Where to put the outer wrap of caps in a Tone control circuit?

[R.G.]

Measuring the output would be interesting. The trouble is it would be a lot of work removing and replacing. Testing each one is required.

It is indeed a lot of trouble, toil and testing. And each cap in every position in the amp is indeed necessary to test - as well as discovering which is the outer foil lead.

I was serious about shielding the capacitor. This removes any issues whatsoever about what is the outside foil. But it's even more trouble, toil, and testing. It makes for a small capacitance to ground added on one or the other lead ( and this is how I'd test for which foil was outside, btw; I was kidding about not knowing any nondestructive means) but this is generally quite small, and probably smaller than the added capacitance of using shielded wires, at a guess. Needs testing.

There are those among us that claim there is a huge difference. I just do it and drive on, which is no problem for a hobby builder. AFAIK this practice was common in tube radios, I have seen it myself.

Yeah, there is a lot of claiming huge differences for things that are not an obvious advantage to the circuits and the amp as a whole. Some of these are real, not formally recognized things. Some are expressions of the human minds' need for there to be real magic. To me, if it can be reliably and repeatably measured in ways that winnow out the human mind's unavoidable biases, there is no question that it's real. If it cannot be measured, it may simply be too small to measure yet.

However, there is a whole lot of hifi tweako canon that says that the human ear/mind can and regularly does hear things that cannot possibly be measured, and/or that the mere act of trying to measure it makes it unmeasurable, or some such misinterpretation of the Uncertainty Principle (which is also regularly trotted out). Of course, the hifi tweako crowd got so burned by their poor performance on double blind tests that they run screaming from any reliable testing.

I'm at the position that if the difference is "huge" it ought to be measurable and repeatably so, when the effect is isolated and properly studied. If it's a "small" difference, I'm OK with it might be both real and too small to measure with today's testing. But today's testing is pretty sophisticated. If it's too small to measure with something like a current Audio Precision setup, my guess is that it can't be a very big audible effect, either.

As for "outer foil" being an issue in tube radios - sure, I believe that on its face. First, all caps except possibly bypasses are order of magnitudes smaller in RF in general, and second, the frequencies are orders of magnitude bigger, so the effect of a "few picofarads" of stray capacitance will also be orders of magnitude bigger. Also, tube radio performance is/was carefully measured in the normal course of things. Those folks will nail down differences. So that is very plausible to me. But for low frequency audio as in a guitar amp? I'm open, but my current estimation is that the chances of it being very much against it making an overall difference in sound. So, for someone who wants to know instead of just believe, show me it's not just wishful thinking.

[Charlie Wilson]

Noise is not the only reason you would play around with outer foil direction in a guitar amplifier.
CW

[martin manning]

Noise is not the only reason you would play around with outer foil direction in a guitar amplifier.

Above I mentioned two possibilities:

Cap orientation has the potential to reduce the introduction of noise from 60 and 120 Hz hum, and also, I would think, the reintroduction of signal from adjacent leads and components.

Are there others?

[Charlie Wilson]

There is a sonic difference. More noticeable in some caps than others. We do talk about this in the Dumble Discussion.
CW

[pjd3]

While on this subject (and something I just contended with on a different thread here),

Can anyone explain what technical dynamics are going on while attempting to do the outer foil test with an amp? When you bring your thumb an finger around the outside of the cap what is actually happening to generate the increasing buzz/noise?

I assumed that the proximity of our bodies increases capacitance (same principle as a theramin?) as one of the effects but how is that increasing the audibility of the buzz? At that point are we tuning the circuit into the range that provides say, a lower impedance to the electromagnetic interference that are happening within that tuned frequency range? Or is there something in our body that gets amplified? I'd be interested in clarifying that dynamic and how the outer foil is interacting with whatever dynamic is occurring.

Thanks,
PJD3

[pompeiisneaks]

While on this subject (and something I just contended with on a different thread here),

Can anyone explain what technical dynamics are going on while attempting to do the outer foil test with an amp? When you bring your thumb an finger around the outside of the cap what is actually happening to generate the increasing buzz/noise?

I assumed that the proximity of our bodies increases capacitance (same principle as a theramin?) as one of the effects but how is that increasing the audibility of the buzz? At that point are we tuning the circuit into the range that provides say, a lower impedance to the electromagnetic interference that are happening within that tuned frequency range? Or is there something in our body that gets amplified? I'd be interested in clarifying that dynamic and how the outer foil is interacting with whatever dynamic is occurring.

Thanks,
PJD3

Our bodies act like an antenna, and then couple with the capacitor, injecting noise.

~Phil

[bmx]

Noise is not the only reason you would play around with outer foil direction in a guitar amplifier.
CW

I currently have a 3 channel build that I did a very compact layout for (Unfortunately). I originally oriented all my outer foils to the low impedance side of things. I had an issue with a 330nF capacitor used to D.C. isolate the tone-stack that was coupling signal into the input capacitor of the PI. Capacitor to capacitor coupling. And really, the Phase Inverter input coupling cap is high impedance on both ends. This coupled signal was bypassing the tonestack, effects loop and global master volume! So, I couldn't actually turn the master volume to zero and get a full mute. Not only that, but the coupled tone was ruining the tone of the amplifier. The Zero master volume tone of the amp was trash until you get the master up high enough that the straight tone drowns out the coupled tone. to find the problem, I grounded a piece of copper tape and put it between my capacitors that had close proximity. Now I know why you will see that sometimes on the very dense mesa layouts for example. It does work! In the end, I was lucky and could move capacitors without making a mess. I have thought about flipping the outer foil to the high-impedance side to make the cathode follower a less good radiator of square waves. At 415V on the plate, this thing has some swing when clipping! And there is still some residual sound left with the master on zero. Anyone else ever use the outer foil to high impedance to remedy unwanted coupling?

[R.G.]

@ Phil: That is of course correct. But I'm still puzzling over why "outer foil" would make much difference in such a test. The inner and outer foils are connected together with the nominal value of capacitance. If the measuring connection is to the inner foil, the signal is transferred through the nominal capacitance, which is nearly always bigger than the capacitance from the universe to the body picking up the noise, or from the body to the outer foil. It's not clear to me why that would not completely swamp out any outer foil effects.

Now grounding the unused lead, that would cause an untested outer foil to shield the inner foil effectively (if and only if the construction is cylindrical wraps), while only showing the nominal capacitance to ground when the inner foil is being grounded. Still, the small capacitance to the outer foil would make this hard to measure. I think. Not sure, which is why I'm till puzzling.

Of course, stacked-layer constructon means there is no outer foil, only a top and bottom foil.

I'm still back at "how the devil can outer foil make an audible difference in sound or noise out of a guitar amp?"

@bmx: Good on you for testing shielding on caps. It does work. A turn of copper tape around the cap and a wire from the tape to ground is a huge step in intercepting external fields before they can get in the cap. It's the same reason we use a grounded shield on input cables.

The problem with using an outer foil as a shield is that to do any good as an electrostatic shield, it must be connected to signal ground by a "low" impedance. Unless your circuit connects that outer foil to a low impedance point, it's going to pick up external signals again. Outer foil to high impedance is unlikely to do any good.

[BobSimpson]

I do not waste a lot of time aligning capacitor leads.
I would bet $5USD Leo and Bran didn't either, after the markings printed on the cap became untrustworthy.

I would think a really conscientious builder might need to test and measure every possible combination of capacitor lead positioning.
And I would think the results might be different with every amplifier built.
And every listener would prefer a different combination.

Life is too short.

Bob

Wait a minute. Is there a government grant available for this kind of time wasting?

[R.G.]

I do not waste a lot of time aligning capacitor leads.
I would bet $5USD Leo and Bran didn't either, after the markings printed on the cap became untrustworthy.

I would think a really conscientious builder might need to test and measure every possible combination of capacitor lead positioning.
And I would think the results might be different with every amplifier built.
And every listener would prefer a different combination.

Life is too short.

Bob

Wait a minute. Is there a government grant available for this kind of time wasting?

Well, as I remember there was a grant for investigating where your lap goes when you stand up.

I agree all points. What if the GREAT sound of a given amp happens only when ONE of the caps is outer-foil reversed? One really have to try the number of caps factorial to get the best-est sound from outer foil orientation? What if some of them offset others? What if the outer foil reception is adding very subtle positive or negative feedback that makes the amp sound GREAT?

I do think that the time would be better spent trying to ever more deeply separating the flavors in a good single malt.

[pompeiisneaks]

...

I do think that the time would be better spent trying to ever more deeply separating the flavors in a good single malt.

YES indeed

~Phil

[BobSimpson]

Or a glass of Red Breast Irish?

Bob

[R.G.]

Me favorites are Tullimore Dew if I'm feeling Irish, and Cragganmore if I'm in a Scots mood.

But really - if you get the chance, get yourself a bottle of Suntory's Hibiki Japanese Harmony. It's worth the cost.

[BobSimpson]

Recommendation taken.

Thanks!

Bob

[sluckey]

Never knew about Tullimore Dew other than this...

https://www.youtube.com/watch?v=qUgUcVnT3NI