Written by Grzegorz "gsmok" Makarewicz,
This description relates to a headphone amplifier based on a very popular White Follower scheme. There are plenty of amplifiers of this type you can find in electronic books and Internet. Among the most popular there may be mentioned circuits described on HeadWize site dedicated to headphone amplifiers. The description is not completed yet. I left it unfinished deliberately, as a place to conclusions accompanying this here THREAD on TRIODA Forum. Because of this, description is a bit chaotic, and I apologize for that all visitors. Ultimately these shortcomings will be removed.
A characteristic feature of the described amplifier is a very strong dependence of technical parameters and listening feelings on headphones used. For fixed values of the components, the amplifier can play delightfully controlling some headphones and embarrassingly controlling others. Obviously, the higher impedance headphones are the better the chance that the amplifier will play great.
A very useful tool for the selection of the key elements of the amplifier is a software for simulation of electronic circuits. Although vacuum tubes are considered as obsolete parts in the modern electronics, their models can be found in the libraries of virtually all known tools for computer simulation.
The figure below shows a circuit to simulate the family of anode characteristics of 6N1P vacuum tube.
And here is the graphical representation of the mentioned simulated characteristics ...
... and to compare the real 6N1P characteristics from manufacturer's data sheet.
I suggest you to compare the characteristics on the basis of a few selected points. It is found that the theoretical model for negative voltage of the control grid is quite good.
Circuit diagram of the headphone aplifier used for simulation is shown in the figure below. You can enlarge it by clicking a mouse on it.
As can be seen, the model simulates the actual structure of the complete amplifier circuit, including the supply portion (anode and heater power supply) and items arranged in several electronic parts, which can of course be replaced by a single element of appropriate value. Inquiring people will also notice that the diagram includes not connected DC voltage source. I used it as a anode voltage source during the simulation of frequency characteristics of the amplifier. The value of the voltage source corresponds to the value of the output voltage of the anode power supply in a steady state.
The other day, on TRIODA Forum the issue of large values of C2/C12 i C5/C15 capacitors was discussed. Their impact on the frequency characteristics can be explored using the simulation model shown above. The following figure shows the effect of the value of the capacitor C2 on the frequency response in the low frequency range. The calculations were made for values from 10uF to 1000uF. As you can see from the 470uF effect of increasing the value of C2 on the curve is negligible.
These same characteristics, this time for the capacitor C5 are shown below. I changed the value of C5 from 100uF to 2200uF.
As you can see, just like in the previous case, the increase of capacity above 470uF practically does not increase the bandwidth in any significant way.
To build the OTL headphone amplifier prototype I used two versions of the PCB.
The first version uses one single-sided printed circuit board. Believe me, that the design of the single-sided PCB without jumpers and guaranteeing a minimum level of disturbance was not a simple matter.
Below is the prototype amplifier mounted on PCB.
The second version consists of two circuit boards: on the first PCB elements of the amplifier are arranged, on the second PCB all elements of the power supply.
Here are the diagrams, which were used to design the PSBs.
PCBs can be placed one behind the other (the design resembles then the first version of single-sided board) ...
... or one above the other.
The view of the assembled boards before their twisting is shown in the photograph below.
The following photograph shows the amplifier after fixing with spacers.
Here you can see more detailes of electrical connection of the power supply and amplifier board with a few pieces of wire.
The side view showing that the length of spacers provides a safe distance from the elements of the amplifier from the surface of the power supply board.
Here is the view from the top side of the prototype.
List of elements (designations for the version II)
Here are two prototypes side by side. You can compare the dimensions. Electrically both systems are equivalent.
Unfortunately, my cat Bulma was not interested in any of the versions of the amplifier.
Continuation will happen sooner or later...
Added on February 8, 2014
Here's the photograph of PCBs with solder mask.
Written by Grzegorz "gsmok" Makarewicz,