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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
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Old 8th September 2015, 01:13 PM   #11
Turbon is offline Turbon  Sweden
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Listened the previous night to my very original O2 and it struck me - damn how good it sounds . Well, I will not put the oda in place before I decide on the knobs... Risk is that I will be handling raw pots ever after... 15-17mm diam/20mm lenght. AL with red line or small red LED (might maybe do this one myself as I have bought two old Unimat lathes). I will turn them into one CNC lathe and one CNC mill. But as I need the knobs earlier - please give me ideas on what would work with the dimensions stated .

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Old 8th September 2015, 04:13 PM   #12
agdr is offline agdr  United States
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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
Quote:
Originally Posted by adydula View Post
Also would like to listen to it in comparison to my fairly new BH Crack, which is a tube amp full of THD!
I would be *extremely* interested in how it compares sonically to the Crack! One of the reasons I've used FET-input op amps is to better match the characteristics of tubes. The success of the Crack amp just goes to show that in the end "lowest THD" and "lowest noise" may not be what sounds the best to some folks' ears.

BTW, thanks for all the feedback on the amp over the last few months! Alex is the main reason why this one wound up with the front panel 1/4" jack, rear RCA jacks, and in the B5-080 case. I had it designed to fit the B3-080 case, the taller version of the O2, and to put an ODAC in the top slot along with the batteries. But Alex has convinced me that most folks are using their O2's as desktop amps. So this one is like the O2 in that the ODAC replaces the batteries. I posted a poll about it a few weeks ago. Last I looked it was like 5:1 desktop vs. portable.

There is still some space on the PC board over by the RCA jack. I'm thinking of adding another op-amp unity gain buffer to buffer the output of the ODAC on its way out the RCA jacks, if someone wants to wire up their RCAs that way. With the buffer the RCA cable capacitance won't load down the ODAC output. I'm also going to have to notch out that corner of the board and use panel-mounted RCA jacks. The one on the board in the photo was a test for spacing.

Which reminds me - this one will need front and back panels drilled and etched, so two panels to buy from Front Panel Express vs. just the one in the O2.
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Old 8th September 2015, 04:39 PM   #13
agdr is offline agdr  United States
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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
Turbon - good points! Yeah with this one there actually is a chance, no matter how small, that it could sound different from the O2 due to the inverting stages lowering common mode rejection error and even harmonics. Which reminds me, back on Nrik's question, here is a short one-page tech paper from Analog Devices on common mode rejection ratio in op-amps:

http://www.analog.com/media/en/train...als/MT-042.pdf (opens PDF)

At the bottom of page 1 it says "Note inverting mode operating op amps will have less CMRR error. Since both inputs are held at a ground (or virtual ground), there is no CM dynamic voltage." One op-amp input is tied to ground and the other is held at zero volts WRT ground by the action of the op-amp feedback. CMRR is also why I tied the non-inverting input to ground through a resistor rather than directly, to make the impedance looking back out that pin as similar as possible to the one looking out the inverting input.

Now whether any of this is audible is something else entirely. Like you say, NwAvGuy's O2 Headamp's numbers are already most likely below the range of anyone's hearing. But who knows. I'm still just 50% on measurements. In the end I personally think the "how it sounds" part has to be rolled in there too, hence all the postive comments the Crack amp receives, as per Alex's post.

Lol, that revised power management circuit was a bit of a trick. I posted a thread about it in the power supply forum a month or two ago. Ever since he released the O2 I've thought it needed separate comparators on each battery. I can easily come up with a situation that would "fool" the O2's PM circuit into thinking the batts were OK when they are not. If one batt was fully charged to 10.15V (1.45V per cell) while the other battery was shorted and just producing 4.35V (I've had TWO of those Tenergy batteries do this!) the total would be 14.5, above the 14.0V PM threshold on the O2, and it would remain on. The new circuit checks each battery individually to make sure it is above 7.0V. Then I added my latch circuit to each to make sure it doesn't motorboat when the battery is low, like the O2 often does.

But it adds quite a pile of parts. one comparator on each battery, plus a transistor circuit to "wire AND" the two comparator outputs, then the optical SSDs to control the power flow from the batteries. I put the whole thing under the battery area as SMD parts, since the comparators involved are SMD. SMD 0.1% resistors for the sampling voltage divider are cheaper than 0.1% through hole. I really like the fact that in the new arrangement the AC power doesn't go through the PM circuit, or SSRs, at all, That stuff is now only on the batteries. I'll bet one of the circuit wizards here on the forum can come up with some slick way to acheive the same per-battery PM function with half the number of parts!

Last edited by agdr; 8th September 2015 at 04:47 PM.
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Old 11th September 2015, 11:21 PM   #14
sgrossklass is offline sgrossklass  Germany
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Cool idea. I can't say I'd have heard about LME49880 and OPA2140 before, but you live and learn...

The configuration that I'd find most interesting would be an inverting (or twice inverting if you value your absolute phase) input stage with a non-inverting buffer.

You know what I'd be most interested in as far as measurements of the stock O2 go? Distortion at 2 Vrms input for 2.5x gain, or generally shortly before clipping. High and only input-dependent levels in the gain stage are the concept's Achilles heel in practical use, and I bet a number of people who ended up disliking the amp were accidentally clipping theirs. The 6.5x gain setting in particular is further impeded by somewhat pathetic output driving in the NJM2068, as load impedance ends up around 1k5 then.

(Well, and it won't be the ideal amp for some HE-6s or other power-hungry planars. It would certainly be interesting to see what it does at 5 Vrms into 50 ohms or so.)

There's a reason why other 2-stage amps tend to have some gain in the output stage, like 8+/-2 dB. That makes it much harder to clip them on the input side while still keeping noise to reasonable levels.
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Old 12th September 2015, 03:02 AM   #15
agdr is offline agdr  United States
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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
I completely agree that folks clipping their O2s without realizing it has likely been the source of some bad reviews. The O2 topology pretty muich demands having a clipping detector in there, which I did with the ODA but was going to leave off on this one since the O2 didn't have one. Hmmm, I should re-think thiat. Maybe I can whip one up with fewer parts. Seems that I've seen several window comparators on-a-chip.

Good obverservation about the NJM2068 outout. I had noticed that, the datasheet specifies a maximum 2K load, which likely means they knew the THD would get worse past that point, especially with larger swings. In the new amp here the LME49880 is THD rated down to 600R, just like the LME49990. The feedback resistors have to be huge due to the inverting topology, so that pretty much leaves just the pot for load, which I've downsized to 5K to reduce Johnson noise. I would have gone to 1K like in the ODA, but I would have dissipation concerns with the dual package. In the ODA I have one separate LME49990 per channel for more dissipation.

Yeah I didn't know about the OPA140/2140 series either, I learned about that one from Sergey888 when designing the O2 booster board. I've found one interesting potential hiccup with the chip though. The open loop output impedance graph (fig 19 in the datasheet) kind of goes nuts below 1KHz (zooms up from 10R to 1K). Which would be less of a problem with the chip in a direct feedback loop, but I'm still pondering the effect with it driving a buffer (the BUF634 here or the LME49600 in my O2 booster board) since the loop is around both parts. I *think* the net outcome will be that it doesn't matter given the high input Z of the current buffer. I'll have to do some frequency response tests of that pair of parts. This particular issue is why I've gone to recommending the OPA827 in the booster board. The 827 shows low open loop output Z through the entire audio band (datasheet figure 27), completely flat in fact.

Hey excellent thoughts about having some gain in the output stage. Sold! I'll do it. In this one it will just mean upping the value of the buffer loop feedback resistor.

I wanted to implement this 13 year old comment from Nelson Pass about throwing some closed loop gain away to keep the op-amp further way from the unity gain stability point. Seems to make a lot of sense. But I just can't figure out how to implement it for the life of me. That is what the 10K resistors to ground on the inverting inputs were originally for, to form a voltage divider, but then later realized that is a zero volt point. A voltage divider isn't going to work, at least not without an input cap and the signal ground being different than the system ground. Then even later I relized that 10K to ground does have to be in there anyway to handle the case when nothing is plugging into the input - an input bias current return path to ground. I'm sure it is like looking at an elephant through a magnifying glass, the implementation of Nelson's comment is probably obvious, but it has me stumped. That whole thread is about how inverting op-amps sound and is pretty interesting - the first post is here.

Last edited by agdr; 12th September 2015 at 03:23 AM.
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Old 12th September 2015, 03:41 AM   #16
agdr is offline agdr  United States
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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
Hmmm - after thinking about what I just wrote above about the 10K resistors from the inverting inputs to ground, that isn't correct. The op-amp (inverting) input bias current will still flow through the feedback resistor if there is no input. In fact, with no input, the inverting stage turns into a unity gain buffer with the input grounded.

So those 10K resistors to ground I have on the various inverting inputs are doing absolutely nothing and can be eliminated. Good, that is 4 less parts.

Last edited by agdr; 12th September 2015 at 03:44 AM.
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Old 13th September 2015, 09:37 PM   #17
agdr is offline agdr  United States
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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
Default Oops - RF filter

I've just realized that the input RF filters in this amp will suffer the same fate as the 10K resistors to ground on the inverting input in the post above. Things are a bit different with inverting amps.

The simple fix is to split the input 10K resistor into two 4.99K resistors with the RF filter capacitor between them, rather than at the inverting op amp input. The corner frequency winds up around 640KHz, vs. the 2.6MHz corner with the O2's RF filter.

I've used the OPA140 in the sims below since I've yet to find a LME49880 Spice model.

* The first plot is the result of an inverting -1 unity gain stage just with the low pass filter formed by the cap across the feedback resistor. Does a reasonable job through the 11MHz bandwidth of the op-amp, but the maximum rejection is around 60dB.
* The second plot is with the RF filter. The RF rejection now continues past the op-amp's bandwidth and is deeper.
* The last two plots are of the O2 headamp's non-inverting stage and RF filter. Note that unlike the inverting amp, without the RF filter cap the amount of RF rejection in the non-inverting amp is miniscule.
Attached Images
File Type: jpg Inverting O2 no RF cap, f past OPA140 BW.JPG (121.4 KB, 384 views)
File Type: jpg Inverting O2 split input R plus RF cap, f past OPA140 BW.JPG (119.5 KB, 356 views)
File Type: jpg O2.JPG (121.1 KB, 346 views)
File Type: jpg O2 no RF cap, f past OPA140 BWJPG.JPG (126.9 KB, 341 views)

Last edited by agdr; 13th September 2015 at 09:47 PM.
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Old 14th September 2015, 08:59 AM   #18
sgrossklass is offline sgrossklass  Germany
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Is R3 (10k) actually needed in the inverting circuit? I suppose it might be beneficial WRT input capacitance, other than that it just adds noise.
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Old 15th September 2015, 01:50 AM   #19
agdr is offline agdr  United States
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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
Maybe not! I know that resistor is usually added to reduce output offset caused by the input bias current going through the input resistances. But in this case that issue doesn't matter due to the cap coupling after the first stage, plus the op-amps are FET input with nearly zero bias current.

I added the resistor thinking it may help with common mode rejection, having the impedances looking out the two op amp inputs similar. But you are quite right, the noise penalty in adding those resistors probably outwieghs any benefit. I will probably eliminate them.

Thanks for the suggestion! That eliminates four more parts from the board.
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Old 20th September 2015, 01:02 AM   #20
agdr is offline agdr  United States
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An inverting version of the NwAvGuy O2 headphone amp vs. the original: THD+N
Default clipping indicator

Here are bunch of modifications and additions on this project.

I had an idea on the new input select switch, the second image below. I've added PCB holes on both throws of the switch (JP16), including the one that is hardwired to the front panel 3.5mm jack. This allows for a whole bunch of fun optional combinations of parts:

* switch in: ODAC input (PCB hole pair 1); switch out: rear panel RCA input (PCB hole pair 2). Anything plugged into the 3.5mm would be in parallel with the rear RCA, so use one or the other.

* switch in: ODAC input (PCB hole pair 1); switch out: front panel 3.5mm via the hard wiring (nothing in PCB hole pair 2). Then ODAC out also run to the rear panel RCAs for ODAC line out in parallel with the amp input.

* No ODAC, batteries. switch in: rear panel RCA input (PCB hole pair 1); switch out: front panel 3.5mm jack via hardwiring.

etc....

Then regarding sgrossklass' good suggestion in post #14 to have some gain in the output stage, I've implementing it but in a special way. I want to keep the overall amp voltage gain in the low gain switch position unity (1x). So although I'm bumping the output stage votlage gain up to 1.33x, I'm actually decreasing the input stage gain to 0.75x = attenuation! This makes use of a poorly understood (at least it was by me a year ago) feature of inverting amps: unlike non-inverting stages they are stable below unity gain, all the way down to zero voltage gain in fact (full attenuation). Here is a great discussion about it on TI's engineering discussion forum:

https://e2e.ti.com/blogs_/archives/b...is-it-unstable

The net result then is 0.75x voltage gain in stage 1 (a 7.5K feedback resistor with the 10K composite input resistor), times 1.33x gain in stage 2 (a 33K feedback resistor with the 24.9K input resistor) = 0.99x overall voltage gain (current buffer-only mode, same as with NwAvGuy's O2 in the low gain position).

Then in the "high" gain switch position the new feedback resistor is 18.7K in the input stage for a first stage voltage gain of 1.87x. Multiplied by the new second stage gain of 1.33 gives (still) 2.5x overall voltage gain, same as before and same as the most used high gain position in NwAvGuy's O2 headamp.

Then regarding sgrossklass' interesting suggestion to elminate the resistors to ground on the non-inverting inputs in post #18, I'm chickening out on this one and leaving the pads for the resistors. LTspice is having trouble converging with those inputs tied to ground directly that it doesn't with some resistance there. Probably just a simulation artifact, but there is also something rattling around in my head from way back when about trouble in tying some FET input op-amp inputs directly to ground. I can't remember why now, but I think it may have been voltage-to-substrate issues, in older FET inpu op amps. At any rate a jumper can simply be used in place of the resistor if anyone wants to tie those non-inverting inputs directly to ground. I am going to decrease resistors to 1K from 10K in the input stage and 5K from 24.9K in the output stage, which should at least help there with noise reduction.

I've completely ripped up and re-done the layout around the headphone relay. In the old V3.0 layout in the first post in this thread a lot of the traces had to snake way to the left to get to the relay. Now the relay is centered, which shortens all those traces. The relay control circuitry is now on the left of the relay.

Plus I had a bit of a brainstorm on the relay circuit. In the ODA I have the relay coil looped through one pole of the on/off switch to peform the "no thump at turn off" function. It works wonderfully. There is no greater "advanced notice" of an amp power-down than the power switch, of course. Well here both poles of the switch are in use by the main circuit, preventing me from looping the relay coil through. Sure enough, although the turn-on thump is gone, there has been a small turn off thump. 3 pole right-angle PC-mount toggle switches exist, at least in the catalog, but not in stock at Mouser or Digikey. The 3 pole switch is also conisderably wider and wouldn't fit.

Well it hit me that one pole on the unused "thow" on the power switch in NwAvGuy's switch-in-the-middle design here can be used to shut down the relay control mosfet and nuke the output thump, similar to what I did with the ODA! The source on that relay control mosfet is already tied to the negative rail, which goes to one pole on the power switch. That switch is non-shorting, so tying the gate of that mosfet to the other "throw" on that pole causes the gate to be shorted to the source when the power switch is turned off. Mosfet goes off and so does the relay.

Then finally I've implemented another one of sgrossklass' suggestions in post #14, the clipping indicator (the stuff in the green circle, still a work in progress, but at this point at least I'm sure it will fit). I've used the clipping detect circuit from the ODA, which in turn came from various internet posts out there. I've made a compromise for space though. The ODA clipping circuit is "stereo", sensing both channels. Here I''m just going to make it "mono" on one channel, under the theory that if someone's source input is high enough to cause clipping it will probably be happening in both channels. Sensing the clipping on just one channel requires just a smaller a SOIC 8 chip instead of SOIC 14, plus saves two diodes.

Plus now the clipping circuit is going to be on the output stage, vs being on the input stage in the ODA. That works because of the first point above, the output stage now having gain, so it will be the first to clip as sgrossklass noted. Good stuff!

I've notched out the right rear corner of the PC board since the RCA jacks will have to be rear-panel mounted vertically to fit. No way to make them fit with either one PC board mounted. These are the REAN/Neutrik panel mount units with the 15.5mm insulating washers.

In the LTSpice plots green is the input source, blue is the output of the first stage, and red is the output of the second stage. In the transient plot I've used a 36K feedback resistor instead of 33K to bump the gain up slightly so the red trace would be visible vs. the input green trace. With 33K they would be on top of each other, of course. The peak of the input trace is 1.0V, the first stage output is 0.75V, and the output would be 1.0V again with 33K feedback. Note the output of the first (inverting) stage (blue trace) is 180 degrees out of phase vs. the (green) input, just like it is supposed to be, while the resultant (red) output trace is back in phase with the input after the second inversion.

In the AC plot the initial gain on the left axis is 0dB, while th initial phase on the right axis is zero degrees.
Attached Images
File Type: jpg layout.jpg (1,003.6 KB, 306 views)
File Type: jpg input select switch.JPG (70.5 KB, 60 views)
File Type: jpg output and clip indicator_1.jpg (231.3 KB, 57 views)
File Type: jpg input stage with 0.25 atten _ output stage 1.33x.jpg (121.1 KB, 101 views)
File Type: jpg transient plot.jpg (71.7 KB, 93 views)
File Type: jpg ac plot.jpg (42.8 KB, 41 views)

Last edited by agdr; 20th September 2015 at 01:24 AM.
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