Headphone amp design

[momaka]

Re: Headphone amp design

The stack of caps would look nice in the ghetto mod thread.

[lti]

Re: Headphone amp design

Here is the APA3541 board in its current state. The green wires are the inputs and will go to the filter board.

[lti]

Re: Headphone amp design

I finally built the filter board. I didn't have any 0.01uF caps besides the good ones I was saving for breadboard use, but I had a ton of 0.047uF ceramic caps. I changed the resistor value to 1K ohm to get the same -3dB point. Unfortunately, that attenuated the treble too much. It sounds fine most of the time, but some songs sound horrible because there isn't enough treble.

The output coupling caps are too small as well now that I have some 24 ohm impedance headphones. I don't know if either amp chip is powerful enough to give those headphones decent bass. They sound best on a TDA2822M with 470uF coupling caps and a 5V power supply (aside from the background noise), which is rated for slightly less power than the TDA1308 with a 32 ohm load and a 5V supply. That MS6308 will probably make a nice amp, but I will need a through-hole adapter (or build my circuit dead bug style) to use it.

Also, for some reason, the resistor in series with the power LED (R703) is getting warm. That used to be the activity LED, but I connected it to the power input for the APA3541. It was nice of Samsung to put those pins next to each other in the ribbon cable.

[momaka]

Re: Headphone amp design

So, any pictures of this??? I've been busy with some amplifier stuff myself as well and curious to see what you got .

[redwire]

Re: Headphone amp design

I made a TDA2822M (dip-8) pcb for a low power, general purpose chip-amp. For headphone use, it's too noisy and has too much gain (39dB).

For a hi-fi headphone amp, I am looking at doing a pcb using the TI TPA6120A2 which has excellent specs: 0.0006% THD+N at 32 ohms and 100mW.

I also use this Lehmann amp clone (from eBay) but I can't get the distortion low enough. It colors things way too much despite looking nice.

[lti]

Re: Headphone amp design

So, any pictures of this??? I've been busy with some amplifier stuff myself as well and curious to see what you got .

It's just the same board with a little filter board soldered to the green wires. I haven't tried to make it sound better since then. I also added a mono/stereo switch so I can use a mono audio source.

I need to remove the MS6308 from the board (a dead motherboard that is identical to the one in my computer) and put it in a DIP adapter before I can do anything with it. It will be used for something that needs better sound quality.

[Logistics]

Re: Headphone amp design

Iti, which headphones have you been using with which chip-amps?

[lti]

Re: Headphone amp design

I used Philips SBC-HL150 and Sony MDR-ZX100 headphones for all amps. They aren't very good, but the Sony headphones are good for testing lower impedance loads. The Philips headphones are what I normally use, and they sound good with a bass boost.

[redwire]

Re: Headphone amp design

I use AKG K240 Studio's (600ohm, discontinued) and they are great because many op-amps drive them directly. So far, I haven't found a headphone amp as clean as direct-drive.

[Logistics]

Re: Headphone amp design

I find the MDR-ZX100's interesting. I have MDR-V700's which have similar specs, but I have yet to utilize an amplifier which takes advantage of their giant 1-watt power handling. I have some chip-amp projects I keep meaning to build in order to do this. I refreshed a Sound Blaster 16 for this purpose, but it uses a 9V regulator so at 24-Ohms the power-output is somewhere between 300-400mW or so, which is plenty loud, lemme tell you. I also considered modding an SB16 to use with my 16-Ohm SkullCandy IEM's, but I would have to change the voltage regulator to 3V, and even then it would likely be much higher output than the 70mW rating they have.

[lti]

Re: Headphone amp design

I got the MS6308 amp mostly working. The chip is still soldered in the board, but I disconnected the input and connected it to an inverting amplifier.

I currently have the circuit designed for a single supply with no gain on the MS6308 and an inverting amplifier with a gain of 2. I modified my old failed CMoy amp into the initial inverter/gain stage. So far, I can't really tell how it sounds since I haven't connected it to a decent audio source, but I can tell that the potentiometer I'm using is a piece of crap (the volume level in each channel channel is always different) and there is a loud hissing on the output when the volume is turned all the way down. Also, it sounds like the signal is still inverted.

[lti]

Re: Headphone amp design

The hissing was fixed by adding a filter cap. I don't know why that helped.

Also, I screwed up and made the amp have negative gain. I put the resistors in the wrong spots. That might end up being a good thing when I finish it and start using it. Strangely, it isn't as quiet as it should be compared to my laptop's audio output.

Is there any way to tell what value of coupling caps is required on the input of the op-amp and MS6308? I'm currently using 1uF caps there, along with 4.7uF caps on the output of the inverter and 1000uF caps on the output of the MS6308.

I find the MDR-ZX100's interesting. I have MDR-V700's which have similar specs, but I have yet to utilize an amplifier which takes advantage of their giant 1-watt power handling. I have some chip-amp projects I keep meaning to build in order to do this. I refreshed a Sound Blaster 16 for this purpose, but it uses a 9V regulator so at 24-Ohms the power-output is somewhere between 300-400mW or so, which is plenty loud, lemme tell you.

The MDR-ZX100s aren't that good. The sound is kind of muddy with a strange emphasis on very high frequencies, and there is almost no stereo separation.

[lti]

Re: Headphone amp design

I finished the MS6308 amp today. I just need to get some more audio jacks. I'm using a 78L05 with a diode in series with the ground pin to supply 5.6V to the amp. That isn't very good for battery power, but that was what I had. A 9V battery is nearly dead anyway when its output voltage gets low enough for the regulator to drop out.

I'm getting a ton of bass in those Philips headphones. I didn't know they could play such low frequencies. The sound is also a little strange. It's distorted in a lot of the audio range, like there is some noise mixed in with the audio signal or it is always clipping. That might be why the MS6308 had caps in parallel with the feedback resistors on the motherboard. I remember those caps doing something to reduce distortion in higher frequencies.

[budm]

Re: Headphone amp design

The cap is for limiting the high frequency bandwidth of the amplifier, as the frequency goes up the Xc goes down so you get more negative feedback as high frequency goes up so the gain is reduced. You should look at the output with scope to see if it has any high frequency Oscillation.

[lti]

Re: Headphone amp design

I don't have access to an oscilloscope, but your explanation makes sense. I could try a multimeter with a frequency measurement function, but that might not work. I wonder if both op-amps will need that cap.

[Logistics]

Re: Headphone amp design

Definitely, if you're using output caps as large as 1,000 uF you should bypass them with a film to regain high-frequency output. I would probably try a 1.0 uF mylar or polystyrene to begin with, but end with a metallized polypropylene.

Also, I would try non-polarized output caps before you do any bypassing to see what sort of difference you hear.

[budm]

Re: Headphone amp design

From looking at the board, I cannot tell how you have the ground for the input signal and the Ground for the output connected, proper ground routing is very important.

[lti]

Re: Headphone amp design

Definitely, if you're using output caps as large as 1,000 uF you should bypass them with a film to regain high-frequency output. I would probably try a 1.0 uF mylar or polystyrene to begin with, but end with a metallized polypropylene.

Also, I would try non-polarized output caps before you do any bypassing to see what sort of difference you hear.

The high-frequency output wasn't a problem. I think most of the distortion actually came from the headphones and the audio source. It sounds like aliasing, but I don't hear it when the same headphones are connected through my normal headphone amp.

From looking at the board, I cannot tell how you have the ground for the input signal and the Ground for the output connected, you should use star ground.

I don't have the input or output wires connected yet. When I get the input and output jacks, I'll get as close as I can to a star ground.

[lti]

Re: Headphone amp design

I soldered in some good input and output wires and set the gain correctly today. However, when I was done, the sound quality was horrible and there was no bass at all. It sounded a little like what happens when the ground wire from the headphones gets disconnected, but there were no missing portions of the audio signal like there would be if that happened. I did notice that the resistance from the ground connection on my audio cable to the computer's case (I was using the line out from my computer) was 9 ohms, but I didn't see anything else wrong. If anything else could cause this, let me know. I currently have 4.7K ohm feedback resistors and 3.3K ohm resistors in series with the inverting input, giving a gain of roughly 1.42.

[lti]

Re: Headphone amp design

Apparently lowering the resistance of the feedback resistors means that the coupling caps need to be bigger. I put a 10uF cap in parallel with the 1uF input coupling caps and got most of the bass response back. I'm going to look for even bigger caps.