[cyrian]

i'm building a cmoy, and looking for a good opamp (and possibly more ideal cap values) for a good quality sound, currently all i use are my cx300b's but i'm soon investing in either a pair of sr-60's or MS-1's.

I've heard good things about the AD8397 opamp and also varying opinions on what are good cap values for various applications.

if i could get a bunch of opinions on what you all think of this configuration that would be great, also if anyone has other suggestions to improve quality that would be awesome aswell.

oh yeah, i'm also useing my Zune as a source.

 

[AudioCats]

the main problem with using a SMD chip (which is the only package type the 8397 comes in) is over heating. In a commercial product they use large copper pads on PCB to draw the heat away from the leads, but in DIY we don't really have that option.

 

[tomb]

If this is your very first CMoy, stick with an OPA2132 or an OPA2227. Make sure the CMoy works and you have everything operating normally. Be sure to use a DIP-8 socket, then you can try rolling in other opamps once you're sure it works. Unfortunately, an AD8397 will probably never work. It's a BJT opamp and the CMoy was not designed for it. There are some who claim a few bypass caps to ground on the power pins will stabilize it, but there are just as many others who will tell you that even the trace lengths can throw it into oscillation. It's only available in SOIC-8, too, which means SMD soldering and a Brown Dog adapter - both beyond the appropriate reach of a beginner.

If you're really interested in the AD8397, finish your CMoy with basic parts for the experience and then you might be ready for the Mini3 V2. Amb should have it ready in the not-to-distant future, and it should blow the socks of any CMoy and many commercial portables, to boot.

 

[cyrian]

i cant seem to find a parts list for the mini³ v2, is there one available? approx cost of production?

 

[MisterX]

No input caps (replace those with 1K resistors)...Replace R3 with a 1.5K resistor, R4 with a 3K resistor, add some bypass caps and the AD8397 will most definitely will work in a "CMoy" amplifier.


Quote:

i cant seem to find a parts list for the mini³ v2, is there one available? approx cost of production?

http://headwize.com/ubb/showpost.php...=60270&fpage=3

 

[Filburt]

In my opinion, it's one of the only configurations really worth doing if you're trying to do this sort of circuit (non-inverting amplifier with a simple negative feedback loop). The OPA2134 and OPA2227 are not buffered outputs and the 2227 has high distortion past a few KHz even with an easy load, let alone something like a headphone.

Get an Aries SOIC to DIP adaptor, solder a 0.1uF cap across the rails (pins 4 and 8), solder 2.2uF caps from the rails to ground (pins 4 and 8 again). Balance your bias currents. If you're using a 10K pot, find a value around where you'll probably have your volume. I sometimes just use 2.5K (more or less middle) because it's easy and fairly versatile. If you want gain 6, for instance, use 3K and 15K feedback resistors (e.g. in place of the 1K and 10K resistors of the standard CMoy configuration).

Good luck with your build!

 

[tomb]

With respect, this might be debating the meaning of possible vs. reliably dependable, but getting an AD8397 "to work" in a CMoy is a lot different than a reliable, trouble-free choice.

The PINT was pulled, the Mini3 was delayed for months/a year(?) and were both based on this chip. Even in the "possible" builds, they incorporate the pin caps and often inductors on the output. To suggest that is in the realm of a beginner's first CMoy is pushing it.

That's all I'm trying to say - sorry if that causes a ruckus.

 

[Filburt]

I run the AD8397 in a breadboard with long wires sticking out various places and it's not oscillating. The PINT had problems with oscillation for reasons including having half of the second chip servicing DC at the input ground and the other half servicing the output ground whilst both ran at unity gain, in addition to a problematic feedback configuration on the output ground. That's not going to make the AD8397 happy. In a 'CMoy' configuration you don't have that going on. The only time I've ever had trouble with the AD8397 was in the first PINT I made; anything else I've ever put it in (and that's quite a few things, from amps to portable sources to desktop stuff) has been fine.

If the guy is careful and observes proper considerations when using a high speed op-amp of this sort with BJT input, he'll be fine. It's not that difficult to do and it's a good habit to get into from the onset since it's really a good idea to do things like bypassing with _any_ chip. Secondly, if he uses something like this, chances are it will have more longevity as a satisfying device since the performance isn't going to be inferior, if not particularly dissimilar, from many portable amps out there; they're generally pretty simple devices.

 

[AudioCats]

You can probably just build the whole thing on a brown-dog's PCB, using 1/16 W resistors. come on, there are only 3 resistors per music channel, 2 resistors per ground channel. Plus two caps per chip for bypassing. If a Dip socket must be used, use it to pipe in the power and pipe out the output current, everything else done right on the brown-dog......

each channel can use one brown-dog (1/2 for music 1/2 for ground). The brown-dogs are not that big, right? The reason I don't like to use a brown-dog into DIP socket the usual way, is the the brown-dog's traces are long and have sharp turns, makes me nervous.

But whichever way, over heating is still a major problem, brown-dog still doesn't have the large copper pads to dissipate the heat.

 

[AudioCats]

and tomb is right, 8397 circuit involves a lot of precise work, not the best candidate for a first-time project. Just think about the Cmoy power supply thread that came up a while back, how much headache that dude had to go throught to trouble shoot his board.... and that was even the standard Cmoy design.

 

[fortney]

Everyone commenting above is surely correct.

Let me tell you what I did and say that I think the results are quite favorable.

Use 470 uF for the power supply electrolytic capacitors
Use .22uF metallized polyester or polypropylene for the input capacitors
If using low impedance phones reduce the gain (refer to tangent's article) to about 3-4
Use the OPA 2132PA (the cheaper of the two models offered) op amp and mount it in a socket after all the soldering is done so there is no risk of damaging it from heat

After experiencing the pure joy of having built your amplifier, do what you will. I use mine all the time.

Have fun!!

F

 

[fortney]

Ooops, forgot to mention the matter of whether to use disc ceramic bypass caps at the op amp V+, V-.

F

 

[Filburt]

I simply disagree that such extraordinary care is required to use the AD8397. It's not like it's the AD8099 or something which truly requires a substantial level of care. I find the AD8397 fairly easy to use for the most part; bypassing and balancing bias currents is about all that is needed, and that is easy to do and inexpensive in both time and money. Chances are this fellow is going to be running the chip somewhere within the range of gain 4 to 10, and the chip should be plenty stable around there.

I think instead of trying to scare people away from trying what is really a fairly simple modification to the project, we should be dedicating our time to helping make sure it's a successful endeavour.

 

[fortney]

I don't disagree with you, but I was making the assumption--maybe wrong--that the original poster is new to all this. If so then early success is important.

I think if you read the posts above there was a lot of terminology and configuration recommendations that could be confusing.

If he wants to do it your way I think you would need to offer assistance in surface mount soldering and techniques for biasing and balancing currents in op amp circuits. Then he would have a high probability of success.

F

 

[cyrian]

i actually think i'm gonna wait a bit for the mini3 v2 to come out, if the pcb's are for sale i'll buy one and just do that amp. i have a fair ammount of electronics knowledge, just not so much with the op/amps. also i have my fathers help if i encounter any problems, he knows a lot more than i do about electronics, schematic is like a second language to him, lol so i dont forsee any problems there. thank you for all the input, and i'll keep my eye on the mini3 v2 and wait for that to complete.

*is excited for the mini3*