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Old 5th April 2011, 04:18 AM   #71
qusp is offline qusp  Australia
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i reread my post and want to make it clear that my role and any input in any of these projects i mentioned is as a simple pick and place builder, nothing more, i just happen to have built quite a number of the projects you mention plus more.

i also didnt mean to harp on 'the wire' so much, i only offered it as an example of the national buffer driving about as difficult headphone load as can be imagined (apart from stats and some rather high impedance older cans of course), to offer confirmation that the lme49600/10 does not produce any offset in this design, plus provides the 250ma with the right layout as far as i can tell, negating the need to stack them Imo and to serve as a warning not to underestimate the power supply requirements when using these buffers.

i have build 2 of PA's qrv08 and to me they lack in comparison to 'the wire', particularly in bass performance, perhaps its the wimpy onboard transformers, the sound is very very clean, but does not provide the same authority

step down transformer to drive multidriver iems jcx? really? i wont right that off before i actually try it as i have been meaning to, especially coming from you, but do you own any iems like this? like little low impedance difficult to drive speakers, not linear in the slightest

Last edited by qusp; 5th April 2011 at 04:21 AM.
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Old 5th April 2011, 04:40 AM   #72
Bonsai is offline Bonsai  Europe
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"DIY SMT, especially with analog, is risky. "

I think if you are prepared to invest in a decent soldering iron and some optics (figure $150 to $200 for both), some tweezers, and you are careful, it really is not that bad. I started using SMD about a year ago and have been surprised at how quickly I got the hand of it. Also, saves a lot of drilling on big projects! to solder a 1206 resistor or cap, I don't think the iron is in contact with each pad longer than 1-2 secs. The IR reflow oven is I think equally as stressful, if not more so. Take a look at SuzyJ's amp - she used all SMD to create a very compact neat looking result.
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Old 5th April 2011, 06:23 PM   #73
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Ok, Peranders has revised the Wiki and apparently his website (or I was blind yesterday). You can find the QRV09 web page here:

Sjostrom QRV09 Webpage

And a ZIP file with a PDF of the schematics and board layout here:

For an extra $8 or so, he'll pre-solder the TPA6120 to the board hopefully solving that tricky problem. That adds to the DIY-friendly factor.

The QRV09 schematic shows local feedback around the TPA6120 and around the input stage with no global feedback. Unfortunately, it also shows the TI recommended 10 ohm resistor in the output (along with an optional 110 ohm resistor). TI says:

The series output resistor should be kept at a minimum of 10 Ω. It is small enough so that the effect on the load is minimal, but large enough to provide the protection necessary such that the output of the amplifier sees little capacitance.

IMHO, they're dead wrong about the "effect on the load is minimal" if said load includes most balanced armature IEMs and even many 16 ohm conventional dynamic headphones. They will show significant frequency response deviations from that much series resistance. It will also degrade the damping factor and raise the system Q hanging the bass performance (generally for the worse if the headphone were designed, as many are, for a zero ohm source).

I have a board on order with Peranders and one of my first goals will be to hopefully replace that 10 ohm resistor with a parallel resistor inductor combination to drop the output impedance in the audio band while hopefully still keeping the design stable. But, I have to confess, this is another thing I don't much like about the TPA6120. Being an extremely "fast" current feedback device it's much more prone to stability problems. I have to agree with JCX it would be a bad idea to try and put gain options in the TPA6120 feedback loop. The data sheet spends quite a bit of time on the topic and even suggests using a network analyzer to determine the stability of the final design.

Some don't like series inductors in the output as they can cause phase shift at high audible frequencies. The National LME49600 doesn't require a series resistor or inductor between it and the headphones.

And, while I'm voicing TPA6120 concerns, I'm looking forward to measuring the channel separation under a variety of conditions. TI claims 90 dB, which if true under real world conditions, is plenty good. But because both channels are in a single part, I'm a bit concerned it's not going to be that good in reality. The LME49600, being a single channel device, also has an advantage here.

Of course you can always use two TPA6120's and not use half of each in a "dual mono" configuration. Or you can try to parallel them but that might create other stability problems (something JCX could probably help out with?). But then you can't use the Peranders board.

So, in summary, I have both the Chinese HA TPA6120 DAC and the Peranders QRV09 board on the way. At this point it seems to make the most sense to see how these perform before going much further with a LME49600 design.

Anyone who's on the fence about the QRV09 might want to jump on that bandwagon now before it's too late. If that board measures really well, and I can overcome the TI mandated 10 ohm output impedance, I likely won't do a LME49600/BUF634 design.
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Last edited by RocketScientist; 5th April 2011 at 06:26 PM.
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Old 5th April 2011, 07:23 PM   #74
jcx is offline jcx  United States
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in audio power amps the load isolation series impedance uses an inductor to keep the audio frequency efficiency/damping ratio high while "disconnecting" the amp output from the load (mostly cable C causes stability problems) at high frequency near the loop gain intercept/corner frequency

the TPA6120 is so fast that the load isolation impedance can be lower than you normally see in audio power amps which may have only ~ 1 MHz loop gain intercept and at best 30-50 MHz output Q - requiring a few uH inductance to get load C isolation near 1 MHz and beyond

the TPA can have >50 MHz corner frequency and uses GHz Q so the ~10 Ohm series load isolation impedance doesn't have to fully develop until beyond ~10 MHz so < ~1uH inductance can be OK

lossy ferrite bead can be a good technical fit to the load isolation requirement - you may even find some that physically fit the QRV09 10 Ohm output series R smt pad

to avoid the often claimed audio distortion of output load isolating ferrites I spec beads with Isat ratings ~ 10x of the op amp short circuit current

those really concerned by audiophile tweaker's unverified claims about ferrite "ruining the sound" can always use air core inductor and parallel R - old carbon comp with a winding over the body is one option

even John Curl seems to agree that < ~ 1uH series load isolation L doesn't harm "the sound" with dynamic speaker loads

except for the multi-armature high end iem all other dynamic headphones have much higher Z than loudspeakers, and usually only a single bass impedance peak followed by a slow high frequency Z rise due to coil inductance - this makes their response very insensitive to small series load isolation L

I can't see worrying about the special impedance requirements of multi-armature iem when drive level mismatch, amplified input noise caused hiss from a general purpose amp will be audible - these special iem require a specialized amp design

Last edited by jcx; 5th April 2011 at 07:50 PM.
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Old 5th April 2011, 07:33 PM   #75
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I've made a CF bastard child of the LME49600 reference design using LME49713 (and minus the LME49600). The person I built it for liked it better than the LME49600 I had previously built for them.

Plan to build myself another one soon. I also made a layout that allows switching the LME49600 in/out of the op-amp feedback loop with a jumper. Curious to hear what kind of difference it makes.
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Old 5th April 2011, 07:43 PM   #76
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Thanks JCX. I'll look for a 1 uH (if you're confident in that value?) SMT inductor that might work and perhaps could even be "SMT stacked" on top of a 10 ohm SMT resistor to damp any resonances (ultrasonic, RF, or otherwise). I can also test for inductor saturation issues with the dScope and my Agilent scope.

I agree kludging in an air wound inductor on an old-school carbon resistor would be the audiophile approved solution. But given how fussy the TPA6120 is, that might create other problems with parasitics, longer lead lengths, etc.

We have some time, so if you're bored, please feel free to toss some values in whatever sims you already have of the TPA6120 and see what you come up with for the minimum L? I can also order multiple values and test stability, phase shift, saturation, etc. starting with the smallest and moving up if needed.
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Old 6th April 2011, 12:52 AM   #77
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FYI, I calculate the parts cost from US suppliers for the QRV09 at about $40 - $55 depending on how many different suppliers you want to use, shipping costs, etc. And the PCB is about $20 with shipping from Peranders. So that leaves a volume pot ($2 - ??) head phone jack ($1 - ??), input connectors ($1 - ??) and a line cord/AC input socket ($2 - ??). So that's about $80+ or so... add another $20 - $50 for a decent enclosure and you're at $100 - $130+ for the total project depending on how fancy you want to get with the off board stuff and enclosure. You could save about $5 with the LM4562 vs the AD8610.

That's within $10 of the National reference design I was considering with the same power supply.

And it looks possible, at least on paper, to solve the QRV09's 10 ohm output impedance issue without a very clean modification for those concerned about that.

JCX, a quick follow up on the above... It looks like 500mA is about the most saturation current you can get in a 1 uH 0805 SMT inductor (rated currents go up to about 1.2 amps but that's well into saturation). A typical part is the Taiyo Yuden CB2012T1R0M. That's not exactly a huge margin but should work OK. Again, the right testing will show if there are any issues. I'm hopeful stacking an 0805 inductor with a 10 resistor will do the trick.
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Old 6th April 2011, 01:07 AM   #78
agdr is offline agdr  United States
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Originally Posted by RocketScientist View Post
So, in summary, I have both the Chinese HA TPA6120 DAC and the Peranders QRV09 board on the way. At this point it seems to make the most sense to see how these perform before going much further with a LME49600 design.
I would tend to agree. I think I've lost interest in National's design at this point. I'll be really curious to read your reviews of the TPA6120 HA and P-A's board.
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Old 6th April 2011, 05:39 AM   #79
jcx is offline jcx  United States
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0805 is ludicrous size for something that may see short circuit conditions

from what I can see of the QRV09 schematic thumbnail it looks like you could use the thru hole jumper positions to put a leaded bead core (hair pin bend the lead to fit) in series by shorting one 0805 pad?

I would like to see the Idc impedance curves but the 5A Steward 28L0138-10R may work

I much prefer lossy bead core in this app - wound "inductor" may have too low self resonant frequency and unnecessary Rdc

Last edited by jcx; 6th April 2011 at 05:42 AM.
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Old 6th April 2011, 02:11 PM   #80
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JCX, I think the output impedance jumper block is a useful feature that more headphone amps should have. If someone, for example, has two or more different headphones (I know lots of people with closer to a dozen) and one is designed for zero ohms and the other the IEC 120 ohm standard, they can use the jumper to have their cake and eat it too. Obviously you could also install a switch off board but it's an elegant solution to have the jumper.

So if you meant using those holes, that's a trade off but could work. You mentioned "thumbnails"? Oddly, Peranders has removed the documentation link from the wiki. I'll email it to you so you have the full size images in the PDF.

The datasheet for the CB2012T1R0M 0805 part is here:

The self resonant frequency is 100 Mhz and the DCR is only 0.15 ohms which is plenty low. The saturation current is 500 mA and the rated current is 900 mA.

I agree 500 mA is a bit marginal but it's likely close to the real world current output of the TPA6120. The TI datasheet is rather vague about what conditions it will put out 700 mA (they don't specify the load--is that short circuit current?). And, in real world use, even if it can manage more than 500 mA it's unlikely anyone will need that much current.

On 15 volt rails TI specifies a 12 volt output swing into 32 ohms. That's a 3 volt drop from the rails. So with the QRV09's 12 volt rails, you'll get about 9 volts peak into 32 ohms. That's 280 mA peak of peak current.

Into 16 ohms, It would have to manage better than 8 volts. And on 12 volt rails, I'm betting that's about the limit of the TPA6120. And, regardless, that's 2000 mW!!!

So, real world, with any load over 18 ohms you cannot saturate the 0805 inductor no matter how loud someone wants to listen. And even with lower impedance (or highly reactive) loads, I suspect the TPA6120 will clip at very close to the same point the 0805 inductor saturates. And considering 50 mW into 16 ohms is well into hearing damage territory with any low impedance headphones I know of, even if you multiply that by 10X, you still get 500 mW which the inductor can do with ease.

But, obviously, someone could certainly use a bigger inductor as long as the physical wiring/mounting didn't create a TPA6120 stability problem. Using the jumper block through hole points should work but you either give up on the selectable impedance feature or have to move it off board.

I'm starting to feelthis should be in the Peranders QRV09 thread... but here is fine for now. I don't want to step on any toes in the other thread. Not everyone agrees with my views about output impedance but the trend seems to be towards "zero ohm" for a lot of good reasons. See:
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