Circuit description

The text is for the right channel.


The main part, the operational amplifier

The main part is an operational amplifier, IC1. The opamp is followed by a high speed buffer capable of up to 250 mA which will be sufficient for all headphones on the market. In between the opamp and the buffer I have put a class A circuit, T1 and a few resistors around.T2 and T3 serves only as diodes and can be replaced with any diode or transistor connected as a diode(base and collector tied together, emitter= anode). This circuit will reduce the crossover distortion, but I don't know how much. You can omit the whole circuit if you don't experience any advantage.

Frequency compensation

C6 is for frequency compensation but is not required for most opamps. Fast opamps like OPA627may need this capacitor. Consult the datasheet of BUF634.

Offset voltage

P1 is only used if you don't use a precision opamp. OPA627 and AD8610 have so good offset values which make this potentiometer unnecessary. OPA134 is slightly worse but still good. Weather P1 should be used or not, consult the datasheet of the opamp. If you can, avoid these offset trimming pots. If you use them you will get worse temperature characteristics. Note also that you in this case only can use opamps which has this trimpot connect to positive supply voltage (most common).

Calculating the gain

The total gain is determined by R4 and R5. Gain = R5/R4 + 1. Low values of these resistors is important if you want low noise.

The input filter

The input filter is very universal. You should have a lowpass filter, R3 and C5, but if you don't have any RFI trouble at all, it's not necessary, just replace the R3 with a jumper and omit C5. DC blocking capacitor may be necessary but check with your signal source first. The whole amp is DC connected so you must ensure that you haven't got any DC in your signal. In order to be flexible I have made room for any type of input coupling capacitor, or capacitors. Since this is a SMD amp I recommend the SMD capacitor. I have chosen a type from RIFA but any "polypropulene compatible" type will do. R2 is very necessary if you use input couplings cap. The resistor creates a DC path for the inputs of the opamp, otherwise it won't work. R1 is a pulldown resistor so the input coupling capacitor won't pick up any charges when the amp is unconnected. A charge capacitor can creates a nasty spike at the output when something is connected to input.

The output buffer

The output buffer is a high speed integrated buffer which can be biased with different currents. I have chosen "medium speed" and "high speed". This is only for personal taste. High speed mode consumes rather much power and adds very little in audio quality but some fast opamps requires this mode in order to be stable. If you want to use this amp with batteries you should use "low speed mode", remove R8. This will exclude fast opamps like OPA627.

The output current limitation

R9, R10 or R11, R12 aren't necessary but those resistors creates current limitation which means that both low impedance och high impedance headphones can be connected. Without these resistors it's very easy to damage low impedance headphones but if you have full control over the situation, omit those resistors. I should mention that many headphones are designed to be driven from a certain source impedance. Some people claim that this impedance is 120 ohms. Test this if you want to optimize the listening experience. Using R9, R10 creates the possibilities to use long cables with no (or little) loss of the higher frequencies. R11, R12 is the classical way of driving headphones. I have made an option for changing "drive modes". With original cable lenght I can't hear any difference between these two modes. Some people claim huge differencies though.

The power supply

The power supply is not very exotic, rather straight forward. It's designed to use two transformers, one for the positive voltage and the other one for the negative.

The regulators are cooled by the groundplane and of a copper area on the upper side. If you plan to use AD8610 you must lower the voltage to 13 volts! AD8610 is made in a advanced high speed process which has 27.3 V as absolute maximum. I have used AD8610 for a while with +- 15 volts and the amp works still and will probably work but I don't recommend using the opamp beyond it's limits. Remove R27 and change R26 to 2.7 kohms. This will lower the voltage.

R25 is a bleeder resistor. The regulators won't discharge the capacitors completely when the have reached the voltage of 0,6 V. The resistor creates a clean start at power up.

C21-C24 are for reducing EMI caused by diode switching.

The transformers are short circuit proof by design, therefore you don't need fuses.

R33 is for transient suppression.