Getting a quality negative rail in an audio project? - Page 1
Forum section: electronics / projects

author: kmm
author_rank: Regular Contributor
timestamp: December 22, 2016, 03:15:45 am
content: Howdy all,Apologies in advance if this is a dead horse, it probably is, but I'm mostly looking for opinions.Lately I've been messing around with headphone amps, and while my amps are working fine I'm wondering what your opinions are on how to get a (clean!) negative rail.The obvious options, with the upsides and downsides I see:Batteries - Convenient and portable; adds consumables and/or costTransformer+linear reg - Rock solid, easy; expensive, large, heavy, requires mainsDC-DC converter - Compact, flexible, runs off batteries or a wall wart; NOISE EVERYWHEREFrom my perspective, wanting something that can run off mains via a wall wart or a lithium, pack the DC-DC converter approach seems optimal from every perspective....except for perhaps the most important for a headphone amp, clean power rails.There's also the concern of asymmetric behavior under load if I have both positive and negative rails provided by a DC-DC.So I guess my core question is would you, as a designer probably more experienced than I, even approach designing something "hi-fi" class that includes an analog rail coming out of a DC-DC?Or is it just a setup for a nightmare of dealing with ripple under load and tracking down noise sources?Total power out desired is a watt or less, I've been running my lash-up with a pair of 9 volts.

author: DaJMasta
author_rank: Super Contributor
timestamp: December 22, 2016, 03:31:17 am
content: If you have the luxury of mains voltage for it, that's an easy choice - very straightforward design and it will perform with similar noise levels as the positive supply side. This is also great if you need high voltage, like 48V phantom power which is a pain to generate with commercially available small boost converters from a smallish DC voltage.That said, it's totally doable with a switching converter, just pick one with a high operating frequency. I recently used an LT1931 for a -5V supply in an audio design and I've seen no issues with the stability or noise levels of the load. I was aiming for fast switching frequency and good line regulation, and this one switches at over 1MHz and advertises 1mVpp ripple. At least in my design, it's been keeping pace (at least, from the best I can tell) with a decent quality linear regulator on the positive side (REG113).

author: mmagin
author_rank: Frequent Contributor
timestamp: December 22, 2016, 03:32:12 am
content: I think that with a dc-dc converter circuit where the switching frequency stays well in excess of audible frequencies (including when lightly-loaded!), you shouldn't have that much trouble.As an aside, if you don't need that much power, there are some headphone amp ICs designed to work off 2-5 volts from TI and such that look pretty good. The TPA6133A2 seems to use an integrated charge-pump (well, it requires an external capacitor).

author: mmagin
author_rank: Frequent Contributor
timestamp: December 22, 2016, 03:35:49 am
content: Also, I might add, if you aren't quite prepared to design your own and want a good dc-dc converter module, don't go for some random cheap ebay/aliexpress/etc. thing, buy made by someone who knows what they're doing, like Murata or one of the usual power supply brands.

author: kmm
author_rank: Regular Contributor
timestamp: December 22, 2016, 03:42:44 am
content: Quote from: mmagin on December 22, 2016, 03:32:12 amYeah, I suspect the current upscale off the shelf silicon offerings will likely be better amps than most of what I make, but my intent is mostly to have a little platform for playing around with different op-amps and transistors.I'm not aiming for pocket sized (more multimeter-sized). I think much of it will be done in through hole for easier hackability, so I think I can have space to put all the power handling stuff on a daughterboard.I don't plan on making many of them unless I find a golden combination of components and can sell finished units to audiophiles for far too much money.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 22, 2016, 06:03:05 am
content: Another solution is to make a good 12V or so rail and then split it up to +5/-7 or so.https://github.com/NiHaoMike/OpenDAC-HD/blob/master/OpenDAC_sch_dig.pngThe latest in headphone amplifiers nowadays are actually scaled down versions of digital amplifiers built onto a single chip. (The real power savings is in eliminating a lot of the analog circuits, not the efficiency of a digital amplifier per se.) I have a USB-C "DAC" that sounds remarkably good, quite a marvel given it is all built into the plug.

author: kmm
author_rank: Regular Contributor
timestamp: December 22, 2016, 06:55:12 am
content: Quote from: NiHaoMike on December 22, 2016, 06:03:05 amSo take a 12V rail and make another rail with a linear reg hanging off it somewhere in the middle, call that the analog reference ground, and the 0v rail becomes the analog negative?Interesting idea, might be a pretty reasonable solution for running off either a lithium pack or external brick.

author: Someone
author_rank: Super Contributor
timestamp: December 22, 2016, 07:12:27 am
content: Quote from: kmm on December 22, 2016, 03:15:45 amThe noise only couples where you let it get to, there are low noise switching topologies (especially transformer coupled/isolated) that can do very well in the audio band. A well designed switching regulator (which you might buy as a ready made module) should be entirely adequate with some filtering.

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 22, 2016, 08:14:03 am
content: Is your amp a stereo amp?This complicates the issue as both headphone channels share the same "ground" , so bridge-tied single supply amps won't work, and rail-splitters have to be sufficiently stiff to avoid cross channel coupling.Conventionally one would use e.g. a LM386 as a single supply amp and couple the output with big capacitors.

author: tszaboo
author_rank: Super Contributor
timestamp: December 22, 2016, 09:48:03 am
content: Quote from: kmm on December 22, 2016, 06:55:12 amNo. A linear regulator can only source current. Your virtual ground needs to be able to source and sink current. Hence the BUF634. That is basically an opamp. Only downside for the audio part is the increased output resistance of the ground.Half wave rectifying a single tap transformer is another option.

author: tatus1969
author_rank: Super Contributor
timestamp: December 22, 2016, 10:26:25 am
content: I have built up an all-switching 600W PA with Hypex modules (2x UcD400, SMPS400). It was all about a proper grounding scheme and closed-loop shorting of RF coupling paths. The amp does well on its own, when you see the picture you'll notice very thick ceramic isolators. That is to minimize capacitive coupling between transistor and heat sink, keeping the RF in a small area.Even at this power level, it worked out well right from the start. I was expecting more problems from RF coupling and parasitic demodulation at some nonlinear element. Mentioning this, this is one thing that you need to be aware of. Even when all unwanted switching frequencies are way beyond the audible range, parasitic demodulation can shift it back into the audible range. This is similar to the first diode detector based AM radio receivers. Any nonlinearity can do this. A BJT or MOSFET input, a diode, and even an opamp - when the design is not fully symmetric close to the transition frequency, both + and - inputs "perceive" your RF differently and a DC offset will be the result. I had to learn this painfully myself when designing a strain gauge amplifier. The aliasing filter was missing (I used a sigma-delta ADC, so I didn't care), so RF could enter the preamp and created a DC offset.I had more problems with asymmetric (cinch) signal feed and hum coming from all the different AC/DC converters in the setup. Short fix was a thick copper connection between all of them, long term plan is to add symmetric/XLR input, and a "virtual symmetric" cinch input modification that basically disconnects the shield from GND at amplifier side, and feeds both signal and shield into a diff amp. That should theoretically eliminate all hum. To keep the shield "signal" within the diff amps common mode range, the circuit will still need a resistor (maybe 10 .. 100 ohms?), possibly a PTC, plus a zener diode to GND for protection.

author: Buriedcode
author_rank: Super Contributor
timestamp: December 23, 2016, 01:00:11 am
content: Headphone amps generally don't require high power, nor high voltage (unless they're 'high impedance' headphones, or you're making a class A?).Most are also based on opamps - which should have a good PSRR, and as its true bipolar power supply, you shouldn't need to connect anything else to this rail... even a 40kHz charge pump with high ripple shouldn't be much of a problem.Whilst I used to use single supply for headphone amps, the large output caps and thus the dreaded 'pop' meant I went to integrated solutions. For a morning project to test a DAC I knocked up a nice sounding ( = transparent, to my ears at least) amp using a TS922 and a TPS60403. The charge pump had pretty large ripple on its output, but given its frequency and the PSRR I couldn't hear anything.If this isn't going to be portable then you have even more options because of less restrictions (efficiency mostly) but if you're going to use a standard mains transformer then that will of course require heavy filtering. I know there's a whole range of headphone amps use 'boutique' parts (usually old/outdated) and 'cmoy' amps where some feel the need to use RF opamps () but the KISS principle should always apply. All depends on the application.

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 23, 2016, 01:07:51 am
content: Hi Tatus,The remote ground is very helpful in removing ground noise, schematic below is an ultralow noise mic preamp, that works in a camera housing with SMPS PSU's and PWM servos , and PWM LED driver on the mic PCB.The reference terminal of the SSM2019 is set for about 6V at DC , but at AC its voltage is determined by the remote gnd , R385 ensures there is a galvanic connection . Pin 3 and pin1 of J372 both connect to the groundplane of the main PCB.There is also fairly aggressive EMI filtering, and phantom power.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: December 23, 2016, 05:17:12 am
content: I have had a lot of success running instrumentation off a zeta converter (AKA "inverse SEPIC") for the positive rail and Cúk converter for the negative rail. With non-isolated topologies the negative rail is actually the easier one to implement, as the Cúk is less demanding than the zeta. If you go with something transformer-isolated, of course, there's no difference. Follow up with a high PSRR type linear regulator for best performance.If it works for instrumentation, I can't see audio being any challenge at all.

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 23, 2016, 07:10:30 am
content: Isn't the Zeta converter topology capable of generating a negative supply voltage by hanging a diode off the first inductor?(assumes a 1:1 coupled inductor)

author: tatus1969
author_rank: Super Contributor
timestamp: December 23, 2016, 07:25:14 am
content: Quote from: salbayeng on December 23, 2016, 01:07:51 amthanks, that's exactly my intention in reverse, good to hear that it should be effective.

author: tatus1969
author_rank: Super Contributor
timestamp: December 23, 2016, 07:32:31 am
content: Quote from: salbayeng on December 23, 2016, 07:10:30 amthat is possible but that addituonal rail would be unregulated. that would be the path that i would take, with good emi filtering and maybe ldo's in a second stage. the zeta topology is better than cuk because the current into the output capacitor is triangular and not pulsed, giving better ripple characteristics.

author: JPortici
author_rank: Super Contributor
timestamp: December 23, 2016, 11:17:11 am
content: Quote from: kmm on December 22, 2016, 03:15:45 amgo tell that to audio interface manufacturers

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 23, 2016, 11:26:47 am
content: Tatus , et al.The SSM2019 is a pretty impressive chip, at gain =100 it has 1.5dB Noise figure, CMRR and PSRR ~ 100dB and bandwidth to 1MHz, THD < 0.01%Slew rate is 15v/us . Offset <100uV . The only bad thing is the bias current ~ 3uA so you need a reasonably low DC impedance on the input (10k), the DC and 1/F performance are nearly as good as an OP27So it pretty much leaves any other op amp in the dust, and with the reference terminal you can reject noise on the ground line.http://www.analog.com/media/en/technical-documentation/data-sheets/SSM2019.pdf

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2016, 11:44:39 am
content: Quote from: salbayeng on December 22, 2016, 08:14:03 amYes, I tend to avoid a dual supply whenever possible. DC bias the op-amps at half the supply voltage and connect any AC paths to either power supply rail via capacitors.The main disadvantage is the pop generated at switch on but many modern amplifier ICs include circuitry to suppress it.

author: mariush
author_rank: Super Contributor
timestamp: December 23, 2016, 12:43:25 pm
content: How about some of those voltage doubler chips, configured in inverting mode. Take 5in and get -5v at 25-100mA with just the chip and a couple of ceramic capacitors.For a headphone amp, it should be enough, and the efficiency isn't that bad at around 80%+Something like LM2664 or LM276*Running at 50kHz and higher, i don't think they'd affect headphones amplifiers that much.

author: tszaboo
author_rank: Super Contributor
timestamp: December 23, 2016, 10:10:45 pm
content: Quote from: Hero999 on December 23, 2016, 11:44:39 amThe disadvantage is that capacitors don't have the nice flat curves in the audible range. If you want low frequencies, you need a big cap, than it is an elco. ESR and volatage dependency will be a problem. If not on the input, then on the output.Quote from: mariush on December 23, 2016, 12:43:25 pmThose are only OK, if you want -5V, not so much for higher voltage. And they are not regulated, so they have some ohms of output resistance. They are also noisy. There is an exception to this, OP can try this actually:http://www.linear.com/product/LTC3260

author: BrianHG
author_rank: Super Contributor
timestamp: December 24, 2016, 12:54:47 am
content: Arrgg, running off batteries....If you dont mind draining your batteries a little, if you want a +/- supply for audio, though it may cost a little more, a balanced +/- isolated DC-DC converters are easiest to filter their outputs. When driving headphones with that dynamic load, unless you are using Class A where the supply load is pretty constant, drawing from a higher current positive supply where your negative supply comes from a voltage inverter which is powered and generating delayed loads on that supply as well can lead to unpredictable situations. Now, I am not saying that you cant accommodate and filter this out, but, if you want a consistent response, equally quality + and - rail, I would use a quality +/- center taped isolated regulated DC-DC convertor, tying the center 0v out to my ground from my power source and use ultra-low ESR caps right in-between the +/- outputs and the center ground pin. If I really need more filtering, I would add a series inductor on each output followed by a second stage of ultra-low ESR caps combined with 2 x 47000uf caps each on the + and -. This is where you'll get your big filtering....Take a look here:http://www.digikey.ca/products/en/power-supplies-board-mount/dc-dc-converters/922?k=dc-dc+converter&k=&pkeyword=dc-dc+converter&pv1472=1&pv1472=76&pv1472=37&pv1472=105&pv1472=7&pv1472=21&FV=fff40042%2Cfff800df%2C1140050%2C114016f%2C11401ab%2C11401c9%2C15c0003%2C8f40015%2C8f40016%2C8f4001e%2C8f40025%2C8f4002b%2C8f4002e%2C8f40036%2C8f4005f%2C8f40095%2C954002b%2C954002c%2C954002d%2C954002e%2C954002f%2C9540030%2C9540031%2C9540032%2C9540033%2C9540034%2C9540035%2C9540036%2C9540037%2C9540038%2C9540039%2C954003b%2C954003c%2C954003d%2C954003e%2C954003f%2C9540040%2C9540046%2C9540047%2C9540048%2C954004c%2C954004d%2C954004f%2C9540065%2C9540066%2C9540069%2C954006a%2C954006b%2C9540078%2C954007c%2C16fc0002%2C16fc0008%2C16fc000b%2C16fc0014%2C16fc0016%2C16fc0020%2C16fc0027%2C16fc0028%2C16fc0057%2C16fc0069%2C16fc0085%2C16fc0097%2C17000001%2C17000004%2C17000007%2C1700000c%2C1700000f%2C17000015%2C17000025%2C17000044%2C1700004c%2C17000054%2C17000069%2C17d4002c%2C17d40036%2C17d4003e%2C17d40071%2C17d40092%2C17d40096%2C17d400b6%2C17d80002%2C17d80006%2C17d80012%2C17d80015%2C17d80016%2C17d8001a%2C17d80025%2C17d80026%2C17d80027%2C17d80044%2Cffe0039a%2C1f140000&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&pbfree=1&rohs=1&quantity=0&ptm=0&fid=0&pageSize=500Now, I've included 1 watt units, but, I personally would use 2 watts at least. With a +/- 5v unit, that's +/- 200ma continuous draw.Surprisingly, these are not too expensive. 20 years ago, I'd expect to pay upwards of 75-100$ per unit.For a little more money, but, you have 3watts, and, a sully voltage anywhere from 4.5 to 9v, take a look at these:http://www.digikey.ca/product-detail/en/cui-inc/PQMC3-D5-D5-S/102-2838-ND/4009762+/- 5http://www.digikey.ca/product-detail/en/cui-inc/PQB3-D5-D12-D/102-2799-ND/4009722+/-12http://www.digikey.ca/product-detail/en/xp-power/JCB0305D09/1470-1795-5-ND/4488175+/-9Note: All prices are in CDN dollars.Also, the lowest 0% load current input I've seen is I believe the Murata 12v in, +/-5v out 1watt unit at 11ma.