SMT passives for audio

[mediatechnology]

Correct. I need to see if I can find the brands that "weren't as good."

[SUBass]

The audio specific Susumu resistors are impossible to find. RRS series I believe it is. They look like their regular RR series with gold ends.

Anyway...I rolled my first SMT board and "cooked" it on a hotplate yesterday. I'm amazed at how well that works. I'm still learning how to make PCBs...But my first go at it wasn't too painful.

I have an issue though. It might be a bit of a dream at the moment...But I would like a coupling capacitor that's surface mount that doesn't mess with the sound too much. I tried a Kemet SMT tantalum and that was not so good (despite having used a through-hole part of theirs with desirable results before). Checking out Panasonics catalog...Looks like the best SMT cap they have (for this type of app) is their FP series.

I'm putting together an order for some various caps to try out. If you have any suggestions or parts that you're interested in...let me know I'm limiting my value to 47uf and application to audio coupling. Currently I've got an AVX Niobium Oxide, Panasonic FP Aluminum Electrolytic, Murata ECAS Aluminum Polymer, Nichicon FS Aluminum Polymer in the cart. Of these, only the Niobium Oxide is touted as being a good cap for audio coupling. Is there a reference cap that I should put in the mix? Panasonic FC or FM maybe?

The test circuit is just a unity gain opamp (AD8512) with no resistors. I don't have any scientific means of testing the devices so it would strictly be based on my sense of sonic aesthetic. In my world...It's going to change the sound...The questions become, How much does it change it and does it change it in a good or bad way?

[SUBass]

I'd be interested in seeing the resistors that weren't as good. It's also nice knowing how far from good they were. For through-hole I used the Vishay/Dale RN or CMF series.

The Susumu RR series seem to be readily available but the "audio" version RRS is pretty much unobtainium.

[mediatechnology]

The Susumu RR series seem to be readily available but the "audio" version RRS is pretty much unobtainium.

That was my experience too.

I'll see if I can find Dave's original research on the resistors "not as good" and then ask for his permission to use the data.

EDIT: I found the doc and have sent the e-mail to Dave asking for permission to reproduce it in it's entirety.

Check also RCD BLU-0805-1002-BT25W (an 0805). As good as Susumu in the tests. The next runner up was Panasonic in 1210. The Pana 0805 did not measure up.

[JR.]

Damn.. 1210 is huge in the context of smt resistors. I've seen 0201.. Well I couldn't really see them.

JR

[mediatechnology]

Yep, in that Panasonic line the 0805 vs the 1210 had an order of magnitude greater THD. So in that one particular case bigger was better. Susumu/Digi-Key have this tiny little figure that may show why the Susumu is better. It was once mentioned to me that they may have some magic in their annealing process.



Not sure what shape the Susumu "comfort sound" versions have. Could be a sinewave But made of unobtainium...

[mediatechnology]

Dave Hill of Crane Song was very kind to permit me to reproduce his SMT resistor findings. Thanks, Dave!

http://www.waynekirkwood.com/Images/pdf ... nesong.pdf

At the end of the document is an update he posted this year.

[SUBass]

That's pretty fascinating. It seemed as a generalization, physically larger resistors performed better and, as previously mentioned, the Susumu and RCD resistors performed best. I was kind of hoping the Vishay/Dale TNPW resistors would be mentioned as they're readily available in a plethora of values.

Thank you for posting this Wayne and give Dave Hill a thanks for putting this out there.

[mediatechnology]

It seemed as a generalization, physically larger resistors performed better and, as previously mentioned, the Susumu and RCD resistors performed best.

Glad I saved that e-mail.

Bigger is better seemed to be particularly true with the Panasonics he tested. As I previously mentioned there are some spots in a mic preamp where TH seemed to be the only viable alternative. There were SMT parts that might have worked but they were custom, long lead time and generally unobtanium. So I couldn't test them.

If you look at Dave's second oscillogram you'll see a strong THD-3 component. The self heating mechanism is THD-2 because it occurs, when the thermal mass is small, on every half-cycle. It's, in effect, an RMS detector with a short time constant and a lot of ripple. That THD-2 related variation in resistance then seems to multiply, or modulate, the signal to produce THD-3.


Image courtesy of Dave Hill, Crane Song

As to aluminum electrolytics I often see a mixture of SMT caps and TH with electrolytics being TH and bypass and smaller values SMT. Most of the larger value aluminum electrolytics almost look to be TH with a SMT carrier. Unless you're assembling gazillions of them I'm not sure they make sense. Even in the gazillions there's probably a cost advantage to using TH or Creative Labs et al wouldn't be using TH aluminum electrolytics.

[SUBass]

Well...I got the caps in..

AVX Niobium Oxide
Nichicon Aluminum Polymer
Panasonic FP in both 6.3V and 16V
Murata Aluminum Polymer

Both the Murata and AVX appear to be environment sensitive as they were shipped in sealed bags with dessicant and a humidity gage to see how much humidity they were exposed to. That makes me a little uneasy about using them in a piece of gear that will be used in non-controlled environments. That's a real pity too because they were my favorites.

Each one of the caps changed the spectrum and dynamic response of signal going through it. It's mainly a subtle difference, but in spending time with each cap I could distinctly pick out their subtleties. I have a specific application that I tired these out in, coupling from a buffer to a couple of preamps in my bass rig. At first I populated a cap or two on the buffer board and started playing. Swapping different caps to different pres, and comparing to no cap at all as well as bass straight into the amp. After getting a feel for the caps, I tracked a DI bass track into my DAW and setup a reamp to feed the buffer. That way I could put each cap into the signal path, track, then sit back and listen more critically.

Going through the list...

The AVX rolled off some bottom (perhaps need a bigger value) and compressed the signal a bit. It also had a slightly forward midrange around 2.5k. It's possible that the apparent midrange boost is really due to the attentuation of the lows. Signal through the AVX didn't seem as "relaxed" as signal without a cap. It did make things seem "tighter."

Nichicon Al Polymer. This was a really interesting cap. It tended to preserve transients better but made things "sit back" a bit. Again...There was a seeming shift in really low end of the sound...But it was a "tighter" "smoother" sound.

Panasonic FPs. As should be expected...There was not detectable sonic difference between the 6v and 16v parts. They sounded the same. The transient response and midrange/top end of the FP was pretty good. The bottom end was a little weak though. This again makes me think the value of 47uf is a little low.

Murata Al Polymer. This was perhaps my fave of the group. Many times when I was blind AB'ing I picked it as my preferred sound over the no cap path. It sacrificed the bottom like the other caps but didn't seem to do so as much as the others. It also did the same "smoothing/tightening" thing that the Nichicon did as well as a bit of the mid boost of the AVX. The transients were a slight bit smeared which gave the no-cap path a slight edge.

I'll pull the exact model of all the caps if anyone is interested.

How does one go about calculating the appropriate size for a coupling cap?