SMPS For USB Headphone Amp - Page 1
Forum section: electronics / projects

author: vvanders
author_rank: Regular Contributor
timestamp: January 22, 2013, 02:51:17 am
content: I'm in the process of building a headphone amp, just for the hell of it and I'm at a quandary for what to do about my voltage regulators.Full details:TI PCM2706for USB -> I2S duties. It'll do 44kHz 16 bit which I'm sure will outstrip my analog design abilities.Already got a PCB out for fab and should get that back shortly w/ .1 pitch headers for plugging in configurable DAC/AMP pieces:It can be powered from external power or USB via a small relay that detects external power. I'm probably short in large decoupling caps but it's a first rev that I'm sure I'll make another of.I2S header has outputs for I2S from the chip and power/gnd. HP headers has inputs for the headphone jack(to the right) and outputs from the DAC on the TI chip for quick verification that I have audio coming through clearly.I've got a quick passive amplifier as per the datasheet on the PCM2706 so that I can just make sure I've got the USB part working well:At some point I'm thinking I'll throw anADAU1702after it for some DSP fun(why keep a project simple when you can make it complex!) and it also seems to have a nice DAC. I've played with the provided software and it's quite slick.I've also been prototyping an amplifier circuit in LTSpice and breadboard, it's a pretty simple circuit that is just two op amps per channel for an analog volume control:I've breadboarded the above with two op amps I'm considering theLT1722from above and theAD8599. The LT1722 is a bit more forgiving bit the AD8599 seems built for low noise audio.I've got a decent bench positive power supply, however having prototyped this with aADM660inverter I quickly realized that SMPS, more specifically switch capacitor supplies like to generate a lot of nasty noise. I put a linear -5V regulator after it but most of the high freq noise persisted. I've done a bit more reading and have some ferrite beads on order along with a couple other ideas to tame it.However with this is all prototype stuff, I'm not sure which approach to take when actually putting the thing together off of a 5V USB supply. If I go with the LT1722 I've got a lot of headroom with almost rail to rail output. The AD8599 however is a lot more picky. I can get within around 3v of my rails, but at -5V, 5V that's not a lot of headroom. Ideally I'd want to push 2.5V PtP on 38 Ohm.In my search for supplies I came across theLT1533. I've never heard of push-pull supplies before. I know about the standard inductor, flying capacitor, etc. I'm thinking I'm going to need at least -/+ 7V, I'm not sure if I should look at a standard inductor step up, switched capacitor inverter and a good filtering network or if there's better options out there.Thoughts? I'm doing this as a bit of a self learning project so I don't mind if the final solution is fairly involved.

author: ve7xen
author_rank: Super Contributor
timestamp: January 22, 2013, 09:11:22 am
content: Have a look at ecp'sHPDAC design. He uses a TI TPS61040 boost regulator followed by a linear to get 8V and runs the amps from the single supply. I use an HPDAC2 pretty regularly, the power supply isn't absolutely silent, if I really concentrate I can hear the noise floor, but it's pretty good for a small form factor USB powered device.I think with all the stuff you want to throw in here though you may as well just go with a more traditional separate supply, you're going to be beyond the size where USB powered is really a benefit since it's not going to be very portable.2.5Vpp only requires +/-1.25V if you're going with a bipolar supply, I don't think you need 10Vpp unless you're driving really inefficient headphones.I don't have a lot of time to look at the circuit you've posted, but I would suggest some R in the feedback loop of U2 and some output R before the load as well. You can sense after the R and the opamp will cancel out the increased output impedance, but it will limit current in fault condition, common when plugging/unplugging phone style jacks. Input R of 5M is really high and you'll probably pick up a ton of noise, I would lower this to 10-50K.Why have you put the pot in the feedback loop? Contact noise will create horrible spikes in the output as you adjust it, and it will greatly lengthen the feedback loop (and increase its impedance), picking up more EMI into a high gain part of the circuit. What is it adding that's worth those downsides, plus all the extra parts (including an entire opamp!). Using a fixed gain with the pot attenuating the input will almost certainly work better and be much simpler.

author: KeithBrown
author_rank: Contributor
timestamp: January 22, 2013, 12:56:41 pm
content: As far as the choice for op-amps did you consider the OPA134? There has been a lot of experience using this amp to drive headphones, search out C. Moy, he has his own site: headwize.com. As well, the LT1722 seems a rather high speed part for a headphone amp.I don't think you should worry about having a zero offset adjustment for your output stage. The gain is low so any input offset voltage, spec'd at 100s of uV for that part, will not burn up your headphones.As far as the noise out of a switched cap inverter--yup, a lot! This is now the time to learn about poles and zeroes and esr...in other words you should be able to clean up the -ve rail but it will take careful design.