Toyota OEM microphone adapter for aftermarket receiver

When I upgraded the stereo of my 2013 Toyota Sienna (although I don't own the car anymore), I found that most of aftermarket receivers are not compatible with the stock hands-free microphone. So, it is usually recommended to give up the stock microphone and install a new one. However, I really wished to re-use the stock microphone and did some research myself. Actually, doing the research, I found that I was not the first who thought about re-using the OEM Mic for the aftermarket receiver. However, to my surprise, none of the precedents seemed to get successful outcomes.

The issue I identified was that, the OEM Mic system uses balanced output, which is generated by an integrated amplifier within the Mic module. On the other hand, the aftermarket Mic system uses unbalanced input. Also, since the aftermarket receiver uses a capacitor Mic, there is phantom power supply to the Mic.

The wiring diagram of Toyota stereo is shown below. We can see the two balanced output terminals MCO+ and MCO- from the Mic module.

Pin	Description	Test condition	Test value
MACC	Microphone power supply	Ignition switch off	Below 1V
		Ignition switch ACC	4 to 6V
SNS2 (SGND)	Microphone connection detection	Always	Below 1V
MIN+	Microphone voice signal	-	-
MIN-	Microphone voice signal	-	-

Another issue I noticed was that the Mic module requires 5V supply, not 12V. 

Okay, so to summarize, I needed the following functionalities:

• DC 5V supply
• Balanced to unbalanced conversion
• Phantom power blocking

So, I designed a converter circuit (schematic is shown below). It worked greatly for my aftermarket (Sony) receiver, so I was so happy with that.

Parts:

• 1:1 audio transformer

• Note: the transform ratio may need to be adjusted depending on the receivers. In my case, 1:1 transformer turned out working fine. However, for some receivers, it may saturate the AGC amp of the receiver, and distort the sound. Alternatively, a potentiometer can be connected in parallel at the output stage (Mic jack) for volume control.
• LM7805
• Note: if the receiver has a USB port, the 5V supply could be tapped out from the USB port (I haven't tried this though).
• 50uF x 1
• 10uF x 1
• 1uF x 1
• 50k potentiometer x 1

Schematic:

9V dummy battery

Never worry about depletion and replacement.

AC source noise filter for audio amps - activated by USB

I own a pair of AudioEngine A2 speakers and I really like them. To enhance my love for them, I planed an AC power source upgrade project.

I planed this project for two things:
1. Suppress the AC source noise - this is the primary goal.
2. Whenever I turn my PC on and off, I want the speakers to be turned on and off together.

The second goal, however, was a small concern, in fact. If you can say you are an audio guy, you must have heard some people say that turning amps on and off without turning down the volume may damage the delicate circuitry. However, at the same time, the others say that today's amps are advanced enough and they no longer need such circumspection. I believed the latter is true. But, just for peace of mind, I sent an email to the Audio Engine customer service and asked them to assure two things for me:

1. Unplugging the speakers directly from the AC source will not damage the speakers.
2. Even if it causes damage on the speakers, it will not void the warranty.

OK. The green light is on. The following are my AC filter design and implementation:

DC source noise rejection filter for DAC/headphone amps

When I first purchased my DAC 4 years ago, it came with a 15V DC linear power supply. As you know, linear power supplies are very heavy and bulky, especially when their power rating is high. On the contrary, switching power supplies are light and small. But there are always pros and cons. Despite all the excellence over the linear counterparts, switching power supplies usually have high output ripple and noise. The output noise is quite intrinsic due to its mode of operation and very hard to avoid. Therefore, at least for applications where the noise is a big concern, linear PS is a good choice.

If for some reason there is no other choice but using a switching power supply, a hybrid type power supply can be used. For example, let's say that we want to get 15V DC source. Then we start with a 18V switching PS. Because the output may have a lot of ripple, we can trim it out with a linear regulator of 15V output. There are bunch of linear regulators out there with superior ripple rejection characteristics, so we have very high degrees of freedom in the part selection.

The followings are my design:

Parts:
LM1084 x 1
capacitors
  10u x 2
  100u x 1
resisters
  150 x 1
  5k x 1 (variable)
panel mound DC jack x 1
5.5/2.1mm male DC connector x 1
heat sink x 1
volume knob x 1

The most challenging part? Well, it was so tricky to pack all the part within a tiny case.