# Speaker impedance and voltage

There is a device which has output impedance 150ohms. If my headphone has either lower(30ohms) or higher (400ohms) impedance can I use it or sound will be too low/high and it will be unsable?

Also, the volume higher/lower is being done by the device by changing the impedance or by changing voltage?

Thanks

EDIT: device outputs: 35mW in headphone output jack, impedance 150ohms.

-Can i connect my headphone of it will damage it?

-Will it work?

-When it says 35mW what does it mean? 0mW no sound, 35mW volume max?

EDIT2:

If output is 50mW, impedance 150ohm then P=V^2 / R , so V= 2.74 ??

• What device it is? What kind of output it has? Commented Apr 28, 2020 at 16:39
• what do you mean by what output it has? its a vhf receiver/transmitter Commented Apr 28, 2020 at 16:58
• Since you're using headphones, it doesn't matter that it's a rec/trans, since you'll be interfacing the audio output. Knowing what impedance the audio out supports means checking the specifications of the device. If you don't have them, or can't check that, then it's a matter of trial and error. Changing the volume is done by changing the signal level, not the impedance, and (caution!) for some amplifiers, using too low an output load can result in severe deterioration. In short: either you have the spec and know, or be careful when checking, otherwise nobody can guess the answer. Commented Apr 28, 2020 at 18:53
• how can i know the voltage output of the device? should it be on the specs of it or its always the same for all devices the voltage output? Commented Apr 28, 2020 at 20:48

With a 150 ohm output impedance...

• 30 ohm headphones: volume will be low (not enough current probably, but often this works anyway with a normal op-amp line output)
• 400 ohm headphones: volume will be low (not enough voltage)

In either case, you can accommodate the mismatch by using an impedance matching audio transformer, or an active buffer.

• would 30ohm headphones destroy the device that has ouput 150ohms? Commented Apr 29, 2020 at 19:07
• Probably not, at least with the power level that a headphone driver can make. Commented Apr 30, 2020 at 1:07
• Low voltage with 400 ohm load and low current with 30 ohm load? I expected it to be way around. Low voltage with 30 ohms load and low current with 400 ohm load. Commented Apr 30, 2020 at 2:41

If my headphone has either lower(30ohms) or higher (400ohms) impedance can I use it or sound will be too low/high and it will be unsable?

Yes you can use either, but the high output impedance is not optimal, and is often the sign of a poorly designed device. To summarize one of the better write ups on this issue (which is worth reading):

The greater the output impedance the greater the voltage drop with lower impedance loads. This drop can be large to enough to prevent driving low impedance headphones to sufficiently loud levels.... As output impedance increases electrical damping is reduced. The bass performance of the headphones, as designed by the manufacture, may be audibly compromised if there’s insufficient damping. The bass might become more “boomy” and less controlled. The transient response becomes worse and the deep bass performance is compromised (the headphones will roll off sooner at low frequencies). A few, such as those who like a very warm “tube like” sound, might enjoy this sort of under damped bass. But it’s almost always less accurate compared to using a low impedance source.

Also, the volume higher/lower is being done by the device by changing the impedance or by changing voltage?

Volume control is implemented by increasing/decreasing voltage. On many common consumer devices, this is implemented by having an array of voltage dividers that apply a variable attenuation to the signal. The output impedance of the device is usually determined by the electronics that follow these attenuators and is therefore independently determined.

When it says 35mW what does it mean?

It means that into some unspecified load impedance the device can output 35mW. If you know the load impedance you could calculate the maximum voltage the device could output (P=V^2/R). If you don't, a low value like 16 or 32 ohms was probably used.

0mW no sound, 35mW volume max?

Correct, although you will only get 35mW into whatever impedance they tested with, so that number probably won't be correct for your load.

• is it important to know what is the voltage output of the device to my speakers? i thought impedance was the only important thing.. or all devices have same voltage output? Commented Apr 28, 2020 at 20:30
• @ChrisT The voltage across the load determines the volume, so if you knew that and the sensitivity you could calculate the maximum volume. Commented Apr 29, 2020 at 0:44
• If I don't know the vostage outout but only the impedance (150ohm) can I still plug my headphones with impedance match circuit between or too much voltage output might destroy them? Commented Apr 29, 2020 at 13:21
• i just found out the output where i connect headphone is 35mW.does that help at all? Commented Apr 29, 2020 at 15:35
• @ChrisT What is your question? Consider editing your original post if you have new questions or information. Commented Apr 29, 2020 at 15:40