The design goals for my amplifiers were:

• no regulator tubes are used such as 6080 and 6C33C-B, because they are considered less linear
• low output resistance (about 33 ohm)
• the bias current of the output stage should be enough for any music peak (20Vpp on 300ohm)
• very good sound
• low power consumption
• small size (30 x 40 cm)
• some choices for the power supply

Figure 1 - E182CC SPICE Simulation

and for the 5814 (ECC82 military type):

Figure 2 - 5814 SPICE Simulation

Figure 3a

Figure 3b

Figure 3c

The headphone amplifier circuits (figures 3a, 3b and 3c) use a single-ended triode class A gain stage and a paralleled-triode cathode follower output stage. The first version with E82CC/5814 should be considered the starting point. It has good sound but with some distortion because noval (new 9-pin tubes) tubes are not very linear. I have tested the first version with a 5814 JAN Philips. It could be that other tubes of this same type such as the E82CC/ECC82 Mullard or Jan Philips will have better sound. Each section of 5814 is biased at 4.42mA and dissipates 3.76W. To reduce the distortion of the first version with E82CC/ECC82/5814 tube, I don't have a bypass on cathode resistor to leave some local feedback, but you can try to bypass it with 220uF, 16V ELNA Cerafine electrolytic capacitor as with the other versions.

The second version with the 3A5 (I used a RCA 3A5), a direct heating tube, gives less distortion and has a very liquid sound as only direct heating tubes can do. The 3A5 MUST have a DC filament supply, such as the 6.3VDC supply shown in figure 7. The third version with the a Jan Philips 6SN7 (that I now prefer) has a less liquid sound than the 3A5, but the distortion is very low and the bass response is fantastic! Sometimes during listening sessions with the 6SN7 I thought that the high frequencies were missing, but this is not true. Note: the 3A5 is sensitive to microphonics (if you touch the table where the amplifier is, you will hear a "gong" in the headphones). I have mounted the 3A5 on a floating socket. Tube dampeners might also reduce the microphonic effect, but I have not tried them.

In all versions, the output tube V2 is biased at 26mA per section (52mA combined) for a total dissipation of 6W (2x3W). Use only Mullard or JAN Philips tubes.

Figure 3d

In the last years, two trends have emerged about the capacitors to use in a high-end amplifier: MKP polypropylene (output stage and interstage), electrolytics (in the power supply) and paper oil (interstage). The MKPs in many tests rank at the top for low distortion, low internal resistance and maximum speed under dynamic conditions. Electrolytics have higher distortion and lower speed, but very good electrolytics are now available, such as Black Gate and ELNA Cerafine, both designed for audio.

Figure 4

Figure 5

Figure 6

About the power supply, the filaments can be powered by AC or DC except that the 3A5 (version 2 of the amplifier) requires a DC filament supply. Each supply has a timed relay that mutes the audio output until the tubes stabilize. I have designed 3 different power supplies - regulated, passive and cheap passive:

The audio output relay prevents turn-on thumps and allows the tubes to stabilize by muting the amplifier output for about 150 seconds or 2.5 minutes (RC = 330K * 470uF) after startup. The relay is a 12VDC DPDT (7-10A max. load) with a coil resistance of about 150 ohms. See the amplifier schematics (figures 3a and 3b) for details about wiring the relay connections.

Figure 7

This amplifier runs quietly without a DC filament power supply, but for a more stable filament voltage, I suggest the slow turn-on supply (figure 7) to replace the filament supplies in figures 4 and 5 (the cheap passive supply already has the slow-on circuit). Also, this circuit is excellent for the 3A5 of version 2 of the amplifier, which requires a DC filament supply. The slow-on supply will also increase the life of tubes! The MJ15004 should be heatsinked to dissipate about 12W.

Some achievements of my amplifier (regular version):

• Gain = 21.5db
• Rout = 33 ohm
• THD = 0.88% with Vout = 9.5v on 300 ohm => 200 mW

  0.40% with Vout = 5.9v on 300 ohm => 80 mW "MAX POWER FOR Sennheiser 580"
  0.17% with Vout = 2.9v on 300 ohm => 15 mw
  0.07% with Vout = 1.2v on 300 ohm => 2.5mw
  

• Freq = 0.7Hz to 1GHz

Figure 9

The graphs in figure 9 are the THD (distortion) decays for the standard amplifier at 9.5V, 2.9V and 1.2V: PERFECT! They show a linear harmonic decay (and the lower the power, the lower the distortion).

9/17/98: Updated figure 4 regulated supply: replaced single MOSFET with MOSFET/BJT Darlington pair to better handle surge currents from large output capacitors. Updated figures 4 and 5 power supplies: grounded filament supply and removed connection point "E" from amplifier schematic (figure 3). Also added a "cheap" version of the passive supply (figure 6).

9/22/98: Added version 2 of amplifier schematic (figure 3b) and revised power supply schematics (figures 4, 5, 6 and 7).

9/24/98: Added Rin to both amplifier schematics (figures 3a and 3b) and removed 6VDC output from slow-start power supply schematic (figure 7). Also corrected value of R7 and fixed spelling of DCC90 name for V1 tube in version 2 of amplifier.

9/29/98: Updated version 2 amp schematic (figure 3b) - added R8, R9. Updated slow-start filament supply (figure 8). Added layout diagram for mounting 7x22K resistors (figure 3c) and suggested chassis layout diagram (figure 7). Also added paragraph on capacitor selection.

9/30/98: Added PC board layouts and chassis layout.

10/15/98: Added 6SN7 version of the headphone amplifier. Also added notes re heatsinking BUX48 and MJ15004.