National LME49600 Reference Design Project

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There have been several threads here talking about the National LME49600. The biggest is this one:

The Wire Ultra-High Performance Headphone Amplifier

I'm checking if there's any interest from others in at least PC boards for a LME49600 design. I suspect most got their itch well scratched with the above project, but I wanted to at least ask.

For those not familiar with it, the LME49600 is an ultra low distortion output buffer that National pairs with one of several of their ultra low distortion audio op amps for a complete servo controlled headphone amp. The design can be found in the LME49600 datasheet here and in their application note AN-1768:

National LME49600 AN-1768 Application Note

The design I have in mind would be virtually identical to the one in the app note above with perhaps a few useful additions. It would be significantly less expensive than the OPC "Wire" version as instead of 6 expensive op amps you only need 2 dual op amps (there's no issue sharing the servo with the gain stage for each channel in a single package as National did in their reference design). And the dual parts are barely more expensive than the single versions. So that eliminates almost $20 of op amps.

I also plan to use through hole parts for everything but the LME49600 (and it's fairly big and relatively easy to solder). The dual op amps used in the National design are available in an 8 pin DIP and could even be socketed for those who like to experiment with different op amps.

I intend to fully measure the design with a Prism dScope III, conduct ABX blind listening tests, and use AudioDiffmaker with my Benchmark ADC1 as well as analog (i.e. "Hafler Distortion Test") audio differencing to evaluate the design under real world conditions.

Part of my interest in the LME49600 is to serve as the "ultra low distortion" entry in a comparison between genres of headphone amps. I hope to conduct measurements, blind ABX listening, and audio differencing on the National LME49600 reference design, a popular lower feedback discrete Class-A design (i.e. Gilmore/Kumisa/Balkishan), and a lowly single IC "cmoy" design using a decent op amp like the OPA551.

If the National reference LME49600 amp is of interest for others wanting PC boards, please let me know? If there's enough other interest it will change how much effort I put it into the PCB and documentation as well as my purchase quantity for the PCBs.

If there's a lot of interest I could also consider a group buy of parts and/or even having the boards fully made at the assembly contractor I use. If the volumes are really high, then a full SMT version can be done on their mask/pick/place/reflow lines and lower the price.
 
If you want to do a budget version then why not use the Buf634P as that's about half the price of the identical(?) 49600.

Otherwise, my own findings using my ears, are that the plastic chips are grossly inferior to the TO99 version of the 49720 which is very inferior to two single TO99 49710HA.

So unless someone really does not care about quality and needs bargain basement, why sink the ship for a few $ of tar?
 
I'm in! :D I would also likely be interested in a group buy if one occurs.

I've been messing around with the LME49600 for several months, including that headamp circuit in the data sheet. I did see that National ap note on the eval board, which contains the details of the parts specifications.

Unfortunately I saw the "wire" amp thread too late to order, but opc put together a fine amp there. My uses are mainly for unbalanced input through.

My random thoughts on some of the comments and discussion so far (in the "wire" thread about this)...

I've read several postings of BUF634 vs. LME49600 and the LME seems to consistently come up as the winner. I have a BUF634 headphone amp, the previous version of the HA-INFO NG98. Sounds good, but I think the LME sounds better, just imho.

See my post in the "wire" thread about paralleling two LME49600s per channel. I've read at least one posting where that was tried and (subjectively) sounded better. The BUF634 apparently has 10R on the emitter of each output transistor. The LME49600 shows a similar resistor but I've yet to read a value. Paralleling may get that impedance down looking into the output port. At least having the option for the second chip per channel would be nice. For what it is worth, that Jim's Audio LME49710+LME49600 board on eBay (no DC servo), which seems to suffer from layout problems (noise) by some posts I've seen, has two LME49600s in parallel per channel. Would be interesting to hear it with servo and with a really good layout. Some thing to experiment with...

I have read those posts before by the (ex) National guy saying the metal can versions sounded better. I think he wrote that he was about to look into it when the staffing "bomb" went off there at National. From that little bit of info in the post I would agree that it may be good to socket the LME49720 so that a metal can version could be stuffed in. Those HAs are about $20 last I looked. Pricey, but hey still in the ballpark if they make better sound, and it would be optional anyway. But I would definitely agree that the circuit board should be able to use just the simple, cheap, plastic DIP LME49720 as an option regardless. A lot of the "sounds better" stuff seems pretty listener-specific, even if it is backed up with low distortion, low noise, etc tests. So if someone likes the sound of the low cost part, more power to them in my book.

Having the option to use 2 metal can LME49710s instead of the 20 would also be interesting, but I can't think of a good layout way to make that work offhand, while still preserving the ability to use one plastic dip 20. I'm never a fan of super packed-in circuit boards - I'm kind of heretical there and like some space between my components even if it makes the board larger - so maybe one solution is to put in holes for for the DIP socket and the can in parallel, then populate one or the other.

I've seen a few posts where people building the headamp circuit in the LME49600 datasheet wound up with 10-14mV offset on the output despite the servo circuit. Mine was about 8mV. The "wire" amp didn't seem to have that problem from what I read in the thread. I haven't tried the theory yet, but maybe much more accurate (as in 0.1%) resistors in a few crucial places could solve that problem, and/or offset trim for the input op amps. I've actually read at least one post where someone thought the VR1s were offset trim. :)

The datasheet circuit shows the various chip's decoupling capacitors going from rail(s) to ground. My best understanding is that is not best practice anymore, rail to rail is preferred as close to the chip (?). I'm sure someone will correct me if this isn't the case.

There actually are heatsinks for the TO-263 with solder rails that solder down next to the chip

Digi-Key - DA-T263-101E-ND (Manufacturer - DA-T263-101E)

I have a few here but haven't tried them yet. They may reduce that foil surface area needed for the LME49600 chips. The formula is given in the datasheet, of course. If using two paralleled LME49600s per channel the power would be divided and the heatsinks might really shrink things down.

Good luck with the project!
 
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Good point - I have low impedance (44R) Shures. For a high(er) impedance headphones the buffer paralleling may not add anything. But again... you could make it optional on the board. If you had pads for two buffer chips per channel the builder could decide to populate one or two. I'm a fan of population-optional board designs to allow one board to cover many different near-similar configurations.

My comment about "sounds better" being listener specific also applies to me. :) My comment above about the LME49600 sounding better than the BUF634 is pretty meaningless since I've heard them in different amps with different front ends and different layouts. I notice the pinout is the same for both chips in the 5 pin surface mount versions and most of the parameters are similar for either. How about a layout that would allow either buffer chip to be used at the builders discretion? IanAS has a good point about the price difference. If the BUF634 sounds good to any given builder, then great!

I have also read about the stacking option IanAS mentioned with the BUF634s in the DIP version - been meaning to try it myself in my NG98 - and that is really just paralleling those chips, of course.
 
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I'm in! :D I would also likely be interested in a group buy if one occurs.
Great!

Unfortunately I saw the "wire" amp thread too late to order, but opc put together a fine amp there. My uses are mainly for unbalanced input through.
I agree he did a good job. Had I made it in on this buy soon enough, I probably would have just bought one of his. My Benchmark DAC1 and Squeezebox Transporter both have balanced outputs. But I think balanced sources are less common for the majority looking for a reasonably priced headphone amp.

I've read several postings of BUF634 vs. LME49600 and the LME seems to consistently come up as the winner. I have a BUF634 headphone amp, the previous version of the HA-INFO NG98. Sounds good, but I think the LME sounds better, just imho.
Good to know. They're pin compatible so anyone would be free to experiment if they want, or save a few bucks and use the BUF634F. I might even be able to work the TO220-5 through hole footprint into the layout but it would require a heatsink.

See my post in the "wire" thread about paralleling two LME49600s per channel. I've read at least one posting where that was tried and (subjectively) sounded better. The BUF634 apparently has 10R on the emitter of each output transistor. The LME49600 shows a similar resistor but I've yet to read a value.
Yeah, the vendors often are intentionally (and understandably) very vague with their "equivalent circuits". But I do wonder to what degree random pairs will play nice and reasonably share the load. That would be something that's easy enough to explore experimentally with some very low value series resistors to enable reading differential currents on a scope.

I generally like the idea of adding the pads for parallel output devices for those with especially low sensitivity or low impedance cans who like their music loud.

How well they share current might be subject to random matching, temperature, etc. It's also possible National may use different fab lines which may have slightly different processes (especially over time). So, at the least, it would be best to use devices with the same date/lot code. The good news is the devices won't self destruct even if they don't share the current at all.

I'm also a bit concerned about the loading to the gain stage with paralleled buffers. You get roughly twice the capacitance and half the impedance. It's a direct connection to the gain stage with no series resistance and paralleling will change the open loop transfer function. But the effects should be easy enough to measure.

I'm a huge fan of "measuring as I go" and have all sorts of custom tests and scripts predefined in the dScope software. It's not unlike writing software. I usually get a much better end result by testing regularly along the way.

For what it is worth, that Jim's Audio LME49710+LME49600 board on eBay (no DC servo), which seems to suffer from layout problems (noise) by some posts I've seen, has two LME49600s in parallel per channel.
Yeah, I found that design and his "modular diamond buffer" board earlier on eBay. Neither is the National reference implementation (as you said no servo), and both seem kind of "random". So I'm not that surprised people have had problems. I've yet to measure any audiophile DIY eBay PCB/kit from Asia that didn't have at least one serious flaw. I don't understand why more of them don't just copy the reference designs and at least try to follow the reference PCB layout?

The KECES amps out of Asia using the LME49600/49710 pair sell for around $400 and have received some fairly good reviews. But when I read you can upgrade to a "pure silver" IEC AC power socket for "even better dynamics" for only $90 more it's hard to take "KECES" seriously and trust their design (and business) objectives.

I will confess I've thought about buying one of the cheaper eBay headphones amps in a nice KECES-like custom enclosure, throw away the main PCB, and just use the chassis, transformer, hardware and other bits. :)

I have read those posts before by the (ex) National guy saying the metal can versions sounded better. I think he wrote that he was about to look into it when the staffing "bomb" went off there at National. From that little bit of info in the post I would agree that it may be good to socket the LME49720 so that a metal can version could be stuffed in.
That would work. I also might be able to squeeze some extra pads in with the DIP layout for those who want to solder in the TO-99 version without mangling the lead wires as much.

A lot of the "sounds better" stuff seems pretty listener-specific, even if it is backed up with low distortion, low noise, etc tests. So if someone likes the sound of the low cost part, more power to them in my book.
Yeah. Personally I think much of it is purely psychological. But, if it doesn't cost much extra for more piece of mind, and it brings more listening pleasure, why not?

Having the option to use 2 metal can LME49710s instead of the 20 would also be interesting, but I can't think of a good layout way to make that work offhand, while still preserving the ability to use one plastic dip 20.
I would be reluctant to share the gain stages between the two channels in a single package (despite that fact it's done all the time in even expensive high-end audiophile gear). But sharing the servo and gain stage within a channel is much less an issue. There's just not much going on with the servo for it to cause any problems that I can see. And National's reference design helps prove that out.

There are various links around the web of golden ears who heard the national LME49600 reference board and gushed praise all over it. The National guys were seriously obsessive with those designs. And if there was a significant performance gain from using single amp devices, they probably would have done so. It's a $270 board so I don't think cost was much of an issue when they can source the op amps internally.

I'm never a fan of super packed-in circuit boards - I'm kind of heretical there and like some space between my components even if it makes the board larger - so maybe one solution is to put in holes for for the DIP socket and the can in parallel, then populate one or the other.
I agree--especially if there's the possibility of modifications. It's nice to leave room for bigger caps, etc. And it also helps with crosstalk and sometimes other performance areas (although large layouts can also create stability problems).

I've seen a few posts where people building the headamp circuit in the LME49600 datasheet wound up with 10-14mV offset on the output despite the servo circuit. Mine was about 8mV. The "wire" amp didn't seem to have that problem from what I read in the thread. I haven't tried the theory yet, but maybe much more accurate (as in 0.1%) resistors in a few crucial places could solve that problem, and/or offset trim for the input op amps.
Hmmm... that might need some investigation. < 15 mV doesn't bother me that much but I know there are some cans that are very sensitive. What does bother me is a fully direct coupled design with no dc blocking caps anywhere and no protection--especially when the source could be most anything. I have to wonder if some of the offsets people have seen are from their source, RFI (especially with the input floating), poor grounding schemes, etc?

As was discussed in the Wire thread, DC protection might not be a bad idea and that's one of the options I was considering at least laying out if the extra real estate wasn't a problem. The protection scheme could take many forms depending on cost and real estate. It can be done without any series relay contacts in the signal path. Another option might be an LED that just warns of excessive offset.

I've actually read at least one post where someone thought the VR1s were offset trim. :)
It often happens when sharing DIY projects. That's one reason sites like Headwize and AMB are so great is they form a better repository of information--more of a wiki--than some thread with 80 pages and 800 posts in it. Few want to wade through that much text.
The datasheet circuit shows the various chip's decoupling capacitors going from rail(s) to ground. My best understanding is that is not best practice anymore, rail to rail is preferred as close to the chip (?). I'm sure someone will correct me if this isn't the case.
I think that partly depends on the PCB, your grounding scheme, and the device itself. I'm not sure there's any one right answer. To me it goes back to "measure as I go". The right kinds of measurements often show the subtle differences from various decoupling options, grounding options, etc. That usually makes it easier to figure out what's best for a particular design rather than following some generic rule of thumb. Of course if they all yield identical measurements, then you fall back on audiophile legend.... ;)

There actually are heatsinks for the TO-263 with solder rails that solder down next to the chip
Calculating real world power dissipation in a Class-AB output stage is always a bit of a dice roll. In this case it's even worse as different power supply voltages might be used, the loads will vary widely, etc. But even when all that's known, different music still is a big variable with radically different peak to average ratios. I trust the engineers at National did their homework but they do hint that extreme conditions may limit the current available due to high temps. It's easy enough to torture test the finished design and monitor the tab temp and outputs for sine of thermal induced current limiting. But I agree heatsinks, and/or parallel devices might help shrink the design.

Thanks again for the great ideas! I would encourage others to join in. I've done searches here on LME49600, etc. but I'm sure there's more I've missed, haven't thought of or perhaps wrongly dismissed.
 
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I've got a few of those in my parts drawer and it's always good to put them to use.

I've tried one design, with a simple board from Ebay, that works quite nice but it's got a bit poor grounding scheme. Uses metal-can LME49710 version of the opamps.
EDIT: Right, the one you're talking about up there. I get about 3-5mV DC offset with that one.

With two of those buffers in parallel it can drive small speakers without issues however, I'm not sure there's any benefit of paralleling those. They really mean business.
 
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chip's decoupling capacitors going from rail(s) to ground. My best understanding is that is not best practice anymore, rail to rail is preferred as close to the chip (?).
I've done both here on this sound card. 100uF Polymer caps from ± rails near the chip(s) to ground and a 0.33uF//0.022uF from rail to rail.

The 0.33 increased the transient response, the 0.022 took away a coarseness. The 0.33 also raised the perceived loudness from the lower mid range downwards, improving (with that system) the tonal balance to a warmer more 'musical' balance.

One 0.33uF was a lot better sounding than three 0.1uF of the same cap type. Also tried 1, 2, 4 and 5 of the 0.1uf and also the 0.33 with 2 x 0.1uF. Up to 0.5uF sounded wrong, dull somehow, soft, blurred. Less than three of the 0.1uF's and there was not as much increase in warmth or transient. A single 0.33uF was cleaner, faster, better transient edges.
 

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Gradually developed it from a good sound card into an outstanding one. :)

Every change carefully done with extensive listening tests at every position of capacitor of chip change. I didn't just throw a bunch of caps onto it. I tried standard and NX Black gates in most positions and preferred the Nichicon Polymers, better more informative bass. Very good transient response, very clean. The BGs bass was larger but muddled.

http://www.avsforum.com/avs-vb/showthread.php?p=19865855#post19865855
I posted further info lower down and on the next page.

The sound card is a work in progress. I have some Vishays to throw on in place of the surface mount chips and also, just in case it helps, replace the chip caps with polystyrene, much larger but I think can be fitted.

Then I thought I'd try and fit a couple or four LME49600, or eight if in push pull, or BUF634 and what ever is needed, to make a headphone output onto a daughter board. Or, a multiple transistor output stage with a Vishay for each emitter resistor.
 
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Oh lord, what have you done to the poor Auzentech ;(
To each their own. As I said earlier... if it puts a bigger smile on your face... :rolleyes:

I'm thinking of spinning a quick and dirty board to use as a test mule to explore things like the parallel buffers. Besides the sighted listening tests, the only engineering reason I can see for paralleled buffers might be thermal. National implies at high temps the LME49600's current limiting could be a problem. Adding the heatsinks Agdr mentioned might be another solution if that excessive dissipation proves to be a real world problem (increasing the foil area only helps so much--especially with 1 oz copper). And as he suggested, it might just be a matter of altering the footprint/pads to accommodate optional heatsinks.

I plan to make measurements both ways (single device and paralleled devices). If nothing else that should at least yield a bit more objective information for others to base their decisions on.

So... dare I bring this up? I'm curious what others think is best for the power supply? National uses batteries in the reference design and I'm guessing most don't think 9 volt batteries are ideal. The main options I'm considering:

  • Just do an amp PCB with pads for the DC rails and leave it up to the user how they want to power it.
  • Include a relatively simple power supply on the board that accepts AC from a transformer. Users who don't want to use it can simply not populate the components.
  • Design it around a $35 external "brick" multi-output DC power supply such as the Meanwell P25A Series with a mating connector on the PCB.
  • Provide for an on-board split rail DC-DC converter module so it will run from a variety of inexpensive DC wall transformers or other DC sources and use a standard 5mm/2mm DC barrel power connector on the PCB.
Any of the above allow for using an off-board elaborate power supply if desired. All but the first choice provide a convenient single board solution. IMHO, with the right PCB layout there's little advantage to putting the power supply on its own board. It just adds expense and hassle to the construction.

The Meanwell power supply comes with a DIN connector and the PCB could accommodate a mating connector making for a very simple plug-and-play solution that's also fully "agency approved"--no wiring, nothing to mount to the chassis, little to go wrong, no exposed dangerous AC, etc. The same is true of the last solution with a DC wall transformer.

It also depends on what voltage rails a person wants (how much output swing you need from the amp). The Meanwell and DC-DC solutions would already be regulated and could just be LC filtered to further lower the noise and ripple. Or, the rails could be regulated again (say from +/- 15 down to +/- 12 (LDO) or +/- 9 volts (7809/7909) with on-board regulators at the expense of some output capability. The National board works very well with 9 volt rails.

The Meanwell triple output supply also has the benefit of a 5 volt supply to operate things like DC protection relays and associated circuitry keeping the split rail supply "pure" for just audio use. The same could be done with the on board DC-DC solution. The raw DC input could power the protection circuit.

The last two solutions also keep AC power completely out of the amp which should mean less hum and noise.

For those skeptical of switching supplies and/or DC-DC converters, it's worth pointing out a lot of high-end audio gear is now "going green" and using switching supplies. And some well reviewed DACs (like those from HRT) use DC-DC converters to generate split rails from USB power. Properly used, switching sources can provide less noise in the audible range than conventional linear supplies. But I know some audiophiles have a psychological bias against them.

I plan to size the board to slide into a reasonably priced enclosure such as the Hammond 1455Q and also have mounting holes so it can be easily mounted in other enclosures.

For a solution with external DC power, all the connections could likely be kept along one edge to require only one panel to be machined and minimize point-to-point wiring. Anyone could skip installing one or more of the PCB mount connectors and use any sort of chassis and wiring arrangement they wanted.

Overall I like the idea of a self contained board that's fairly usable (like the National eval board) "as is" without a bunch of Rube Goldberg wiring required. The last 2 power options would provide an elegant plug-and-play solution without needing an enclosure, off-board connectors, etc.

Thoughts?
 
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Besides the sighted listening tests, the only engineering reason I can see for paralleled buffers might be thermal
And to lower the output impedance.

Thermal is not an issue is it? 'The Wire' had no heat sinks. If it becomes an issue, then use one, or a bigger one. The solder down tab is about saving money in large scale production.

My Senn HD650 are 300 ohms only in some places. Nearer to 500 ohms in others. I don't really know, but I saw some graphs which suggested that the higher the output resistance, the quieter the mid bass and treble as those are the 650's higher impedance areas. Output Ohms ÷ coil Z = Less current at the higher Z areas? That's probably not what's going on.

Maybe at say 10 Ohms output resistance, there is no further benefit. I don't know. With power amps, the lower the better.

I've read a few times that paralleled output devices produced more and better bass on headphones. Just the same as power amps.

I plan to make measurements both ways (single device and paralleled devices). If nothing else that should at least yield a bit more objective information for others to base their decisions on.
That will be interesting :) Maybe it's a back EMF thing. Lower output impedance reducing the effect on the feedback.
 
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RocketScientist: Good work! You are being thorough. That is always a good thing. :)

I finally remembered where I had seen that 10R output resistance figure for the BUF634. From this Burr Brown app note on using an op amp with the BUF634

http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sboa065&fileType=pdf

Some good stuff in there although they discuss a 50 ohm output video amp. They go through some math with one op amp powering 3 BUF634s in parallel. From page 2, "The output resistance of the BUF634 is about 10 ohms."

I'm also a bit concerned about the loading to the gain stage with paralleled buffers. You get roughly twice the capacitance and half the impedance. It's a direct connection to the gain stage with no series resistance..

Good thought! From that app note, which also directly coupled the op amp to all 3 paralleled BUF634s:

"No series resistors were used between the output of op amp A1 and the buffer inputs since they would form a low pass filter in combination with the input capacitance of the buffers. Any phase shift resulting from this low-pass could cause the entire circuit to oscillate..."

So you are right, no series resistors between stages at least for the BUF634, and double (or in this case triple) the load capacitance presented to the output of the op amp. Something to look into.

Yep, definitely do some tests on single buffer vs. paralleled if you have the time. The results would be really interesting. My expectation would be that 10 ohms in the chip in series with the 44 ohm resistive part of my Shure's impedance over much of the frequency range = not so good in terms of power delivery. But the same with a 300 ohm set of cans, probably no significant difference.

How well they share current might be subject to random matching, temperature, etc.

Now this is really interesting. In that app note they go through the math to calculate the compensation current passed between paralleled BUF634s for the minimum and maximum input offset voltages, given the internal resistances that help balance the group up. Good grief, the BUF634 has a +/- 100mV max input offset voltage. I knew the LME49600 had a typical around 16mV, but its max is +/- 60mV. At those offsets they come up with several mA (3mA - 10mA).

I would agree with your comments about sharing the gain stage and servo in a single package. The servo half should be operating at a very low frequency vs. the gain stage. Shouldn't be much interaction there on the die or the shared power supply leads. And I also agree that is probably why National did it that way - they certainly did have access to chips for that eval board as you noted.

On that DC offset, I agree that 10mV or so is probably not going to affect the headphones much, but why is it there? Seems like that circuit should be able to do better. Probably time for some circuit measurements to find out what isn't working the way it should. Or it is working exactly the way it should and my expectations are off. :)

One thought is that huge +/-16mV -> +/- 100mV input offset voltage for the LME49600 and BUF634. Maybe that is pressing up against the edge of the design to compensate. Might also be interesting to throw the circuit into SPICE and do some sensitivity analysis by varying each part's value a few percent for effect on the output offset.

On that heatsink, I was mainly thinking in terms of reducing the amount of heatsink board foil needed while keeping the chip temp the same. However, you bring up an excellent point. There is going to be some temperature distribution on the foil from the chip out to the pad edge. If it is like most things I would guess gaussian. Adding that heatsink would likely flatten the curve, reducing chip temperature, given the recommended amount of foil (from National). That is probably better. I would take a cooler chip temp over saving some board space any day, and it preserves National's full recommended foil area.

IanAS: I love that skywire picture with the two metal can LME49710s! That works. I'm going to have to skywire a couple of parts myself on a project this weekend.
 
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Hello,
$0.02 worth for grins. For reference I did a perf-board headphone amplifier inside a computer AB switch box. The amplifier was a BUF 634 installed inside the feedback loop of a LM 4562 Op-Amp. This little amplifier built in a couple of hours is the reality standard and for a few dollars in parts beats most in a heads up contest. This design was cobbled from TI example circuits on their page. No ownership here.
I recommend some simple basic amplifier as a reality standard to compare to the product being created here. It is not difficult to build something from a bag of chips.
DT
All just for fun!
 
Hello,
$0.02 worth for grins. For reference I did a perf-board headphone amplifier inside a computer AB switch box. The amplifier was a BUF 634 installed inside the feedback loop of a LM 4562 Op-Amp. This little amplifier built in a couple of hours is the reality standard and for a few dollars in parts beats most in a heads up contest. This design was cobbled from TI example circuits on their page. No ownership here.
I recommend some simple basic amplifier as a reality standard to compare to the product being created here. It is not difficult to build something from a bag of chips.
If I understand you correctly, that's in effect what I'm trying to do. My intent is a relatively low cost design that will at least measure roughly as good or better than most any headphone amp at any price. I don't want to turn this into a silver soldered, $200 power corded, unobtanium wonder.

Also, you may know this already, but the LM4562 is essentially identical to the LME49720 and LME49860. It tends to be cheaper but it's the same, or nearly the same, chunk of silicon as the other parts. This was discussed a few years ago by Audioman54.
 
Thermal is not an issue is it? 'The Wire' had no heat sinks. If it becomes an issue, then use one, or a bigger one. The solder down tab is about saving money in large scale production.
I don't know yet how hot the parts are going to run. The datasheet calls out 3 - 6 square inches of copper per device. I suspect 6 square inches is pushing the point of diminishing returns (i.e. 100 square inches of PCB foil probably wouldn't keep the part much cooler). I suspect, if they get too hot at all, it would be with < 32 ohm headphones that are seriously inefficient playing highly compressed music played at hearing damage levels.

Someone also might set their amp on top of other gear that runs hot, seal it up inside an airtight enclosure, use it outdoors on their deck in the sun, etc. I don't know what sort of ambient temps, ventilation, etc. National was assuming.

And to lower the output impedance.

Maybe at say 10 Ohms output resistance, there is no further benefit. I don't know. With power amps, the lower the better.
Because of the overall feedback, the output impedance of even a single device should be well under 1 ohm as long as it doesn't clip or current limit. Paralleling buffers won't change it much at all. But paralleled devices will give you roughly twice the ultimate current capability. And that could be a valid reason for doing it.

National rates the operating current at 250 mA and the short circuit current at 490 mA both on 15 volt rails. If it can really deliver 250 mA into typical headphone loads, that's plenty for nearly any sane application. But someone with low impedance really inefficient cans who likes it loud and is playing something with really wide dynamic range may hit the current limit on extreme peaks. The data sheet adds:

Thermal dissipation may be the factor that limits the continuous output
current. The maximum output voltage swing magnitude varies with
junction temperature and output current.


The good news is most of the above are easy enough to measure. I can, for example, simulate a worst case load while playing really wide dynamic range music at ear splitting levels and check for current limiting with a single buffer. I can also measure the output impedance of both versions. Etc.
 
I finally remembered where I had seen that 10R output resistance figure for the BUF634. From this Burr Brown app note on using an op amp with the BUF634

http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sboa065&fileType=pdf

Some good stuff in there although they discuss a 50 ohm output video amp. They go through some math with one op amp powering 3 BUF634s in parallel. From page 2, "The output resistance of the BUF634 is about 10 ohms."
Thanks for the link. That 10 ohms is, I think, the effective emitter resistance of the output devices in the BUF634.

Yep, definitely do some tests on single buffer vs. paralleled if you have the time. The results would be really interesting. My expectation would be that 10 ohms in the chip in series with the 44 ohm resistive part of my Shure's impedance over much of the frequency range = not so good in terms of power delivery. But the same with a 300 ohm set of cans, probably no significant difference.
The 10 ohms, in effect, disappears due to the feedback. Any drop is compensated for by the feedback loop. So the actual output impedance should be well under 1 ohm as long as the amp doesn't clip or current limit.

Good grief, the BUF634 has a +/- 100mV max input offset voltage. I knew the LME49600 had a typical around 16mV, but its max is +/- 60mV. At those offsets they come up with several mA (3mA - 10mA).
I need to study it more closely, but on first glance I think what they're talking about is the paralleled BUF634's "fighting" each other with respect to different offsets. Any DC difference in their output will have to be compensated for by the other(s) as the net current must sum to zero if the output voltage is (essentially) zero.

On that DC offset, I agree that 10mV or so is probably not going to affect the headphones much, but why is it there? Seems like that circuit should be able to do better. Probably time for some circuit measurements to find out what isn't working the way it should. Or it is working exactly the way it should and my expectations are off. :)

One thought is that huge +/-16mV -> +/- 100mV input offset voltage for the LME49600 and BUF634. Maybe that is pressing up against the edge of the design to compensate. Might also be interesting to throw the circuit into SPICE and do some sensitivity analysis by varying each part's value a few percent for effect on the output offset.
You're welcome to run a sim if you want. I'm happy to build it and measure what's going on. Generally servos work consistently well until they hit their limit and then things get suddenly ugly. That's why I suspected RFI, grounding, etc.

For those who might not know, RF picked up by audio circuits can easily be rectified into DC by any of the many transistor junctions. The induced DC can look like a valid signal to the circuit. So unexpected DC offset is a rather common symptom of poor RF immunity.
 
If I understand you correctly, that's in effect what I'm trying to do. My intent is a relatively low cost design that will at least measure roughly as good or better than most any headphone amp at any price. I don't want to turn this into a silver soldered, $200 power corded, unobtanium wonder.

Also, you may know this already, but the LM4562 is essentially identical to the LME49720 and LME49860. It tends to be cheaper but it's the same, or nearly the same, chunk of silicon as the other parts. This was discussed a few years ago by Audioman54.

Hello,
OK, hats off, I am with you. That exact same TI pdf is what inspired me.
With a pair of 12 volt gel cells +- 12 volts with the BW pin shunted (15 ma class A) heat was not an issue at the buffer. There was no heat to the touch, with the IR thermometer there were only a few degrees delta t. I was ready to attach heat sinks but they were not needed.
With the BUF in the Op-Amp feedback loop the DC offset is limited to the offset of the Op-Amp only.
A thought about the DC offset, perhaps buy 10 or 20 Op-Amps put them in a jig and select the Op-Amps at the low end of the DC offset range. Save the others for use where there is a blocking capacitor being used.
A servo will add alot of parts and complexity.
DT
All just for fun!
 
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