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Author Topic: Easy Projects -- Headphone Amplifiers  (Read 2002 times)
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str67
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« on: July 24, 2012, 08:57:00 08:57 »

Hi,

as I am a fresh member of this board, I wondered a while which of my past projects could be interesting being shared for a larger community like Sonsivri.
I did not quite come to a conclusion yet, but decided to start with a very simple one, something that might have been tried by nearly everyone in his early days of electronics: a Headphone Amplifier.
I experimented a lot with these some years ago, because beside the fact that at that time I needed a high quality amplifier replacement for my USB sound card, this type of circuit is ideal for experimenting with. It's only some hours (or maybe days) from the idea to the result, and the quality can be judged and compared with the status quo in an instant.
Therefore I attached 2 really simple circuit diagrams, one as a compact solution (only 1 quad opamp) with medium quality, one with only a little more devices, but of an astouding quality.
Of course these circuits are not very spectacular or unique, but I found it very helpful playing around with them in simulation and on a bread board level.
If somebody is interested, I could extend this series with some more sophisticated circuits, or with more detailed data (Eagle SCH+BRD).

- str
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Parmin
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« Reply #1 on: July 25, 2012, 01:20:22 01:20 »

Thank you for your post.
I think this is how a new member should behave, with a share rather than a demand.

======
Below is not a criticism, please treat it as an opinion from this grouchy old bastard.

About your circuits, generally it is good idea to add some more info for them.
For instance, the input voltages, the input/output impedance and maybe a little description on the workings of the amp.
Even though they are simple circuit, many members here are clueless on "simple" electronics, thus your effort would be wasted unless they are convinced that it is easy enough to built.
For the more enlightened members, they may already have a favorite circuit to use, that might use different components that they have handy, again in this instance your effort will be wasted unless you have a clear definition on how much improvement your circuit can offer.


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DreamCat
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« Reply #2 on: July 25, 2012, 08:19:11 08:19 »

BUF634 is a good choice, but it can not drive low resistance speaker of headphone. so the 100ohm above speaker is recommend.
the 1uF capacity of output is too small, it will have 160ohm capacitance impedance under 1kHz. 1000uF is better.

bteween the LM837 and NE5534, which is the better?

btw, there is supply symbol in EAGLE, such as VCC, VSS, GND, etc.

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Jef Patat
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« Reply #3 on: July 25, 2012, 09:03:20 09:03 »

BUF634 is a good choice, but it can not drive low resistance speaker of headphone. so the 100ohm above speaker is recommend.

It has been a long time since I was looking into audio, but I was under the assumption that low resistance headphones are the cheap models, used with PC, portable players etc.  IIRC an expensive model will play on those cheap interfaces, but not loud.  On the other hand they won't be damaged easilly because of the high resistance.

Please correct me if I'm wrong.
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metal
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« Reply #4 on: July 25, 2012, 09:26:06 09:26 »

I think he assumed high impedance headset. OPA134 series is excellent and dedicated for Audio.

I expected to see discrete circuits to be honest :- )
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str67
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« Reply #5 on: July 25, 2012, 12:20:18 12:20 »

Thank you all for replying!

For me this was a test which kind of topics might be interesting in this board.

@parmin:
You are obviously right. A little more information is need to really being able at discussing such a project.

The original idea was pumping up the quality of a common PC sound equipment to a level, which might be something in the range of HiFi. Of course this always depends on the view of the reviewer, whether this level can be achieved or not. However, the first step for me was replacing the PC soundcard with a good quality USB sound card, as the on-board DAC's usually do not really have a chance in performing well. This is mainly due to restrictions in the mainboards. These are perfectly fitted to handle digital signals, but when it comes to analog (like audio is after passing the DAC), the circuits and the environment are just too bad.

After having that part of the audio chain fixed (I can open another project about building a high-quality USB sound card, if somebody is interested), the next problem to solve was how this (now HiFI) Line-out signal can be transported to my ears in a cheap but high-quality way.
For me, the answer to that was using a HQ headphone (like AKG, BeyerDynamic, etc.), as these are affordable in price (compared to good speakers), and have the additional advantage, that I can hear music as loud as I like without having my neighbors beating at their ceiling all the time...

So this is more or less the specification of the circuits. Amplifying a line signal (~1V, >10k input impedance) to a level, that is sufficient to drive a >100 Ohm headphone (as nearly all of these are, or are at least available). The supply voltage is generated from a good quality power supply using a LM317/LM337 pair with +/-12-15V. There is the possibility to use batterie(s), but this was not required for my application.

The first circuit using the 4134 is really a compact solution according to this spec, as it is only one chip and some external parts. The driving capability is limited, though, but is still enough for delivering a good volume. The second circuit needs a bit more parts, but on the other hand has much more driving capability (~1 W), and due to the usage of NE5334 a better noise floor. Especially if you assemble a socket for the NE5334, is is quite easy to play around with different OPAMPs for the first stage (i.e. LME49710, OPA627, AD8610 or similar, @DreamCat: LM837 I did not try). But as NE5334 is quite cheap and still delivers a good quality, I finally chose that for this application. In the end there is no need to make the first stage better than the second one.

As I mentioned, I have tried more sophisticated circuits as well. Especially for the second stage a discrete solution, or a different concept than just using a buffer delivers even more quality, but only for a higher prize.
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metal
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« Reply #6 on: July 25, 2012, 02:02:39 14:02 »

Can you post the more sophisticated circuits, what simulation software do you use?
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str67
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« Reply #7 on: July 25, 2012, 04:45:33 16:45 »

Yes, I will collect something. This might need a bit of time, as this all is a few years old and I have to search for documents.

However, two of the best sounding amplifiers for my ears, that I have build so far, can be found on the net:

1) More or less the same structure as described above, using a two stage OPAMP approach, but replacing the second stage with the brilliant LME49600 amplifier from National (now TI). For the first stage I used an LME49710, which is even better in terms of noise and THD than the formerly mentioned OPAMPs. Of course it always turns out to be a question of personal belief, whether you really can hear the difference between different first stage OPAMPs, but in the end I was quite happy with the sound, and maybe this is because the guys at National simply did a good job in matching the characteristics of these two devices.
I found a schematic on the net, which more or less shows the circuit I used: http://www.twistedpearaudio.com/docs/linestages/ventus_ez_schematic.pdf . You only have to add the volume control in front of the input stage.

2) The QRV07 design done by Per-Anders Sjöström using the TI TPA6120 as an output stage. This is a really good amplifier IC with outstanding datasheet values and a heavily convincing sound. You can find the description at http://sjostromaudio.com/pages/hifi-projects/36-hifi-projects/93-qrv07-headphone-amp?start=2. I bought one of the PCBs from Per-Anders, because they are not that expensive, and the TI chip is definitely not suited for a breadboard approach. First of all it's SMD package only, and second it needs a good PCB layout to function correctly. As a pre-amplifier I chose AD8610, but I did not much experiments for this particular first stage OPAMP.

The simulation software I used for the headphone amplifiers on board level is PSpice. I wrote some small scripts to convert the netlist generated by Eagle into a suitable format, but this definitely did a better job for me, than directly using Orcad for schematic entry.
Lately at work I had to build some boards using Altium, but until now did not try to use their simulator. It is always a bit of work to get the models delivered by the IC vendor (if they exist) into the correct format for the simulator. This worked quite fluid with PSpice.

For more elaborate circuits combining Digital and Analog I tried some AMS software as well, especially Mentors ModelSim/Eldo and Cadence's NCSim/Spectre. But these are Linux based tools without a graphical user interface, and therefore I cannot recommend using them without the intention of really digging deeply into the usage of these tools.
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