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Old 30th September 2014, 10:38 PM   #1
matdogg is offline matdogg  United States
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Default problem with my first lme49723 amp

Hi everyone, I'm building my first ever headphone amp very closely based on Chu Moy's pocket headphone amplifier but using the lme49723 opamp instead. When I test my circuit it was horribly distorted. Can hardly tell it's music! I then experimented with changing the resistor between input and ground, with a little change, but nowhere near right.
At the risk of sounding really stupid, I assume the input ground and output ground are to be tied to the virtual ground of the circuit, but I noticed that immediately upon removing either the input ground or the output ground I get nice clear sound for maybe a second. Then back to very distorted.

What am I doing wrong? Hopefully just a dumb mistake! A quick drawing of my particular schematic is attached. Thanks!
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File Type: jpg IMG_0237.JPG (223.9 KB, 234 views)
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Old 30th September 2014, 11:13 PM   #2
agdr is offline agdr  United States
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Quote:
Originally Posted by matdogg View Post
Hi everyone, I'm building my first ever headphone amp very closely based on Chu Moy's pocket headphone amplifier but using the lme49723 opamp instead. When I test my circuit it was horribly distorted. Can hardly tell it's music! I then experimented with changing the resistor between input and ground, with a little change, but nowhere near right.
At the risk of sounding really stupid, I assume the input ground and output ground are to be tied to the virtual ground of the circuit, but I noticed that immediately upon removing either the input ground or the output ground I get nice clear sound for maybe a second. Then back to very distorted.

What am I doing wrong? Hopefully just a dumb mistake! A quick drawing of my particular schematic is attached. Thanks!
Chu Moy has an error of sorts in the original article. The 10K in the feedback loop plus the 1K to ground on the inverting op amp input give you a gain of (1 + 10K/1K) = 11 which is just way way to much for anything. Try reducing the 10K to 1.5K for a gain of 2.5x.

The gain formula is here:

Operational amplifier applications - Wikipedia, the free encyclopedia

Desktop sources often only need a gain of 1x (just a current buffer, leave that resistor from inverting input to ground out altogether). Pocket sources like ipods may need the 2.5x to 4x or more. But not 11x! That only was needed in the movies, lol: Up to eleven - Wikipedia, the free encyclopedia

You have another problem too. Your power rails are +/-2.5Vdc and the absolute minimum for the LME49713 chip is +/-5Vdc. You need more voltage...

Another solution would be changing chips. The NJM4556AM is good down to +/-2Vdc power supply rails and the pinout of the surface mount version is exactly the same as your LME49723. It would drop right in. 73 cents at Mouser: 513-NJM4556AM.

Last edited by agdr; 30th September 2014 at 11:37 PM.
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Old 1st October 2014, 04:26 AM   #3
matdogg is offline matdogg  United States
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Ah, thanks for the help! It was indeed a rookie mistake. I guess I read the datasheet wrong... I was planning on running this on USB power. Looks like I will be switching to that NJM chip you mentioned. How does it sound? In the small amount of browsing I've been doing, I don't remember seeing it mentioned.
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Old 1st October 2014, 05:11 AM   #4
agdr is offline agdr  United States
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Originally Posted by matdogg View Post
I was planning on running this on USB power. Looks like I will be switching to that NJM chip you mentioned. How does it sound? In the small amount of browsing I've been doing, I don't remember seeing it mentioned.
The NJM4556A is the chip used in the O2 headphone amplifier in this forum here. There are probably several thousand O2s out there by now. It is also the chip used in Grado's RA1 headphone amplifier.

Please realize though that with only +/-2.5Vdc power rails on your CMOY there will be some limitations on the type of headphone that can be powered. You don't have a lot of voltage swing to work with there. The headphones will need to have a fairly high sensitivity rating, like 115dB/mW vs. something like 92.5dB/mW.

As for USB, you might get a kick out of reading through 00940's creation here to see where that can go on the more technical end of things. A very good design. He is using a DC-DC converter module to take the 5V 500mA USB to +/-9V, filterting out the converter noise, then using a LME49713 gain op-amp wrapped around a current buffer op-amp for each channel.

Last edited by agdr; 1st October 2014 at 05:25 AM.
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Old 1st October 2014, 01:36 PM   #5
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The LME49723 is actually rated for 5..34 V of supply, so it should still work. Input common-mode range, however, is limited to within 2.0 V from both rails typ, so some gain would be needed to make the most out of output voltage swing, which is not quite as limited (about 1.0 V from each rail at limited loading). About a factor of 3 or so.
Very low-voltage circuits tend to employ inverting operation in order to avoid running into input common-mode range problems (as the inputs remain at a fixed potential then - and as an added benefit in single supply operation, the virtual ground is not being loaded).

I suspect that the OP either overdrove his circuit badly or made some mistake wiring up virtual and "real" grounds. This might already show on a complete schematic.
Ah, I read the first post again: I think the problem is that the virtual ground is tied to "real" ground, with the "+/-2.5 V" supply in fact being +5V/Gnd. A virtual ground does not simply turn a single supply into a split supply.
If so, things should be connected like this:
Input ground at the 9k4 goes to "real" ground.
The 100k goes to virtual ground.
The feedback network either connects to virtual ground, or (preferably) is capacitively coupled to "real" ground.
The output needs to be capacitively coupled and connected to "real" ground. (Virtual grounds usually cannot sustain load currents, or at least those typically used in a cMoy. Some circuits employ a dedicated "ground buffer" though. It means higher power consumption and some other potential pitfalls, but you can do away with the bulky and potentially rather non-ideal output coupling caps. Try squeezing coupling caps of 220 F up into a tiny MP3 player. A generally even better approach involves generating your own negative supply with a charge pump on-chip.)
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Old 1st October 2014, 03:59 PM   #6
agdr is offline agdr  United States
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sgrossklass is right - I had the LME49723 and LME49713 datasheets up at the same time and looks like I mixed them up! Good catch.
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Old 2nd October 2014, 01:28 AM   #7
agdr is offline agdr  United States
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Originally Posted by sgrossklass View Post
Very low-voltage circuits tend to employ inverting operation in order to avoid running into input common-mode range problems (as the inputs remain at a fixed potential then - and as an added benefit in single supply operation, the virtual ground is not being loaded).
I was curious how this would simulate. The first circuit and plot is with a 300R load while the second is with a 32R load. Green is output voltage and blue is input voltage on the left scale. Red is op amp output current on the right scale.

I ran both of these up until just before clipping. For the 300R load with the 1V peak input that was around 1.5Vpeak. 1.6 gain = 16K/10K. For the 32R load with the 1V peak input the max output was around 0.75V, an attenuation, but still useful current buffering going on. Both transient plots are done at 20Hz to show low frequency performance given all the caps in the signal path.

I've added the cap in series with the pot on the inverting attenuator to block DC as per the good comments from 00940 in this this thread. As he mentions the 270K is in there just so the feedback loop doesn't go completely open loop when the pot's wiper eventually fails.
Attached Images
File Type: png inverting CMOY circuit 300R.png (34.6 KB, 184 views)
File Type: png inverting CMOY AC plot.png (11.1 KB, 172 views)
File Type: png 300R load 1Vpeak input 16K pot.png (13.8 KB, 160 views)
File Type: png inverting CMOY circuit 32R load.png (34.9 KB, 161 views)
File Type: png 32R load 1V peak input 9K pot.png (15.4 KB, 35 views)

Last edited by agdr; 2nd October 2014 at 01:42 AM.
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Old 2nd October 2014, 02:02 AM   #8
matdogg is offline matdogg  United States
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Wow lots of stuff for me to read through already! Thanks guys.

agdr, thanks for the info on 00940's USB amp. I might have to try something like that. The headphones I'm planning on pairing this with are 32 ohm and 104 db/mW. That falls right in the middle of your given range but I think I may try the dc-dc convertor and buffer anyway (however, one step at a time). If I remember correctly, a buffer is recommended for low impedance headphones, right?
I may need you to explain your most recent post though... Are you showing that even with my low input voltage I still might have useful voltage swing at the output? Pardon my ignorance, I'm a mechanical engineering student, not electrical.

sgrossklass, I caught one mistake I made from your suggestions. The 9.4k resistor was indeed tied to virtual ground. That's fixed now. Capacitive coupling is something I also don't have. Can you explain the purpose of the coupling capacitor? If I remember, they will let AC current through to ground, but not DC current. I'm curious, this seems to me like the opposite of what we want to drive speakers...
Regardless, looks like I'll be making yet another digikey order!
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Old 2nd October 2014, 03:17 AM   #9
agdr is offline agdr  United States
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Originally Posted by matdogg View Post
Are you showing that even with my low input voltage I still might have useful voltage swing at the output?
Amazingly enough your headphones are sensitive enough that you should be able to hit very loud musical peaks just with that inverting CMOY circuit and 5Vdc USB power, unless I've missed something in the math.

Take a look at the spreadsheet below (PDF file). That is a headphone power calculator that was posted on Head-Fi last year. Enter the headphone impedance, sensitivity, and loudness in dB (Sound Pressure Level) that you are shooting for and it calculates the voltage swing (rms) and current requirements needed. Ignore the last spreadsheet section about how load the amp will drive your headphones - those are numbers left over from a different amp.

The inverting CMOY simulated at around 0.75V peak into 32 ohms. 0.75V peak is 0.53Vrms. Entering your 32R and 104dB in the spreadsheet and fiddling with loudness levels to hit 0.53Vrms comes up with 113.5dB. At that point the current requirement is 17mA(rms). The chip is good for around 25mA peak, which is around 17.5mA(rms), so that even works out. The power dissipation in the chip also looks OK if I've done the math on that correctly.

Take at look at the "hearing damage" chart at the bottom of that spreadsheet. 113.5dB SPL is pretty loud and should only happen in short peaks anyway. Average listening level will be way less. Musical peaks are often 2x to 3x the average voltage levels.

It also looks like I screwed up my previous description of 00940's amp too. I looked at it a bit more today and it looks like he is using the LME49713 for the current buffer and another chip for the gain chip in a multiloop configuration. The gain chip is FET input so the thinking there may be that it wouldn't pull any significant current through the pot wiper, which can cause scratchiness and lead to pot failure. And it looks like he wound up with a +/-12Vdc converter rather than the +/-9Vdc in his first post.

Which reminds me - putting 100uH chokes (coils) on the power leads coming from the USB in that inverting CMOY wouldn't be a bad idea to keep the PC digital noise out of the amp. All the resistors should be 1% metal film (if though hole) or 0.1% thin film if surface mount to keep noise down. That circuit could also be run off a 9V battery but the capacitor voltage would have to increase to 16Vdc which would make them a little larger physically. Those three 220uF caps should ideally be the new low ESR organic polymer electrolytics, like Mouser #667-6SVP220MX or Digikey #P16598CT-ND.

You are right, a "current buffer" is a chip or transistor circuit that is used mostly to source/sink current and not voltage. Low impedance headphone loads are the usual culprit for needing a lot of current, unless the sensitivity of the headphone's drivers are high enough where the voltage swing involved can be small, which is the case with your headphones. Remember ohm's law, voltage/resistance = current. If you are able to keep the voltage swing small then even a low resistance results in low(er) current flow.
Attached Files
File Type: pdf 32R 104dB_mW 120dB.pdf (104.7 KB, 13 views)

Last edited by agdr; 2nd October 2014 at 03:46 AM.
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Old 2nd October 2014, 06:16 AM   #10
agdr is offline agdr  United States
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Whoops - I left the "u" (micro) off the inverting CMOY output capacitor in the circuits above - 220 Farads, lol. With that fixed the organic polymer electrolytics have to go up to 470uF to be perfectly flat at 20Hz, and 3 have to be in parallel for the output, but the good news is those are only $1 each at Mouser these days with ESR=0.008R to boot.

I also forgot to capacitively load the output to see if the 4.99R isolation resistor is working. This one has 500pF in parallel with the 32R load. I've also split up the 10K input resistor to add an RF filter to the input. The 3.3uF film caps can be the cheaper 50Vdc/30Vac units since the voltage swings involved are small. 100uH coils on the USB inputs turn out to not be a good idea - they resonate. Ferrite beads would be better if EMI is a problem.

Here is a revised schematic with real-world (Mouser) part values filled in for everything.

sgrossklass - your idea is pretty interesting!
Attached Images
File Type: png inverting CMOY schematic.png (69.7 KB, 51 views)
File Type: png inverting CMOY AC plot 470uF.png (10.5 KB, 46 views)

Last edited by agdr; 2nd October 2014 at 06:41 AM.
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