As the title states this post defines the correct way to bypass the TSX amp.
After reading several posts on the subject I realized that there was a lot of confusion and half right information on exactly how to get a noise free install. First off my hats’ off to all of the people that provided the wiring diagrams, pin outs and disassembly instructions, without that info I would not have endeavored to mod my system.
If you are wondering who the new guy is, I have over 30 years experience in electronics and built my “parts bin” amp (60 watt tube) when I was 17. I keep busy repairing pro audio and musical instruments amps and mixers. I know a lot about electronics.
Before connecting you new amp to the factory HU you have to determine what type of input circuitry you new amp has. If you have an amp like the JL of the Rockford Power series with differential inputs follow the first set of instructions. If you have an amp with regular (un-balanced) line level inputs follow the second set of instructions. In any case read on.
Lets start with the head unit outputs. They are full differential current limited AC coupled outputs. This is very important to know if you want a proper interface to your power amp. Lets define some terms (apologies to those who know this already).
A differential output is different from a standard RCA jack coaxial hook up in that it doesn’t use a signal ground. Instead it sends the signals over two (a pair) of wires with one signal (the minus pin) being 180 degrees out of phase with the plus pin. Differential transmission of any signal is referred to as Balanced. Differential signal transmission is far superior to un-balance signal transmission in terms of noise rejection. This is why the output of the factory HU uses differential and not un-balance line outputs.
A little more boring tech stuff before we get to the how do I get a clean signal.
We all know that when you run audio lines in a noisy environment like the car you can pick up a lot of noise. An un-balanced line level connection (your RCA cables) use a coaxial cable with a shield to block EMI and channel it to ground. This we know puts a great dependency on cable quality and grounding. A balanced differential connection runs over a two conductor (usually twisted) pair with a shield. The signals are labeled plus and minus (+ -) with the minus being 180 degrees out of phase with the plus. The shield is not a ground and is not used to carry any part of the signal. So how does this set up provide a clean signal you ask? The secret is the differential audio inputs on that fancy amp you have. Now for some simple Algebra.
If I have a 2-volt audio signal and I want to send it over a balance line I would first create an inverted copy of it. Then I would send the original in phase signal out on the plus (+) lead and the inverted signal out on the minus (-) lead. This is what the factory HU does. When the balanced signal hits the differential audio input the minus signal is again inverted bringing it back in phase with the plus signal. The two now in phase signals are added together (or summed) by the differential amp to produce a 4-volt signal. Wow! Did the cable just give use gain? No, the differential amp did, but the secret to the balanced signals noise immunity can now be reviled.
Lets transmit that 2 volt signal again, we send it as a +2 and a –2 over our balanced two conductor twisted pair audio cable. During the trip the cable passes over other electronic equipment (noise sources) and picks up a whopping 3-volts of pure noise. This swamps our poor 2-volt signal and it looks like a real problem. But wait, our differential audio amp to the rescue! Since the cable is balanced the plus and minus signal wires are the same size and take exactly the same path, therefore any noise picked up by one side is picked up equally by the other. This means that the noise is equal and in phase over our twisted pair cable (remember the audio signal is out of phase over the cable) By the process of inverting the minus side with the differential input (diffamp) the noise on the plus and minus lines are now out of phase +3-volts and –3-volts for the noise and +2-volts and +2-volts for the signal. When the plus and minus inputs are summed the +3 and –3 = 0 for the noise and the +2 and +2 = 4 for the signal. So we have a 4 volt zero noise signal for our power amp. This property is called CMRR (common mode rejection ratio) and is measured in decibels. I think the JL or RF amps spec it at 50dB. This is a good number (the higher the better). The noise picked up on each lead is called common mode noise because, well, its common to each line.
If you were to try to connect the differential out put using an RCA coaxial cable you would spoil the balanced properties of the differential amp. The reason this happens is the shield of the coaxial cable will not absorbed the same amount of noise as the center conductor due to location and area. If we redo out example using the coaxial method this is what happens.
We send our audio signal with the +2-volt signal on the center conductor and the –2-volt signal over the shield of our RCA cable. Passing over our noise source the shield picks up the same 3-volts of pure noise but the center conductor only picks up 1-volt of noise because the shield protects it somewhat. When that signal hits the diffamp this is what happens.
When the plus and minus inputs are summed the +1 and –3 = -2* for the noise, and the +2 and +2 = 4 for the signal. So we have a 4 volt audio with 2-volts of noise defeating the purpose of the diff amp.
*Note, the (– minus sign) indicates the phase of the noise relative only to the original noise. The important number the absolute voltage, in this example 2-volts of noise).
Connecting an amp with Differential inputs.
The correct way the connect the factory HU to an amp with differential inputs is to use shielded twisted-pair cable between the 14 pin connector (unplugged from the factory amp of course) all the way to your new amp. At the 14 pin connector you will connect one wire of the pair to the + and the other wire to the – output. This is important; you will also connect the shield of the shielded twisted pair cable to the shield pin for that channel. Make sure you keep the phasing correct (remember what color you used for the + and – signals and connect the shield to the correct pin) each channel has it’s own shield (THIS IS NOT A SIGNAL GROUND!) and it must stay with it’s own channel. Then at the amp strip the cable and remove the shied, it is not connected to anything (If you ground it at the amp you will create an antenna for noise). I used a piece of shrink-wrap over the end of the cable so the shield would not touch anything. Next connect the twisted pair wire (the color you connected to the HU + at the 14 pin connector) to the center pin of your RCA jack. Connect the other wire of the pair to the other side of the RCA plug (in the unbalanced coaxial world this would be the signal ground).
Where do I get shielded twisted pair audio cable? Radio shack sells a 25 ft roll of two conductor shielded audio cable for about 10 bucks. It is not twisted but it works. I caution you that it is great audio cable but it uses cheap insulation that will melt when soldering, this can be a real PITA.
Connecting the amp this way gives the added benefit of reducing or eliminating turn on pop. Since the + and – outputs pop in the same polarity (plus during turn on and minus during true off) due to the nature of the HU coupling caps, the diffamp will cancel them just like the noise.
Connect a amp with normal Unbalanced line inputs.
To connect a regular amp you would use the shielded twisted pair cables from the 14 pin to take advantage of the noise characteristics of the balanced signal. The only thing you have to do is convert the balanced signal to unbalanced before you connect it to your amp, and you must do this just before the amp. You need a device called a BALUM. Unfortunately they are not readily available for this application.
So now what? Do you spring for a big bucks amp? No, We have two methods to do this, One is to use an audio coupling transformer (not a GLI or LOC). I am investigating this right now; I want to be sure the HU can drive an inductive load before telling people to use one.
The next solution is to use an active (audio op-amp) circuit. I can build a prototype if the HU can’t drive a transformer. Using a BALUM active or passive will give you all the benefits of the high price diffamp set up including turn on pop rejection.
I will post some pictures of what I did when I get some more time. I put this post together on my lunch break so I don’t have access to any of my photos.
I installed this on an 06 TSX/navi using the JL 300/4 (picked for diffamp inputs and fancy power supply).
I get zero noise and no pops I played the silence track of my test disk with the engine running and everything I could think of on.
I hope to barrow a normal amp to verify the BALUM solution.