The Lavry DA11: For your ears only - Page 5

Ideally, the waveform is the same for all analog audio loads (destination device). The waveform “tracks” the air motion (the sound). While the ear response to air motion. The air “moves” with the music, the motion changes over time. The electronics itself utilizes electric signals such as voltage variations over time (moving electrons, not air molecules).

But keeping the wave shape identical is where the similarity may end. One can “keep the wave shape” and do so at different voltage levels, current levels and power levels.

Devices that need to regenerate sound (convert electrical signals to air motion) require more power then devices that simply receive and process audio. Passive speakers are real power hungry devices. Headphones are second in line. The input ports of power amp, mixers, AD’s, EQ, reverb and similar gear do not require much power.

The impedance range of headphones is very wide, but all have much lower impedance then a line level and pro gear device, thus they calls for more current drive then an ordinary OP amp can supply. They also typically operate at lower voltages then most other audio devices.

A typical OPamp device is a higher voltage and lower current device, thus not suitable for headphones. There are “specialty” semiconductor devices that can drive lower voltage and higher current, but I did not find one that is good enough, so I made my own from discrete parts.

For low power applications, there is nothing inherently wrong with OP amp, and there is nothing fundamentally advantageous about discrete. The difference is mostly about packaging. But when making higher power circuits, semiconductor solution cause very high temperatures (a lot of localized heat), and discrete circuits spread the heat over many devices. Spreading heat over larger area and many devices lowers the temperature very significantly. High temperature is not a good thing for best performance and for reliability.

But the main reason I made my own circuit is the headphone itself:
Most audio loads are resistive. When a driver is “looking at” a load that behaves like a simple resistor (or nearly so), the wave shape of the voltage and the wave shape of the current are nearly identical for any audio signal.

But headphones (and passive speaker) are anything but a resistive load. That makes the designer’s life much more complicated. The relationship between the voltage wave shape and the current wave shape is complex. The load (headphone) is not resistive, it complex, it “varies with the music”. The driver circuit must be able to supply the required voltage wave shape while accommodating some other current waveform. That poses additional requirements not normally encountered in transmission of analog audio signals.

I wish I could find a “good enough ready made device”. Instead I ended up with some 2-3 dozen parts. It sure takes some doing to drive a truck, as if it were a race car, but it was real fun circuit to design.

Regards
Dan Lavry

The DA11 manual (page 14) tells you details about the headphone level at various volume settings:

Volume=56 ---> headphone level=18dBu
Volume=55 ---> headphone level=17dBu
Volume=55 ---> headphone level=16dBu
.
.
Volume=50 ---> headphone level=12dBu
.
.
Volume=46 ---> headphone level=8dBu
.
.
Volume=56 ---> headphone level=18dBu
.
.
Volume=01 ---> headphone level=-37dBu
Volume=00 ---> headphone level=OFF

18dBu is the maximum level, enough to make your head "ring". The voltage equivalent is 6.14V, which is 17.36V peak to peak. I do not recommend listening to HD650 at 56 setting (My own personal headphone is HD650)

The point is that DA11 can supply the current, even for very low impedance loads.

There is also a built in output protection circuitry, to protect the DA11 circuitry from breakdown in case of accidental shorting of the output. Such feature is very desirable, and not uncommon, but some headphone amps have the protection cut in too early, which may distorts the peaks (the DA11 will not do that).

So 300Ohms impedance is very fine. Even 60 Ohms is fine.

Regards
Dan Lavry