Okay, it has four pins, and appears to include a capacitor wired to two of those four pins.

But what is it, and what does it do ???

Not sure immediately, but can we see more of the circuit around it?

Got a photo of a physical one or are you just looking at a schematic?

Maybe a three-lead feedthrough capacitor? I don't know if that even makes sense.

It looks more like a two-way sneeze-through wind vent. I don't know if that even makes sense.

Well, pins 4,2 are both grounded, and pins 1-3 are a feed through of line-level audio, just before a final series resistor.

That's all I can say, really.

???

Is it some kind of top secret design we're not allowed to see?

In that case I'd say it's simply some kind of shielding intended to protect the audio signal from any outside interference. I'd guess the capacitor has a metal casing to go along with that.

My own guess is that it's some kind of high-transient filter device, but I've never seen/heard of these things until now. The actual surface-mount component looks like an elongated surface-mount fuse, with two side tabs. No metal sheilding.

Peculiar.

I'm just asking out of curiosity, not any particular need.

Cheers

Hi,

Judging from the capacitance value (0.0022uF), it is a feed-through capacitor.

The reason they are there is to remove the high frequency noise caused by clocks, strobes, data buss activity, etc from the audio leads to meet EMI compliance and prevent parasitic oscillations. They can also be used for power filters, oscillator power filters (with care), etc.

If you look near the Audio RCA jacks on the Empeg Mk2a motherboard, you will see 4 of them near the white cable connector. They are the row of 4 brownish rectangular shaped SMT components.

The center of the component is pins 2 and 4 and it wraps around the sides which is the outside body of the capacitor. It is connected to the lowest potential point in the system (Ground or Chassis Ground).

The center pins 1 and 3 are the signal pins and are in fact part of the multi-layer ceramic capacitor (alternate layers of the ground side pins 2 & 4 and the center pins 1 & 3). That's where the 2.2nF comes from.

The feedthrough filter strips off common mode and differential EMI noise. To meet EMI requirements, (FCC or CE-Mark) there is a limit as to how much noise is allowed to be radiated from connected cables as well as the Empeg cabinet. The Empeg housing provides some EMI containment (it has alot of open edges and slots and the wiring can still pick up common mode noise), and when a cable is connected to a connector like the RCa or Docking connector, noise couples to the cable conductor (if it has a crappy shield bond to the mating connector), or cable shield and may radiate noise. Since the RCA connectors do not go directly to the chassis there is a potential difference between the RCA connector shell and Chassis ground. There is also a ground potential difference between the Empeg and the receiving device it is connected to. The potential difference between the two allow an RF voltage to occur across the interconnect cable which radiates noise to the surrounding area.

The source of the noise on the cable is the energy conducted on the lines exiting the Empeg cabinet containment shell. Conducted noise on unintended lines is generally a source of radiated emissions non-compliance.

The capacitance value must be selected as not to attenuate the pass-band (audio frequency range in this case) but, attenuate the unwanted higher frequencies. A 2200pF capacitor at 20kHz is around 3.6k Ohms. The frequency attenuation roll-off and turnover frequency depends on the source and destination Impedances.

There is a similar filter near the power coaxial connector (the verticle dark looking box near the 4 pin power connector). Since there are higher currents involved and conducted noise limits are strict, is has more filtering and requires larger components.

If it's not this then I have also seen the schematic symbol used in TV Horizontal Output circuitry and it is referred to as a Safety Capacitor. It offers protection if the capacitor or other circuitry fails.

I know, too much information.

Ross

And here I had MST3K:The Movie playing in my head...

"Where do we start?"

"Right here." (Points to symbol on alien schematic.)

"... at happy clown face."

At a rough guess, and it IS only a guess, it's a Murata NMF40R series 3 terminal capacitive feedthrough filter, part number MFE31PT222Z1E9L, Farnell order code 952-8172, Page 941 of book 1 of the 2007 catalogue.

Bottom left of the page.



pca

Wow.. SWMBO & I are, like, totally speechless.
(and yes, you nailed it).

Yes, but much appreciated by us commoners!

Cheers