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[Russell Ault]

I'm on a show where I'm using a CP200 with a fist mic. During the show I plug one of those surveillance earpieces (the one with the little tube) into the jack on fist mic. This works well, but the earpiece isn't particularly comfortable. I have a set of custom-moulded IEMs that I find very comfortable, so I thought I'd switch to using those (well, the left one, since the fist mic only has a TS jack, but that's no problem).

For reasons I don't entirely understand the IEMs sound pretty bad. There's a lot of distortion, even with the volume practically off, and the CP200s AF amplifier clicking on and off—which is faintly audible using the tube earpiece—is annoyingly loud.

I'm aware of products that would solve this (and many other problems), like the AdaptEar (etc.), but that does a lot more than what I'm looking for (and costs a lot more than I was hoping to spend).

I'd like to build an adapter that will take the 3.5mm TS output from the CP200's fist mic and make it earbud-compatible. I checked bestaudio.com and didn't see a suitable design (but I might have missed it, too). Does anyone know what kind of passive circuit it would take to make this work?

Thanks!

-Russ

[Scott Helmke]

Basically you just want a resistor pad.  The incoming signal from the radio goes through a resistor (guessing at maybe 100 ohms), then another resistor (make it the same for a 6dB reduction in volume) to ground.  Pick your earbud signal off the junction between the two resistors.

I'm very poorly describing a voltage divider, which is the same thing that a guitar volume control uses.  You could even use a pot for adjustable attenuation.

[Russell Ault]

Basically you just want a resistor pad.  The incoming signal from the radio goes through a resistor (guessing at maybe 100 ohms), then another resistor (make it the same for a 6dB reduction in volume) to ground.  Pick your earbud signal off the junction between the two resistors.

I'm very poorly describing a voltage divider, which is the same thing that a guitar volume control uses.  You could even use a pot for adjustable attenuation.

Something I should have added: the distortion I'm hearing is present even when the overall level isn't very loud (i.e. I'm not simply overdriving the IEMs' driver); I'm not sure a pad will solve that, but I can try it. Am I correct in assuming that, given an IEM impedance of 29 ohms, the resistors in the pad should be much lower than that?

A pad also won't solve the clicking. I've seen a suggestion to use a pair of Schottky diodes paralleled back-and-forth between the two leads, but I'm wondering if some kind of filtering (low-pass?) would be a better bet?

Thanks!

-Russ

[Scott Helmke]

The big question is what would be the best impedance for the radio's output.  You could afford to give up quite a bit of power if you're going into earbuds.

[Ike Zimbel]

Something I should have added: the distortion I'm hearing is present even when the overall level isn't very loud (i.e. I'm not simply overdriving the IEMs' driver); I'm not sure a pad will solve that, but I can try it. Am I correct in assuming that, given an IEM impedance of 29 ohms, the resistors in the pad should be much lower than that?

A pad also won't solve the clicking. I've seen a suggestion to use a pair of Schottky diodes paralleled back-and-forth between the two leads, but I'm wondering if some kind of filtering (low-pass?) would be a better bet?

Thanks!

-Russ

I think you're on the right track here. My guess is that you are hearing stuff that is already there beautifully reproduced by the hi-fidelity IEM.

[Brian Hancock]

I’d suggest you reach out to the guys at racing radios / diversified electronics ... they helped pioneer in car communications for racing and if I’m not mistaken do some of the in helmet stuff in football.

They can make you custom molds or I’m sure an appropriate adapter.

Coincidentally we made a box a wile back that had radio, intercom, and board phono audio and combined them I’ll see if I can find the schematic ... it had a Ptt for com but could easily have been a toggle to key the radio. 

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