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It's a two-wire capsule () -- AFAIK the ground terminal is connected to the capsule body in that capsule. I have continuity between capsule body and plug sleeve. It just seems like the connection between the cable and the capsule (maybe 5 or 6mm) where the wires are exposed is enough to admit hum?

Not in normal circumstances. Are you sure that the body in which you put the capsule is properly grounded?

No: there is no body -- well, no metal body; it's inside a bakelite antique wall-mounted telephone mouthpiece. I was under the impression that a 2-wire PiP capsule acts as its own shield so I didn't need shielding, but apparently I'm wrong and the join from cable to capsule does need to be shielded?

Yes indeed.

Got it, thanks for the education.

There is a splice near the capsule, now shielded, and a splice near the plug which is not shielded. The second splice does not cause hum -- is that because it is further from the capsule?

I don't think so. It looks like there's more EMI/RFI near the capsule. Why, IDK. It depends on too many factors. Or perhaps the splice near the capsule is more exposd than the other.

-c

¿ªÔÆÌåÓý

It's a two-wire capsule () -- AFAIK the ground terminal is connected to the capsule body in that capsule. I have continuity between capsule body and plug sleeve. It just seems like the connection between the cable and the capsule (maybe 5 or 6mm) where the wires are exposed is enough to admit hum?

Not in normal circumstances. Are you sure that the body in which you put the capsule is properly grounded?

No: there is no body -- well, no metal body; it's inside a bakelite antique wall-mounted telephone mouthpiece. I was under the impression that a 2-wire PiP capsule acts as its own shield so I didn't need shielding, but apparently I'm wrong and the join from cable to capsule does need to be shielded?

Yes indeed.

Got it, thanks for the education.

There is a splice near the capsule, now shielded, and a splice near the plug which is not shielded. The second splice does not cause hum -- is that because it is further from the capsule?

-c

¿ªÔÆÌåÓý

It's a two-wire capsule () -- AFAIK the ground terminal is connected to the capsule body in that capsule. I have continuity between capsule body and plug sleeve. It just seems like the connection between the cable and the capsule (maybe 5 or 6mm) where the wires are exposed is enough to admit hum?

Not in normal circumstances. Are you sure that the body in which you put the capsule is properly grounded?

No: there is no body -- well, no metal body; it's inside a bakelite antique wall-mounted telephone mouthpiece. I was under the impression that a 2-wire PiP capsule acts as its own shield so I didn't need shielding, but apparently I'm wrong and the join from cable to capsule does need to be shielded?

Yes indeed.

-c

¿ªÔÆÌåÓý

It's a two-wire capsule () -- AFAIK the ground terminal is connected to the capsule body in that capsule. I have continuity between capsule body and plug sleeve. It just seems like the connection between the cable and the capsule (maybe 5 or 6mm) where the wires are exposed is enough to admit hum?

Not in normal circumstances. Are you sure that the body in which you put the capsule is properly grounded?

No: there is no body -- well, no metal body; it's inside a bakelite antique wall-mounted telephone mouthpiece. I was under the impression that a 2-wire PiP capsule acts as its own shield so I didn't need shielding, but apparently I'm wrong and the join from cable to capsule does need to be shielded?

-c

¿ªÔÆÌåÓý

Thanks Chris and Jerry:

Jerry wrote:

It seems to indicate that the capsule body is not properly grounded.
In 3-wire connection, there is no need to shield teh capsule, as long as the body is connected to teh shield. Check your connections.

It's a two-wire capsule () -- AFAIK the ground terminal is connected to the capsule body in that capsule. I have continuity between capsule body and plug sleeve. It just seems like the connection between the cable and the capsule (maybe 5 or 6mm) where the wires are exposed is enough to admit hum?

Not in normal circumstances. Are you sure that the body in which you put the capsule is properly grounded?

Chris wrote:

It is not stereo, it is standard PC microphone wiring of a TRS jack, sleeve is shield + signal and power return, tip is signal, and ring provided power for a capsule with built-in FET.

If I use my separate stereo PiP mic, which is wired one capsule to tip and one to ring, both mics work in this interface (meaning, their input shows up in the mono output)... does that contradict anything that you said above? E.g. it seems that ring isn't just for power but is actually receiving a separate signal (which is mixed to mono before going to the computer).

Also, currently the mic is just wired signal-to-tip, shield-to-sleeve, so ring isn't connected to anything, and it works (with hum), implying that the bias isn't only on ring?

However, most of the information I could find about PC standard microphone connections indicated that both the resistor and the DC-blocking capacitor are typically in the audio interface, in which case I don't see why connecting tip and ring together at any point would not work.

I may have been unclear: connecting tip and ring does work, it just didn't eliminate any hum so it didn't solve the issue.

I am not familiar with"PiP," does it match that connection described above?

I admit being mystified at the myriad ways there seem to be for 1/8" jacks to be wired, so I don't know if it matches or not, but PiP is just "plug in power" referring to lower voltage supplies on (usually 1/8") mic inputs.

I see that there is another response which came in while I was typing which indicates that you did add better shielding using conductive tape.

Yeah that seems to solve it completely.

-c

¿ªÔÆÌåÓý

Thanks Chris and Jerry:

Jerry wrote:

It seems to indicate that the capsule body is not properly grounded.
In 3-wire connection, there is no need to shield teh capsule, as long as the body is connected to teh shield. Check your connections.

It's a two-wire capsule () -- AFAIK the ground terminal is connected to the capsule body in that capsule. I have continuity between capsule body and plug sleeve. It just seems like the connection between the cable and the capsule (maybe 5 or 6mm) where the wires are exposed is enough to admit hum?

Chris wrote:

It is not stereo, it is standard PC microphone wiring of a TRS jack, sleeve is shield + signal and power return, tip is signal, and ring provided power for a capsule with built-in FET.

If I use my separate stereo PiP mic, which is wired one capsule to tip and one to ring, both mics work in this interface (meaning, their input shows up in the mono output)... does that contradict anything that you said above? E.g. it seems that ring isn't just for power but is actually receiving a separate signal (which is mixed to mono before going to the computer).

Also, currently the mic is just wired signal-to-tip, shield-to-sleeve, so ring isn't connected to anything, and it works (with hum), implying that the bias isn't only on ring?

However, most of the information I could find about PC standard microphone connections indicated that both the resistor and the DC-blocking capacitor are typically in the audio interface, in which case I don't see why connecting tip and ring together at any point would not work.

I may have been unclear: connecting tip and ring does work, it just didn't eliminate any hum so it didn't solve the issue.

I am not familiar with"PiP," does it match that connection described above?

I admit being mystified at the myriad ways there seem to be for 1/8" jacks to be wired, so I don't know if it matches or not, but PiP is just "plug in power" referring to lower voltage supplies on (usually 1/8") mic inputs.

I see that there is another response which came in while I was typing which indicates that you did add better shielding using conductive tape.

Yeah that seems to solve it completely.

-c

It seems to indicate that the capsule body is not properly grounded.
In 3-wire connection, there is no need to shield teh capsule, as long as the body is connected to teh shield. Check your connections.

powered from that interface via PiP.

?

When I connect ring to ground (which I think is what a mono plug will do, right?) the mic goes totally dead.

Progress! Aluminum foil wrapped around the capsule body (but not covering the face of it), which covers the wire connections to the capsule from the wire, makes the hum disappear. I'll solder in some copper foil to the situation.

Is this standard best practice for PiP mics that I should have known about?

Get a mono plug. It'safer.

I think we've shown that it makes the mic go dead, though? When I connect ring to ground (which I think is what a mono plug will do, right?) the mic goes totally dead.

How is the capsule shielded? Cover the capsule with an empty can. If the hum is gone - you have improperly or insufficiently shielded capsule.

Per above, this indeed seems to be the issue!

-c

Meant to also reply to this:

Or just connect the mic to both inputs.

If I understand you correctly, I did try this; from previous email: "tried joining signal to ring (in addition to tip) but it didn't seem to make a difference."

Thanks,

-c

Could be that the mic power gets shorted out.? You could try a big electrolytic cap instead, shorting the hum to ground but not the DC.

Thanks -- recommended spec for the cap? I took a 16V 47?F, tied the negative lead to the shield and when I touch the positive lead to the plug ring wire the mic goes totally dead, as before.

(Or is that cap too big so it's still effectively shorting out?)

Or just connect the mic to both inputs.

@Gerwin:

I have tried to get rid of this hum many times, both with ultrasonic mic or normal audio. It is the raspberry Pi and the cheap sound card that will make it impossible. There are two solutions.

Thank you -- I have done this before with a Pi and was moderately successful when I covered the audio interface in grounded foil, and surrounded the mic itself with grounded foil (not the front of the mic of course)... that had almost no hum (and I wasn't even using shielded cable for the mic-interface connection -- long story). I also note that when I plug my twin PiP mic into this interface (tip and ring wired to different capsules, shared ground) there is no hum at all (though the overall quality is poor).

-c

So, is a preamp needed in such an application and, if so, what kind of preamp?

_________________________________________________________

NO. You should not require a pre amp for the SM58 and your transceiver. A pre amp is likely to overload the input state on the transceiver - inline pre amps typically provide between 20 and 30 dB gain, which is not required for this application.

The SM58 has a medium-hot output level (open signal voltage on the product data sheet) and it should drive the rig well enough without assistance. Do not worry about matching impedance - that is rarely needed on modern circuits (google this and you will learn a lot!) Also, avoid - do not use - the 5v BIAS (not phantom) bias voltage intended to power FET type computer gaming headset microphones and the like. You will likely have an unbalanced connection, so the bias voltage does not become or act or even seem like phantom voltage which only works as a phantom on a balanced cable/input. I say unbalanced, because pins 4 and 5 carry mic signals, without a third pin to ground. The schematic indicates one is signal and the other is shield, although I rather doubt that is the case as the cable has 8 internal twisted pair conductors, and none are shielded per se.

The FT900 has a front panel 8-pin modular, RJ45 network type connector - you need to be careful how you wire that . It should expect a dynamic cartridge per spec sheet - see link below. I believe it uses the standard Yaesu Hand Microphones MH-31A8J hand mic, which has a dynamic cartridge. Your Yaesu user manual should have pin out charts for all connectors, which will guide you in building a suitable cable / connector. Otherwise, vendors on eBay.com and elsewhere sell suitable cable adapters to mate the XLR on the SM58 with the 8-pin modular, RJ45 connector on the rig. You will also have to pay attention to the PTT connections, as the microphone on-off switch (if present) will only disconnect the mic from the rig - it will not activate the transmitter as a PTT switch - at least not without substantial re-wiring!

K8JHR

Schematic for your mic can be found here:

Could be that the mic power gets shorted out.? You could try a big electrolytic cap instead, shorting the hum to ground but not the DC.

Or just connect the mic to both inputs.

-Scott

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¿ªÔÆÌåÓý

An in-line preamp such as the Alctron or FETHEAD is certainly NOT suitable, if only because they need 48V phantom power.
Why do you think you need a preamp? Have you tried connecting directly the SM58 to the input? It should work. Now, maybe if it works but has not enough "juice", you may have to add a preamp, but you would not need much gain. The SM58 delivers about 2mV at normal speech level at a distance of about 2ft, so with close talk, it should be OK for the required 7mV for full excitation.

¿ªÔÆÌåÓý

Thanks Jerry and Scott! -- when I tie the unused wire from the plug to ground as suggested, the mic goes silent - meaning totally dead with no signal at all being recorded.

So, currently the live channel is going to tip, and ring is disconnected, resulting in hum, but joining ring to ground it goes dead, so I assume a mono plug won't help.

I have crudely tried joining signal to ring (in addition to tip) but it didn't seem to make a difference. I also crudely tried a 10k resistor between ring and ground (with no signal connection to ring) and it also made little difference (maybe like 2dB less hum, but didn't really help.) But I did both of these things in a hack way; just holding the exposed wires (using a plastic-handled tool so my hands weren't pinching the joint directly). I can try a more "real" version if it would seem useful.

Any ideas on a next move?

Some other notes in case they reveal that I am doing something dumb:

- mic cable is two conductors inside a braided copper shield; I use both of them joined to carry the signal from the mic (since I don't need both)

- mic is not shielded besides its factory metal enclosure, which AFAIK is adequate for PiP mics

- joint between mic cable and the pigtail leading to the plug (maybe 10mm) is not totally shielded: maybe this is my issue? Similarly, where the cable joins the capsule is not fully shielded (maybe 6mm). I had assumed these spots (which are small) might be irrelevant, but maybe I was naive.

- I mention again in case it's a clue: a different stereo PiP mic (one mic wired to tip, one to ring) works without hum, though the signal level seemed poor

- no idea what voltage the PiP supply is but I could measure if useful

Thanks for any further thoughts!

-c