Impedance matching for DACs and Amps?

Discussion in 'Sound Science' started by audio jester, May 8, 2014.
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  1. Audio Jester
    Apologies, this has probably been answered many times but the more I read the less clear things become.
     
    I have a DAC which has the following specs:
     
    Output Signal Level: XLR 3.0 Vrms @ 0dBFS each pin
    Output Impedance: 75 ohm
     
    Now I have read that it is recommended that you have a 10:1 ratio (minimum) between the Preamp and Amplifier, or possibly 20:1 is better?
    So I want to connect this DAC's balanced outputs to an amplifiers balanced inputs.
     
    The amplifier has the following specs:
     
    Input Sensitivity: -10dBu
    Input Impedance: 10k ohm
     
    Sadly I have not yet grasped the formulas involved, but it looks to be like the ratio that this DAC and amp will have is something closer to 133:1
     
    Is this a good or bad thing?
    Alternatively, Could I just assume that the DAC would be fine to plug into any amplifier (or preamp) that has an input impedance of 750ohms - 1500 ohms (10:1, 20:1 respectively)
     
     
    I think my brain needs a reboot and a new hamster...it used to work better I think?
    Thanks for the assistance.
     
  2. 00940
    The ratio of 10 is a minimum, a lot more isn't a problem usually. The ratio is more useful to see if an output impedance of a DAC/preamp is too high; it's not very helpful when the output impedance is very low.
     
    Btw, the output impedance alone is not a good indication of the ability of the DAC to drive an amp. Many DAC have rather weak output buffers (or even no dedicated buffer) and are not able to drive loads under 10k properly or, if they can, distortion will rise significantly at lower loads.
     
    Professionnal gear is usually designed to drive 600R inputs if needed, hifi gear is more usually designed for a minimum of 10K.
     
  3. Audio Jester

    Thanks for the response.  I guess for someone like me who does not know much about electronics that it becomes a simple case of "try it and see how it goes".  I didn't even realise that output buffers were important... I hope my DAC has a decent one?
     
  4. superjawes
    Your 10:1 amplifier rule is really for devices that need power. There are electrical dampening issues that can pop up, but what happens is the output impedance of one device will create a voltage divider when paired to the input impedance (or driver impedance) of another device. If the impedances of your headphones and amplifier are the same, then the voltage output of the amplifier will essentially be cut in half. From a power perspective, this makes it more difficult to drive a pair of headphones even if your amp can supply a lot of power.

    With a DAC, you actually want a theoretically infinite input impedance on the amp (input not output). The output voltage of the DAC is already small, and this pairing ensures that the signal isn't degraded by the connection. At 133:1, the pairing will be good.
     
  5. Audio Jester

    Thanks Superjawes.  I guess this means there's one less reason for me not to buy more audio gear. [​IMG]
     
  6. Speedskater
    My thoughts:
    a] The 600 Ohm matched output to input system is ancient history from the broadcasting & pro audio industries. The only equipment made in the last 50 years using this system are legacy or boutique units.
    b] The typical concern with the 10:1 ratio is when using high output impedance vacuum tube equipment to drive low input impedance solid state equipment.
    c] There are no 'electrical dampening issues' in any interconnect system.
     
  7. cjl
    Ideally, you want a preamp/dac to have a much lower output impedance than the input impedance of the thing being driven. Also, most preamps/dacs aren't designed to drive low impedance loads. Because of this, the higher the input impedance on your amp, the better. In other words, a 10k input impedance is much better than a 750 ohm input impedance would be, for your use.
     
    (Basically, I'm joining with the chorus here - I agree with everything people said above)
     
  8. Steve Eddy

    :confused:

    What DACs are you referring to, specifically? Hell, even the cheapest opamps can drive a 600 ohm load to a few volts without any significant increase in distortion.

    se
     
  9. SilverEars
    Ok, question.  I have orthos that is purely resistive with flat impedance and phase graph.  Does the ratio rule still apply?  For power delivery, I can see it.  If you rule out power delivery, I can see the larger output impedance benefiting the resistive orthos, in terms of noise.  Or will the presense of the impedance between the amp and the phones skew the FR.  Within the audible range, there really isn't much changes that happens with output impedance right?  Only for resistors at very high frequencies correct?  But yeah, we are talkin impedance so it's probably complex.
     
  10. cjl
    Assuming they don't rely on electrical damping to clean up any low frequency driver resonance (I don't know enough about orthos to know if they do or not), it shouldn't matter.
     
  11. 00940
     
    Plenty of cheap stuff around here with direct output from ICs like pcm2702 (min 10K load) or es9023 (min 5K load from memory).
     
     
    And while it is true that some cheap opamps can drive a 600ohms load, that's not true of all opamps. You wouldn't want to try that with a tl071 or similar for example.
     
  12. Steve Eddy

    Oh, stuff like that is for toys. I'm talking about stuff that people here would likely own.



    TL071 can do a bit over 10 volts max into 600 ohms. It would do just fine with a few volts.

    se
     
  13. ab initio

    Orthodynamic drivers and regular dynamic drivers aren't all that much different in their mode of operation. It's a slight rearrangement of the winding and magnets, but the mechanics of the transducer remain. You want electrical damping. It lets the amplifier control the motion of the speaker, whether it's a dynamic or orthodynamic. Otherwise, your sound could end up pretty farty at the transducer's mechanical natural frequencies.
     
    Cheers
     
  14. SilverEars
    [​IMG]
     
    Planar magnetic transducers typically consist of two main components: a diaphragm with circuit and magnet arrays. The “planar” in planar magnetics refers to the magnetic field that’s distributed in the same plane (parallel) to the diaphragm. Planar magnetic diaphragms are thin and lightweight compared to much heavier moving-coil or dome diaphragms found in “dynamic” drivers. This thin diaphragm is suspended in the magnetic fields created by the magnetic arrays.

    Unlike a dynamic driver with a cone or dome attached to a voice coil, a planar magnetic diaphragm has a printed circuit spread across the surface of a thin-film substrate. When the circuit is energized with an audio signal it interacts with the magnetic field and produces an electromagnetic force that moves the diaphragm back and forth creating sound.

     
  15. 00940
     
    It can... badly. Have a look here, fig. 4.32, 4.33. And that's 3Vrms into 1K.
     
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