headphone amplifier

Dual power supply, op amp buffer, transistor push-pull amplification

The LED constant current sets the bias voltage, and the resistors r9 and r10 adjust the cross current.
There are no requirements for ddq1, hhq1, these two transistors, just choose one with low noise.

hhq2, ddq2, these two transistors can be used or not. If not, just short-circuit their base and emitter. At this time, only one LED is used (just install one of LED1 and LED2)

You can also use a diode to adjust the bias voltage.

The push-pull part at the back uses two pairs in parallel, r17, r18, r24, r23 to adjust the quiescent current.

Op amp reverse input buffer, forward amplification, r4, r5 adjust the amplification factor, adjust according to needs 

The related resistance of the push-pull part can be adjusted according to actual needs.

The static power consumption of the whole machine is about 4w, 5w, and can be smaller.

The power is above 20w, but the loudspeaker is too loud, so I haven’t tried it yet.

Regarding the dual power supply, I use a dual voltage toroidal transformer.

Sound quality part

    Can be used with headphones or small speakers

    You can basically choose any op amp, just pay attention to the power supply voltage. A cheap lm358 will work, but it will distort large signals. A ne5532 is enough. No matter how good a 49860 or a 2604 is, you can use it freely.

    For input capacitance, 4.7uf is sufficient. It is not recommended to be too low or too high. I used cheap 47uf aluminum electrolytic. It is recommended to use non-polar capacitors. It is recommended to use new ones.

    For push-pull transistors, it is recommended not to use too poor ones. For push-pull speakers, it is recommended to connect two pairs in parallel and use tubes with a larger power factor. For bd139 and bd140, it is recommended to choose 16-type ones.

There is basically no noise. This circuit is a reference to a certain headphone amp, which I added in parallel. It is mainly made to compare with the original version. The sound quality of the original version is indeed better in subtle ways, but this board is a little bit worse. It is estimated that the original versions of the LEDs and cross-current transistors have not been found, which affects the details of the signal path. But I usually can’t hear it (hahaha)

It is estimated that bd139 and 140 use type 16 high amplification to reduce the output current of the op amp.

When welding this board for the first time, I only used the ordinary type bd139140. The one with the lowest amplification factor could not reach the bass of the speaker, which caused a lot of pressure on the op amp. The second time I connected the reserved Darlington and expanded it by one level, the bass was enough.

I borrowed an oscilloscope and took a look. 

x-axis 5us

in out 49860 200hz

in out 49860 20k Hz 

in out 49860 50k Hz

49860 5532 20khz 

I just did it casually for fun, I will make it into a patch and shrink it later when I have time.

I don’t remember the rest, I’ll fill it in later if I think of it.

You are welcome to take a look and criticize and correct me.