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O2 headamp output booster PCB
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Old 17th February 2014, 09:03 PM   #111
agdr is offline agdr  United States
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O2 headamp output booster PCB
Sergey888 - another idea. I've been wanting to build a notch filer + high gain amp pair to use as the input to the oscilloscope FFT. The new scope has a vertical sensitivity of 500uV/div, so a gain of 50x or so should do the job.

I'm thinking of stuffing an OPA637 in after the notch filter as the optional stage. The 637 is the uncompensated high-gain version of the OPA627 of course. As for compensation, I've used your 2nd order network. I've arranged the stage to be inverting, since current drive from the previous stage is no problem. I've added an optional zero ohm bypass around the 637 stage so that stuff can simply be left unpopulated, in which case the board reverts back to just a notch filter. As per your suggestion I've left the 10R's off that stage's power lines.

A lot of layout cleanup yet to do - may even put the parts on the board bottom - but here is a rough-in. Top layer is route, next is Vee, next is Vcc, and bottom is ground since the outer layer foil is twice as thick with Seeed's process.

I was able to get the ground on the C5 anc C6 decoupling on the same via. Wasn't able to share a via with C8 and C9 but at least I got them in the vicinity of each other. I'm still pondering the power routing.

Last edited by agdr; 17th February 2014 at 09:09 PM.
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Old 19th February 2014, 02:46 AM   #112
Shaq888 is offline Shaq888  Ukraine
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agdr

It is highly likely it will be unstable with a decompensated opamp like OPA637. In this case you can use something called phase lag-lead compensation, by placing series RC network in between inputs of the opamp. I'll find and example and post it later.

Also you may need a bandpass of low pass filter for you sig gen to get rid of harmonics.
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Old 19th February 2014, 03:22 AM   #113
agdr is offline agdr  United States
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Sergey888,

Sounds good, please do post any examples. I nearly tried some phase lead compensation - just a cap between the op amp inputs - while trying to wrap chips around the NJM4556A earlier in the thread.

Hey I've had an idea. Maybe the AD797 would be a good candiate for the high gain chip. I've stayed away from that chip since I've read a great many posts over the years from people trying to stop it from oscillating and eventually giving up. The chip apparently has a 110mHz GBP just like the LME49990, but the 49990 is unity gain stable. From one of the datasheet voltage follower diagrams it looks like the AD797 is unity gain stable, in a fashion, requiring matching resistances in the feedback loop and input. Probably not a good candiate for the Fliege filter op-amps since those circuits wind up at unity gain.

In the datasheet I see they consider the chip optimal for source resistances 1K or less, which would be the case for that inverting output stage. The data sheet also seems to like the inverting configuration for the chip. I recall having read something in the datasheet months ago about the chip's input bias current cancelling circuit needing some special care and feeding on the input, certain minimum input resistace, which is likely why that voltage follower configuration needs the resistances. I'll have to go back and read the whole datasheet again.

At any rate the AD797 would be cheaper, at $9.40, than the OPA637 at $25.
Attached Images
File Type: png Fliege 1khz notch filter circuit.png (88.9 KB, 233 views)
File Type: png Fliege 1khz notch filter layout.png (384.9 KB, 228 views)

Last edited by agdr; 19th February 2014 at 03:43 AM.
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Old 19th February 2014, 04:08 AM   #114
agdr is offline agdr  United States
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Quote:
Originally Posted by Sergey888 View Post
Also you may need a bandpass of low pass filter for you sig gen to get rid of harmonics.
I forgot to say that I agree here, it certainly couldn't hurt from the AP photos that jackinnj posted of the QA400 output here, if I'm understanding that test correctly

http://www.diyaudio.com/forums/equip...ml#post3815567

A bandpass would help peel away the few remaining harmonics. If the filter is placed on the output of the generator it should be possible to use a purely passive bandpass, then just crank up the generator level to compesate for the filter insertion loss.
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Old 19th February 2014, 05:29 AM   #115
Shaq888 is offline Shaq888  Ukraine
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AD797 has its own tricks. Most likely it will need a small damping resistor in series with a capacitor from an output to an inverting input to keep it happy.
You can try OPA211 or OPA1611(2). They are quite linear and have reasonable current and voltage noise.
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Old 19th February 2014, 06:11 AM   #116
Shaq888 is offline Shaq888  Ukraine
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There is an example. It is pretty crude, but gives an idea. Don't pay attention to C1 - it is just a NE5534 model imperfection compensator
Basically you just manipulate a noise gain of the circuit on HF without affecting a signal gain. If you plot a loop gain of the circuit you'll see a section with 12dB/Oct roll off.
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File Type: png example_1.png (37.3 KB, 244 views)
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Old 19th February 2014, 06:34 PM   #117
agdr is offline agdr  United States
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Sergey888 - thanks! I'm heading off on a business trip and that will give me something to ponder on the plane.
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Old 23rd February 2014, 04:36 AM   #118
agdr is offline agdr  United States
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Quote:
Originally Posted by Sergey888 View Post
You can try OPA211 or OPA1611(2). They are quite linear and have reasonable current and voltage noise.
You were right. The OPA211 looks like a good way to go after spending some time with that and the AD797 data sheets.

Below is the lead-lag comp with the OPA211. I had been working with non-inverting circuits so long it wasn't until I graphed it and saw the phase that I realized a compensation capacitor from output to input isn't going to work with inverting. Pretty much would feed 180 degrees out of phase back to the input = oscillator. What was I thinking. So I definitely see the need for the compensation on the input leads.

470pf + your 2.1K seems to yeild a gain curve that is just slightly underdamped, flat through 50kHz with the 50x = 33dB voltage gain. The second plot is with 1000pF instead, a bit closer to critically damped.

The input circuit to the QA400 fries if fed more than 5V. I may add some clamp mechanism like running the input power through +/-5V regulators then to the op amps to limit their swing.
Attached Images
File Type: png Fliege 1khz notch filter circuit.png (87.7 KB, 48 views)
File Type: png Fliege 1khz notch filter layout.png (391.2 KB, 32 views)
File Type: jpg LT Spice OPA211.jpg (219.0 KB, 32 views)
File Type: jpg LT Spice AC plot OPA211.jpg (39.5 KB, 28 views)
File Type: jpg LT Spice AC plot OPA211 1000pF.jpg (39.7 KB, 12 views)

Last edited by agdr; 23rd February 2014 at 05:06 AM.
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Old 23rd February 2014, 04:55 AM   #119
agdr is offline agdr  United States
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O2 headamp output booster PCB
Default Booster board also provides a 6x slew rate improvement

A recent set of postings about the O2 amp over on head-fi just made me realize the booster board provides another potential benefit: the slew rate is nearly 7 time faster than the O2's NJM4556A chips. The NJMs have a slew of 3V/uS, vs. 20V/uS for the OPA140. The LME49600's slew is something insane like 2000V/uS, but the OPA140's would be the limiting factor in the combination, of course.

The O2 amp's NJM2068 gain chip's slew is 6V/uS would would actually be the end-to-end limiting factor now. But... just pop in a LME49720 or pair of LME49990's on a dual SOIC8-to-dip adaptor and you now only get 20V/uS slew end to end, you get lower THD+N than the NJM2068 chip that also can make it through now due to the booster board chips.

NwAvGuy made a pretty good argument on his blog that no more than the NJM4556A's 3V/uS would ever be needed for headphone audio, given that is about the maximum present in any real-world recordings which limited by the signal processing chain. But there are some interesting arguments to the contrary out there. NwAvGuy's only negative comment about higher slew was opening up the potential for oscillation, which I have the scope shots to show isn't the case in this deisgn, at least out to 200mHz.

I'm experimenting with some SOIC8-to-dip adaptors for two LME49990 chips. If I can find one that fits properly in the U1 space on the O2, and can prove with the scope that it doesn't oscillate (or add bypassing as needed), I will probably make that the preferred gain chip upgrade to use with the booster board to take the O2 measurements to the next level.

Last edited by agdr; 23rd February 2014 at 05:01 AM.
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Old 9th March 2014, 05:55 AM   #120
agdr is offline agdr  United States
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Default Fliege notch filter needs input R and current buffer

Here is some interesting info for anyone attempting the Fliege notch filter circuit posted above, in this case for the QA400 audio analyzer. I received my PCBs back yesterday and discovered a couple of things. Goes to show the importance of building and testing circuits! Stuff that looks good on paper can have some surprises in real life. I've never seen either of these issues mentioned about the Fliege notch filter circuit in all the Googling I've done to date.

First off it turns out some input resistance is required, otherwise the output will shoot down to the negative rail voltage when the input BNC is disconnected. I'm adding 10K across the input which results in about 1.2mV of DC offset. 100K results in about 16mV. Shorting the input, or hooking it to a headphone amp output with nearly zero impedance like the O2 or O2 booster board, results in just 600uV of output DC offset from the notch filter.

Since headphone amp outputs have the very low output impedence the problem doesn't show up when the Fliege notch filter input is connected to a headamp output. Also doesn't show up in the LT Spice sim since ideal voltage sources also have zero impedance. But when the filter input is disconnected on the PC board, or the filter input voltage source removed in Spice, the filter output zips down to a couple of volts above the negative rail, the maximum swing of the LME49990 chips if that input resistance is absent.

The reason is due to the capacitors in the Fliege filter being open circuits at DC (input signal removed from the filter). That effectively breaks one of the internal feedback loops and causes the op amps to act more like comparators. The polarity (negative rail) is due to the op amp inputs sourcing input bias current rather than sinking.

The other Fliege notch filter discovery is more serious. The LME49990 op amp doesn't have enough capacitive drive capability to power the 0.047uF cap in the middle of the Fliege filter circuit. The op amp is only rated for around 100pF of output drive capability before ringing effects start showing up. That would probabably be true for most of the 100mHz-or-so gain bandwidth op amps needed for the notch at 20kHz. The problem shows up as a distorted sine wave on the output of the Fliege notch filter, along with a modulation envolope effect at about a 250mS rate.

This particular problem is a little tricky to find in that it only shows up in the transient simulation of the notch filter. The notch vs. frequency in the AC simulation looks just fine either way. Also shows up on the scope looking at the filter output.

The solution is shown in the circuit below for a V2.0 of the Fleige notch filter. A BUF634 in the DIP8 package is now fed by the middle LME49990 op amp to provide adequate capacitive drive ability for that 0.047uF cap. The other solution would be to insert a small resistance (20 ohms does the trick) in series with the 0.047uF cap (after the op amp output) to isolatate the capacitance. Unfortunately that also affects the Q of the filter and shallows the notch. Using the current buffer chip preserves the notch depth, and the buffer is rated for the 110mHz GBP needed for the notch filter at 20kHz.

Some other improvements are using low noise TPS7A voltage regulators set for +/-5V to power everything. That way the maximum output level is limited to 5V, preventing the QA400 input (maximum 5 volts) from frying. The pre-amp stage is switched over to non-inverting so that a higher input impedance can be used while still keeping the feedback resistors small to limit Johnson noise. That high input impedence (10K) in turn allows a 4.7uF film coupling cap to be inserted at the input, blocking any DC offset coming from the Fliege notch filter stages and/or the headamp under test. Blocking the DC becomes especially important give that the stage voltage gain is 33dB (50x). A relatively small DC offset going into the pre-amp stage would result in a large DC output going into the QA400. It would also more or less negate the nice low 125uV input offset voltage of the OPA211 chip.

The BNC connectors are now mounted upside down on the bottom of the board to free up more top board space for parts, as is the new 4.7uF coupling cap. A switch is added for the incoming power. The incoming power filter caps are shrunk since they are no longer needed with the TPS7Axx voltage regulators and their low output impedance across the audio spectrum.

In the transient plot green is the output of the notch filter, about 33mV with a 200mV signal input at 1kHz, and the blue is the output of the 50x pre-amp fed by the notch filter. In actual practice using two of the Fliege notch boards in series would help drop the notch depth even further. The pre-amp circuit would be unpopulated on the first board.
Attached Images
File Type: jpg IMG_2366.JPG (67.5 KB, 32 views)
File Type: jpg IMG_2367.JPG (63.2 KB, 37 views)
File Type: jpg IMG_2370.JPG (62.9 KB, 42 views)
File Type: jpg Fliege V2.0 Spice.jpg (233.8 KB, 53 views)
File Type: jpg Fliege V2.0 AC plot.jpg (35.1 KB, 38 views)
File Type: jpg Fliege V2.0 transient plot.jpg (97.8 KB, 16 views)

Last edited by agdr; 9th March 2014 at 06:24 AM.
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