Quote:
| For a first build, try to avoid SOIC form factor chips |
Not to be avoided.
No new opamp designs will be offered in anything other than SMD so it is either get on board or be left behind as the DIP integrated circuit is made obsolete.My problem is not the typical SOIC but the even
newer trend to chips that will sit on the tip of a pen ! Someone say "build with a microscope " ?
For compatibility with designs already built with the DIP Op-Amps there are the Brown dog and other adapters available for not a lot of loot.
Quote:
| The ad8397 is a very fast bipolar-input opamp |
actually it is quite slow by modern standards and what makes it especially suited to audio use.Also why I have a suspician it was designed for audio specific use when all the specs are weighed against other modern opamps which are geared to networking or test equipment use so ultra-high-bandwidth and ultrafast.Plus they actually spec out poorly at lower "audio bandwidth" frequencies
the specs of the AD8329 vary depending on power supply voltage but under all cases exemplary for battery operated audio use :
69 MHz bandwidth (G = 1, -3 dB) 53 V/µs slew rate (RLOAD = 25 Ω)
High linear output current
310 mA peak into 32 Ω on ±12 V supplies while maintaining -80 dBc SFDR
Fast is a relative number and in direct comparison to the chips in regular use around here not "fast" at all but "dead on" for audio use.Look at the graphs for speed vs. gain factor vs. ps voltage.
There really is no directly competing single op-amp chip all specs taken as a whole
Quote:
| Confused yet? Turns out it's worse than that, read Tangent's article about cranky opamps if you want the real dirt. |
You obviously have not yet used this part."cranky" it is not and is more easily implemented than many JFET input Op Amps I have tried.
Quote:
| Also, since it is bipolar, the feedback loop resistors must be carefully calculated to prevent a high DC offset in the output, which would have a very real chance of cooking your headphones. |
Again not the case in actual use.Read the spec sheet section under "DC PERFORMANCE" where the Vs is +/-12VDC which is the
worst case condition for DC offset but best for ultimate voltage and current output delivery.
Also read the "OUTPUT CHARACTERISTICS" section and you will see this baby for what it is and that is a killer single-stage self-contained audio headphone amp or line driver for battery operation.as the batteries drain it will still outperform most chips at full power and for sheer current delivery and opamp/buffer pair as the batteries fall below 5 VDC.
And this is a good thing becuase if there is a limitation it is the quiscent current draw which could be better until wieghed again against TOTAL current draw of every device in the opamp/buffer type amps.When compared directly the 8329 comes up ahead.Even when the numbers even out there is no denying a single device is always preferable to multiple devices.Simple over complicated.
The data sheet graphs tell the tale and the content not hard to decipher at all plus there is this :
Quote:
Quote ADI :
The AD8397 has two voltage feedback operational amplifiers capable of driving heavy loads with excellent linearity. The common-emitter, rail-to-rail output stage surpasses the output voltage capability of typical emitter-follower output stages and can swing to within 0.5 V of either rail while driving a 25 Ω load. The low distortion, high output current, and wide output dynamic range make the AD8397 ideal for applications that require a large signal swing into a heavy load. |
Pretty damn impressive for an opamp that behaves in an audio device
Quote:
| Just build a Dynalo or an M3 and be done with it Well, actually, I'm not joking. But if you must persist with the AD8397, take a look over on headwize for Tangent's MINT V2.0 discussion, as well as another (at least one) discussion of the AD8397. |
Why ? Do you not think the Head-Fi gang is capable of making an amp ?
If you actually check the archives at
Headwize you will find that I am the first "known" person in these circles to have used this chip as a headphone amp and have been using it since I received advance product samples.
I announced the availability of the part last spring here in a thread but for some reason no one took notice until now so other than a few metions mostly figured folks would rather just use what they know and what is popular and made no more mention.
It is only recently that Tangent and others (xin) have looked into using this device not that it means anything.I think the more folks exploring its use the better when it is the simple CMOY Pocket amp style amps being pursued.Many minds means many ideas.
To use this with a buffer stage or "doubled up" in a 47 type amp would be dumb since there are better choices when the extra current is not only not needed but would be redundant.Simple,easy,all in one tidy package that if done right can produce a very tiny amp capable of blowing away many opamp/buffer style amps on pure kill power along.Sonics are pleasant enough that fancy "EQ'ing" need not be done with boutique parts and the gain envelope fine for everything from X2 to X10 so pretty much a universal part for high current needs (including limited use as a network cable driver.Limited by banswidth and there are better choices like the AD8000)
Quote:
| In short, I'm mainly asking for your help in this new venture of mine. I've always had an interest in electronics so I'm open arms when it comes to learning. If you don't have the time please point me in the right direction. |
My first build was straight from the data sheets and my copy of "Op Amp Hardware and Housekeeping Techniques by Walt Jung" ,an ADI publication and was built on one of the ADI universal opamp EVAL boards which eliminate a
lot of work when you need to get a design from the schematic to working device in short time.Nothing beats pencil to paper and paper to working device in less than an hour if you are trying many options.
If the EVAL boards have a limitation it is all the pads are SMD but the SMD parts
are catching up with their bigger counterparts (and some better them) in all ways until we come to audio signal path coupling caps which are
ALWAYS big if they sound good (at least better in direct comparison to the the smaller caps though i am looking into some options there too.Brand new and RoHS compliant) and that means off board mounting if the EVAL pcb is used (a true bargain in price BTW,check one out or better yewt buy a bunch
)
Don't read too much into the data sheet layout considerations which are geared more to frequencies out of the audio band.For an audio opamp stage simply placing the bypass caps right at the +/- pins of the opamp will be fine and is standard parctice for all such devices even far slower chips (slow is actually good)
SIDEBAR :
The reason you bypass close to the pins is because the RF bypass cap,the 0.1 uF ceramic can only shunt the RF to ground where it sits and if there is a gap between the "shunt" and the actual power supply pin there is room for RF to creep in.RF is exactly what it says "Radio Frequencies" and that means AIR transmitted just like any AM/FM radio signal.With a Radio you want to receive these signal but any other audio device you want to eliminate these signal from entering and causing contamination.
The larger bypass is meant to lower the impedance of the power supply delivered to the chip and determines how fast the chip can respond to changes in that curent delivery in relation to the voltage.Low ESR means low impedance which again translates to low "resistance".Again the word means what it says and the lower the resistance the less the capacitors RESISTS passing along the current.Low means fast,high means slow beciuase it is an impediment to the delivery.
The reason you want it right at the pins also si anything other than air has its own resistance value and that means even a board copper trace will add its own "resistance" to the cap ESR which in a worst case can make a low ESR cap a high ESR cap !
Back OT :
Because this IS a bipolar transistor and not JFET input amp there are lmits on the actual input impedance.If you build as an inverting stage (audio input to negative terminal) this almost become infinate being a virtual ground but since we are using the non-inverting (audio input to positive terminal) there are limits to what we can do.
The simple answer is :
1-The Op-Amp Input Network
ual Audio Taper 10K pot to 1.0 uF Metal Film Type input coupling cap to 100k input load resistor from the "+" opamp input.
2-The Op-Amp Feedback Network :Rf=470K and Rg=100K for a gain of close to X5 (4.7 actual) or 10dB.If the cans will only be Grados cut that in half by making Rf 220K
3-The Op-Amp Output Network : 10 ohm resistor inline with the output to protect from capacitive load feedback.Do NOT put this inside the feedback loop but directly from the output to the output jack where it will actually isolate the "load" from the feedback loop thus avoiding potential loop instability.
3-The Power Supply Bypass Network :A "Multi-Capacitor-Parallel-Group" made up of X2 10uF or single 47uF Solid Tantalum "slug" capacitors and a single 0.1uF high quality NPO ceramic (NOT a disc.AVX COG are good here) at each power input pin of the chip and by "at" I mean directly to the pins.
4-Wiring : Connect the input and output jacks,power switch and LED assembly,battery clips and two nine volt batteries.Whether to add an artificial ground circuit or not is your choice and can be even added later
5-double then triple check all wiring then check it again
6-power that sucker up and enjoy
Edited part value typo.C1 was listed as 0.1uf and should have been 1.0uF ! Yes my typing skills suck
Edit #2-expanded on the parts list for more clarity
