I Have Noticed Some Odd Microphonics From My Tube Pre-amp.

Discussion in 'Audio Hardware' started by SandAndGlass, Oct 19, 2017.

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  1. SandAndGlass

    SandAndGlass Twilight Forum Resident Thread Starter

    I have a Constellation tube pre-amp from Audio Electronics, a now defunct division of Cary Audio.

    [​IMG]
    It is about four years old and runs off of 6SN7 tubes. It is currently on its 2nd set of tubes.

    I used to shut it off at night, but now I leave it on, usually 24/7.

    I have found good replacement tubes that are surplus Russian military grade at very reasonable prices on eBay. I have stocked up on these tubes (I have no issues with the sound or build quality).

    So here is the issue, it is a minor one, but I am bringing it before this forum because, I can not reason why there are some microphonic issues. I will explain.

    I have this pre-amp on a shelf, directly behind a 2nd sofa, where I sit.

    I keep the remote, between the rear of the tubes, up against the plate which covers the transformers.

    I keep a simple flat remote that controls the LED lighting strip on the left side against a rear amplifier that sits on the left and against the pre-amp.

    A few months ago, I was replacing the remote back on the top plate of the pre-amp and I heard sounds of the remote being placed against the top of the metal plate.

    The sound was like a had a small microphone placed against the metal plate and the noises that it was making, were being reproduced through the pre-amp and subsequently through the Altec A7's.

    I know, the tubes are a couple of years old and this pair of tubes have been run 24/7, they are getting long in the tooth, are going microphonic and simply need to be replaced.

    Those were my exact thoughts.

    But, then I turned up the stereo a bit and played some music and there was not enough vibration from sound pressure from the speakers to cause them to go microphonic. So I noted that I should be replacing them soon.

    A few days later, I went digging through my supply of new Soviet made 6SN7's and brought out a styrofoam package of four tubes. I sat them on a nearby speaker.

    When I had some free time, I decided that it might be a good time to check and replace the tubes, with the new set.

    Before beginning, I picked up the LED remote and used it to tap against the top plate. Tap, Tap, Tap.

    No microphonics, through the speakers, just the sound that your ears hear from the plastic remote, taping against the metal top case.

    I thought, that was odd?

    No further microphonics.

    That was, until two days ago, when the same thing happened. I was replacing the lighting remote on top of the case again, and then, the tapping sound was coming out of the A7's again.

    Then, yesterday morning, I had the tube PrimaLuna Prologue Five on and was listing to streaming Pandora One. I paused the stream and took the remote again and tapped on the case.

    No Microphonics!

    There is no microphone anywhere in the room. There is nothing that I know that could produce that tapping sound, except for a tube(s) going microphonic.

    I am trying it again, as I type this, and nothing (amps are on music was paused).

    What I am failing to comprehend is, how can a tube that is going microphonic, suddenly not be microphonic?

    This has happened twice over the past two months and then completely went away.

    I have plenty of new replacement tubes, no big deal to change them out.

    Can anyone weigh in on how a tube can be microphonic one day and not the following day.

    Thank You, for any input that your might have concerning this matter.

    Just curious.

    S&G
     
  2. Davey

    Davey NP: Electrelane ~ No Shouts No Calls (2007 LP)

    Location:
    SF Bay Area, USA
    Most of the parts in your preamp are microphonic to a certain degree (afterall, they make microphones and speakers from capacitors), so it just depends on the signal level they handle whether it might show up in the output. Could be the tubes, but could also be a capacitor or resistor, or even a wire touching the top plate, or on a circuit board, I'm not familiar with the insides of that unit. Parts maybe move around a bit as the piece warms up, and so sometimes become more prone to being vibrated by that tapping. You could try some of the silicone o-rings or other dampers on the tubes as an experiment. Some people like them, others don't. Not sure if it's very easy to take off the top plate for a look around.
     
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  3. SandAndGlass

    SandAndGlass Twilight Forum Resident Thread Starter

    Who knows, but sounds like it might be a more logical of a reason, as any that I can come up with.

    If it continues and becomes an issue, I might have to crack it open and take a look.
     
  4. TarnishedEars

    TarnishedEars Forum Resident

    Location:
    The Seattle area
    You are almost certainly hearing your tubes, not your capacitors. While one can sometime measure some very slight microphonic behavior from some capacitors with a scope, the effects are normally so insignificant as not to produce a result that you will actually hear as sounding microphonic.

    More often than not, micophonic tubes start-out as microphonic. Although it is entirely possible that with age that some tubes of lesser build quality may also develop microphonic tendencies with use. And while some of the Russian tubes can sound great, there can be no denial that their build quality is not nearly what tubes used to have back in the 60s and 70s. It is also possible that simply after warming up that the tubes internals loosen-up a bit due to the differing thermal expansion constants of differing materials, and that this could be resulting in more microphonic behavior.
     
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  5. SandAndGlass

    SandAndGlass Twilight Forum Resident Thread Starter

    I would tend to agree with you that the microphonic effects of capacitors should have a negligible effect on microphonics and I have never witnessed a microphonic resistor, or anything other than a microphonic tube.

    I do realize that unshielded components can be subject to RF interference and feedback, but microphonics is due to physical vibrations of the grids within the tubes themselves. They basically recreate the the vibrations, which alter the flow of electrons, within the tube, just like a electrical source would, resulting in the microphonic sounds.

    It is not the microphonics within the tubes which has me puzzled, it is how the transient microphonics have manifested itself.

    Meaning, most tubes that are microphonic, are microphonic, right from the box, just as you have said.

    A tube will become microphonic later, if a grid becomes loose and the the tube becomes sensitive to physical vibrations, which usually come from the speakers.

    But once a tube becomes microphonic how can it become non-microphonic?

    As I have pointed out, the pre-amp has been on mostly continuously for the the past two years. While I turn the tube power amps off, when I leave the premises for a extended period of time, and mostly, whenever I leave, as a safety precaution, I am not as concerned, when it comes to a pre-amp.

    The sound is like someone tapping their fingers against a microphone on a podium. And, this was just from lightly tapping a lightweight plastic remote against the top plate, a few inches away from one of the tubes.
     
    Last edited: Oct 19, 2017
  6. reb

    reb Money Beats Soul

    Location:
    Long Island
    OP

    If the microphonic tube pre-amp noise is not causing any audible distortion while playing music. Then its not an issue,

    A microphonic tube stays microphonic, it never returns to being non microphonic.

    I understand you're frustration in trying to solve this issue.

    For now, stop tapping on the pre-amp to check for microphonics and just listen to music.
     
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  7. sturgus

    sturgus Forum Resident

    Location:
    St. Louis Mo
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  8. Davey

    Davey NP: Electrelane ~ No Shouts No Calls (2007 LP)

    Location:
    SF Bay Area, USA
    I agree that the most likely cause is the tubes, but you can easily hear the microphonic effects on capacitors just by tapping on one of your interconnects. It's a more drastic change in capacitance than you would get by tapping on most solid capacitor bodies, but it is there, and some cables are designed to minimize the effect. Not really applicable here, but in abstract :)
     
  9. SandAndGlass

    SandAndGlass Twilight Forum Resident Thread Starter

    My point exactly. As long as they continue to play well, all is good.

    Just thought that this was strange, because, as you say, once a tube goes microphonic, it should always remain microphonic.

    I accidently discovered it, putting the remote LED strip remote back to it's usual resting place.

    Meanwhile, I am keeping the spare set close by.
     
  10. Bill Hart

    Bill Hart Forum Resident

    Location:
    Austin
    I know in the case of the Allnic phono stage I use, the H 3000, Mr. Park uses a special gel socket. When I first got the unit several years ago, if you played music but muted the output of the line stage, so you just hearing the signal going through the phono stage, you could hear the tubes singing sympathetically with the music. Mr. Park suggested that as the tubes aged, this characteristic would diminish, and so it did (though I did replace the tube set at one point, they aren't fancy tubes). This is all precis to a slightly different point- that is, if the tube may be inherently microphonic given how it is used and mounted (I had this problem with the vaunted ARC SP-10 mk ii--finding quiet tubes was beyond difficult), I wonder if it has to do with the way the tube mounts- the socket or the physical connection of the socket to the chassis. In other words, the tube is microphonic, but it is attenuated by how it is mounted. Just a wild stab.
     
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  11. Davey

    Davey NP: Electrelane ~ No Shouts No Calls (2007 LP)

    Location:
    SF Bay Area, USA
    Technically speaking, I think what you are describing isn't exactly microphonics, it's the opposite, transforming electrical energy into mechanical vibrations, more like a loudspeaker. For instance, anyone that has done much amplifier testing with the normal resistor/capacitor load has probably heard the capacitor singing along with the input signal as the energy flexes the plates.
     
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