John Curl's Blowtorch preamplifier part II

[DF96]

Originally Posted by john curl

I should go out of my way to . . . Have I gotten this right?

No, not really. You seem to be exaggerating a minor point.

All this talk of hole size wrt wavelength partly misses the point. We don't have a hole, we have a hole plus wire poking almost through the hole. Inside the cage there will be a rapidly decaying (evanescent?) field from the hole, but it doesn't become zero imediately. Poke a conductor into that space immediately behind the hole and you can sample the EM field just outside the cage. To make a good Faraday cage you need small holes (wrt wavelength) and nothing inside the cage too near a hole. A regular array of holes is called a diffraction grating, so you could get effects which depend on exactly which direction you hold your phone.

A plastic light tube would probably be fine, unless it acted as a dielectric resonator. Isolating the wires would work - chokes or feedthrough capacitors. Making the circuitry itself relatively RFI immune would also help. I think it is unlikely that adding a panel LED to a well-designed circuit will seriously damage the sound. If it does, then the circuit needed better EMC performance anyway so the LED was a useful diagnostic tool.

[SY]

Originally Posted by Wavebourn

3 mm is approximately 1/20 wavelength of 5.8 GHz. I have chassis made of perforated aluminium, full of such holes. Still, no radio reception.

Likewise. Quite a few classic and excellent amps even have tubes on top of the chassis.

Then again, I don't use my cell phone while doing critical listening, its presence next to my left ear unbalances the imaging.

[PMA]

Originally Posted by DF96

We don't have a hole, we have a hole plus wire poking almost through the hole. Inside the cage there will be a rapidly decaying (evanescent?) field from the hole, but it doesn't become zero imediately.

But he has INSULATED I/O connectors on the rear panel with all those cables going "through the hole" inside the preamp and this is fine??

You guys are exaggerating problems that do not exist and overlooking obvious issues, this is so usual in audio design.

[dennis h]

Originally Posted by PMA

But he has INSULATED I/O connectors on the rear panel with all those cables going "through the hole" inside the preamp and this is fine??

You guys are exaggerating problems that do not exist and overlooking obvious issues, this is so usual in audio design.

Thanks PMA , a simple search of PMA as the author and "connnector" as the subject brought me a whole new world to ponder...
dennis h

[john curl]

What I am being 'criticized' for not adding an LED. '-)

[jacco vermeulen]

Mr Curl,
looks fine to me, without the LED.

[SY]

Originally Posted by john curl

What I am being 'criticized' for not adding an LED. '-)

No, your questionable reasoning was criticized, not you.

[DF96]

Originally Posted by PMA

You guys are exaggerating problems that do not exist and overlooking obvious issues, this is so usual in audio design.

No, just explaining why an LED in a high RFI environment apparently giving rise to audio problems does not mean that LEDs damage sound but it could mean that the wires to the LED are breaching the screen. This is to correct two opposing views:
1. LEDs damage sound. (No they don't, but the wires might if there is poor EMC performance elsewhere in the circuit.)
2. A panel LED can't possibly damage the sound. (True for the LED itself, but it needs wires and they might have an effect.)
I agree that it is likely to be a small problem.

Of course, other wires might cause problems too but we are not talking about other wires.

[john curl]

My questionable reasoning has made me very well known and respected as an audio designer. The Vendetta Research phono stage had the same dilemma and we never added an LED, as well.

[PMA]

Originally Posted by DF96

Of course, other wires might cause problems too but we are not talking about other wires.

It would be nice to distinguish between important and unimportant issues. As usually, unimportant 'issues' attract much more attention than necessary and important issues are not discussed.