A big "hello" and a little capacitor question...

[rog951]

Hi guys,

This is my first post so I thought I'd say "hi." I just stumbled onto this place and it looks like tons of good info is being exchanged. I'll try not to be too annoying with my questions!  :wink:

I'm building a Foxx Tone Machine clone and using the info on "generalguitargadgets.com" as a reference. Is says there that all of the non-polorized caps can be either ceramic or film. BUT, I just found an older shematic for this thing in my big "pile o' junk" and it shows a couple of the caps as "mylar film only." So, here's my question:

When and why is it important to use a film cap in an audio circuit? What parameter(s) of a film cap's specs make it "better" than a ceramic?

In this design, I'm using suface mount packaging for all my caps. I'd planned on using 0805, 10%, X7R caps for all my non-pols, but now I'm second-guessing myself because of this whole film thing. The capacitance tolerance on the film caps I'm finding are generally no better than 10% so I'm hoping somone can explain the difference. I'd appreciate any info you guys could provide!

Thanks,

Rog

[petemoore]

For the most part in the signal chain [well sometimes electros]
 I don't use much ceramics [unless I 'have to] opinions very on whethere the tiny amount of 'undesirable' or 'desirable' distortion can actually be heard in ceramics [which by the way are generally low tolerance, but I never notices ... I admit I use them teeny ceramics for high end roll off etc and have liked them there just fine.
 If the schematic says Polarized I use Polarized [and make sure + in the ckt IS + as shown on the schematic].
 Other than that only 'minute' differences can be heard [opinion again] [Distortion in ceramics read at GEO] and as long as they meet and or exceed the Voltage requirements and are good they [any of them] should work just fine.

[petemoore]

Good to 'see' ya'!!!

[rog951]

Thanks for the info, Pete! For the first attempt, I guess I'll just stick with the ceramic caps (since I already ordered the PCB and the parts!  :oops: ). I'll just see how it sounds with those and if I'm unhappy with the results, then I'll start substituting in some film caps.

Incidentally, I'm using surface mount aluminum electros for the 8 polorized caps in the design. Also, the transistors will be BC237B and I found some NOS 1N34a germanium diodes to use for the clipping diodes. The board is gonna be super tiny...I sure hope it sounds good!

Thanks again!

Rog

[Peter Snowberg]

Welcome Rog,

Cool, it's always nice to see more folks who work in SMDs.

I'm from the "capacitor type makes a big difference" school of thought. I use film caps for everything I can. After doing some A/B comparisons with film and ceramic caps using a toggle switch, I prefer film. Films are not microphonic like ceramics, they don't absorb humidity like uncoated ceramics, and when pushed to the top of their voltage range I would say ceramics have more of a "gritty" sound character while films are more true. I suspect this comes from a frequency response characteristic rather than any kind of first order distortion characteristic.

The whole "sound of caps" is such a debate with too much voodoo on one end and too much academic myopia on the other. Somewhere in that middle lies reality. Build two small tube amps with different technology components, taking care to match values, and the difference can be striking. (I did it with a 12AU7 amp)

When you consider that capacitance and max working voltage are only two out of at least 12 parameters, it opens up some possibilities for differences that would be under the RADAR most of the time.

I would say that the place where films are most important is in the coupling positions. Treble bleed caps can be X7R, but I would use films for everything that touches the audio path unless the value is too small where I prefer silver-mica. When it comes to polarized caps, I prefer electrolytics over tantalums for sound, but tants win when it comes to lifespan. You can't have everything.... well, except that you can get 20uF film caps if you can deal with the size.

In the end, as a person who loves SMDs, I would actually advise using SMD for everything except audio caps in the film and silver mica size range. For those positions, any decent plastic film cap will work great. Unfortunately that means through hole unless you want to trim leads and just solder to pads. I use cheap Xicon polyester films and really like 'em.

I just hit preview and saw your reply to Pete's post. If the board is on the way, not much you could do for layout. Cheap xicons are still easy to solder to 0805 pads if you have a good iron and don't drink any coffee that day.

Best of luck with your build. I hope you post some pictures.

Take care,
-Peter

[rog951]

Peter,

Hey, is EVERYBODY in here named Peter?!

Thanks a lot for all the good info! I really appreciate your insight. I do SMT stuff all day long in my day job as a pcb designer; I don't have the "fear" that some people understandably have. I do generally stick with 0805 as the smallest package to use though. With 0603s, my old-man eyes start to get blurry. On this Foxx clone, there are a couple 4148 diodes in the circuit and I found a bunch of SMT 914 diodes to pop in their place...the only problem is they're in a SOD-323 package. Those guys are TINY! I'll put them in first.

Regarding the film caps; I might get lucky in the case of the Foxx clone. Most of the caps in the direct signal path are 10uF electros. There are only two or three that I've designed in as 0805 ceramics. It looks like most of those connections get wired to something off-board too. So, if I do substitute film caps for those guys, I'll be able to solder them directly to a big thru-hole pad, or a transistor lead, instead of to a little 0805 SMT pad, in most cases anyway. Good thing too cuz I can't live without my coffee!

I have a polyester film cap kit from Mouser so I'll use those for any substitutions. Of course, they're all 100V caps and they're all HUGE but what fun is life without a little challenge?

I'm also glad to hear that you feel that aluminum sounds better than tantalum, especially since I'm using the little aluminum SMT "cans" on this design. I found 10uf, 25V in the size-A footprint (I believe the smallest of the cans). I'm no big fan of tantalum anyway, although they do come in smaller packaging I guess.

Thanks again for your help and I'll definitely post some picks as soon as I can!

Rog