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Thread: Hammond AO-29 Frequency Response

  1. #1
    p Piano stefanv's Avatar
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    Hammond AO-29 Frequency Response

    I'm in the midst of a ground-up rebuild of a Hammond AO-29-13 amplifier, as used in a 1962 M-100 series. I actually bought a second AO-29 amplifier, and am using the chassis, most of the tube sockets, and the percussion transformers from that, taking only the power and output transformers from my original AO-29 once I'm ready.

    As part of the rebuild, I'm doing extensive before and after measurements of the performance of the amplifier, so that (a) I can see the rebuilt one is working right, and (b) see if anything changed. I thought I'd share some of my findings here.

    First, here is the end-to-end frequency response of the stock amplifier (electrolytics are new), with the pedal at full volume, and the Volume Soft switch up (i.e. volume not soft):

    EntireAmplifierPedalLoudVolumeLoud.png

    You can clearly see the 6dB/octave roll-off at 3.5kHz, beginning gradually at about 2kHz, that Hammond designed in to minimize key click.

    More to follow in later posts ...
    Last edited by stefanv; 05-12-2016 at 11:56 PM.

  2. #2
    p Piano stefanv's Avatar
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    Here is the end-to-end frequency response with the pedal at the lowest volume,

    EntireAmplifierPedalSoftVolumeLoud.png

    and here is the difference in frequency response between the two:

    EffectOfPedalSoftWithVolumeLoud.png

    You can see that reducing the volume with the pedal changes the tone, de-emphasizing the mid-range.

    All plots are normalized to a peak of 0dB. These are not absolute levels.

  3. #3
    Moderator Wes's Avatar
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    Signal generator or tonewheel generator input?

    Very awesome graphs, btw!

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    ff Fortissimo enor's Avatar
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    Nice work! I love it when science confirms what my ears tell me...

  5. #5
    p Piano stefanv's Avatar
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    Quote Originally Posted by Wes View Post
    Signal generator or tonewheel generator input?
    White noise, piped into the "A" input terminal. I wanted to measure the characteristics of the amplifier, not the organ as a whole. The tone generator of course gets louder as you go up in frequency, to compensate for the amplifier's roll-off.

  6. #6
    p Piano stefanv's Avatar
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    Volume Soft

    Here we have the response with the pedal loud, and the Volume Soft switch down (on),

    EntireAmplifierPedalLoudVolumeSoft.png

    and this is the relative effect that the Volume Soft switch has:

    EffectOfVolumeSoftWithPedalLoud.png
    Last edited by stefanv; 05-13-2016 at 03:34 PM.

  7. #7
    Moderator Wes's Avatar
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    Excellent methodology! Nice to have you back aboard, Stefan.

    Wes

  8. #8
    p Piano stefanv's Avatar
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    Vibrato

    All the results above have been from the "A" input to the speakers (equipped with a line-out tap). I also did some testing from the "B" input, which feeds into a preamp much like the "A" input, but then passes through the vibrato system before entering the intermediate amplifier. The following thus include the "B" input preamp, the vibrato delay line, and the scanner.

    Please keep in mind that my vibrato delay line is modified, so a stock M-100 will have slightly different results. I wish I'd had the foresight and equipment to do these tests before the modifications.

    EntireAmplifierVibratoPedalLoud.png

    EntireAmplifierVibratoSmallPedalLoud.png

    EntireAmplifierVibratoChorusPedalLoud.png

    EntireAmplifierVibratoCeleste3PedalLoud.png

    The last one is interesting. The Celeste settings feed some of the output from the terminated end of the delay line back into the input. Because of the delay, this means that some of the fed back signal will be exactly 180 degrees out of phase with the original, resulting in cancellation. The effect is a comb filter.
    Last edited by stefanv; 05-13-2016 at 07:34 PM.

  9. #9
    p Piano geoffbrown's Avatar
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    What are you trying to achieve with your rebuild ? is it to give a more B3 like sound or something else?

  10. #10
    p Piano stefanv's Avatar
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    Quote Originally Posted by geoffbrown View Post
    What are you trying to achieve with your rebuild ? is it to give a more B3 like sound or something else?
    The old amp is very noisy (scratching and rumbling noises), I suspect due to old resistors. Rather than replace them one at a time, I just decided to bite the bullet and start from scratch.

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