know if I am way off base here or not...If I am please correct me as a lot of this stuff can get very confusing. thanks jt
 1st October 2001, 12:22 AM #3 Pete Fleming   diyAudio Member   Join Date: May 2001 Every CD player I have come across has 2V standard output, but as mentioned most put out more than that so they compare favourably in A/B comparisons. Cheers, Pete

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