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Mixing 2 inputs of variable impedance

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exomic

Member
Hi,

I'm flying allot and I'm trying to find a way to use my Bose QC20 earbuds during my flights with my aircraft so I can listen to both my music and the airplane Audio. I'm planning to make a microphone in the future if this project work well but for now let's just try to make the listening side of it.

I'm currently looking to mix 2 inputs to one output).

Input #1 : Audio coming from my aircraft's headset jack PJ-055.
Input #2 : Music from my iPhone 3.5mm jack

Output #1 will go into a 3.5mm female jack that my Bose QC20 earbuds will plug in.

I'm quite new to this and I made some research but I'm still not quite sure if I missed something so feel free to correct me if I'm wrong.

The Output #1 is my QC20 earbuds which to my understanding has a Low Impedance.
The Input #1 is my airplane audio from the headset jack which is made for aviation headset with High impedance.
The Input #2 will be my iPhone 3.5mm jack which is made for consumer headset with Low impedance earbuds.

My goal is to make a circuit that will merge both input and allow me to control the output volume with a logarithmic (Audio taper). What are the challenges that I have to face with the impedance difference of my inputs? Is there a way to build a electrical circuit that doesn't need a external power source like a battery and simply use the inputs power?

Thanks!
 

crutschow

Well-Known Member
Most Helpful Member
The Bose QC20 earbuds are powered so I expect their input impedance is high enough (probably at least 10k ohm) to be used with aircraft headset output.
Do you know what the impedance of the aircraft headset is?
 

MikeMl

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Most Helpful Member
Nowadays, I just use my Noise-Cancelling Lightspeed Aviation headsets which have music inputs, including BlueTooth...

Back in my David Clark days, I built a stereo amplifer which mixed monaural aircraft radio, monaural Garmin *****-in-a-box with a stereo music source, and fed it to the DavidClamps rewired for stereo...
 

MikeMl

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Most Helpful Member
Do you have an example of the circuit for your amplifier?
It is on another computer. I'll dig it out when I go over to my lab...
 

AnalogKid

Well-Known Member
Most Helpful Member
Are you talking about building a circuit from scratch, or wiring together some ebay modules? What is you skill set for assembling and soldering on perfboard.

ak
 

exomic

Member
Are you talking about building a circuit from scratch, or wiring together some ebay modules? What is you skill set for assembling and soldering on perfboard.

ak
I’m thinking about building a circuit from scratch and once the testing is completed I’m thinking about ordering a pcb based on the circuit and wrap all that into a small enclosure or box for the jack and volume knob.
 

AnalogKid

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The standard approach is to have an opamp stage for each input (either a voltage follower or with a little gain or filtering), followed by the mixing/combining stage, followed by output drivers. Some of these functions probably can be combined to reduce circuit size/complexity/parts count/cost, but keeping them separate makes things much easier to design, debug. and modify in the future.

What is the power source to run the circuit?

ak
 

throbscottle

Well-Known Member
Uhhhhhhhhhh ......................................................... no.
Given a powerful enough audio source it might be possible to harvest enough power from the signal (from the iPhone I suppose) to drive a low powered version of the circuit. I think it would be quite challenging and you might end up with bad audio though...
 

MikeMl

Well-Known Member
Most Helpful Member
I found the circuit on my other computer. What I built is permanently installed in my Skylane, and is in use every time I fly.

When I originally built it, I needed a mixer so that stereo music from an 1/8" stereo cable (Tip, Ring, Sleeve) is combined with three existing monaural sources, one being the HeadPhone bus coming from the aircraft's KMA-24 audio panel, the second being a monaural 4-place Sigtronics SPA-400 intercom, and the third being the Garmin Navigation system "Bbitch-in-the box".

The existing aircraft headphone audio comes the monaural headphone output from an installed King KMA-24 audio panel. I installed the Sigtronics 4-place intercom. Originally, the four headphone jacks permanently installed in the aircraft where monaural, and the previous installer made the cardinal sin of grounding each headphone jack to the airframe, and you could hear alternator whine in each headphone.

As part of the stereo mixer installation, I isolated the four headphone jacks in the aircraft using isolation washers, and rewired them using shielded cable to the output of my home-brew mixer. Look at the attached LTSpice simulation schematic. It shows only one channel. Find the LM386 in the upper right-hand corner.

The LM386 provides a default gain of 20 with a differential input, so effectively sums the signal applied to the non-inverting input to the signals applied to the inverting input. The LM386 is capable of driving a small 8 Ohm speaker, but in this application, even if all four headsets are plugged into the aircraft, I figured that the impedance that the LM386 has to drive is about 37 Ohms (See R6). I determined that a good listening level at the aircraft headphones is about 2Vpp, which the LM386 can easily do since it is powered with 14V. C7 is just an output coupling/DCblocking capacitor. R10-C6 is the required snubber network (see the LM386 data sheet).

The music input to the mixer comes from an 1/8" stereo cable (tip, ring and sleeve). It can be fed from an XM Roady, CD player, IPOD, IPAD, Music Player, or Smart Phone. All of those devices have a stereo jack capable of driving ear buds, which have an impedance of ~35 Ohms, and produce about 0.5Vpp of audio across that load.

Since my XM Roady is powered from the aircraft's electrical system, that effectively connects the audio ouput jack on the Roady to the airframe, which can cause a ground-loop to mix alternator whine into the music. For this reason, for the stereo music input, I used an Argonne AR-170 audio transformer to break that potential ground loop. The music source goes the 40 Ohm winding of the transformer. The schematic only shows one of two channels. There are two of those transformers.

The 8 Ohm winding of the music input transformer goes through an attenuator consisting of R2 against the intrinsic input resistance of the LM386, shown as R3. C4 is there to roll-off the highs a bit... R8 is not real; it is there because all parts of an LTSpice schematic have to have a path to ground...

My Garmin navigation system has a separate audio output which conveys alarms like "obstacle ahead, pull-up, PULL-UP!" in a female voice. I call her the Bbitch-in-the-box. There is a separate audio coupling transformer for that input, which you likely do not need. Ignore all of the stuff on the Left side of the schematic, and ground the Grn wire on the music coupling tranny.

The aircraft VHF radio traffic comes to the mixer on a single wire (See V2, HeadPhone Bus). At the same time I built the mixer and rewired the headphone jacks, I installed a monaural four-station intercom (Sigtronics SPA-400), and its output comes in on another wire (See V4, Intercom). So I effectively have two 0.8Vpp monaural sources to sum into both channels of the stereo headphones. I sum these two signals using a passive resistive summing network (R4, R13, R9, and R1 inside the LM386). There is plenty of level to do this passively because the LM386 has a gain of 20. The summed audio goes to the Inv input of both of the LM386s.

I never drew a real "schematic". I just wired it from the LTSpice sim. The sim was done so I could balance all of the levels, and do some frequency shaping (C1, C3, C4, C7).

This was installed in my Cessna in 2006, and has been in use continuously since. With the Lightspeed Zulu stereo noise-cancelling headphones, the music is very high quality. The transformer-isolated inputs (and proper wiring of the headphone jacks) prevent hearing any alternator whine. The aircraft comms is heard only while there is activity on the radio, the VOX circuit in the intercom mutes any cabin noise when nobody is speaking...
mix.png
(click to enlarge)
 

crutschow

Well-Known Member
Most Helpful Member
Since the Bose headphones should have a high-impedance input, is there any reason you can't just connect each source to a pot and sum them resistively to the Bose input?
Or just use one pot to balance the input levels, with resistive summing to the Bose?
Or just resistive summing with the iphone volume adjustment to balance the levels.
 

MikeMl

Well-Known Member
Most Helpful Member
The input impedance to aircraft headphones is not that high. At the time I built the mixer, I wanted any of the pilot, copilot or passengers to be able to hear the music, so had to drive the four jacks with a low-output-Z amplifier (voltage source) , so that the levels do not change as additional headsets are plugged in.

With modern, high-end, Bose or Lightspeed noise-cancelling headsets, this project is unnecessary. My LightSpeed Zulus have two ways of connecting to a music source, 1/8" stereo cable, or Bluetooth. Those did not exist in ~2005 when I built the mixer.
 

MikeMl

Well-Known Member
Most Helpful Member
...
I'm currently looking to mix 2 inputs to one output).

Input #1 : Audio coming from my aircraft's headset jack PJ-055.
Input #2 : Music from my iPhone 3.5mm jack

Output #1 will go into a 3.5mm female jack that my Bose QC20 earbuds will plug in.

I'm quite new to this and I made some research but I'm still not quite sure if I missed something so feel free to correct me if I'm wrong.

The Output #1 is my QC20 earbuds which to my understanding has a Low Impedance.
The earbuds about 30 Ohms.
The Input #1 is my airplane audio from the headset jack which is made for aviation headset with High impedance.
This comes from an amplifier that has a low output impedance. It will drive up to ~4vpp into load that is ~50 Ohms or higher.

The Input #2 will be my iPhone 3.5mm jack which is made for consumer headset with Low impedance earbuds.
Any similar music source has an amplifier capable of driving at least 0.5Vpp into 30 Ohms.

My goal is to make a circuit that will merge both input and allow me to control the output volume with a logarithmic (Audio taper). What are the challenges that I have to face with the impedance difference of my inputs? Is there a way to build a electrical circuit that doesn't need a external power source like a battery and simply use the inputs power?..
You might just try a passive resistive summing network consisting of two resistors for each channel. Try about 10 Ohms between each channel of the Iphone and the respective earbud. Then put about 22 Ohms from the aircraft audio to the earbud end of the 10 Ohm resistor. This may require you to crank the level of the aircraft radios to a point where a copilot or passenger might complain, but should work if you are flying alone.

I would just use the volume controls on the Iphone and on the aircraft radios to set levels.
 
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