Quote:
Originally Posted by komebudu /img/forum/go_quote.gif
I increased the volume up.. till my ear drums feels uncomfortable.. then I can hear distortion… something like crete signal or like the current was insufficient to deliver such full impact…
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This is not a defect, per se. Remember, the cMoyBB is a cMoy with switchable bass boost and several power supply enhancements (a TLE2426 virtual ground instead of a resistor divider, decoupling capacitors, and a low impedance return plane). These features give the cMoyBB an edge over the base cMoy circuit, but it's still a cMoy.
Assuming a stable amplifier such as the cMoyBB, distortion at "uncomfortable" volume levels can be the result of one (or several) of the following situations:
1) Insufficient supply voltage. This can be expected when operating high impedance headphones at very high volumes with a 9V battery. When the volume is raised too high, the opamp is forced to clip the signal. Bass boost exasperates the problem by demanding an even stronger signal voltage. An 18-24V power adapter will significantly raise the positive and negative supply rails, therefore, the opamp can then produce higher voltage signals (i.e., louder music) before being driven into clipping.
2) Insufficient current. Typical opamps supply 20-30mA current, which is generally enough to attain reasonably loud volumes. Efficient headphones will reach dangerously loud volume levels before consuming too much current. Even moderately inefficient headphones can reach high volumes without overloading a cMoy. However, it is unrealistic to expect 20mA to be enough to drive
all headphones at absurd volumes. Some headphones simply demand more current than a cMoy can provide; they may barely reach the maximum volume of an iPod. Nothing can be done about this in a cMoy, else it would not be called a cMoy.
3) Physical limitations of the drivers. Every headphone driver (speaker cone) will reach a point where it cannot physically respond to the voltage received from the amplifier. Most commonly, this occurs when listening to strongly amplified bass. It is rare for a cMoy to overexert headphone drivers, but it can happen.
komebudu: You are exactly right, I suspect the amplifier is running out of current at those "uncomfortable" volume levels with your CX500's. At only 16 ohms, it's extremely unlikely that voltage is an issue. An 18V+ power adapter may help slightly with your AKG K-701's, but from my experience, they also need more current to achieve painful volumes (volume is quite subjective, though).
For anyone new to the scene, it is important to understand the goals of headphone amplification. Refined sound quality is the chief concern! The ability to reach higher volumes comes secondary.
Some general advise: If you haven't acquired it already, spend some time reading about hearing loss and
tinnitus. Most people here are well aware of this risk, but I always make a point to warn anyone who chooses to listen to loud music...
Health risks aside, current limitations can only be overcome by adding a current buffering stage to the amplifier and its ground channel. As mentioned above, however, we're then talking about redesigning the cMoy.
Happy New Year, everyone!
--JDS
) to you too.
