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Headphone amp... the poweramp method

Discussion in 'Pro Audio Equipment' started by koolaola, Jun 12, 2006.

  1. koolaola

    koolaola Guest

    Well well.

    I know this has been discussed before... best headphone amp blablabla.

    However, this is my senario. In August I'm going to purchase some equipment to build myself a portable recording rig. All the preamps are going to be mounted in a rack case together with my audio interface. From this rig I'm going to have a multicable with a stagebox to which all the connections to and from this rig can be accessed, this include mic to preamp but also headphone amp to headphones.

    I must say that I really hate the fact when you have to dial past 12o clock on the headphone volume to get a decent level in your cans, and what I've seen in studios is that they use a regular poweramp to drive the cans, and I'm somewhat suprised that the headphones doesn't melt being plugged in to a poweramp. But as I discovered, there seems to be a method for reducing the juice going to the headphones.
    In this thread ((Dead Link Removed)) Ms Remy describes the lowdown of such project, althought I didn't get the whole thing. :roll:

    So this would then be the problem I might encounter, if I choose to use a poweramp as a headphone amplifier, won't I fry the multicable in my rig? Can I use some other cheapo poweramp like the german thomann brand (http://www.thomann.de) or a samson, instead of the tried and tested Crown D75?
    The keyword here is 'level', and using a poweramp just makes me imagin that all my worries will go away. The other side to this coin is that I currently use a pair of DT-100, the 400Ohm version, and I know, they soak up all the juice of any of the regular headamps. So just maybe, My problem will be solved buying the 16Ohm version and one of those "behr&%# :twisted: Gaaaahhh.... can't even say that name out loud" headphone amp.
  2. RemyRAD

    RemyRAD Guest

    Sep 26, 2005
    OK then. Thank you for your questions.

    The answer is yes, you can easily blow up and destroy all of your headphones and your hearing if certain precautions are not observed.

    The basic rule of thumb here is that headphones generally cannot cope with anything much over 1 - 2 W of power. The impedance of the headphones is not as an important factor as the efficiency of the drivers themselves. Different drivers have different levels of efficiency i.e. XYZ of power input, produces XYZ of decibel output.

    In the system that I have described in previous posts, the headphone system requires numerous current limiting resistors installed on both the amplifier output (along with proper loading resistor) and within the "headphone boxes" themselves. These current limiting resistors prevent the maximum power the amplifier is capable of from ever seeing the headphones. 1/2 W potentiometers (volume controls) can therefore generally be used without a fear of burning out since those are also protected by a 1/2 W fixed 50 -100 ohm in-line resistors to the volume control, in the headphone box. That coupled with the 5 to 10 W 50 ohm resistor on the hot side output of the amplifier along with the appropriate 8 ohm 5 to 10 W load resistor so that the amplifier never sees anything much above or below 8 ohms loading. The load resistor is really only important for certain brands of amplifiers that must see a "proper" load. Some amplifiers such as the old Dynaco 120 watt transistor amplifiers had a tendency to blow out almost instantly without a proper 8 ohm load. So I would first include the 8 ohm 5 to 10 watt load resistor, followed by the 50 ohm 5 watt buildout hot side series resistor. The Crown series of amplifiers are so rugged, they could operate all day, with or without a proper load, from open to short. And therefore is one of the amplifiers that I recommend for headphone use. You can use any amplifier as long as you observe certain precautions as I have mentioned. If you don't know whether the amplifier will blow up without a load, always include a load resistor of at least 5 -10 W minimum at 8 ohms.

    While you might hate the fact that many headphone amplifiers require you to dial in a volume setting beyond the 12 o'clock position, most professional designers know that the 2 o'clock position is generally your nominal gain position (like the 0 position indicator on your linear slide faders and your standard operating starting point). I have no objection to those "headphone amplifiers" for a bedroom oriented desktop project studio/control room when you or maybe one other person might be recording within close proximity to the recording rig itself in your control room. My system is more appropriate where a centrally located power amplifier and a requirement for as many headphones as needed for a symphony orchestra is required. Completely appropriate for the project studio where from 4 to 10 musicians might require headphones at any one time during tracking and overdubs.

    As far as the cabling is concerned, the output cabling from the amplifier along with the associated cabling from the amplifier to the headphone box should always be done with 3 to 4 conductor, unshielded, always unshielded electrical extension cords like those orange ones you find at the hardware store. You don't want to load an amplifier into a shielded microphone style cable anytime! That can cause an improper inductance at the output of the amplifier which many amplifiers don't like and can cause failure of the amplifier output section.

    Now you can breathe easier as you will not be required to purchase any of those BareAnger amplifiers. Let me know if you have any further questions?
    Ms. Remy Ann David
  3. Kev

    Kev Well-Known Member

    Nov 13, 2001
    I think people miss the point about power available and power used.

    A 40W to 50W amp may be able to deliver some amp to a n 8 or 4 ohm load BUT it can't inject power into a 4k load.
    the laws of physics still apply

    using the Analog Devices
    Interactive Design Tools: Utilities : VRMS / dBm / dBu / dBV calculator


    a typical high signal level from a modern I/O box might be 21 dBu
    = 12.3 Volts peak = 8.7 volts RMS
    and into a 600 ohm load would DRAW 125.9mW
    this is = 21 dBm


    if that load were a 4 ohm load, the current demand would be higher and lead to
    1.888e4 mW = 18880 mW= 18.9 W


    if you wanted only 3 dB of headroom the level required would be 24dBu and so the power demand for the 4 ohm load would be 38watts.

    these number get even higher if you would signal levels that were typical in years gone by of say 26dBu and headroom of 6dB ... etc

    No-one is suggesting that 40watts is required by any headphone or that continuous signal levels of 26dBu is ever required.

    It is all about headroom and the ability to drive multiple headphones without interaction between or loss of signal levels as more headphones are added to a circuit.

    Think of it as a very capable Audio Distribution Amplifier ( ADA ).
  4. koolaola

    koolaola Guest

    Ahh an Ozzy...
    Were in Australia for about 14 months... came home just before x-mas... miss it as hell... not to talk about the surf... Mmm :(

    Well Thanks both of you... I'll try to get my head around the info you've posted... then I'll probably be back with further questions!


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