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Help amateur with speaker load and power supply questions

Brad Karal
Prodigy 40 points
Other Parts Discussed in Thread: LM3886

Hi everyone:


Newbie here- to electronics, to everything related to audio. So please, I'd appreciate simple answers in plain English, if you

don't mind. I don't understand electronics nor do I understand much technical jargon.

I am building an audio amplifier from LM3886 chips. Now, the speakers I'd like to drive have the impedance curve

you see in the attached graphic. I really would like to be able to use two of these in series (Yes, there is a good reason).

The problems is, that drops load impedance very low. On a chart such as the one below, what would a knowledgeable person

judge to be the impedance of one of these speakers? That being said, is it relevant that the impedance drops really low

(to about 5 ohms) only below about 20 Hz. In other words, can I assume the "impedance" of this speaker is really higher

than 5 Ohms, or should I assume it's about 5? Cause of course, I'll almost never be playing music which contains content at 20 Hz

or does that matter?

I would still like to be able to at least experiment with connecting two of these speakers in parallel (total impedance?) If the total impedance

of two of these drivers connected in parallel is not too low, what effect would it have on the sound? Am I likely to just run out of adequate current

and voltage or will other things come into play?

If two of these wired in parallel would be usable, what voltage would I want to select in a power transformer? The TI design guide and Excel

spreadsheet only allows calculations based on a minimum impedance load of 4 ohms.

I hope these questions are clear. Thanks in advance for any help.

  • 0
    TI__Guru**** 189401 points
    Hi, Brad,

    Based on your plot, I would call this a 5-ohm speaker.

    You mentioned putting them in series, and then later in parallel. I think series was a typo based on the rest of your post.

    Putting these in parallel will drop the effective impedance to 2.5 ohms which is probably too low for the LM3886 to handle. Check out Figure 33 - the current capability really drops off below about 4 ohms.

    You could use two separate LM3886s - one to drive each speaker.

    -d2
  • 0 in reply to Don Dapkus
    Prodigy 40 points

    Wow! Thanks for that clear and helpful answer d2!  As I understand it, my main problem would just be a lack of current.

    So...I forgot to mention that I'm going to be using a setup with an active crossover. (For those newbies out there, that means you

    run a crossover before the amplification stage and so each amplifier drives a different driver or set of drivers. One amp for woofers,

    another for mids, etc.) I assume the amp would stay reasonably stable if I just drove the mids and tweeters with it? Aren't most

    mids and tweeters fairly high impedance? What's the upper end of the impedance curve that the LM3886 handles well?

    Thanks

  • 0 in reply to Brad Karal
    TI__Guru**** 189401 points
    Hi, Brad,

    Unfortunately, the LM3886 isn't spec'd for higher load impedances (doesn't mean it won't work, just means we didn't publish any info when this device was developed). You will need to check stability of your system and add some compensation externally if it oscillates.

    -d2