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Primer: Impedance Matching 101

Discussion in 'Amps and Cabs [BG]' started by bgavin, Aug 20, 2002.

  1. I'm starting this thread with the intent it can be referenced from the archives, so I'm looking for info about how/why to identify and correctly match impedances of various equipment.

    Example #1:
    High impedance inputs for passive bass pickups

    Example #2:
    Preamp 1/4" TS unbalanced out to Power Amp in.

    Example #3:
    Preamp XLR balanced out to Power Amp in.

    Example #4:
    Preamp 1/4" TS unbalanced out to Sound Card in.

    Passive pickups require a high impedance input, or the tone degrades. Multiple pickups, volume and tone pots in a passive circuit reduces the impedance and degrades the tone.

    QSC specifies input impedance as 6K unbalanced, and 12k balanced. How does a typical unbalanced preamp-out operate with this?

    Rane typically uses a 20K input impedance, but only a 100 ohm output impedance. How does this 100-ohm output impedance interact with the 12k input of the QSC PLX?

    Going DI from the preamp into a high quality sound card should sound good, but often does not. Is the problem simply a voltage mismatch, or is there also an impedance issue? Pro audio and consumer equipment use different dB systems. Matching voltages is only part of it. How are they interfaced correctly?
  2. geshel


    Oct 2, 2001
    A 100-ohm output impedance will be just dandy feeding a 12k-ohm input impedance.

    Roughly, the voltage lost will be proportional to the percentage the output Z is relative to the sum of both.

    Veff = Vout * (Zin) / (Zout + Zin)

    This is because the output impedance of a voltage source is defined in series with the source. So the current goes through the output impedance, then through the input impedance of the other device, creating a voltage divider.

    If the voltage ranges between the two devices are matched, then you would lose 6dB of dynamic range (or SNR, take your pick) if Zin == Zout.

    In the Rane + QSC case, the voltage loss would only be .8%, or -.07dB.

    The question of tone is most important with passive devices, where the resonant frequency changes with the load impedance, and where they might not be able to supply enough current should the impedance drop enough (leads to compression/distortion).

    Most preamps, and definitely solid-state ones, should have a plenty low enough output impedance (less than 1k?). The Alembic F-2B (and F-1X?) is a bit of an exception.
  3. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    A general rule of thumb in audio is to seek at least a 10:1 ratio of input Z to output Z. So a device with a 100-ohm output impedance would have a good signal transfer driving an input impedance of 1K or higher, assuming it's got enough oomph to supply the current at any possible output voltage.

    With speakers, you generally want a ratio somewhat higher than 10:1, though. ;) At least 50:1 would be good.

    What kind of sonic problem do you get on the sound card? What I've encountered most often is distortion due to overdriving the card's input circuitry with too high a signal voltage. A simple two-resistor voltage divider could provide the attenuation necessary. Unfortunately, the sound cards I've had never specified what the maximum input voltage was.
  4. I'm using either the TS or XLR outs of my SWR IOD and not getting enough voltage to drive the card. I have to have the card mixer all the way to max, and it generally sounds bad.

    Turtle Beach tech support says the LINE IN spec is -10dBV. This is 0.316v RMS as I understand it. The IOD puts out 0.775v RMS with a 140mV input at 100 Hz.

    What I don't know is the impedance for the -10dBV sound card input. This is a typical consumer electronics input spec, but I don't know the typical impedance of this spec. Nor do I know the output impedance of the IOD.
  5. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hi Bruce,

    Hmm. Could it be that the card's nominal input voltage is 0.316 v RMS, and 0.775 v is the IOD's typical max output voltage?

    In what way does it sound bad? That could be a clue. In another thread you said you have a Rane crossover. How about trying that in between the IOD and the card, and use it to boost the signal going into the card.
  6. The sound card is most likely nominal at -10dBV, and the IOD is definitely 0.775 V maximum at 140mV input.

    I have to crank hell out of everything to get any recording volume. I suspect the strained tone is a consequence of overdriving the IOD and turning the card recording input value to MAX.

    The IOD is completely divorced from the remainder of the Rane gear in my rack when I run to the sound card. I wanted purity as much as possible. I supposed I could include the EQ at flat, then use it to amplify the IOD, because the EQ has greater than unity gain.

    All I want is a simple method of DI recording of my basses through a sound card. This kinda hijacked the intent of this thread, because I'd also like to learn about baluns, etc. Rane makes a Balance Buddy that converts between pro and consumer audio levels.
  7. geshel


    Oct 2, 2001
    I found the level a bit low when I plugged my bass directly into the line in. But all things are handled by the wonderful tool that is a Behringer 12-channel mixer. For <$200, hard to go wrong. :)

    I am suprised though that the IOD wouldn't have the voltage to drive the line in. 0.775v isn't enough to drive a power amp very far, even, is it? I'm not sure if 140mv is typical bass (that's your passive, right? Though many active basses have the same output voltag as passive ones now.).
  8. The DiMarzio passive put out between 150 and 250mV for the J, P, and UltraJ models.

    The IOD won't drive my PLX to clipping without the aid of the other Rane gear in the signal chain. I use greater than Unity gain on the Rane to pump up the signal. My PLX requires 1.7 volts to reach full power.

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