Leaderboard

Peaceful and relaxing. Sort of an 'end of the adventure' tune that'd roll over the credits. Good job, you saved the thing! Now go enjoy a beverage of your choice. You've earned it!

Well, that guy isn't "technically" wrong but there was a lot glossed over in that simplified explanation. So, now frequency response is just one specification of an amplifier that you should pay attention and really it is one of the most useless for actually telling you much about it. Beyond telling you what frequency ranges the amplifier will amplify it also implies the phase margin of the amplifier. Essentially, the amplifier has a more phase aligned output throughout the actual audible band in "theory".* As I said before you don't "technically" need additional line outputs to use a headphone amplifier. There are disadvantages to using one set of line outputs, but it isn't necessary. If you're using the built in volume control on your audio interface then you've got two stages where you can introduce either gain or attenuation before you actually hear it. Right or wrong really is a semantics kind of thing on paper. In practice the fewer things in your monitoring signal path the better because there are less things to get in the way of what you're hearing. I know before I bought my current audio interface, RME FireFace UFX+, I considered a lot of audio interfaces, but I also had some different requirements than you did. However, I also don't use the audio interface for monitor control. That is done via an external unit in my case and I just pipe over an unattenuated signal to my monitor controller, which has a banging headphone amp lol. So, the Tascam is generally well liked for what it offers. The Steinberg stuff is always pretty highly liked too. I like Focusrite stuff too so something like the 6i6 or even 18i8 are also quite comparable. But another possibility is Presonus. There are lots of audio interfaces out there. You have to figure out your requirements to see what it is you actually need and may need in the future. I can't tell you what those are. You spell out your requirements and select an audio interface based on that. If for example you want to use Universal Audio plugins then you may actually consider getting something like the Universal Audio Arrow or splurging for an Apollo Twin mkII. I only brought this point up because I find it smarter to spend however much money you need at first to get what you actually need. * - Extra Geek talk. Now, I don't really need to read much into the specifications of the Lake People G103-P because I know what it is built around, a LM1876. That is the datasheet to the device at the heart of the G103-P. Now, this device is designed to drive 4-8 ohm speakers. However, because it is driving such high impedance loads the chip's performance will be much better in several aspects. If you look at the datasheet on page 10 for Figure 10 to Figure 15 you'll see a bunch of graphs that are titled THD + N vs Output Power (THD + N stands for Total Harmonic Distortion plus Noise). Now, I doubt you're going to be running this anywhere near 1W output power. More than likely you'll be running it around 50-75mW. So, you're going to want to pay attention to that first decade on each graph which represents 10mW to 100mW. It is also important to note that these are specified at particular loads and frequencies, so pay attention to what it says in the upper left of each graph. So, it looks like THD + N will probably be around 0.05%-ish, which is quite good actually. There is also Figure 19 on page 12 that specifies the amount of output power into a given load. It only goes 40 ohms, but it looks like it is starting to level out. The maximum output power is probably going to be ~4W. There are other important factors at play here that aren't specified often that you should always look for. These include the Slew Rate of the amplifier, which given that this is a power amplifier is quite high at 12V/us** (this is a worse case figure). One factor that is a bit lacking is the Crosstalk though, at a typical 80dB, but again should be much better because it is very unlikely that you're going to be driving the headphones with a nearly 11Vrms signal. That is just absurd. There are plenty of other things to keep in mind too. Figure 29 is a good one for showing how the phase of the amplifier varies, Figure 28 is also helpful for showing how good the amplifier is at rejecting common mode noise vs frequency. Like from this datasheet you can pretty much guess exactly how good this amplifier will be in terms of specifications This is why I say specifications are important. Now, they could improve the CMRR of the LM1876 by using an external opamp and running the output of that into the LM1876's non-inverting input, but I can't really find much more on the G103-P other than it uses a LM1876. For further reading to better understand these specifications I highly suggest reading Rane's Note on Audio Specifications. So, why do I say the guy is "technically" correct? Well, because like most things in life there is a grain of truth to what he is saying, but it isn't the whole truth. See, by his logic then by extension higher impedance speakers need more powerful amplifiers, which isn't actually the case. See any power amplifier is really just trying to swing the output voltage to the voltage rails of the power supply. That is to say if you're running a 60V power rail then it can produce a voltage difference of 60V across the load, aka the speaker. So, then you may quite rightly ask, "Then why do power amplifiers need these massive heatsinks if all a power amplifier is doing is changing voltage?" Well, this is where Ohm's Law comes into play. In a nutshell Ohm's Law says that if you have resistance and there is a voltage difference applied across that resistance then current must flow as a consequence. See, you know the resistance of the speaker, which say for the sake of simplicity and the like is 8 ohms, then that 60V across that 8 ohms requires a certain amount of current to flow. According to Ohm's Law, the Current (I) is equal to the Voltage (V) divided by the Resistance (R) or I = V/R, I = 60V/8 ohms = I = 7.5 amps. That is why an amplifier needs all that heatsinking, because it has to be able to source A LOT of current as a consequence of Ohm's Law. Now, the main issue with many power amplifier is that they actually become current limited. This means that in order to sustain that 60V across the load the power amplifier can no longer source enough current to maintain the relationship between Voltage and Current as required by Ohm's Law. Headphones must obey this as well, and why I originally said that the problem with most headphone outs is that the output device is simply inadequate to drive the desired load. Looking at a common audio opamp, such as the NE5532, there is a specification called Output short-circuit current labeled as Ios. This spells out the ABSOLUTE maximum amount of current that the NE5532 can deliver. There are three figures listed for it, which are "Min", "Typ", and "Max". Assuming the "Typ" or Typical value of 38mA along with the Maximum peak-to-peak output-voltage swing (pretty self explanatory, but says the maximum voltage difference the opamp can produce) you can actually figure out the maximum amount of resistance that the NE5532 can drive using Ohm's Law. In this case R=V/I = R= 24V/0.038A = 631 ohms. Further, since we know both the voltage and the current, we can also estimate the power, which is about 912mW (basic estimates are easy as P = IxE, where E is actually Voltage but is based on slightly different terminology). So, for your DT 880s, a NE5532 could drive them quite well The problem is most headphone outs don't use a NE5532 because while it is a cheap opamp, it costs more than a NJM4558, which is often used. In 10,000 quantity the NE5532 is about $0.29 vs the NJM4558 at $0.10. Why spend an extra 19 cents, when you don't have to when the output is just "adequate"? The simple answer is they don't. ** - Super duper extra Geek Talk Slew Rate is an interesting specification. It is basically how fast the amplifier can change the voltage or current in certain situations. So, for the LM1876 that is 12 volts per microsecond or V/us or for fancy folks V/μs. This means that if the LM1876 is at say 0V and all of a sudden it needs to jump to +12V then it will take it 1 us to do that. Now, say the LM1876 needs to move to -12V then it will take 2 us for it to move to that. 1 us to move to 0V and 1 us to move to -12V. This specification fundamentally limits the maximum frequency that amplifier can reproduce in a linear fashion. Assuming that the LM1876 is running on +/-15V (a common value for audio circuits and I'd be surprised if the G103-P is running much higher or lower than this) then that means the maximum peak voltage the LM1876 needs to be able to swing is 30Vpk. So, if it is quoted to be able to reproduce 150,000Hz then to do so in a linear fashion requires that the LM1876 have a Slew Rate of 28.278V/us. Now, the LM1876 could very be able to operate at that kind of slew rate depending on the load that it is driving. I'm just using 12V/us as a worst case scenario kind of thing. There also isn't a graph to show how the Slew Rate changes vs anything so I couldn't even hazard a guess at how it is performing. However, that worst case figure gives an upper bound of frequency right around 63,700Hz anyway. But it still pails in comparison to a lot of other parts. The NE5532 for example is about 9V/us, the TL07X series have 13V/us, and high performance opamps like the OPAx134 series are 20V/us or OPA161x is 27V/us. Also, super high slew rates aren't necessarily a good thing either as they can lead to opamp instability and may require frequency compensation to stabilize the opamp, typically in the form of a compensation capacitor. A classic example of this is with the NE5534 which requires a compensation capacitor whenever the gain is below three, typically a 22pF. And even then I'm not going to enter into the topic of Gain-bandwidth product or GBW and how it is also a factor to consider with amplifier stability and how all of this comes together to help explain whether an amplifier is actually stable. Definitely, a lot of good reading on the topic if you want to explore more into it though.