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Old 16th June 2012, 09:59 PM   #91
agdr is offline agdr  United States
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Default O2 current buffer mod for AKG K550s - input only 80mV

Quote:
Originally Posted by sgrossklass View Post
How do you intend to "unparallel" the 4556 halves without PCB surgery? The two noninverting inputs are tied together after all...
Good thoughts! To unparallel I'm just unsoldering R11 and R18 - the op amp inputs remain connected. So the second half of the op amp on each channel will get a signal, the output just won't go anywhere. Then I'm shorting R10 and R15 on the remaining (connected) sections for zero ohm output impedance.

I was wondering about those large feedback resistors in the RA1 schematic! You are quite right, the result is a CMOY with a gain of 1, the power management circuit, and a really good PCB layout (which is half the battle right there).

I received the K550s today and I have to say I'm impressed! These things totally rock IMHO.

But... I can also see that my input voltage assumptions in the writeup above for the current buffer are too high by an entire order of magnitude. I've measured reasonable listing levels with the K550s at around 60mV to 80mV. That matches up with the "hearing damage warning" on the box, for the wide band characteristic voltage (here: AP High Performance Audio Analyzer & Audio Test Instruments : Service & Support) to produce 94dB, at only 135mV.

Good grief. These things (K550s) don't need an amplifier, either voltage or in many cases current buffer. 80mV(rms) into the 32R is only 2.5mA, easily handled by many standard op amps. No super high current buffer op amps needed here. In fact, with the mod outlined above, LME49720s (or possibly the NJM2068 if unity gain stable) could be used in place of the NJM4556s for lower noise and distortion. RocketScientist points out that the NJM4556s were a bit of a trade off for high current capability that many phones need. The 49720 is good for 26mA or so per channel.

About the only remaining reasons to even use a current buffer with the AKG K550s would be (1) lower output impedance for better damping, if the source Z(out) is not << 32R or (2) if the source really is expecting 10K input, probably like many DACs, where even 2.5mA would be an order of magnitude too much current to supply. But I would think that most any source with a "headphone" output will be able to power the K550s with no amplifier of any kind needed.

Last edited by agdr; 16th June 2012 at 10:03 PM.
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Old 17th June 2012, 06:07 AM   #92
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The O2 has a way of quieting poorer audio equipment's noise and it has a lower output impedance, as you noted, which add up to a better listening experience overall.

If I understand correctly, the O2 has such good performance in part because it divides the audio across two NJM4556 opamps and therefore has lower distortion (for the operating demands of this circuit, at least). I'm also under the impression that lowering the voltage raises the noise floor in the opamps. I don't remember what voltage is optimal for the NJM4556, but I seem to think that it's 12-15V.

One last thought: input impedance imbalance. Is the pot absolutely consistent in terms of input impedance? If it gets imbalanced (possible poor channel matching), won't that have some negative effect on the source?

Moving right along... the next mod I'm thinking up is variable gain but still with the hi/lo switch and the low position being a voltage divider circuit. Thoughts?
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Old 18th June 2012, 03:37 AM   #93
agdr is offline agdr  United States
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Quote:
Originally Posted by ethanolson View Post
If I understand correctly, the O2 has such good performance in part because it divides the audio across two NJM4556 opamps and therefore has lower distortion (for the operating demands of this circuit, at least). I'm also under the impression that lowering the voltage raises the noise floor in the opamps. I don't remember what voltage is optimal for the NJM4556, but I seem to think that it's 12-15V.
You are quite right about the voltage vs. noise! Here is a good recent article: AV: Low-Voltage Audio Products: Power &amp; Noise - Pro Sound Web . I'm assuming though that the noise floor will still be way below audible levels, even if it is up a bit. That is something that would have to be verified with the good equipment though, RocketScientist's dScope or an AP. With any of these measured parameters I'm always of the mindset that if it is below my threshold of hearing, it doesn't exist even if I can measure it. The voltages and currents involved here with the AKG K550s are so low that it might be worth looking into some +/-5V op amps, if any exist with good audio specs.

RocketScientist definitely did some smart things with the O2 - stuff that never would have occurred to me. Separating it into two stages was one. Going from unity (1) gain to anything higher on op amps seems to degrade performance. RS did some of his dScope measurements on the NJM4556 at gain=1 on his website, if I'm remembering right, then again at gain=2 and the numbers decreased. By moving the voltage gain off to a separate stage he was able to use a (lower output current) lower noise, lower distortion chip for the gain (and the chip didn't have to be unity gain stable). Then the current buffer could run at gain=1. So the paralleling of the NJM4556s isn't what helped reduce noise and distortion, I don't believe, that just increased output current. Moving the voltage gain to a separate stage with a better chip was the key. But here we don't need any voltage gain at all with the K550s so one stage with unity gain will do the job.

One good exception to the two-stage design is compromising a little bit to reduce parts count, like AMB did with the Mini-3. Distortion and crosstalk levels may be higher, although I'm still not sure they are above audible threshold levels, but the result is a super-small amp by making the one single op amp do voltage gain and current output. Two stages produced better measured numbers but require more chips and twice the space.

Having that 40.2K resistor across the 10K pot wiper will probably (a guess, haven't done the math) at worst make a 1K or so difference in input impedance with the current buffer mod. So as the pot is turned the input impedance may vary between 9K - 10K. I should plot that out from 0K to 10K on the pot wiper for fun and see what it really is. For sources with fairly low output impedances that shouldn't matter. I haven't done squawk with tube based amp design, so really can't speak there, but I keep seeing mention of tube sources being more sensitive to loading. Maybe it would matter with a tube based source.

Good thoughts about the pot imbalance issue. The natural tracking imbalance that occurs between the two pot sections (left and right channel) will affect any audio device, including the original O2. Even the good analog pots like the Alps or Bourns Pro Audio only track within 20% or so between the sections as I recall. A person would have to go to a step attenuator with 1.0% or 0.1% resistors to do better. So that tracking issue between pot sections shouldn't affect this current buffer any more or less than the regular O2 or any other amp, at least from what I can see now.

Well I spent three hours listening to the AKG K550s last night with all kinds of music. Hard to put them down. These things are fantastic. They don't make my left earlobe sore from rubbing (the K550s don't rub at all) after just 30 minutes as happens with my Shure SRH940s. Headphones shouldn't take off skin!! I also find that I don't need bass boost anymore with the K550s, either the circuit mod or to eq in bass boost in the player, like I've had to do with the Shure 940s.

Last edited by agdr; 18th June 2012 at 04:06 AM.
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Old 22nd June 2012, 05:37 AM   #94
agdr is offline agdr  United States
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Default O2 AKG K550 current buffer mod - DC offset reduction

Here is another interesting issue related to running the new AKG K550 headphones with a unity gain current buffer, like a modded O2 amplifier. With the high sensitivity of these phones - just 60mV to 80mV for standard listing levels - DC offset in an amplifier output becomes a bigger percentage of the swing voltage.

For the typical 3mV DC output offset in the O2 amp with a less sensitive headphone that uses, say, a 1Vrms swing for normal listening levels, that offset is just 3mV / 1000mV = 0.3% of the swing. But with the K550s a 3mV DC offset now becomes 3mV / 60mV = 5% of the output voltage swing and results in a greater chance that the applied DC moving the voice coil off-center will matter. 3mV of DC is still probably within design specs for the headphones - I have an email into AKG to find out - but lower offset can probably be had in a couple of ways.

One way to get lower DC output offset is with chips that have lower input bias current and/or offset voltage specs. Earlier in the mod thread here I did some measurements with the LME49860 replacing the NJM4556 as the output chip(s), the high voltage version of the LM49720, both of which have less than 1/10 the input bias current (10nA vs. 180nA) and 1/5 the input offset voltage. In the actual tests I posted the DC offset voltage was 0.3mV, 1/10 as expected, on one channel and 0.0mV on the other - below the LSB of the meter.

If the LME chips are used the 1R output resistors should probably remain for stability and the sections paralleled, as in the unmodified O2, since the rated capacitance direct-drive capability of each chip section is just 100pF, less than the typical headphone cable. The LME chips also have built in short circuit protection like the NJM4556 so they should also survive use with the TRS output jack.

The other way that should work was posted by Regal earlier in the thread. The PC trace that directly connects the output on each NJM4556 chip section to the inverting input, to form the unity gain buffer, can be cut and a 40.2K resistor soldered in. SMD resistors would probably work well. With this mod the input bias current into the inverting input should cause roughly the same voltage drop across the new 40.2K resistor as the non-inverting does with the original 40.2K resistor. The two voltage drops should then roughly cancel out in the differential input.

Last edited by agdr; 22nd June 2012 at 05:54 AM.
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Old 22nd June 2012, 08:27 AM   #95
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with sensitive phones my experience is DC offset can be a significant impact to SQ. It makes sense when you think about how a driver works, and your numbers show it is a significant deflection, has to affect fr resonance, waterfall plot, etc.


I have to admit that my O2 has turned from serving headamp amp duty to tool duty. I just don't like the O2's treble as much as my tube SET. And where my SET fails (power for orthos), the O2 does fails also.

The O2 design is really much like an instrumentation amp. With a multimeter I use it to easily make great soundcard measurements of audio gear with up to the ~9v rms input clipping) this allows measuring say a small power amp volume cranked up to 20w into 4 ohms. So it serves as a almost negligible distortion and noise buffer with the Asus soundcard+art as the signal generator. Poor man's dscope.
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Old 22nd June 2012, 08:57 PM   #96
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I put in an email to RocketScientist/NwAvGuy about the DC offset issue and use of my Shure E2c IEMs and his response is as follows:
-------------
Yes, you can experiment with cutting the 4556 feedback traces and using a resistor of around the same value as is on the other input pin. Unfortunately you need a resistor for each op amp section (4 total) which there just wasn't room for on the PCB while still maintaining an otherwise decent layout. If you only need one op amp section per channel that simplifies things a bit.

If you don't want that big of "hack" you can also just drop the value of the input resistor to around 10K which will proportionately lower the offset and raise the -3 dB cut off frequency which is current so low it can move up with no harm.

But, to be honest, I really don't think 3 of 4 mV of offset is anything to worry about even with Sure IEMs. The FiiO E10, for example, has 13 mV in low gain mode and over 30 mV in high gain mode.

The "driver offset" will be negligible at < 4 mV. I don't think it will significantly change the driver performance. I have a "fake ear" (damped vinyl tube for my calibrated lab microphone) that I use for some headphone tests. I played around briefly with offsets up to tens of millivolts and didn't notice any difference in frequency response or distortion performance from my IEMs. I didn't test everything but I really doubt 4 mV will make any audible difference.

The reason is the linear range of the IEM driver is much greater than the offset. So you're just ever so slightly shifting the operating point of the driver and the only effect will be an ever so slight change in the overload behavior as you approach the limits of the driver (very slight asymmetrical "clipping" for lack of a better description). But, we're talking such a small amount the mechanical tolerances of the drivers probably create bigger asymmetrical differences. And we're also talking about hearing damaging volume levels.

-------------

So... I'm still not content to have an imperfection, so I'd like to get something better worked out. I suspect a trim resister on each output opamp and tuned in manually would be the most ideal. It might be easier to dial in with series resistance and a lesser value trimmer.
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Old 23rd June 2012, 04:39 AM   #97
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Default O2 current buffer offset reduction mod tests

Well, I did it. Mod built and tested below, complete with pictures, using both regal's feedback resistor and a LME49720 in the same circuit. Either way worked well, below.

regal: Interesting thoughts about the offset! I agree the O2 would probably make a good instrumentation amp, given the low noise floor and distortion. With the coupling caps removed it could even be used with DC things like strain gauges. I'll bet it would be very linear from that -9Vdc t0 +9Vdc.

ethanolson: thank you for getting RocketScientist's input!

This is using an O2 that is built up the standard way with the two output op amp sections still in parallel on each channel. For just testing offset reduction here I didn't want to also mix in unparalleling them yet.

I replaced one NJM4556 with a LME49720. For the other one I built up regal's feedback mod, putting a 40.2k resistor between pins 1 and 2, and another between pins 6 and 7 after disabling the existing PCB short between those pins for unity gain. In actual practice that turned out to be a bit harder than it seemed, since the PCB traces that connect 3 of those 4 feedback loops (both 4556 op amps) are on the top of the PC board, covered up by the IC sockets if you all ready have the board built, which would require removing sockets to cut the traces.

Instead I bent pins 2 and 6 on a NJM4556 up and soldered the two 40.2k feedback resistors on top of the chip. Bending the pins up disconnects them from the PCB traces (they don't go into the socket holes), of course. The first picture below is the NJM4556 with the two pins bent up. The second picture is the resistor from pins 6-7 crimped on before soldering, and the third picture is both resistors soldered on. In the final picture that is an LME49720 in the rightmost NJM4556 position, and of course the modified NJM4556 in the left position.

The LME49720's DC offset actually measured 0mV, below the LSB of the meter. The the NJM4556 measured 1.5mV, down from the 3mV I measured before modding it, so the feedback resistor modification worked well.

The interesting thing was the listening test. I could hear (completely subjective, your mileage may vary!) that one channel seemed to have a bit better sounding bass than the other. I assumed the good bass was probably the NJM4556, since it had the best current capability. So to save from tracing wires to find out which channel was which chip, I just pulled out the LME49720 chip and powered it up expecting the better bass channel to be remaining. To my surprise that was the wrong channel. The LME49720 was the one with the better sounding bass. So I pulled the modified NJM4556 out and stuck in another LME49720 and listened for an hour. That is completely subjective though, so take it with a gain of salt. The offset measurements are quantitative and what count here. The LME49720 would not work at all with many phones since it isn't capable of a lot of current. It is just sort of luck that is will work here due to the low current needs of the sensitive K550s.

So in summary either method worked well for reducing the DC offset, using a LME49720 or using regal's unity-gain feedback resistor mod. If someone was building an O2 up from scratch it would be easy enough to cut those three top traces (and the one on the bottom of the PCB) before the NJM4556 sockets were soldered on, then the 4 feedback resistors could be soldered on the bottom of the PCB.
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Last edited by agdr; 23rd June 2012 at 04:53 AM.
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Old 23rd June 2012, 11:15 AM   #98
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Would it be possible to put a pot in the 4556
feedback loop and tune for 0 offset and then
measure the pot resistance ? The 4556 data sheet
shows a minimum input R of 300k and a typical
of 5m, quite a wide range of resistance and
current. Seems like every opamp is going to need
a different R in the feedback path to exactly null
out the offset. If the offset is stable after being nulled,
wouldn't that allow safely reducing or eliminating
the output R's for the paralleled buffers ?
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Old 23rd June 2012, 08:00 PM   #99
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Quote:
Originally Posted by agdr View Post
The LME49720's DC offset actually measured 0mV, below the LSB of the meter. The the NJM4556 measured 1.5mV, down from the 3mV I measured before modding it, so the feedback resistor modification worked well.
So, was it a 1.5mV swing or a 4.5mV swing? Pardon my ignorance on this. I assume the former, and if so, what's it going to take to null it out?

Also, once the magic number is determined, is this something that can be employed universally on the NJM4556 and get extremely close to null every time?
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Old 27th June 2012, 07:53 PM   #100
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bada bing and ethanolson: good question! The DC offset remaining is 1.5mV. The feedback resistor took care of the offset voltage created by the NJM4556 input bias currents through both inputs (inverting and non inverting) passing through those 40.2k resistors, then reflecting back to the output of the buffer stage. Looks like that was about 1.5mV worth of the total 3mV DC offset.

But... that still leaves the inherent input offset voltage of the op amp itself, which for the NJM4556 is given as 0.5mV - 6.0mV in the data sheet. Looks like it may be around an additional 1.5mV in this case. Some op amps have "null" pins to hook a pot between to null out the differential amp. In this case it could be done externally by putting a pot between the power supply rails, with fairly large resistors between the pot and each rail to reduce the full wiper range to around 0.1Vdc (5mV per turn on a 20 turn trimmer pot), then a resistor from the wiper to the inverting input to supply a small DC offset voltage. Then the pot can be turned to null the output to zero, and it should stay relatively stable with just a bit of temperature drift. See figure 1-42 here, along with the associated writeup: Op Amp Applications Handbook - Walter G. Jung - Google Books (no R1 in our case). The LME48720 was 0.0mV since the input offset voltage is also much lower than the NJM4556, along with the input bias current being much lower. Took care of both issues.

I'm probably not going to mess with the additional circuitry to fully null, since I'm really finding an amp just isn't needed at all with these AKG K550s and the sources that I have. If I do I'll pop in the LME49720s in place of the NJM4556s. I've spent some time now A/B-ing between a DAC + O2 current buffer and just the headphone output jack on a Sony laptop. I'm finding the sound difference so small I can just eq it out. Not worth the hassle of even firing up the amplifier. But if someone does add the circuitry to null the NJM4556 out the rest of the way, please do post! If the additional circuitry was added for a full null, I would still leave those output resistors on the NJM4556 if they are to remain in parallel, but they could probably be reduced to 0.5R or even 0.1R. But with the low current needs of the K550s I still think a single half of the NJM4556 would do the job - no need to parallel - in which case the output resistor on the remaining section could be eliminated.

Last edited by agdr; 27th June 2012 at 08:05 PM.
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