Microphonic Effects on Solid State Circuits

Microphonic Effects on Solid State Circuits



  1. RobHolt
    As promised some time ago, here's a little test for microphony in solid state audio circuits.

    This is not presented as a case that such effects do no exist under any circumstances - it is simply a practical test that can hopefully inform further discussion.

    When doing these sorts of tests I like to make the conditions as extreme as reasonably possible, on the basis that real world use is very likely to be kinder and any results can be scaled accordingly.

    In this instance we are looking at a a pre amp. I have a few models knocking around but to satisfy the above it seems sensible to chose something with complex circuits, containing lots of active and passive components and importantly, something with plenty of gain. My Quad 34 fits the bill - lots of active op amp stages, capacitor coupling, arguably too much gain and with no attempt by the manufacturer to protect the circuits from either structure or airborne interference. It is a circuit screwed onto a light chassis and slipped into a thin steel sleeve.

    See here:

    1.jpg

    The pre amp was placed onto a loudspeaker (Rogers Export) bypassing the rubber feet and place on-end so as to expose the underside to the output of another speaker placed 12 inches away. This ensures that the unit receives vibration from the speaker below, and is exposed to the SPL of the speaker sitting to the right.

    Talking of SPLs, as this is a test the speakers were fed with wide band pink noise with the SPL meter measuring around 105dB at the test position.

    See here:

    2.jpg

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    The pre amp CD input was selected and volume (remember this is active with gain) advanced to maximum. The output was taken from the pre amp out and captured via line in on a Macbook Pro using Audacity. Line gain on the Mac was again set to maximum.

    Once the file was captured into Audacity, a further 50dB of amplification is applied. I did say this was an extreme test!

    The result is as follows:

    Here is the amplified WAV file showing the pre amp residual noise:

    4.jpg

    Here is the 30 sec WAV file for download and listening:

    http://www.mediafire.com/file/2i9e91cpcwdpj2e/Microphony.wav

    The pink noise was applied part way into the recording - so can you hear or see when this happened?
    No, me neither.

    Food for thought when it comes to the purchase of special feet, cones, spikes, racks and other assorted accessories for solid state gear - certainly at line level.
    We'll look at valve circuits, TTs and high gain phono sections in parts 2 & 3.

    Part 2 - higher gain circuits and valves

    In response to some of the points raised the first test is repeated here using a drum track as requested, and the casing removed as requested.

    Before going any further I'd suggest downloading and familiarising yourselves with the test track:

    http://www.mediafire.com/file/0jevlyubb7cfky8/02 Improvisation.mp3

    The test was repeated with the same Q34 but this time without the outer steel casing. The component side of the pcb was placed sqaure onto the bass driver of the loudspeaker at a distance of 20cm. So a little closer than before and the transient nature of the test track meant that SPLs of 107dB could be produced at the test position. See below:

    5.jpg

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    Set to line as per the initial test and full gain on both pre and recorder, these are the results obtained:

    9.jpg

    Sound recording:

    http://www.mediafire.com/file/urr0tbwcuwmpp6r/drum micro LINE.wav

    If you then amplify the WAV as per the first test what you get is this - still residual circuit noise (ignore the two spikes which are static ticks from the exposed circuits):

    10.jpg

    So now lets look at circuits with far higher gain.

    I inserted a high gain MC card into the Q34 - 200uv of the type suited to low output MCs. This gives overall 60dB of gain for the stage IIRC.

    No change to test conditions gives this:

    11.jpg

    This is the sound recording:

    http://www.mediafire.com/file/yjxbnk4in46c7op/drum micro MC.wav

    Gets interesting now. You can just about hear something down in the noise, more at HF a little further into the track. I'll leave you to determine any significance on this one.

    Now it gets very interesting - tests with the WD Valve MC stage.

    Same test method as before except that the output is taken directly into the Macbook and full gain applied. This is 3 tube (ECC83) stage similar to the EAR circuit. The active stage is MM and 12:1 ratio Sowter step-ups are in place to match low output MCs.
    it is built to the highest standards I could conceive short of a fresh design, so high end fast diodes in the PSU, Balck Gate PSU caps, NOS Mullards, AudioNote paper in oil caps etc. In comes in two boxes with a steel perforated cover as seen in the pic.

    12.jpg

    This is what we get:

    13.jpg

    This is the sound file:

    http://www.mediafire.com/file/a0r7ppmpchg4sv7/valve micro.wav

    In summary I would suggest:

    - Valves can be microphonic as expected and that is hardly news. You do however need to consider the extreme test condition used, because if the unit is placed a couple of meters away from the sound source and a normal listening level is used, the trace and audio recording sink to zero. Performance would seem largely dependent on the tube used as other stages are clearly very microphonic. You can also see from this test that a slightly different result was obtained for the two channels - again not unexpected with valves.

    - High gain SS phono stages are microphonic, but to micro degrees, if that makes sense.
    The output, given the massive amount of gain applied here is remarkably tiny. Put the case back on and move the unit only a couple of feet away and again you have flat line traces and nothing on the recording.

    - Lastly the line stage. You can make up your own minds but I for one will never be the slightest bit concerned about these things, or having such electronics inside the loudspeaker cabinet.