DAC + 3886 As Headphone Amplifier?

[uzernaam]

Forgive the newbie for his newbishness...

I've built a 3886 amp. Figured out how to ground it for low-noise.

Now I want to move on to more ambitious projects.

The next thing I want to build is a single chassis which will take the digital output of my sound card and convert it cleanly into a pristine analog signal, and then amplify it for my Sennheiser Hd650 headphones.

Could I modify resistor or ps voltage values to lower the gain of a 3886 amp to make it appropriate for this? I am also considering a low-watt class A design intended for headphone use.

I have read a lot of threads on this forum about various dac kits, some available on ebay for creating a nice analog signal from a digital source. I think I have narrowed it down, but if anyone would like to provide further insights, I am all ears.

Thoughts?

[sangram]

Minimum gain of the 3886 is a little higher than I would feel is safe for headphone users. Enthusiastic use of the volume control may cause hearing damage, and it would be a waste of power and the 3886's capabilities to limit the input (and the output) to what is safe for cans (1W would be my guess).

Sometimes, DACs will have output stages capable of directly driving headphones. In any case, building a current buffer for headphone use should not be too difficult, plus there are lots of nice headphone amplifier projects on head-fi and other similar resources.

I've built something similar to what you propose with a PCM2702 'Alien DAC' and a NE5532 buffer. It does have issues driving low impedance phones, but is good enough for consumer-grade 32 ohm cans. I suppose I could use more opamps in parallel for better current output.

[ratza]

Go with low watt class A.

[Redshift187]

If you already have an LM3886 amp you like, it's almost free to add a headphone output. I added the following to mine and it sounds great: Headphone Adaptor for Power Amplifiers

Not as good as the other options, but it'll only cost you about $5.

[DcibeL]

I once looked at an old Technics amplifier. It had an STK chip in it for the amplifier, and the headphone output was simply a voltage divider from the speaker out. The amp was shot, so I never did get to hear how well this method worked, but it sure is a quick and simple way to get a headphone output from a power amp. Just some resistors are needed. [edit: yes, very much like the ESP link above]

Similar to what you are suggesting (build a good proper amplifier for the headphones from the DAC output), I am in the midst of putting together a simple high current headphone amplifier myself. It is a quite simple circuit, a BUF634 and LM4562. Such circuit can be built on simple perf-board (point to point wiring) and powered from the supply that powers the analogue output stage within the DAC.

The basic circuit is in Figure 5 of the BUF634 datasheet. I've set the gain differently, and added 100 ohm in series with the speaker output. With this resistor, if the output is shorted it is shorted through 100 ohm, not zero. Use good poly cap for input coupling.

[Redshift187]

Originally Posted by DcibeL

The basic circuit is in Figure 5 of the BUF634 datasheet. I've set the gain differently, and added 100 ohm in series with the speaker output. With this resistor, if the output is shorted it is shorted through 100 ohm, not zero. Use good poly cap for input coupling.

Not to mention, many commercial headphones are designed to be driven from a 120 ohm output, although some people feel the really low impedance output directly off of a chip sounds better.

[pacificblue]

Of course it sounds better directly off the chip, because any protective resistor forms a voltage divider with the headphone impedance. The frequency response with resistor therefore becomes a scale model of the headphones impedance.

The trouble with headphones is that their jacks usually produce shorts when they are plugged or unplugged, so you absolutely need that resistor.

Another resistor in parallel with the headphone will make the impedance more linear over frequency and make for a good compromise between safety and good sound.

[DcibeL]

Sorry this is Off topic a bit.

Powered up the headphone amp, works great! I was sure there would be some AC hum and I would need to put a can over the circuit, but it is dead silent. 120V AC is only about an inch away. Now I can get rid of the hiss from my computer, turn the volume up to get SNR to a reasonable level, then adjust volume at this preamp thing I built. Good stuff.

[AndrewT]

Usernaam,
as a first stage add the resistor ladder to generate your low voltage output for the headphone channel.
Try 100r 5W wirewound for the series resistor from the speaker output (speaker Red terminal).
Connect a 10r 500mW metal film from the 100r to audio ground (speaker black terminal)

Now take a wire from the junction of these two resistors to the tip contact of the heaphone socket.

Do the same for the second channel. But this time the wire goes to the ring contact of the headphone socket.

Finally connect the sleeve terminal of the headphone socket back to the the Audio ground.

Listen to this for a while.
Decide if you want/need more quality.

[pacificblue]

A 5 W wirewound will even survive an extended short circuit at the output during very high output signal level. A metal film resistor with a lower power rating iwill give a better balance between safety and sound quality. You usually find 1 W ratings for the series resistor in commercial amplifiers.

10 Ohm in parallel may lead to a bit too much of attenuation. With the 300 Ohm Sennheisers you can probably use a much higher resistor value in that place, if any at all.