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Monitoring the signal going into a non-grounded speaker?

Discussion in 'Electronic Repair' started by N_Cook, Feb 11, 2013.

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  1. N_Cook

    N_Cook Guest

    Without differential scope amplifier/probe, audio out is missing something
    in the bass range about 70-80 Hz. Line-out signal faithfully follows the
    bass guitar input signal . Shure microphone, capable of those frequencies,
    monitoring the sound from the speaker, does not match the input at those
    sort of frequencies. But is it a failing of suspension or something of the
    speaker or some inductive/capacitive interaction with the amplifier when
    loaded with the speaker?
    Not yet monitored the main rails to see if dipping on this bass amp.
    I will then be subbing the speaker, as a test, but it still would not
    determine whether an amp or a speaker problem , speaker is original to the
    combo. Any simple component to put in series or parallel with the speaker to
    check if an output matching problem ?
  2. N_Cook

    N_Cook Guest

    Volume , tone controls etc are in the DSP sextion and as line out is fine ,
    I assume not a problem in that piece of magic.
  3. Geo

    Geo Guest

    Battery operated (AC) DVM?
    Or the green/yellow wire in the scope plug...
  4. tm

    tm Guest

    NEVER do that.
  5. mike

    mike Guest

    Can you be more descriptive about what you're doing?
    If you're connecting a sound card to the line side of a switcher,
    I can't imagine anything more in smoke/fire/death.
  6. N_Cook

    N_Cook Guest

    I suspect a current transformer would affect any PA/speaker/Zobell
    resonance/interaction effect. I do have a scope with differential inputs but
    as its not been used for 15 years I would bet good money on it having at
    least corroded switch contacts.
    I may sort out replacement rechargeable batteries for an ancient Tektronix
    212 hand held scope
    This is a Roland Cube Bass 30. There may be a faulty , going open, "shape"
    switch and may sometime settle into the default at the bottom of the
    resistor chain which is the Octave Bass setting which , may explain the odd
  7. whit3rd

    whit3rd Guest

    A properly loaded currrent transformer can be ten millliohms, or a few
    hundred microohms. There needn't be any significant effect on the system
    being measured.
  8. N_Cook

    N_Cook Guest

    So you would pass a speaker lead through the hole, ignoring the primary
    sense turns.
    In the meantime I may as well try a solenoid without pole piece
  9. N_Cook

    N_Cook Guest

    I tried a solenoid but not enough output and poor at sub 100Hz. Do you know
    what the response is like below 100Hz on your pass-thru Tx ?
    Dug out a couple of microphone Tx and checked they worked down to 40Hz ,
    taking readings, less response but not too much. Then some more important
    jobs came in.
    One for the phones out line ( so same f response) and one with series 1K or
    so over the speaker, should work fed to a normal dual channel scope
  10. Guest

    All this ado about not much......

    First of all just float the scope. For the signal levels and impedances youare dealing with just don't worry about it. Now of course this requires diligence to keep Murphy's law from being enforced, but in my 30+ years on the bench I have determined that it is usually better not to have things grounded on the bench.

    What's more you don't even have to do that. This is all fine for an academic discussion but in reality there are a few things to consider before getting one's bonnet in a beehive. At least 99.999999 % of the time the same thing is going to be on the + and - side of that speaker, just inverted. If not you likely have a problem in the output stage after the phase splitter. If it is OK from that poit you can measure either speaker wire and derive anything you want, just do a little mental math.

    Now since we are such the academic group here, there are some things about measuring speaker current which makes this inductive pickup method not so desirable. Unless this is some wierd tube (valve) amp with no damp(er)ing factor at all, the output is a voltage source. That means the voltage at the output is supposed to follow the voltage at the input, relatively regardless of the specific current drain at any given frequency. On the other hand the speaker is a complex load, in a bass bin or subwoofer it will be inductive with some resistive component. This affects the current drain at varyingfrequencies and makes measurements of amplifier perfomance near useless.

    In practice the inductive component of the woofer(s) partially copensates for the fact that the radiating surface is much smaller then the wave it radiates. The lower the frequency, the less efficient it is, however the lowerthe frequency the higher current the voice coil draws. It somewhat offsets..

    Tell you what, test the +/- theory just by getting two resistors, say a thousand ohms or so and put them in series across the speaker wires. Stick a scope at their junction and crank that baby up. If you see relatively a flatline and hear sound that means the output stage is OK and you can proceed.Now just scope one side or the other, doesn't matter. If you need to calculate output power just double whatever voltage you read, or quadruple the calculated wattage.

    When dealing with this type of audio, I recommend aa 1 Khz square wave. It can tell you alot about frequency response. Any decent amp should reproduceit quite faithfully, at least at a glance. To get an idea how to read it, put it through an audio equalizer and watch the effect. You will get the idea of what it looks like when low frequency response is lacking, or whatever. Looking at the flat parts of the output waveform is almost like looking at a frequency response curve in reverse. That is with the highest frequencies showing at the beginning and the lowest at the end. Looking for these characteristics with the scope should allow you to isolate which stage is causing the problem.

    If this is like most of this modern junk it probably has eighty-one digitalpreset EQs that are modelled after some blues player's amps from 1956 or some shit, but if the response deficiency is in all of them, the problem is most not within that little universe/mess. It would also be nice to know ifthe problem came on all at once or gradually.

    People who write regulations are not techs. I do not want anything earthed on my bench. Nothing. I have good reasons for that which are numerous. I can understand the concerns of people who believe that every square inch of this world should be safe enough for a four year old to play unsupervised but I do not espouse that bullfuckingshit. This is my bench and keep your hands off. If you happen to get shocked, see rule number one. In fact you guysare lucky with the single ended 240 volt mains, over here we have to deal with voltage doublers which make floating anything much more exciting. I try not to do it, but I will do it when needed. Suffice it to say that isolation transformers are needed more in the US for a couple of reasons.

    I would like to float everything, even the antenna/cable lines in the shop.Put an isolator on each one. In my plug mold I not only didn't connect thegrounds, I cut the green wires between each and every outlet. Of course this was at my shop, where I am now doesn't have that luxury and I am too lazy to do it.

    Maybe I am crazy, but it hasn't killed me yet.
  11. N_Cook

    N_Cook Guest

    I tried a couple of physically larger microphone Tx and the response is
    essentially flat from 2Hz (the bottom limit of my sine gen ) and about 20KHz
    , rapid tail-off after 30KHz.
    So will try one or both of them when I get back to this speaker/PA problem.
    RTFM time : the "shape" switch on a Roland cube 30 bass does not switch in
    out the COSM selection. I think there was a problem in this sw and when the
    wiper disconnects then the function stays with whatever was selected before,
    the control lines stay high.
    Incidently if o/c at switch on then disables the amp totally. The speaker/PA
    is perhaps fine after all , but will try comparing at some point , to at
    least see if microphone Txs are useful for this
  12. N_Cook

    N_Cook Guest

    Mic Tx and dropper worked well, no difference other than amplitude between
    line out and speaker input in normal low colouration amp types (perhaps
    10Kdropper for a more powerful amp). I will not be subbing the speaker as
    the owner went through the same thinking process as me, thinking the "shape"
    switch disabled any type selection.
    Not only that but he usually uses this amp flat with no reverb or effects,
    with an acoustic bass. To switch out reverb and flanger etc is a matter of
    turning the pots fully ACW in this DSP amp. He did the same with amp type
    following the same reasoning (plus shape off) but ACW selects the octave
    bass , an effect rather than colouration and the source of the non existent
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