Zoom UAC headphones output

[orkidea]

Hi there! I have Zoom Uac 2 audio interface which has 33 ohms headphones output impedance. I'm planning to use Avantone Mp1 pro headphones with 16 ohms impedance. According to information from reddit impedance of headphones output on the interface should be 8 times smaller than impedance of the headhones.
So i was just wondering if i should use a dedicated low impedance headphones amp
Thanks for any help!

[BertKoor]

The factor 8 or 10 is usually quoted when talking about matching the impedances of line level equipment, microphones etc.

A device with a standard line level output will be happy to provide 1 volt at about 1mA. That is an output impedance of about 1000 Ohm. Telephone lines were designed around an output impedance of 600 Ohm btw. If the device consuming that current wants to draw much more than 1mA, then it's kinda short-circuiting so that's not good. If it consumes less than 1/10th of the maximum, then that's ideal. So on line level inputs you'll see in the tech specs they have an input impedance of 10 kOhm or more.

But with amps designed to deliver power it works a bit different. They are designed for all power produced to be consumed. On big power amps I've seen specs that say eg. 100W at 8 Ohm speakers and 200W at 4 Ohm speakers. That is because the voltage is kept steady and the impedance then dictates the current, in ideal circumstances.

On the tech specs of the Zoom UAC-2 I see this:

PHONES OUTPUT
TYPE: ¼" stereo phone jack 20mW + 20mW (into 32Ω load)
MAXIMUM OUTPUT LEVEL: +8 dBu (@0dBFs)
OUTPUT IMPEDANCE: 33Ω

So it delivers 20mW into a 32 Ohm load. So they quote it will be happy to feed a low impedance headphone. Yes, 32 Ohm is low. 10 to 60 Ohm is low, 200 to 600 Ohm headphones are high. They are not implying you can only use cans of 300 Ohms and higher at all.

I got a calculator made for this type of things: http://www.bertkoor.nl/VawoCalc/
0.02 Watt into 32 Ohm equals 0.8 V @ 25mA.

Now what happens if you plug in 16 Ohm cans on outputs designed for 32 Ohm? It won't blow up for sure. Solid state components can handle that, whereas tube amps cannot. If the headphone amp still delivers 0.8V to a 16 Ohm load, then that's 40mW / 50mA. Possibly the current gets limited to 25mA, then it's 0.4V / 10mW. In practice it will be somewhere inbetween, maybe 0.48V - 30mA - 14mW - that's also 16 Ohm impedance.

The only thing which worries me is that 20mW is on the weak side. Far too weak for really high impedance cans. So with your 16 Ohm cans you're lucky.

[orkidea]

The factor 8 or 10 is usually quoted when talking about matching the impedances of line level equipment, microphones etc.

A device with a standard line level output will be happy to provide 1 volt at about 1mA. That is an output impedance of about 1000 Ohm. Telephone lines were designed around an output impedance of 600 Ohm btw. If the device consuming that current wants to draw much more than 1mA, then it's kinda short-circuiting so that's not good. If it consumes less than 1/10th of the maximum, then that's ideal. So on line level inputs you'll see in the tech specs they have an input impedance of 10 kOhm or more.

But with amps designed to deliver power it works a bit different. They are designed for all power produced to be consumed. On big power amps I've seen specs that say eg. 100W at 8 Ohm speakers and 200W at 4 Ohm speakers. That is because the voltage is kept steady and the impedance then dictates the current, in ideal circumstances.

On the tech specs of the Zoom UAC-2 I see this:

PHONES OUTPUT
TYPE: ¼" stereo phone jack 20mW + 20mW (into 32Ω load)
MAXIMUM OUTPUT LEVEL: +8 dBu (@0dBFs)
OUTPUT IMPEDANCE: 33Ω

So it delivers 20mW into a 32 Ohm load. So they quote it will be happy to feed a low impedance headphone. Yes, 32 Ohm is low. 10 to 60 Ohm is low, 200 to 600 Ohm headphones are high. They are not implying you can only use cans of 300 Ohms and higher at all.

I got a calculator made for this type of things: http://www.bertkoor.nl/VawoCalc/
0.02 Watt into 32 Ohm equals 0.8 V @ 25mA.

Now what happens if you plug in 16 Ohm cans on outputs designed for 32 Ohm? It won't blow up for sure. Solid state components can handle that, whereas tube amps cannot. If the headphone amp still delivers 0.8V to a 16 Ohm load, then that's 40mW / 50mA. Possibly the current gets limited to 25mA, then it's 0.4V / 10mW. In practice it will be somewhere inbetween, maybe 0.48V - 30mA - 14mW - that's also 16 Ohm impedance.

The only thing which worries me is that 20mW is on the weak side. Far too weak for really high impedance cans. So with your 16 Ohm cans you're lucky.

Thanks for the help! This is a really clear explanation!