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  • MADC 2160 Speaker Distortion

    In previous threads I sought answers for an Allen 2160 that was given to my church. Happy to say it is now installed and is working well... just a few minor tweaks to volumes and console mechanics. (A shout out to Ken Mervine, the Allen installer in Denver for a job well done). It's debut was Christmas Eve. After the services that night 2 different men, engineers by trade, came up to me and told me they were hearing..."Intermodulation Distortion – and that’s likely to be caused by an impedance mismatch between the output of the amplifier and the load presented by the speakers. The result is reflected energy back into the output transformers and subsequently into the amplifier electronics." (a quote from one of them, who also happens to be in my choir). So... what is the remedy for this? More speakers? A limiter on the amp? A bigger amp and is that possible on a 2160? Thanks.

  • #2
    Intermodulation distortion can be caused by several things but it is probably caused by an impedance mismatch. If you are using one or two HC-12 or HC-14/15 speakers on each channel and the speakers are in parallel you are within the operating parameters of the amplifiers. If the gain is turned up too high on the amps it could cause intermodulation distortion.

    Also, one or more of the gain settings on the tone generator boards could be set too high.

    Cone break-up on speakers can also cause IM, and the solution for that is to turn the gain down.

    Also, check the woofers and midrange drivers for foam rot on the surround. It's unlikely to cause IM distortion on the woofers, but could, and can easily cause IM distortion on the midrange. The tweeters could also be blown.

    So the first steps are to reduce gain and check physical condition of the speaker drivers.

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    • #3
      The electronic air sound can sound like distortion, too. Is it adjustable on this organ?

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      • #4
        A dirty spot on one of the PCB TG trim gain pots or the amp gain pots can also cause this effect wherein it sounds like IM, but it's really just a dirty connection.

        I had this issue in one stop group of the ADC 5000 over Christmas. Narrowing the distortion down to one stop group or amp channel, and then exercising all the pots associated with those stops either fixes the issue or eliminates it as a possibility.

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        • #5
          There are no "output transformers" on any modern Allen amplifier...
          John
          ----------
          *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

          https://www.facebook.com/pages/Birds...97551893588434

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          • #6
            Toodles, did you mean to say "NOT caused by an impedance mismatch"?

            The concept of an "impedance mismatch" is not even valid for a modern transistor power amplifier because its output impedance is extremely low. The speakers are being driven from a nearly ideal voltage source and are not being "matched" to anything. And the concept of energy being "reflected" back into the amplifier is strictly true only if the energy is traveling as an electromagnetic wave on a transmission line. The wavelengths of audio-frequency signals are so long that these signals are not identifiable as waves on typical-length speaker cables.

            Those two so-called engineers should stick to driving their trains.

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            • #7
              Originally posted by don60 View Post
              Toodles, did you mean to say "NOT caused by an impedance mismatch"?
              Yes, Thanks for catching that. The only way I could see an impedance mismatch causing intermodulation distortion is if it results in a non-linearity of some form. Impedance mismatch usually results in reduced power distribution, which is not inherently non-linear.

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              • #8
                Interesting commentary here - I suspect that there is a legitimate problem if both these engineers hear something independently. As stated above it sounds like their theory for the cause is a bit out of date, given that it applies to items like my tube Hi-Fi gear and not any of these modern amplifier circuits.

                Given my previous ownership of a 2160A, I'd clean all of the pots and check to make sure there aren't things turned up excessively high. Especially if the organ has reverb, see if the problem occurs without it active as well. There can be distortion from having too high of a signal level. Saving that, there could also be damaged drivers on the speakers. That would be the most likely cause. If that doesn't explain it there could be a damaged amplifier or something else less likely.
                Corey

                Allen MDS-41-S with MIDI-DIVISION-II
                Schulmerich Carillon Americana - 61 notes Flemish / Harp / Celesta / Quadra / Minor Tierce
                - MIDI Retrofit finally underway & Moller console in need of refurbishment
                Schulmerich Campanile Digital Carillon (Cast & Harp)

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                • #9
                  As to solving the problem, which is probably real, given that two people mentioned it on first hearing the organ, you do need to consider the possibility that the organ is undersized for the church and is perhaps being driven harder than usual. I would say though that I've seen and heard some simple two-channel models, no larger than the 2160, installed in churches seating maybe 300 people, and they can produce plenty of sound. Of course a LOT depends on the acoustic setting. If the church seats 300 and is also somewhat padded with carpet, pew cushions, other acoustically absorbent materials, then a two-channel organ might not be capable of filling it with undistorted sound.

                  You may have told us in the previous thread, but please remind me as to the size of the church, the acoustical situation, and how many amp channels and speakers you have on the 2160.

                  Rotted midranges can certainly create very ugly distortion, and one that has been repaired can do it too if the voice coil somehow did not wind up centered properly. ADC amplifiers can deteriorate and produce unacceptable levels of distortion. I have had to replace a few ADC amps in organs even newer than your 2160.

                  And of course there could be audio trouble related to the infamous mini pots on the tone generator board or even to corrosion on the RCA plugs and jacks in the audio path. If you have antiphonal relays on the organ, the relay contact points are also a known source of distorted sound. To rule that out, you can completely bypass the relays by mounting the speaker wires directly to the amplifier's output terminal strip.

                  So, remind me of the situation and I might have another suggestion to offer.
                  John
                  ----------
                  *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

                  https://www.facebook.com/pages/Birds...97551893588434

                  Comment


                  • #10
                    And, too, it may not be intermodulation distortion at all that they are hearing. It could be some sort of noise or some of the digital artifacts that are audible but usually not noticed.

                    True intermodulation sounds horrible--a clipping amplifier causes this sort of distortion. It also can burn out tweeters and midrange units very easily.

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