Equipment Supports and Turntable Microphony Testing

Equipment Supports and Turntable Microphony Testing



  1. RobHolt
    Many will have read the claims of equipment support manufacturers and witnessed the stand wars that spring up from time to time.

    At shows you may have seen enthusiastic salespeople flinging bits of kit around the demo rooms to the equally enthusiastic nods from those gathered.

    So before we start with the test lets look at what functions the support needs to fulfil.

    There are surely three:

    - Basic support functions and making the equipment accessible, while meeting the aesthetic considerations of the owner.

    - Isolating the equipment from external influence, be they generated by the loudspeakers in the room or sources outside the room. In other words, the inflow.

    - Providing a pathway for energy generated by the component being supported. In other words, the outflow.

    The test concerns itself with the inflow of energy and will measure both the effectiveness of the support at minimising adverse effects and also the more general effects of equipment placement within the room. For this test we are using a turntable but you can just as easily use any other piece of equipment by connecting its output to the measurement rig.

    You'll need a PC, soundcard and some good recording software such as Audacity which is a free download.

    Place the turntable on the support and place the support about one foot away from one of your loudspeakers, preferably in front.

    With the TT drive switched off place the stylus onto the record.

    Connect TT via your phono stage to the soundcard.

    Now play another source through the system such as CD at the loudest level at which you would normally listen and start recording the output from your TT onto the PC. If nothing appears on the visual trace, increase recording levels until you see a signal.

    Repeat the above as many time as you wish using different supports, different equipment mechanical interfaces and different position within the room.

    The goal is to have nothing showing on the trace, or audible on the resulting recording. Using this method you can easily identify both the true effectiveness of the support and the ideal room positioning.

    As mentioned earlier, you can also use this test to to test for microphony in other components and it is especially useful for testing valve equipment.

    Many claim that SS electronics are sufficiently microphonic to be influenced by support and positioning. Well, test the claims.

    Connect your pre amp to the line input on the soundcard are repeat the test, using as much gain as you wish. An extreme test is to actually place the kit under test directly onto the loudspeaker cabinet.

    Two further points.

    The TT test is of course applying RIAA to the signal from the TT and as you aren't actually playing an LP this isn't desirable. It doesn't detract from the purpose of the test but it does make analysis of the audio recording - and that is important but will be covered separately - more difficult.

    So you might want to consider reversing the RIAA EQ and you can do that easily in Audacity by simply applying 18db of cut at 20hz and 18db of boost 20khz with straight line EQ between the points. Quick n dirty but it works for these purposes.

    Here is an example of what you can expect:

    Rega P9 on a Sound Organisation support spiked onto wooden floor at approx 1ft from the speaker, sound peaking at around 90db:

    Near.jpg

    ... and about 3m away to the side of the room.

    Far.jpg

    Have fun.