Headphone amps

dvv wrote: I've lost touch a little, so how about a short explanation of the circuit topology in the headphone amps?

Hi dvv,

I assume you mean Graham Slee headphone amps?

First some background.

Headphones have been traditionally driven from the attenuated outputs of integrated stereo amplifiers for as long as I can remember. The headphone user may therefore think a power amplifier of the sort used to drive loudspeakers is required by headphones, and by the looks of things, many designers think that too.

But when you look at a typical headphone specification you find that's not the case. The convention for rating the sound pressure level (SPL) which tells you it's loudness, is based on its ouput at 1mW (that's 0.001 watts!). A comfortably loud listening level will be somewhere between 80 and 100dB, so with a 96dB SPL headphone (like many are), a 1mW output will just about suffice. However, it is good to have some headroom (and cater also for less sensitive Grado's) - let's allow 10dBW which is 10 times the power (10mW), and sounds twice as loud.

For the 10mW into a high impedance headphone of 300 Ohms (Sennheiser HD600/650/250/etc) an rms voltage of 1.73V is required and the rms current will be just short of 6mA. This can be easily done by small signal transistors or even the ouput of an op-amp (operational amplifier IC). However, a small signal transistor can't handle a dead short that could be possible inserting a phones plug into a headphone jack. Therefore some series resistance is required. And on hearing shouts of "what about the damping factor?", I must ask why? The tiny headphone transducer hasn't much mass at all, and compared to an average 8 inch loudspeaker drive unit, you may as well call it none. At this point I could be accused of over simplifying the argument, but I'm used to that, and I wouldn't want to loose our less technical readers too early in tech-speak.

When driving lower impedance headphones like the 32 Ohm Grado's and 25 Ohm Denon's most headphone amps that opt for a "power amp" approach, will use a gain switch to reduce the output voltage so as to place the volume control in the same respective place. However, by inserting the right value of output resistor, an approximately similar volume can be obtained with headphone impedances from 8 to 2,000 Ohms, and gain switching is not required. And this means small signal transistors can be used instead of power devices. This very approach was adopted by the IEC (International Electro-technical Committee of Geneva) under CEI/IEC 61938.

The Solo MC was designed to that standard and in common with many headphone amplifiers, the Solo uses an op-amp with an output buffer, which in the Solo's case is a class AB output stage using small signal transistors. And going on its reputation and popularity, it works!

However, there are those who will deny what their own ears tell them, and insist IEC61938 is wrong. This could be due to personal bias, or maybe brainwashing by the doctrines of other manufacturers. However, it could be that some headphones are designed for the most popular headphone amps on the market - and unfortunately, the Solo, for some reason, doesn't rank among those. One very popular headphone amplifier actually uses 33 Ohm output resistors. Therefore will some lower impedance headphones sound better fed that way? I haven't found that to be the case with any of the Grado's, but perhaps we need an alternative design to cater for such an eventuality?

The thing is, you don't need anywhere near the gain an op-amp can give to amplify headphones. In fact, with some CD players you'd need negative gain! So is an op-amp fed buffer stage really the answer?

As I said above, small signal transistors and op-amps can drive headphones without a power stage, and if researched-out, will work safely with an output resistor down to about 30 Ohms. This opens up the possibility of either using a simpler discrete bipolar transistor circuit design, or using an op-amp alone and optimizing it by using its ultra-high open loop gain in negative feedback to drive such low impedances whilst keeping its wide bandwidth which is essential to good perceived performance (read Voyager here). The op-amp also lends itself to portable applications where the available supply voltage is relatively low. However, where the supply voltage is considerably higher, the discrete bipolar small signal transistor approach can yield excellent results driving headphones probably designed for lower impedance amplifiers, as well as the higher impedance ones which are less fussy about driving impedance.

One thing I and fellow design enthusiasts have noted is that commercial designs that go for the power amp assisted op-amp approach seldom deliver the sort of performance small signal transistors, and direct op-amp outputs can, if properly designed, deliver. It's a case of too much of everything - voltage gain and current gain - without observing the changes in linearity between low and high frequencies because of capacitance. Too many stages also degrade a signal because, after all the output of each stage is only a facsimile of its input (the signal is left behind at the input) and an nth copy of an nth copy, by negative feedback, introduces more complex distortions than the negative feedback has a chance of restoring (I can produce a paper on this subject).

Global negative feedback can be removed, but still the voltage amp (op-amp) and power amp are using "overkill" amounts of negative feedback which "flies in the face" of the "prior-art" that too much negative feedback isn't good for music. A simple two transistor class AB current stage uses 100% negative feedback - adding further current amplification when it's not really required simply adds more negative feedback.

To conclude this reply, I have to acknowledge the fact that the Solo MC (my original headphone amp design) is one such high negative feedback design. The difference with the Solo design is the realisation that something "was-up", and the sound it gave prior to its release a few years ago, wasn't as acceptable as I required. With a certain persons suggestions (you'll see him the next time you look in the mirror) quite a lot of tuning work surrounding the type of op-amp and the way its application was optimized, led to its acknowledged superior musical abilities. A further "discovery" by a now sadly "lost friend", led to further optimization op-amp wise, and in its implementation. I can certainly see how "chip-rolling" has become such a popular passtime replacing that of listening to music, but the Solo has remedied that for many.

You said a short reply?

dvv, we have different points of view on how to best implement high performance headphone amplifier designs.

dvv wrote: ANY technology you use can provide poor and good results, it's not so much the technology as the implementing of it that really matters.