geoffmartin

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  • geoffmartin
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    Usually, the audio signal is represented as a change in voltage at the output terminals of the power amplifier. In theory, that voltage is identical at the input terminals of the loudspeaker.

    The instantaneous current through the system is determined by the relationship between the instantaneous voltage and the impedance of the loudspeaker.

    The problem is that this model is too simple. The power amplifier has an output impedance (which is typically independent of frequency, more or less…). The speaker wire has a measurable impedance (resistance and capacitance – therefore it’s frequency-dependent) that is a result of the wire’s construction and its length.

    If the output impedance of the power amplifier is close to (or worse: larger than) the combined impedance of the speaker wire and the speaker (which is highly dependent on frequency), then the voltage at its output terminals is different from whatever it should be. This is because the amplifier’s output impedance is in series with the wire and the loudspeaker, and therefore the system acts like a voltage divider (measuring at the amplifier output terminals).

     

    This means that the voltage at the power amplifier’s output will not be correct – but since the load impedance (the wire+loudspeaker) is frequency-dependent, then the error will be signal-dependent (since that’s what determines the frequency).

    Therefore, if you have a loudspeaker with a low impedance at some frequency, then you need to use a power amplifier with a MUCH lower output impedance at that frequency in order for it to behave.

    So, it’s not really the case that the loudspeaker is “difficult to drive” – it’s that the loudspeaker/wire/amplifer interaction makes the amplifer misbehave.

    The simplest way to avoid this problem is to buy a power amplifier with an output impedance as close to 0 Ω as possible. This is a good solution if you have an amp in a rack, and you have no idea what will be connected to it tomorrow. However, if you are buying a “closed” system (an amp, some wire, and a loudspeaker) that won’t change, then you don’t need to spend quite so much money, since you can chose the amplifier helped by the knowledge of the wire+loudspeaker’s impedance.

    Cheers
    -g

    in reply to: Connecting Beogram 9000 to BL 28’s #6888
    geoffmartin
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    I’m a little confused about the initial problem.

    Are you comparing the sound of a song

    • played from Deezer on a Moment connected to the BL28s
      to
    • played from vinyl played on a Beogram 9000 via a RIAA preamp through the Moment connected to the BL28s

    ?

    If “yes”, then the difference in spectral balance is most likely to be caused by (in order of probability)

    • different masterings of the track (Deezer vs. vinyl)
    • wear on the stylus
    • weirdness in the RIAA filter’s magnitude response curve

    Of course, there’s nothing wrong with connecting the RIAA output directly to the BL28s Line Input, since the fewer devices in an audio chain, the better (generally speaking…), but I doubt that this will make a big change in overall timbre that you’re describing.

     

    If “no” then never mind… 🙂

    Cheers
    -g

    in reply to: MMC20 Genetic manipulations… #6175
    geoffmartin
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    It’s a good question – but I think that shellac (+ limestone) 78s are harder than PVC 33s. Also, the groove is MUCH wider, so there’s plenty of room for error.

    Either way, it’s worth a try. Thanks for the ebay link you sent. This will give me a new project once the days start getting shorter here in Denmark.

    Cheers
    -geoff

    in reply to: MMC20 Genetic manipulations… #6161
    geoffmartin
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    I forgot to add…

    > you may not be aware of, but there is a 20 pages “technical sound guide” about beograms

    I am aware of it… I wrote it. 🙂

     

    EDIT: I didn’t realise it was a joke. I’m a bit thick this weekend, apparently. Sorry… 🙂

    Cheers
    -geoff

    • This reply was modified 1 month, 3 weeks ago by geoffmartin.
    in reply to: MMC20 Genetic manipulations… #6153
    geoffmartin
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    Hi again,

    No need to make excuses… I’m genuinely curious! I’m currently in the process of turning two sick 42VFs into one healthy one – but I don’t have many pickups with the cantilever still attached (the end result will have the 20º tonearm – not the later 15º version, and the SP1-2 pickups are hard to find in good condition…) Given the way I’ll be using this turntable (mostly with my 78 RPM disks) a DIY pickup rebuild would not be crazy.

    I’ll have a look around for the sapphires – maybe I’ll have to call in a favour from an audio geek friend in Paris to look around the next time he’s at FNAC if I can’t find my own around here in the fields of Jutland… 😀

    Thanks for the inspiration! Nice work, in my opinion!

    Cheers

    -geoff

     

    • This reply was modified 1 month, 3 weeks ago by geoffmartin.
    in reply to: MMC20 Genetic manipulations… #6099
    geoffmartin
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    Too bad you’re in France and not next door. I’d be interested to measure this to see how good/bad it turned out!

    On the other hand, I have some broken cartridges lying around. I might just try to duplicate your results by making some franken-pickups of my own…  🙂

    I’m curious about the “chinese bulk saphirs”…

    Cheers

    -geoff

    in reply to: Beograms: tangential or radial? #5895
    geoffmartin
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    I use my 42VF often – but that’s because it can rotate at 78 RPM…

    in reply to: A question for Geoff Martin on BL8000 Damping… #4946
    geoffmartin
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    It was only invisible to me, since I don’t drop by here very often.

    As I said, I didn’t know that B&O made this kit until you asked about it – and the only information I have about it is what you’ve shown here.

    However, if one of the pieces of foam is blocking the entry to the port, then something isn’t placed correctly.

    in reply to: A question for Geoff Martin on BL8000 Damping… #4936
    geoffmartin
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    Hi,

    Sorry it took a while – I didn’t know that we sold these kits so I had to do some investigating…

    After asking all the right people, we’ve agreed that the thick foam is mounted incorrectly. It  has a hole in it that allows you place it so that the bottom of the port is flush with the bottom of the foam. This will also make it perpendicular with the sidewall of the loudspeaker – not angled as it is in the second photo.

    Hope this helps.

    Cheers

    -geoff

     

    in reply to: Flash Your B&O – new(er) thread! #4894
    geoffmartin
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    I read on the Interweb that analogue is better than digital, so I decided to try it out for myself and make up my own mind.

    My latest acquisition is a Beocord 506K, combination wire recorder and 78 RPM record player with built-in loudspeaker and integrated carrying handle. “Flexible living” 1950’s style. There are connectors on the side for an optional external loudspeaker and a radio input.

    As soon as I can get my hands on a compatible power cable, I’ll put on a pair of rubber gloves, plug it into an isolation transformer and see how much smoke gets released. Or maybe I should have a look inside first… Maybe someone already let the smoke out.

    Side note: If anyone else has one of these and is thinking of digging around inside, note the following: “Two UL41 tubes (push pull AF output) are located under the asbestos cover on the left.”

    67372293976__1D00A91A-103E-415E-A419-3AC4628D458F

    • This reply was modified 3 months, 1 week ago by geoffmartin.
    in reply to: Beolab20 Bass issues #3840
    geoffmartin
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    > I do realize that the Avant has in Sound settings an option to enable/disable loudness

    This will “only” increase the levels of the low and high frequency bands at lower volume settings, as is described in the Beovision Technical Sound Guide.

    > as well as clipping protection

    This will “only” prevent the analog and wireless power link levels from clipping due to a combination of high signal levels, increases in the tone controls, and playing at maximum volume.

    It’s difficult for me to diagnose whether or not there is actually a technical problem, or whether this thread is a discussion of a mismatch between your preferences / expectations and the sound design & capabilities of the loudspeakers. Changing the source will not increase the maximum possible amount of bass that the Beolab 20s can produce – but it’s possible that the settings in your current source (which I assume is a Beovision Avant, based on your comment) are reducing the bass level. However, if this is true, then the Avant is configured to reduce the bass at all volume settings.

    In order to make sure that you’re not dealing with incorrect settings in the Avant, create a new Speaker Group with only two loudspeakers (the BL20s) set to Left Front & Right Front – and then choose the Music Sound Mode (and reset it to the default settings).

    Also, check your global Bass and Treble settings on the Beovision Avant to make sure those are set to the default values.

    From there, you can tweak the settings to your preferences. However, if this does not give you the desired amount of bass at extreme volume settings, it’s because you’re hitting the physical limits of the loudspeakers.

    Of course, the placement of the loudspeakers in your room will result in changes in the overall spectrum – but there’s no way to diagnose this or make any recommendations remotely. Generally, the old rule of putting loudspeakers in corners will give you more bass – but it’s typically a very “boomy” and uncontrolled bass due to the coupling to room modes.

    cheers

    -geoff

    PS. I’ve (hopefully) disabled the comments on my website – but I might have missed a page or two here or there by accident.

    in reply to: Beolab20 Bass issues #3797
    geoffmartin
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    It is not possible to disable the ABL algorithm, however:

    Disabling the ABL will not increase the level of bass (relative to the other frequency bands) in any Beolab loudspeaker.

    If you were able to disable the ABL, you would merely get distortion caused by clipping of the signal – either caused by

    • excessive excursion of the loudspeaker drivers
    • reaching the limits of the amplifiers, or
    • reaching the limits of the power supply.

    ABL is there to prevent this clipping (and therefore a short time with distortion, followed by destruction of the loudspeaker).

    There are some people who confuse ABL with a “loudness” function (which boosts the bass and sometimes treble frequency ranges at lower listening levels) – however, these should not be conflated. Beolab loudspeakers (Beolab 28 excepted) do not have a loudness function, since this would be a feature of the source (e.g. a Beovision television) instead.

    in reply to: Connect Mac Mini via network to Beolab 28s #1689
    geoffmartin
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    If you plan on playing two-channel audio only, and only through the 28s when you’re listening to them (in other words, the other loudspeakers in the house aren’t singing along), then one option is to get a reasonably-priced USB sound card with an optical digital output and connect that to the 28s. (This will require daisy-chaining the 28s with a second optical connection.)

    You’ll need to be a little careful about how you’re managing the  volume control – but this is not a big issue.

    Of course, I’m ignoring the “network” part of your question – but if you wish to go that route, then an answer has already been provided.

    • This reply was modified 4 months, 3 weeks ago by geoffmartin.
Viewing 13 posts - 1 through 13 (of 13 total)