Hardware Blog: Doublet Amplifier Part 2

A few weeks back I wrote up a blog about the Doublet Headphone Amplifier which lets two people listen to the same audio source with independent volume controls. I got so many emails from folks who were interested in building one that I decided to make another run of the printed circuit boards - and they've arrived!

If you want one of the PCBs, email me with your mailing address and I'll drop one in the mail to you at no charge Sorry, they're all spoken for.

I also had a chance to clean up the circuit diagram. You can download the schematic in pdf or flash format. The schematic is broken down into two parts, the power section and the amplifier section, as shown below in miniature.

The doublet uses a9V battery along with two TLE2426 railsplitters to generate a virtual  ground and supply rails at +/-4.5V. For power savings, only one side of the power supply and one of the op-amps is active if a single channel is on. A 11DQ03 diode keeps things from breaking if you accidentally put the battery in backwards. This diode has an extra-low forward voltage drop of 0.55V which is nice for battery based amplifier circuits. The dual power supply is based on Tangent's TLE2426 virtual ground design.

The amplifier stage starts at the input jack. The left and right audio channels are each split in two and fed into the volume potentiometers. Capacitors C3, C4, C5 and C6 act to protect the op-amp from any DC offset at the inputs. The op-amps are configured in a textbook non-inverting configuration with 14dB of gain. The amplifier circuit is based on Chu Moy's original Cmoy design.

Here is one set of parts to fill the board:

• Capacitors
  • Signal capacitors 4x EPCOS B32529C5334J(0.33uF)
  • Power supply capacitors: 4x Vishay 2222 150 65471(470uF)
• Resistors
  • 4x Vishay RN55D1001F(1kΩ)
  • 4x Vishay RN55D4021F(4kΩ)
  • 2x Vishay RN55D1002F(10kΩ)
  • 4x Vishay RN55D1003F(100kΩ)
• Railsplitters: 2x Texas Instruments TLE2426
• Diode: 1x International Rectifier 11DQ03
• Audio Jacks 3x Switchcraft 35RAPC4BH3
• Op-Amp Sockets 2x Areva DIP308-001BLF
• Op-Amps 2x Texas Instruments OPA2134PA
• Potentiometers 2x ALPS RK0971221Z05
• Knobs 2x Multicomp MC0616003
• Indicator LEDs 2x Avago HLMP-K150
• Battery Strap 1x SPC 8459-0674

Keep in mind that the parts I've listed here are only suggestions. Though I've listed nice precision resistors for the audiophiles in the crowd, if you have some through hole resistors already, they will work fine (especially the LED resistors which don't need high precision).

There are many nice audio op-amps which will work in this design. In fact, the doublet amplifier can serve as a nice test bed for head to head op-amp listening comparisons.

Finding the ALPS RK0971221Z05 potentiometer in small quantities at a good price can be tricky. Tangent sells them in single pieces for $3.25 each which is reasonable.

Please send feedback on the circuit and the board layout as there is definitely room for improvement in both. The original Eagle files can be downloaded for those of you who are interested in moving the design forward. A few ideas are:

• Miniaturizing the board with SMD components.
• Adapting the power supply design to run off of AA, AAA or some lithium polymer battery.
• Adding a trickle charger for use with rechargeable batteries.
• Clean up the Eagle schematic (not the one shown) with better drawings for the potentiometers.

Assembling the doublet into an Altoids tin will have to wait for another post. Just remember to insulate the bottom of the PCB from the Altoids tin!