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Grzegorz Makarewicz 'gsmok'
(the description comes from archive materials published some time ago on the TRIODA - Grzegorz Makarewicz website)
This description concerns a headphone amplifier based on a very popular scheme using a White follower. The description is not a closed study, but only a place to place important information accompanying the thread on the TRIODA Forum. Therefore, at least at first, it may seem a bit chaotic - for which I apologize in advance. Ultimately, I intend to place here detailed results of simulation calculations, results of measurements using different types of headphones and a description of the practical implementation of the amplifier based on the printed circuit boards designed for it.
A characteristic feature of the described amplifier is the high dependence of technical and listening parameters on the headphones used. For the set values of the components, the amplifier can play delightfully driving some headphones and embarrassingly driving others.
A very useful tool for selecting the values of key amplifier components is software for simulating electronic circuits. Although tubes are, to put it mildly, outdated elements in modern electronics, their models can be found in the libraries of practically all known computer simulation tools. The figure below shows a diagram of a circuit that allows determining the family of anode characteristics of a 6N1P tube.
Here is the family of anode characteristics of the 6N1P tube model...
... and for comparison, actual characteristics from the manufacturer's catalog page.
I propose to compare characteristics based on several selected points. It will turn out that the theoretical model for negative control grid voltages is quite good.
The schematic diagram of the amplifier circuit used for simulation is shown in the figure below. It can be enlarged by clicking on the figure with the mouse.
As you can see, the model reproduces the full structure of the real amplifier circuit, including its power supply part (anode and filament power supply) and elements assembled from several components, which can of course be replaced with single elements of the appropriate value. The curious will also notice that there is an unconnected DC voltage source in the diagram. I used it as an anode voltage source during the simulation of the amplifier's frequency characteristics. The source voltage value corresponds to the voltage value at the output of the anode power supply in the steady state.
At the TRIODA Forum, the issue of large values of capacitors C2/C12 and C5/C15 was raised some time ago. Their influence on the course of the amplitude characteristic can be examined using the model shown. The figure below shows the influence of the value of capacitor C2 on the transfer band in the low frequency range. The calculations were made for values changing from 10uF to 1000uF. As you can see, starting from the value of 470uF, the influence of increasing the value of C2 on the course of the characteristic is negligible.
Analogous characteristics, this time for capacitor C5 are shown below. I changed the value of C5 from 100uF to 2200uF.
As you can see, similarly to the previous case, increasing the capacitance value above 470uF does not practically increase the bandwidth in any significant way.
To build the headphone amplifier prototype, I used two versions of printed circuit boards.
Version I
The first version uses a single single-sided printed circuit board. Believe me, designing such a board without jumpers and ensuring minimal interference during amplifier operation was not an easy matter.
This is what the assembled prototype circuit looks like on a board made using the "thermal transfer" method.
Version II
The second version consists of two printed circuit boards - the first one contains the elements of the amplifying part of the system (the actual amplifier), the second one contains the elements of the power supply part. Here are the schematics that I used to design the boards.
Once assembled, the printed circuit boards can be placed one behind the other (then the design resembles the first single-board version)...
... or one above the other.
The view of the assembled mounting plates before screwing them together is shown in the photo below.
The following photos show the amplifier after screwing together the amplifier and power supply mounting plates.
Here you can see in more detail the electrical connection of the power supply and amplifier circuit boards using a few straight lengths of so-called "silver wire".
Side view showing that the length of the spacers provides a safe distance between the amplifier components and the power supply board surface.
Here's a top view of the twisted breadboards.
Below is a list of the amplifier components (component markings are the same as for version II)
Amplifier board
- R1L, R1R 1.0MΩ/0.6W
- R2L, R2R 100Ω/0.6W
- 3L, R3R 43KΩ/2W
- R4L, R4R 750Ω/0.6W
- R5L, R5R 1.0MΩ/1W
- R6L, R6R 200Ω/1W
- R7L, R7L 220Ω/1W
- R8L, R8R 22KΩ/1W
- C1L, C1R 200nF/630V
- C2L, C2R 1000uF/16V
- C3L, C3R 100nF/630V
- C4L, C4R 470nF/630V
- C5L, C5R 2200uF/16V
- C6L, C6R 100nF/630V
- C7L, C7R 220uF/250V
- C8L, C8R 220uF/250V
- C9L, C9R 470nF/630V
- C10L, C10R 10uF/400V
- H1 33uH/3.5A
- H2 33uH/3.5A
- H3 33uH/3.5A
- L1 6N1P-EW
- L2 6N1P-EW
- L3 6N1P-EW
- A1L, A1R ARK2 horizontal
- A2L, A2R ARK2 horizontal
- A3 ARK2 vertical
- A4 ARK2 vertical
- T1 (L1) Noval type 9-pin socket
- T2 (L2) Noval type 9-pin socket
- T2 (L3) Noval type 9-pin socket
Power Supply Board
- R1 3K9/1W
- R2 2K7/0.6W
- R3 150KΩ/2W
- R4 51KΩ/2W
- R5 0.51Ω...1.2Ω/5W
- R6 470Ω/0.6W
- C1 1nF/1kV
- C2 1nF/1kV
- C3 1nF/1kV
- C4 1nF/1kV
- C5 220uF/400V
- C6 100nF/630V
- C7 100pF/1kV
- C8 100pF/1kV
- C9 100pF/1kV
- C10 100pF/1kV
- C11 100pF/1kV
- C12 33uF/400V
- C13 47uF/400V
- C14 100nF/630V
- C15 10000uF/16V
- C16 10000uF/16V
- C17 100nF/630V
- C18 10000uF/16V
- C19 10000uF/16V
- C20 100nF/630V
- D1 BY228
- D2 BY228
- D3 BY228
- D4 BY228
- D5 SB540
- D6 SB540
- D7 SB540
D8 SB540 - D9 1N4006
- D10 1N4006
- D11 1N4006
- DZ1 91V
- DZ2 91V
- DZ3 75V
- DZ4 75V
- DZ5 15V
- LED1
- LED2
- T1 K2996
Here are the two prototypes side by side. You can compare the dimensions. Electrically, both systems are equivalent.
My cat "Bulma" was unfortunately not interested in any of the headphone amplifier versions.
To be continued...
Added February 8, 2014
And here is a photo of factory-made printed circuit boards.
Prepared by: Grzegorz "gsmok" Makarewicz,