# DAC impedance and voltage rating

#### juliangst

##### Active Member
I'm looking for a small dongle DAC with 2V output to feed my portable amp (Topping NX7).

Because most manufacturers don't state voltage ratings for their dongle DAC I'm not sure which spec I have to pay attention to.
Should I just use the power at the highest given impedance and calculate the voltage?

Example:
According to specs, the Fiio KA1 has a voltage of 1.2V at 32 Ohms and 6.8mW at 600 Ohms which would translate to 2V.
There should be no volume difference between the fiio feeding my amp and a normal desktop amp with 2V feeding my amp, right?

My main problem is that I want something smaller than my BTR5 (which only has 1.84V with no load)
but my Apple dongle (EU version), which has the perfect size, only has about half a volt.

#### solderdude

##### Grand Contributor
The max output voltage of a dongle (assuming SE and no DCDC converters inside) is determined/limited by the internal power supply voltage.
Assuming it works on 5V a max. 4.8Vpp (requires so called Rail-to-Rail output op-amps) voltage can be reached = 1.7Vrms.
Most of them are lower in output voltage though.
One can double this into balanced headphones (3.4V)

OP

#### juliangst

##### Active Member
The max output voltage of a dongle (assuming SE and no DCDC converters inside) is determined/limited by the internal power supply voltage.
Assuming it works on 5V a max. 4.8Vpp (requires so called Rail-to-Rail output op-amps) voltage can be reached = 1.7Vrms.
Most of them are lower in output voltage though.
One can double this into balanced headphones (3.4V)
But I've seen a lot of dongles that claim 2V and 4V (balanced) and the Hidizs S9 Pro even has 2.1V according to Amir's measurements. Am I getting something wrong about the voltage rating?

#### solderdude

##### Grand Contributor
2.1V = 5.94Vpp and even with rail-to-rail output op-amps a 6V power supply voltage would be required while USB output voltage is only 5V (maybe 5.2V in some USB ports).
This is what limits the output voltage.
Some dongle type DAC/amps could have DC/DC converters or charge pumps which could allow for a higher output voltage for high impedance headphones/line-outs.
Most of them will not have this and will be limited in output voltage.

OP

#### juliangst

##### Active Member
2.1V = 5.94Vpp and even with rail-to-rail output op-amps a 6V power supply voltage would be required and only 5V (maybe 5.2V in some USB ports) is available.
This is what limits the output voltage.
Some dongle type DAC/amps could have DC/DC converters or charge pumps which could allow for a higher output voltage for high impedance headphones/line-outs.
Most of them will not have this and will be limited in output voltage.
So should I just stick to my battery powered BTR5?
A small dongle would be nice but if that means that I'm getting way less voltage and thus less volume it's not worth it to me.

#### solderdude

##### Grand Contributor
2.1V vs 1.6V (BTR5) is merely 2.3dB difference in output level.

When you want to go audible louder (+6dB) you would need to go balanced. Not all headphones can be used with balanced amps though.

An option may be the FiiO KA3 = 4V in SE out (has internal DCDC converter).

OP

#### juliangst

##### Active Member
2.1V vs 1.6V (BTR5) is merely 2.3dB difference in output level.

When you want to go audible louder (+6dB) you would need to go balanced. Not all headphones can be used with balanced amps though.

An option may be the FiiO KA3 = 4V in SE out (has internal DCDC converter).
Thanks. I have a balanced cable for my headphones but similar to my desktop A30 Pro, the Topping NX7 only has 4.4mm for convenience and it's not a differential output

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