Weird preamp behavior

Discussion in 'Tech Talk - Electronic Repair Modifications DIY' started by niclaus, May 18, 2019.

  1. Boswell

    Boswell Moderator Distinguished Member

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    OK, that goes some way to explaining why it doesn't work correctly with transformer-input pre-amps. From your colleague's response, it seems that, as I suspected, the equipment was designed assuming it would feed a fully differential input rather than simply a balanced input. Many non-transformer pre-amps have fully differential inputs, so it should not be too difficult to find others that work in this application, maybe even over a wider range of gains. Audio quality would be a separate consideration, and may narrow the field to a very few candidates.

    As far as the RME Quadmic goes, it doesn't feel right that the output noise level should make a significant jump between two gain settings. In the quest to find the source of this, a quick test would be to try it again when connected to the rig, but with the rig powered down. That would give a crude indication whether the noise jump was coming from the rig when powered (probably in the form of an h.f. oscillation), or whether the Quadmic was exhibiting instability at medium gain settings with that input cable. The cause may be more subtle than that, but at least you could get an indication of whether or not your Quadmic itself had a problem.
     
  2. niclaus

    niclaus Active Member

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    when you say the rig, you mean that crazy machine, right?
     
  3. Boswell

    Boswell Moderator Distinguished Member

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    Yes. I wasn't using its technical name.
     
  4. niclaus

    niclaus Active Member

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    Ahahah!! will try that then.

    The hf modulation thing is what this guy seems to think it is as the RME quadmic MK II supposedly has a frequency range of 5hz to 200khz!
    He used to have it working with the quadmic mk I and as he remembers he did not have that problem. We will try and and get a MK I and see if that is better.
     
  5. niclaus

    niclaus Active Member

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    It still reads 0v when the machine is powered down.
     
  6. Boswell

    Boswell Moderator Distinguished Member

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    Sorry, I meant try the noise jump with gain change.
     
  7. niclaus

    niclaus Active Member

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    Oh yeah I tried that.
    No noise when the machine is off.
    I tried with a mic too. No problem whatsoever. It looks like it is working fine.
     
  8. Boswell

    Boswell Moderator Distinguished Member

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    OK, that indicates it's likely to be due to one or both of two things:

    1. In addition to the audio signals, the laser rig is outputting inaudible frequencies (above or below the human audible range), which the Quadmic pre-amp is having trouble dealing with, starting at a certain gain setting. This gain setting could well be the point at which rogue high frequencies in the waveform cause excursions that reach the amplifier's slew rate limits, or that low frequencies cause the signal to hit the rails.

    2. When the laser rig is powered, its output impedance has a complex component that interacts with the microphone cable inductance to cause a high frequency instability in the pre-amp. This might sound far-fetched, but I've had an instrumentation system misbehave in exactly this way, and it was something I found hard to believe at first.

    If you have the time and inclination to persue this further, the diagnostic tool for both of these conditions is an oscilloscope.
     
  9. niclaus

    niclaus Active Member

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    Hey Boswell,

    Sorry for the late reply, I spent the last few days with my head in that thing alongside the inventor. And what I have learned is that for some reason, it works and I should accept the fact that it does and embrace it. I mean when you know how to set it up, it sounds really good. We tried a bunch of things and we have to accept that even though there are problems, if you know how walk around them, it does what we want it to do, and it does it well.
    I wonder what this guy could have achieved working with a knowledgeable guy like you though.

    But, the high frequencies are probably the cause of that behavior since the input signal is almost (if not completely) a square signal.

    Anyway, thanks a lot for your help. I really appreciate your time and effort and I will let you know if at some point we are able to make it better.
     
  10. Boswell

    Boswell Moderator Distinguished Member

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    Most audio pre-amps are not guaranteed to behave impeccably when fed with large-amplitude square waves. That said, it is one of the standard set of tests I do on the pre-amps I have designed.

    If you are able to, I would try feeding the input of the RME Quadmic through a balanced low-pass filter set to (say) 25KHz, just to see if the noise levels then track linearly with gain.
     

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