Help finding components for headphone amplifier

[The Soulman]

Or look at the c-moy op-amp based designs.

Cmoy is best, when one battery dies, and your amplifier is suddenly the headphone killer.

Yes don't use a center tap on your battery's, make some half decent symmetrical regulated psu.

[firstcolle]

Hi everyone!
first of all thanks for all the suggestions!! now I'm taking in consideration the PGA2311 + TPA6120 solution.
I'm ok with the digital section (encoder + spi and OLED display) but I'm a little noob with the analog one. now the stupid questions.. can I connect straight the output from the PGA2311 to the TPA6120 input?? and for the PGA2311 input, do you suggest some buffer to match the impedance? the PGA2311 datasheet says that it works at the best with a 600Ohm impedance input... I know that all the commercial audio equipment may have a line output with low impedance.. so I'm ok with that?

many many thanks!

[firstcolle]

Hi again!
I've done a little prototype on breadboard and I found out that I can connect the PGA2311 direct to TPA6120, all works fine except the Mute by pulling low the Mute pin on the PGA2311, doing that cause an output of 2V from the TPA6120.. probably due to the internal PGA mute circuit. no problem if I load 0x00 in the volume register (software mute).

now I'm looking to design a PSU that must have 5V for digital section, +5V -5V and +12v -12V for the analog section. My idea is to use a toroidal transformer with 2 secondary 0-15 0-15 VAC. The first one for the digital section and the second one for the analog (all the rails). I want to keep the 5V LDO on the first secondary always ON and use a soft switch to switch the digital section on and off and a relay for connecting the second secondary only when the digital section is On.

I attach the first schematic, what do you think?
I'm using LM2940 and LM2990 because I have them at home but I'm not sure if they could work fine in my application

[Zero999]

Hi again!
I've done a little prototype on breadboard and I found out that I can connect the PGA2311 direct to TPA6120, all works fine except the Mute by pulling low the Mute pin on the PGA2311, doing that cause an output of 2V from the TPA6120.. probably due to the internal PGA mute circuit. no problem if I load 0x00 in the volume register (software mute).

now I'm looking to design a PSU that must have 5V for digital section, +5V -5V and +12v -12V for the analog section. My idea is to use a toroidal transformer with 2 secondary 0-15 0-15 VAC. The first one for the digital section and the second one for the analog (all the rails). I want to keep the 5V LDO on the first secondary always ON and use a soft switch to switch the digital section on and off and a relay for connecting the second secondary only when the digital section is On.

I attach the first schematic, what do you think?
I'm using LM2940 and LM2990 because I have them at home but I'm not sure if they could work fine in my application

It looks fine but I find the combination of a linear power supply and class D amplifier bizarre. You have an efficient amplifier, then go and throw that away with a linear regulator, which burns off the extra volts as heat!

I probably would have gone down the route of a traditional class AB amplifier and an unregulated power supply. It would achieve similar, if not better efficiency to the class D + linear regulator combination, without all the disadvantages and potential problems of a class D amplifier.

EDIT: I was talking nonsense. He's not using a class D amplifier, see below. 

[firstcolle]

nice point of view.. what do you suggest for more efficiency but without creating noise and interferences?

many thanks

[tszaboo]

I probably would have gone down the route of a traditional class AB amplifier and an unregulated power supply. It would achieve similar, if not better efficiency to the class D + linear regulator combination, without all the disadvantages and potential problems of a class D amplifier.

The TPA6120A2's current-feedback AB amplifier architecture delivers rms at (16? Load) high bandwidth, extremely low noise, and up to 128dB of dynamic range.

Power supply looks fine. Maybe add some TCS protection or Ferrite beads. Ferrite beads help a lot with the RF interference coming from eg mobile phones.

[firstcolle]

Power supply looks fine. Maybe add some TCS protection or Ferrite beads. Ferrite beads help a lot with the RF interference coming from eg mobile phones.

I'm going to add some ferrite beads between analog and digital GND. may I have to add t also in other points?

I'm also looking at the TPS65131 that offers positive and negative rails up to +-15V from single 5V supply.. but I'm not sure if it can introduce some interference and noise in an audio application (it's rated for audio in the datasheet)

[tszaboo]

Power supply looks fine. Maybe add some TCS protection or Ferrite beads. Ferrite beads help a lot with the RF interference coming from eg mobile phones.

I'm going to add some ferrite beads between analog and digital GND. may I have to add t also in other points?

I'm also looking at the TPS65131 that offers positive and negative rails up to +-15V from single 5V supply.. but I'm not sure if it can introduce some interference and noise in an audio application (it's rated for audio in the datasheet)

The ferrite bead between GNDs... I'm not sure if that is a good idea. It is one of those things that can make things worse, and you might never know. I prefer keeping AGND and DGND tied together, and provide a continous GND plane. This is definetly better for EMC.

TPS65131: While it lists audio applications, I dont know about it. Look at those nasty graphs at page 21 and 22. I see 50mV+ ripple on the rails. I have bad experience with "power saver" SMPSes with precision circuits. They go into power save mode, and suddenly they make audible noise and pollute everything around them with ripple.

[firstcolle]

The ferrite bead between GNDs... I'm not sure if that is a good idea. It is one of those things that can make things worse, and you might never know. I prefer keeping AGND and DGND tied together, and provide a continous GND plane. This is definetly better for EMC.

TPS65131: While it lists audio applications, I dont know about it. Look at those nasty graphs at page 21 and 22. I see 50mV+ ripple on the rails. I have bad experience with "power saver" SMPSes with precision circuits. They go into power save mode, and suddenly they make audible noise and pollute everything around them with ripple.

thanks for the advice on the TPD65131. I'm going with classical Linear PSU.

I thought that Ferrite beads between the GND's was a good idea!! 
where I must put it for prevent RF interferences?

[b_force]

If it's just for headphones, the lme49600 is also a very good candidate.

[tszaboo]

There is nothing wrong with the TPA6120A2 package. It has separate power pins, so it can be seperately powered. The package is not an issue for automated assembly, I run production with several thousand (different) high power opamp in the same package. For DIY, you just create a big holes under the thermal pad and food it with solder. It is good enough. I have a amp with it, and of course, it is as expected. Like not even there.
The only downside of the amp is the output impedance. But I have a HD600 which is 300 Ohms, so not an issue.
And power... With a watt in a headphone you can probably kill someone.

[Hydron]

TPA6120 isn't too bad to solder with hot air - I've made an amp with one using on home etched double-sided PCB (extended the pad underneath to the sides of the part, ran a bunch of manual-bits-of-wire vias to the ground plan underneath).

The main downsides to this chip are it's unnecessary speed (be careful with the feedback - no parallel capacitance and use the datasheet suggested feedback resistor!), and the 10 ohm output impedance requirement.

For my amp I wrapped it within the feedback loop of an audio-specced fet input op-amp (some TI one, nothing particularly amazing, but it was in the parts box...) so DC characteristics were determined by the input op-amp, not the TPA part. As I was running the TPA chip in gain-of-2 mode I was also careful to set the external op-amp's gain above 2 (4 or 5 IIRC) to make sure it was running at at least unity gain for stability. Worked perfectly, I got good audio performance, very stiff current drive capability and low output impedance. There are however a couple of caveats to this method though:
- Any op-amp/buffer inside a feedback loop should be faster than the outer part, as to not disturb it's stability (no issue in this case, the TPA6120 screams along)
- Make sure everything it running at a stable gain
- If the output is shorted then you've got a grunty op-amp dumping the full supply rail into it's output resistors, which will proceed to go up in a puff of magic smoke. To fix this I put an ohm or so in series with the headphones - makes the output impedance worse but at least a short will only apply 10x the normal output voltage over the 10R resistor rather than the full supply rail.

Other notes:
- I used a low noise switcher to make a negative 12V supply from the 12V input, and followed this with linear regulation to ~10V. Can't hear the difference compared to a linear supply (actually a little quieter, see below)
- Be very careful to separate audio traces from other power traces. Star grounding is your friend. You can hear stuff that is invisible on a scope - my initial prototype with a linear power supply hummed because a tiny bit of the 50Hz input power was flowing through an audio ground due to a layout mistake.
- I also incorporated a simple headphone protection circuit - the output is switched through a 5V coil relay with an 15V zener in series. If the total supply voltage drops below ~15V the headphones are open-circuited, hopefully avoiding any DC if one of the supplies fails.

I'd probably do things a bit differently if i built on again, probably using some of the LME parts mentioned earlier in the thread, or at least work out a way to deal with the TPA6120 output impedance requirement more elegantly. Then again, I got everything designed and built in a weekend from parts on hand - the only things I bought were a plastic case and the potentiometer from the local Jaycar store.

[firstcolle]

again, many many thanks for all your suggestions... have many things to think on now 

only the last question for today:
the PGA2311 datasheet report that the amp is capable to drive 660ohm loads but the TPA6120 has a 300Kohm resistance input. Is it better to put a buffer between these 2? I think not because of the TPA6120 absorb no current.. or I'm wrong?

[xani]

- high audio quality
...
- digital volume control (encoder + picmicro maybe IR or BT remote)

IMHO, these two requirements are mutually exclusive  .

I smell audiophoolery 

But you can always go oldschool way and have digitally controlled stepper motor to turn the pot

[Zero999]

I probably would have gone down the route of a traditional class AB amplifier and an unregulated power supply. It would achieve similar, if not better efficiency to the class D + linear regulator combination, without all the disadvantages and potential problems of a class D amplifier.

The TPA6120A2's current-feedback AB amplifier architecture delivers rms at (16? Load) high bandwidth, extremely low noise, and up to 128dB of dynamic range.

Power supply looks fine. Maybe add some TCS protection or Ferrite beads. Ferrite beads help a lot with the RF interference coming from eg mobile phones.

Sorry, I was talking nonsense. I got confused with the part numbers and thought the volume control IC was a class D amplifier IC for some reason. 

nice point of view.. what do you suggest for more efficiency but without creating noise and interferences?

many thanks

The TPA6120 has a wide supply voltage range, +/-5V to +/-15V, so doesn't need a regulated power supply. If you use a transformer with a 9V-0-9V, you can power the TPA6120 directly off the unregulated DC off the rectifier.

[NiHaoMike]

1. USB can get you very good sound. In fact, some best specced DACs are USB ones, but don't expect to get any good USB DAC for <$200.

Expect that to change now that USB-C is becoming a popular standard on smartphones. USB DACs are no longer a niche product, with the latest designs being a very scaled down digital amplifier. I was actually very surprised by the USB-C DAC that came with my Le Pro 3, easily beating the classic PCM2707 and almost managing to match a PCM1792A. (USB IDs seem to indicate a Saviaudio chip, of which it appears to be a custom part just for USB-C DACs.)