Coupling Cap Types and Voodoo Parts?

[makinrose]

I was wondering what people's opinions were on coupling cap brands and materials.  There lots of expensive parts out there that in the fine print seem to be  polyester or polypropylene caps in fancy large packages.  Is that any difference in sound? Can there be? or is it just a money grab? 


My experience has been that I can hear difference between different material types.  To me polyester/mylar caps sound a little warmer and less detailed than polypropylene.  Ceramic tend to more gritty and harsh. New paper type caps sound a lot like polyester but maybe a little less bright.


The other thing I've found I really cannot tell much if any difference between brands and construction types of the same material.  For example the little Panasonic radial polyester caps sound much the same as the Mallory 150 film and foil polyester. For that reason I'm really skeptical that spending $5 to $15 on coupling caps is worth while. Does the size or construction have noticeable tonal effect? 

What's the lowdown on it?   Thanks! I'm really interest to hear everyone's thoughts.

[K O'Connor]

Hi Guys

Polester/mylar adds much more distortion than polypropylene, which is essentially distortion-free. Polyester adds odd-order harmonics which will be heard in different ways insofar as these harmonics become part of the input signal to the following gain stage, which distorts the fundamental and all these extra tones, so "fatter" is a possibility. The extent to which the cap distorts depends on the signal voltage across the cap.

If you want minimal ceramic cap distortion, use NP0 or C0G; for maximum grit use X7R.

Within a given dielectric range, the physical size of the cap will effect parasitic elements contributing to tone, as TUT3 states.

[makinrose]

Hi Guys

Polester/mylar adds much more distortion than polypropylene, which is essentially distortion-free. Polyester adds odd-order harmonics which will be heard in different ways insofar as these harmonics become part of the input signal to the following gain stage, which distorts the fundamental and all these extra tones, so "fatter" is a possibility. The extent to which the cap distorts depends on the signal voltage across the cap.

If you want minimal ceramic cap distortion, use NP0 or C0G; for maximum grit use X7R.

Within a given dielectric range, the physical size of the cap will effect parasitic elements contributing to tone, as TUT3 states.

That  explains my affinity for polyester caps!  
 
The  line  that stuck out to me in TUT 3  was in 4-12 and reads " All that is required to make small parts sound like large ones, is an airy layout".  So what I'm inferring is that that state in conjunction with what you wrote on the post is that the parasitic elements are a function of layout (which is influenced by the size of the parts)  rather than the  size directly?

Which lead to one more question:  when you layout your amps are there rules of thumb that you use to ensure the spacing used minimizes parasitic element?

[K O'Connor]

Hi Greg

Do you mean "you" as me or "you" as anyone?

I don't use a rule of thumb for that but you can look at industry standards that follow a rule of life inasmuch as "empty space does not remain empty, which also applies to time. Take any hand-wired Fender amp. The circuitry is very simple yet it manages to occupy the entire internal space of the chassis, from Champ to Twin Reverb. There is lots of space around the components even though they seem cheek-by-jowl and the main filter caps on nominally "outside" the chassis. If you look more closely there is more space than occupation, so the parasitics in this assembly are low.

In more modern amps, costs are considered differently and most components are physically smaller than vintage types, combining to make tight assemblies with true empty space around the boards. In some cases, the boards are laid out airily to keep wiring short, as in re-issue Fender amps. The tight board has small parts and proportionately the parasitics might not have increased very much, while the airy modern board with small parts likely has similar parasitics to the eyelet board. The board materials will be different but the spacings are also different.

TUT3's guideline is an easy one for any hobbyist to follow, and choosing physically large components can force an airy layout, but could also lead to tight spacing if everything is large. There is a balance when you consider how the component carrier layout influences how the wired connections to the carrier must be laid out, sometimes beneficially and sometimes not.

In a test case, you might have a reference amp who's tone you wish to capture in a new build, but using entirely different component types and assembly methods. Intuitively you might begin with the same circuit, but you might find that you have to add some new circuitry or make modifications to be true to the tone. That is fine. That's what I do as I see each new format as a different amp even if it is the same circuit.

[makinrose]

Kevin,

Thanks for explanation. I had meant you/London Power when I had asked since I'm curious abut your design methodology.
One of the reasons I ask was because I'm in the situation I think lots of your readers are in: as they learn more about proper design they find the common pre-made boards and some chassis for popular amp models won't accommodate the extra de-coupling and galactic grounding. It's just better to build a board and sometimes use different chassis too. The last paragraph is particularly helpful to me. Thank you!

[jmcd]

If you want minimal ceramic cap distortion, use NP0 or C0G; for maximum grit use X7R.

Where do Y5P, SL, and Z5U fall on the ceramic cap grit continuum?

[I suggest this thread be moved to the preamp mods section, or perhaps a new section for components.]

[K O'Connor]

Hi Guys

Ceramic "tone" as it were is not necessarily linear with type. In a broad stroke, you could put them from cleanest to dirtiest corresponding to their temperature stability, where the most stable types are the cleanest.

Also, the higher the capacitance the more ceramic distortion there will be. This is why for the most high fidelity applications, ceramics over 100pF are rarely used.