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Hammond AO-29 Frequency Response

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    #16
    That is going to be very useful Stefan when I rebuild my M100. It'll also help when rebuilding my C3, the amps are very similar.

    roj
    C3 chop rescue - in progress
    M102
    L102
    TTR100
    VK-8M
    FrankenLeslie (in progress)

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      #17
      Nice work, Stefan ! So nice that it really deserved its own thread.
      I especially like the fuse panel; that oughta' save some iron and copper!
      Also, nice enough that I gotta' ask... do you hire out for preamp and amp rebuilds?

      Roger Memphis
      Last edited by Roger Memphis; 05-15-2016, 03:47 PM. Reason: added a word "copper"
      C-3 with O-M, 145, 122RV, 2 PR-40's, PSR-36
      CV with HR-40, 2 B-40's

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        #18
        So the question is what components have you changed from the original amp now in the rebuild?, also what equipment did you use to take these measurements?

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          #19
          Originally posted by Roger Memphis View Post
          Nice work, Stefan ! So nice that it really deserved its own thread.
          I especially like the fuse panel; that oughta' save some iron and copper!
          Also, nice enough that I gotta' ask... do you hire out for preamp and amp rebuilds?

          Roger Memphis
          I'm not sure I want to tackle this again any time soon. I've probably got about 40 hours in this rebuild so far (I'm replacing a few other components in the rest of the organ too).

          Originally posted by geoffbrown View Post
          So the question is what components have you changed from the original amp now in the rebuild?, also what equipment did you use to take these measurements?
          All the components are new except one resistor (a 10K 2W resistor, because when I ordered the replacement, I didn't realize it was 2W), and one ceramic capacitor (forgot to order that one). The one modification (besides the fuse block) was to insert a 3.9M resistor in series with 20pF capacitor C20. This changes the feedback in the initial part of the output amplifier and flattens the frequency response.

          I spent some time building a white noise generator, but never got one working whose noise was white enough (they all generated blue noise). Finally, I just gave in, generated a 60 second white noise .wav file, and used my phone as a test signal generator (can't use .mp3 for this, as it messes up the spectrum). I built a couple of custom cables that plugged into the phone, terminated with either an RCA plug for line-in, or alligator clips for "A" and "B" in.

          To measure the response, I fed the output (via a line-out that I'd built into the organ long ago) into the input of my laptop, running Audacity to record the 60 second long noise burst, and generate the spectrum. I exported the spectrum data to .txt files, and imported those into a custom program I wrote that generates the graphs you see. The program can compute the difference between two spectra (that's how I generated the "relative effect" graphs), and for the non-difference graphs, I actually generated the difference between the measured signal and the raw signal to adjust for any non-flatness in the noise. That's one advantage to using a recording of white noise; it's identical every time.
          Stefan Vorkoetter: http://www.stefanv.com

          1962 Hammond M-111 with Improved Vibrato, Internal Rotary Speaker, Drum Machine,
          Window Seat Tone Cabinets, Completely Rebuilt Amplifier, and Recapped Tone Generator.
          1978 PAiA 1550 Stringz'n'Thingz with many enhancements.
          2017 Raspberry Pi organ-top synthesizer.

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