Speaker amps for headphones

Discussion in 'High-end Audio Forum' started by operakid, Feb 1, 2013.
  1. potterma
    At least it has a 4 pin XLR on the other end. You should either ask the manufacturer of the cable or ohm it out to verify there are no shared connections. I've seen some very strange configurations that, if used as a headphone adapter, could damage the amp.
    Each banana should have a connection to ONE pin and ONE pin only on the XLR.
  2. Sonic Defender Contributor
    How easy is it to have the wrong matchup, and can one visually tell? Sorry for the ignorance, not something I'm as up on as I should be. My main speaker tap cable terminates in a XLR and I have an adapter section that terminates in a TRS so when needed I can use 1/4" terminated headphones. When Trevor at Norne made the cable and PETEREK my adapters they knew I had a common ground as I asked NAD directly. I'm not sure it would be stupidity for people if they made a mistake, more like lack of knowledge. I know that I'm not stupid or careless, however, I could conceivably have made a mistake such as you are talking about as I don't understand enough about the subject, but I do know about other things so I don't think ignorance in one area would qualify people as stupid.
    Last edited: Jul 22, 2017
  3. Armaegis
    Using words like "stupid" or "ignorant" was a bit hasty on my part. I'm just very tired of seeing these kind of adapters, and ordering some random cheap cable from ebay could very well be disastrous if the builder is just slapping something together and doesn't really know what it's for.

    An amp with a shared ground means the two black terminals at the back are actually the same thing. They don't have a signal running through them, they are simply "ground". The majority of amps have this configuration. You can check this with a cheap multimeter: set the dial to the lowest resistance setting and measure between the two black terminals; the reading should be at or very near zero.

    The problem is when you have an amp does does not have shared grounds; i.e. there is signal running out of the black terminals. If you use this with a TRS adapter, basically you've connected the two blacks together to the sleeve of the adapter. That's essentially the same as shorting the outputs on an amp, which is all sorts of bad mojo and will let the magic smoke out.
  4. Sonic Defender Contributor
    I shouldn't have taken you literally as it should have been obvious you were speaking out of frustration and not from the heart per say. No harm no foul and thanks for the great explanation.
  5. west0ne
    Would an adapter such as the TRS shown have any sort of resistor circuit built in? I built myself a Robinette Box which does use common ground on the TRS output after the resistors (I checked my Amp first) but all of the advice I read before going this route suggested that there needed to be some sort of resistor circuit because the Amp was expected a load of 8 Ohm and that without resistance the noise level would be high as well as there being very little room on the power output.

    I can certainly see how the TRS adapter and even a 4 pin XLR into balanced adapter could give the impression that it was a good idea to plug headphones straight into a speaker amp when in reality a speaker amp is likely to have more power than many headphones can handle or at least need.
  6. Hutnicks
    Headphones make wonderful fuses.
  7. west0ne
    Do you just have to hope the headphones blow before your eardrums though?
  8. Hutnicks
    Usually just a loud pop occurs as the driver overheats and disassembles itself.
  9. Sonic Defender Contributor
    I have been driving many headphones (TH 900, TH 600, HE 560, HE 4001, MDR Z7, MDR Z1R, Vibro X, LCD 2F, and many more) from a 180 watt amp right from the speaker output without the slightest hint of trouble. The gain control is a perfect safety device, you just need to use it. A very knowledgeable member here, Stan D assured me that amplifiers like the load a headphone represents, the more the merrier, at least that is what I took him to mean in a PM chat we had a while back on the subject. The fear of too much power is unfounded if the amplifier is properly designed with decent specs and a gain control you have nothing to fear.
  10. Armaegis
    Most adapters do not have the resistors. It's easy enough to do if you're handy with a soldering iron.

    A box with a switch that bridges two outputs... yeah that's an accident waiting to happen.

    Sometimes it's a sizzle and a scream. I have the melted drivers to show for it. :darthsmile:

    In theory there's nothing to worry about if you have good gain control. In practice, you have:
    - accidental bumpage of the volume knob
    - turn on/off thumps
    - very high DC offsets
    - high noise floor

    Solid state amps like to see high impedance loads. Tube amps might not (they'll most likely still work, but perhaps not optimally).
  11. Sonic Defender Contributor
    So what is the impact of a high DC offset and would it typically be audible? Yes, tube amps for speakers are not something I have ever owned so I should be more specific and say solid state amps when I post.
  12. Armaegis
    A small DC offset will push the driver off to one side. This will limit driver excursion and cause non-linear behaviour/distortion.

    A large DC offset will melt your drivers because you've turned the voice coil into a space heater.

    Offset can happen regardless of your input settings/volume. The amount of offset may also change with warmup time. It is also not uncommon to have somewhat large offset surges at powerup/down on speaker amps, especially if it's older.
  13. Sonic Defender Contributor
    That doesn't sound like fun. I have a feeling the offset in my NAD m3 is low enough. So is there a range of DC offset that is considered if not ideal at least very low in terms of either type of potential impact you are describing, eg, not too low or too high?
  14. Armaegis
    DC offset can be easily measured with a multimeter. Set it to the lowest DC voltage setting and plug into the pos/neg terminals. The lower the reading the better. On a speaker amp, you should be reading below 50mV (and a good one will be under 10mV). A headphone amp should be an order of magnitude smaller than that (under 5mV).

    You should be just fine with the M2; NAD makes good stuff and a newer amp shouldn't have any issues. Older amps are more likely to develop offset problems.
  15. Sonic Defender Contributor
    Yes, I have been very pleased with the M3 speaker tap, very pleased. I have driven low Z (16 ohm) right up to 600ohm headphones without a problem of any kind. Of course it is overkill in terms of power, but the driver control you get with two really well designed beefy power supplies and circuits is amazing.

    Here is a picture showing the cable tapping into the terminals. Trevor at Norne did a fantastic job making the cable for me. Top notch craftsmanship. 20170724_110333.jpg
    Last edited: Jul 24, 2017
    Fearless1 likes this.

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