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modest base requirements for a line stage

Posted 22nd June 2012 at 06:44 AM by rjm
Updated 29th June 2012 at 05:38 AM by rjm

Copied from this post, for reference.

Originally Posted by RNMarsh
Here are my own modest base requirements for a line stage (similar for power amp):

1. Open loop BW of 40KHz or more (-3dB)... 20KHz min.
2. IM and THD of less than .001% at 1v rms into 30 ohms for any frequency between 20Hz and 20KHz.
3. No coupling caps on input or output or in feedback path.
4. No use of dc servo circuits to track and correct dc offset and drift.
5. No more than 6-8 transistors (excluding power supply).
6. S/N ref 1 volt rms and without weighting of at least -130dB (input can be shorted or terminated).
7. No significant harmonics above the 2nd and 3rd.
8. Closed loop gain between 12 and 20 dBv
9. Low Zout (less than a fraction of an Ohm at any audio freq).
10. Distortion not be changed by source Z.
11. Transistors should be low cost and not be exotic, hard to obtain, very expensive or no longer manufactured.

There might be a thing or two I missed, off the top of my head, but this is do-able and should be done in all audio circuits IMHO. -RNM
Take both "modest" and "requirement" with large grains of salt. Most pre-amplifiers on the market today, even the really good ones, would fail a plurality of the points above.

They have merit though as guide for audio circuit design. It gets you thinking in the right direction. As such, trying to implement them may be more useful than actually doing so.

I'll try to annotate this list in the coming weeks.
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  1. Old Comment
    Yes to all, but, if the open loop BW are at least 20kHz, don't use feedback and the THD can be 1% or 2% without loss of sound quality as in the darTZeel NHB-458:

    darTZeel NHB-458 monoblock amplifier | Stereophile.com

    And, if well chosen, the coupling capacitors can have the advantage of eliminate any offset problem.

    I hope see your desing. Cheers.
    Posted 24th October 2012 at 02:08 PM by raul_77 raul_77 is offline

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