Yet another TAP6120 based headphone amplifier you’d say. Well maybe you are right. This one started as a group project involving myself and few other guys forum mates. So here it is – the UBIQUITOUS TPA6120 amp.
The TPA6120 chip is a rather strange beast. Looking at the TI’s portfolio of headphone amplifiers you would not find any other amp similar to this. Especially if you look at the slew rate. So why is that? The answer is rather simple. The TPA6120 chip is actually a re-branded high speed line driver that had failed to meet the requirements. This is a common practice among the manufacturers. Many so called “audio opamps” are actually the same thing – lower grade opamps that had failed to meet the criteria.
However failing to comply with the high speed line driver requirements does not make the TPA6120 chip a useless junk. Believe it or not it’s an excellent performer. Many people believe that it can outperform much more expensive headphone amplifiers. I don’t want to put any thoughts on this mater though.
Some facts about the TPA6120 chip – as it’s a high speed IC it requires a careful PCB layout. Any parasitic capacitance may cause the amp to go unstable; – the chip is rather hard to run alone. The simple truth is that you may destroy your headphones if you don’t put any attention to this. The chip will have a severe DC offset at the output depending on the volume pot setting. This can go up to several volts. You wont find this in the datasheet so BE CAREFUL!
This being said I’ve decided to go safe. This design uses an input opamp based stage. It’s not an ordinary two stage project though. This design is based on the so called COMPOSITE opamp design. This approach uses the best of both CFB and VFB design worlds. More info on this matter can be found here:
I went for the non inverting design.
So my input stage is based on a precision opamp. Few candidates here:
OPA2132 – http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sbos054a
I’ve decided to use opamps in DIP package for an easy experimenting. However you could use just about anything you find suitable and use a DIP->SOIC adapters. Just remember that you need a PRECISION opamp in this stage. This would help keeping the DC offset low.
Few words about the project realization. This one is completed on a two-sided PCB using smd components and TH metalization. A special attention was paid on the bottom ground plane to minimize the parasitic capacitance. More info on this matter can be found in the TPA6120′s datasheet:
“A ground plane should be used on the board to provide a low inductive ground connection. Having a ground plane underneath traces adds capacitance, so care must be taken when laying out the ground plane on the underside of the board (assuming a 2-layer board). The ground plane is necessary on the bottom for therma reasons. However, certain areas of the ground plane should be left unfilled. The area underneath the device where the PowerPAD is soldered down should remain, but there should be no ground plane underneath any of the input and output pins. This places capacitance directly on those pins and leads to oscillation problems. The underside ground plane should remain unfilled until it crosses the device side of the input resistors and the output series resistor.
The power supply regulation is complete on the board itself near the TPA chip. This requires a +/-18 to +/-21VDC prefiltered.
The complete schematic for this project can be downloaded HERE.
Here is how the PCB looks like:
Since it was the first batch of PCB’s I had some problems with the silkscreen printing. However it’s not a big issue at the moment. In order to keep the return paths as short as possible I was forced to use jumpers. This could be easily solved with a 4-layer PCB. These come at a price though.
WARNING: This project was brought to the DIY community for free. It is intended for personal DIY needs only. Commercial usage of this project is not allowed!