Design/Build a DIY Audio Attenuator and Splitter

Question

I’m trying to be less of an appliance operator. To that end, I recently came up with a simple project that I thought I could figure out and build entirely on my own. I was wrong.

My goal was to create a splitter with an attenuator that would allow me to connect the external speaker jack output from my Yaesu FT-8900 to the microphone input on an iPhone in order to record, while simultaneously listening to the live audio via headphones.

I’m doing this because I’d like to record FM satellite QSOs as they occur while I’m outside using a handheld yagi (thankfully, building the antenna was within my skillset!). Currently, I just use an old Olympus DM-10 digital recorder Velcro’d next to the built-in speaker, which is less than optimal.

My problem is I get lost in the calculations for resistor sizes needed to get from the higher voltage output on the FT-8900 speaker jack to the millivolt input on the iPhone, as well as well as how to physically wire everything from the mono TRS plug for the radio side to the stereo TRRS plug for the iPhone side, as well as splitting-off for a headphone jack.

For reference, the specs on the Yaesu output is:

• Maximum AF Output: 2 W @ 8 Ω for 5% THD
• AF Output Impedance: 4-16 Ω

Any help would be greatly appreciated. Thanks in advance.

To answer Chris K8NVH's questions:

As far as headphones always being plugged in: The vast majority of the time, I would be using headphones. However, it would be nice to have the ability to use the device both with and without headphones attached.

As far as line level drop: if the drop is such that it doesn't render the resulting audio unusable, it doesn't really matter to me. I don't mind turning up the volume when playing back if that is the only consequence.

I wanted to go with the iPhone over the Olympus for a number of reasons:

1. First and foremost, it is very easy to accidentally hit a button on the Olympus and cancel a recording. This has happened to me in the past. With the iPhone, this would not be a concern.
2. I already use the iPhone when I'm working the satellites outdoors to see times, tracks across the sky, etc. Using it over the Olympus would allow me to eliminate one piece of equipment
3. The iPhone offers better battery life. Its surprising how fast the Olympus goes through AAA batteries.

But, in the end, simple is probably better. If that means sticking with the Olympus and creating a circuit that is designed to be used only with the headphones plugged in, I can accept that for the time being. I can always try a more involved design later.

Answer 1

Pulling some information from Electronics Stack Exchange, https://electronics.stackexchange.com/questions/38452/electronic-aspects-of-iphone-3-5mm-audio-output and https://electronics.stackexchange.com/questions/30841/how-do-i-build-a-trrs-headphone-jack-with-a-jack-that-is-detectable-by-an-iphone and Apple Stack Exchange, https://apple.stackexchange.com/questions/326149/how-to-record-a-audio-with-iphone-via-input-cable-like-aux-from-laptop here is my estimate of what such a circuit would look like. There are some assumptions built in here, as will be explained.

^{simulate this circuit – Schematic created using CircuitLab}

A simple voltage divider is 2 resistors. The values are somewhat arbitrary, but the iPhone seems to need 1.6k Ohm input impedance to recognize the item as an input. Additionally, one of the linked pages states full scale input is 4mA (I cannot verify). The circuit below is expected to provide a max current to the iPhone of about 2mA (1), about 1/2 of full scale amplitude; 3.2 volts. The iPhone needs to "see" 1.6k Ohm, the first resistor can be adjusted either up or down to get the amplitude right (either use a variable resistor or perhaps a small handful of larger resistors and keep adding more in parallel as needed) but that second resistor should stay close to 1.6k Ohms.

Further, microphone input circuits often include a DC bias voltage to power the microphone. To prevent that from causing any issues with the radio, best to place a small capacitor (say, 10 micro-Farads, though probably any capacitor you have laying around is "good enough") in front of this voltage divider. More about this below.

The headphone jack can be wired in parallel to the voltage divider. If your headphones are high impedance, it will have minimal effect on the circuit, though you will hear "pops" when you connect and disconnect. If your headphones are lower impedance, a headphone amplifier would be appropriate (easy enough to buy or build).

Connecting to the iPhone seems tricky, but the steps to do it are outlined and/or linked to in the Electronics Stack Exchange links from earlier. In particular, Wikipedia https://en.wikipedia.org/wiki/Phone_connector_(audio)#TRRS_standards seems to believe the iPhone TRRS connector has Sleeve=Microphone and Ring 2=Ground. There is an odd note in a linked article ( http://martinjohnsoncommunications.blogspot.com/2012/04/iphone-external-mic-connection-solved.html ) stating that some TRRS connectors have a sleeve which will touch the phone case, causing problems --if your connector does that, add a small rubber washer to isolate it. So connect the voltage divide output (that would be the space between the 2 resistors) to the TRRS Sleeve and Ground to Ring 2.

Having said all of this, I am not sure this circuit is completely necessary. Certainly C1 is a Good Idea and R2 seems Important, but R1 might be optional. If 4mA really is full scale with an input impedance of 1.6k$\Omega$, that would mean the phone's max range is really 6.4V and R1=0.

One more pedagogical note. Without knowing for certain what input the iPhone is looking for (+/-5 volts? +/-4mA?) or its input impedance (1.6k$\Omega$? 16k$\Omega$?) or even the actual purpose of R2 (does that somehow signal that Sleeve is a microphone instead of a Line In, adding an additional amplifier to the signal chain, for example?) perhaps it makes some sense to do some quick error analysis. What is the effect of changing the different resistors?

$V_{phone}={V R_2 R_3}/{(R_1 R_2+R_1 R_3+R_2 R_3)}$ - signal voltage seen by the phone $i_{phone}={V R_2 }/{(R_1 R_2+R_1 R_3+R_2 R_3)}$ - signal current seen by the phone

...where V=5Volts, the signal voltage generated by the radio. You can use these relationships to identify what happens if things change. This might help you decide which high values for R1 to keep on hand in case things are not quite perfect the first time.

Last item, I think. The capacitor forms a high pass filter with the voltage divider and the load. The cutoff frequency for a 10 micro-Farad capacitor and an 800$\Omega$ resistance --2x 1.6k$\Omega$ resistors in parallel-- is about 20 Hz. A larger capacitor will decrease that frequency and a smaller one will increase that frequency. Since you are using this for voice recordings, any cutoff below about 200Hz will work well; this corresponds to a 1 micro-F capacitor. For signal processing reasons, there is an incentive to use the smallest capacitor you can get away with but such a discussion is beyond the scope of this answer :-)

(1) Assuming the max voltage put out by the FT-8900 is about 5 volts and the internal impedance of the iPhone is about 1.6k Ohm (apparently Apple changes this with different models).