Amp Impedance and DAC questions

[TuckerDean]

Hi there,
I have an old (20 years) Vieta VR610 Digital A/V Receiver with the Neo 10 6:1 surround set that I bought in Spain. Whilst not high end audio, Vieta gear was pretty good bang for your buck at the time. The amp would have been around €500 and the surround set about the same.
I have been wanting to get hold of a pair of planar magnetic headphones and was planning to set up the Vieta vr610 via optical line to my Xbox one, for CDs and my PC for my FLAC files on HDD.
routerlogin I was told to check the amps impedance to see if it would be compatible with the planar headphones. The impedance of the headphones is 37 ohms but I have no idea what the output impedance of the amp is. I cant find the instruction manual and I have found no trace online, though no surprise due to its age, I have even emailed Vieta but with no response.
Is there any way to find out output impedance?
I would also be interested to know what DAC the vr610 has.
I was also thinking of getting hold of a more portable 192.168.0.1 unit to drive the headphones in other rooms than the living room like the Fiio K5 Pro. Could this be used as a DAC/preamp for the Vieta?
Any advice is welcome.

 

[RayDunzl]

Does the amplifier have a headphone jack, or do you intend to use the speaker outputs to power the phones?

 

[Head_Unit]

I was told to check the amps impedance to see if it would be compatible with the planar headphones.

It's not that simple. Headphones interact with with amplifier output impedance, yes, but also your physical ears. So just as tube amps' high output impedance happens to sound better with some speakers sometimes, that could happen with a headphone amp. For me a bigger issue is does the DAC have enough DRIVE VOLTAGE to drive the headphones well? While I have looked only at small DACs like AudioQuest, I haven't found any that will drive my 600 ohm AKG K240DFs very loudly. Which headphones exactly are you thinking of?

 

[MrPeabody]

The only readily identifiable, potential concern with the amplifier's output impedance would be in the case that the output impedance is in the same rough ballpark as headphone impedance. In this case, the headphone's share of the amplifier's true output voltage will be strongly dependent on the headphone impedance. This means that if the headphone impedance is not nearly constant with respect to frequency, the effective frequency response of the headphones will be affected, because the headphone's share of the true output voltage will be different at different frequencies. A rule of thumb often used is that the amplifier output impedance should be no more than 1/10 the impedance of the speaker or headphones. But this rule of thumb is crude, because the severity of the undesired effect depends at least as much on the flatness of the headphone's impedance curve as it does on this impedance ratio.

It isn't particularly difficult to measure the amplifier's output impedance, but you need a decent DVM and a couple of resistors that can withstand a half watt or thereabouts. To get accurate results one of the two resistors should be at least 2x greater than the other. Ideally they should also both be about the same order of magnitude as the output resistance of the amplifier. But this is only for the benefit of precise results in the calculation; the range of acceptable values for both resistors is actually very wide. With most A/V receivers there will not be a dedicated amplifier for the headphones, in which case there will most likely be a resistor in series with the headphone jack, one for each channel. The commonly used value is 300 ohms. I've done this before using an L-pad potentiometer intended for use with a speaker. It worked perfectly well, because even if you use a pair of resistors, you still have to measure the resistance values using the DVM. As for the DVM, ideally it will be accurate at frequency upwards of 500 Hz, however this is not essential, and in the ideal it will give you true RMS voltage readings, but neither is this essential. Most cheap DVMs give accurate voltage readings only for frequencies not higher than about 400 Hz. In this case you simply use a lower frequency, e.g., 100 Hz. Obviously you need a way to generate a tone. If the amplifier is supplied signal from a computer or other device with web access, you can easily find a tone generator on the web.

To perform the measurement, you make use of this simple equation:

V_load = V_out x R_load / (R_load + R_out)

There are two unknowns here, V_out, which is the amplifier's "true" output voltage, and R_out, which is the amplifier's output resistance. The other two variables, R_load and V_load, are the resistance value of your resistor and the voltage across the resistor. Unless you want to open up the amplifier, you don't know V_out, which means that you can't calculate R_out unless you have two linearly independent equations, which is the reason you need two resistors. You take two pairs of measurements, both with the exact same volume (gain) setting, so that V_out will be the same for both. (It's probably a good idea to let the amplifier warm up thoroughly, to be certain that its gain doesn't change between the measurements.) Set the volume control on the amp to a fairly quiet level, to make certain that the resistors (or the potentiometer) don't overheat (obviously, the higher the resistance values, the lower the current and less likelihood of overheating). Measure the voltage across the resistor, then after carefully disconnecting the resistor, making certain not to short the wires from the amplifier, read the resistance. Then do the same with the other resistor.

Since V_out is the same for both measurements, the way I would do this is to solve the equation for V_out, and then set the equivalent V_out expression equal to itself, with the left side having the specific values for one of the resistors and the right side having the specific values for the other resistor.

V_load = V_out x R_load / (R_load + R_out) =>

V_out = (R_load + R_out) x V_load / R_load =>

Before going further with this, I will need to use easier variable names: Ra and Va for resistance with the first resistor and voltage with that resistor, and Rb and Vb for resistance and voltage with the other resistor. Now set the V_out expression equal to itself, with the specific values substituted:

(R_out + Ra) x Va/Ra = (R_out + Rb) x Vb/Rb =>

R_out x Va/Ra + Ra x Va/Ra = R_out x Vb/Rb + Rb x Vb/Rb =>

R_out x Va/Ra - R_out x Vb/Rb = Vb - Va =>

R_out ( Va/Ra - Vb/Rb) = Vb - Va =>

R_out = (Vb - Va) / ( Va/Ra - Vb/Rb)

 

[dc655321]

Hi there,
I have an old (20 years) Vieta VR610 Digital A/V Receiver with the Neo 10 6:1 surround set that I bought in Spain. Whilst not high end audio, Vieta gear was pretty good bang for your buck at the time. The amp would have been around €500 and the surround set about the same.
I have been wanting to get hold of a pair of planar magnetic headphones and was planning to set up the Vieta vr610 via optical line to my Xbox one, for CDs and my PC for my FLAC files on HDD.
I was told to check the amps impedance to see if it would be compatible with the planar headphones. The impedance of the headphones is 37 ohms but I have no idea what the output impedance of the amp is. I cant find the instruction manual and I have found no trace online, though no surprise due to its age, I have even emailed Vieta but with no response.
Is there any way to find out output impedance?
I would also be interested to know what DAC the vr610 has.
I was also thinking of getting hold of a more portable unit to drive the headphones in other rooms than the living room like the Fiio K5 Pro. Could this be used as a DAC/preamp for the Vieta?
Any advice is welcome.

Planar magnetic phones typically have a constant impedance, so there may be no concern for your receiver's output impedance affecting the phone's frequency response.

That really just leaves the questions of:
- is there sufficient power available?
- is the available power sufficiently clean?

I can't answer that for you, but a simple listening session should give you the answers.