post #31 of 92
6/19/09 at 8:21pm
A zero output impedance is not necessarily the ideal for a headphone amp. It depends on how a particular headphone is designed. There is an international standard IEC 61938 which specifies 5V with 120 ohm output impedance for headphones of the range 8 - 600 ohm.
Yes but consumers have no idea which headphones are designed for what output impedance. I'm sure all headphone manufacturers are aware that consumers use everything from a sound card to a speaker amp. Wouldn't a headphone manufacturer say "optimized for a 120 ohm jack" on the headphone box if they were specifically designing a headphone to sound best with 120 ohms?
Plus, how many amps (headphone, speaker or pro audio) have a 120 ohm output? I can only think of one. I doubt headphone manufacturers expect people to build resistor networks to get the "ideal" sound of their headphones, meaning the FR the manufacturer designed the headphone to output.
|The buffer section, however, sits there as a means to power the drivers. When signal goes up or down, and causes the driver to move, the driver then pulls current, and voltage drops (and exactly how much it does this is not the same for all frequencies!). If it's pulling more than your amp can provide (sink, usually) at any instant, or more total than your amp can provide over time, then you get distorted sound. If a big drum hit comes in, and the drivers go from ~2mA to ~50mA right then, and in no time flat go back to needing ~2mA, that can give a weak little amp some hell. With most typical consumer products being made with cost savings having priority over quality, things like this suffer.|
Yes but the distortion you are talking would probably be audible (depending on how much distortion there is) and would be measurable. The definition of a weak amp would then be an amp that *measurably* fails such a current or voltage swing. But few people on Head-Fi perform measurements so all we've got is a bunch of subjective mumbo-jumbo, a lot of which could not be verified in a blind test.
|If we are talking about pure slew rate, you don't need a very high slew rate for the voltages talked about with headphone amps.|
|So how many devices are there that adhere to this particular "international standard." Can I count them with my fingers? It's a stretch for me to believe that headphone manufacturers are designing to a standard when it seems few amp manufacturers actually follow it.|
Very interesting thread. Even when I auditioned Graham Slee Novo I felt it didn't bring any audible improvement over when compared to iPod's line out feeding my headphones using an in-line volume attenuator. I spent about a week and tried to appreciate the improvements an amp brings but failed. Many people said my source wasn't great. But the same people when auditioned iPod as their source posted different thoughts. I even tried the same with my vintage Sony PCDP and I couldn't hear any difference.
Good point odigg, I neglected to note that his headphones are low-impedance Grados. Bullseye, if you search pro audio sites such as sweetwater or zzounds you can find a huge selection of pro headphone amps. Try the SM Pro Audio HP4 available at zzounds (SM Pro Audio HP4 Headphone Amp from zZounds.com!), it will perform identically to the other amplifiers but it has a 22 ohm output impedance - not the lowest, but it should be low enough for 32 ohm headphones. From my personal experience and observations, so long as the damping factor is a bit above 1.0 (that is, the headphone impedance is at least a bit higher than the amp's output impedance), you should be fine with headphones. Speakers need much more, but (likely due to the small size and power requirements of headphones) this doesn't seem to be the case for headphones.
The downside is that there's no pass-through for a pair of speakers. I bought the HP4 based on that, and I could've probably had the same performance for 20 bucks, but the convenience of having a quick button to switch between headphones and speakers was worth the extra cash. You can find various pro amps at many price ranges, and they all perform the same (given a proper match in terms of impedance), but they have different features.
|Power: 0.25W /channel into 32 Ohms
Output devices: 2 per channel
Output impedance: 5Ω
Total harmonic distortion: <0.005% typical 20Hz-20 kHz
Signal/noise ratio: >109dB ‘A’ weighted
Input impedance: 25KΩ
Frequency response: +0,-1dB 20Hz to 80 kHz
Power requirement: 12V DC 500mA
|Power output 330mW/30ohms, 60mW/300ohms
Headphone connection > 30ohms
Signal to noise 78dB (100dB - A weighted) at full output
Frequency response 30Hz – 20kHz/-0,05dB
THD (IMD) ,005%/30ohms, 0,007/300ohms
Headphone jack 3-pole ¼" (6,3mm Ø) phono socket
Outboard power supply 16V/500mA AC, suitable for your country's mains supply
Power consumption 16V/120mA AC
Dimension W x H x D 103 x 38 x 127mm
Weight 600g without power supply
|Output Power > 10 mW into 30 ohm / 300 ohm loads
Headphone Impedance 30 ohm to 300 ohm
Total Harmonic Distortion < 0.01% at 1 kHz
Signal To Noise Ratio > 70 dB
Power Consumption < 4 VA
Power Supply Requirement 24V DC 150mA (Uni-OBH)
Size W/H/D 130 x 100 x 65mm
I just found out from a friend that SM Pro also makes this headphone amplifier:
SM Pro Audio XPH4 4 Channel Headphone Amp
I haven't listened to it personally, but my friend has and found it transparent - he uses it for music production more so than reproduction.
As for the amps you picked, you can't really go wrong with any of them - most amplifiers work just as well as the next, they just overcharge after espousing "detail," "transparency," "soundstage," and other words that have no fixed meaning. If I were to pick, I'd choose the V-Can just because it's the cheapest one available, though you can find pro offerings with low output impedances (like the one I just linked, 10 ohms is low enough for just about anything) for far cheaper that will give you the same performance.
Pio2001 recently posted a (don't know which one) test of a Pro-Ject headphone amp.
I'll quote his comments on the amp"We also noted the level for each channel. The Pro-ject is quite unbalanced with 0.7 dB of difference between the left channel and the right channel at the listening level for hifi headphones, and 1.5 dB of difference at the level for earphones (11 db lower). And it doesn't feature a balance setting. Too bad for a dedicated headphone amplifier."
At least to me, such an imbalance is unacceptable in a $100+ piece of gear. Even my motherboard's onboard sound has less channel imbalance and my motherboard probably cost less than the amp.