BTL to Balanced Conversion

[dogsbark26]

I am an accountant not an electrical engineer so please don’t laugh at my questions.

I have an in-dash radio/cd player made by Blaupunkt (BMW CD43).

I want to convert the 4 pair of speaker-level outputs to a balanced line for transmission to a Zed Audio ESX 120.4 amplifier.

I do not have the schematic for the input stage(s) of the ESX unit.

If I understand correctly, a balanced connection uses three wires. One wire is chassis ground and the other two wires are for the signal with the signal equal in amplitude in each of these two wires, but opposite in phase. What is balanced is the impedance on the two signal wires in this configuration.

I want to connect a BMW CD43 head unit that has bridge tied load (BTL) audio output. The outputs come from two ST Microelectronics TDA7376B audio amplifier IC's. They are rated to put out a maximum of 40W per channel.
The datasheet is here, if you are interested: http://www.st.com/stonline/products/...7/tda7376b.pdf

The outputs carry a common-mode voltage of ~0.5*V supply, and the - output terminal is inverted relative to the + terminal. It is not a true balanced setup because there is no dedicated neutral line that they are referenced against. The offset between the BTL outputs is ~100mV (spec sheet says it can be up to 125mV). 125mV into a 4-Ohm load is ~16mW (0.016W).

To use these outputs in a balanced line, I intend to add dedicated neutral wires in the interconnect cables.

On the destination end the Zed Audio ESX 120.4 is designed to accept balanced input(s). It uses two 6-Pin Mini DIN female connectors (which, according to the manual, each carry both pairs of a stereo channel). This would be Front Left +, Front Left -, Ground, Front Right +, Front Right -, Ground. So the dedicated neutral wire is predetermined by the schematic on the amplifier end.

I think my questions are:

1. Where should the two dedicated neutral lines terminate on the BMW CD43 head unit end? Those are to be radio chassis grounds?


2. How can I ensure the balance of the impedance in each signal pair?



If I have posted this to the wrong forum will the moderator please move?

Tom

[BigE]

What you want to do makes no sense. Surely you can plug the in dash cd player into an amp some other way.

I'd have a car audio installer make them work together for you.

[theAnonymous1]

Doesn't the ESX 120.4 have two pairs of single ended inputs right next to the balanced DIN inputs?

Just buy a line level converter and be done with it.

If for some reason you really wanted to, you could buy two line level converters and rig up a pseudo-balanced output, but I don't see the point.

[dogsbark26]

While it is true that I could wire the dash unit outputs to some RCAs using only one of each channels pair (+,-) I want to take advantage of the balanced routing of signal even though it may be somewhat excessive, because the signal cables are short. The original purpose of balanced routing is to reject unwanted noise. A second advantage accrues however in the bigger headroom offered. The noise issue may be of lesser importance.

So I do understand I can use standard "quick and dirty" method for connecting these two units just like everyone else that uses these amplifiers.

My questions remain can I take advantage of superior circuitry?

[dogsbark26]

Originally Posted by theAnonymous1

Doesn't the ESX 120.4 have two pairs of single ended inputs right next to the balanced DIN inputs?

Yes. It does.

Originally Posted by theAnonymous1

Just buy a line level converter and be done with it.

If I were to go that route, I think the line level converter would be unnecessary. That is, it would provide no advantage, unlike using a balanced connection.

Originally Posted by theAnonymous1

If for some reason you really wanted to, you could buy two line level converters and rig up a pseudo-balanced output, but I don't see the point.

[theAnonymous1]

You want to take the output of a power amplifier and feed in into another power amplifier. I think you can toss any advantage of using a balanced connection between the two right out the window.

A single ended connection is more than sufficient in this scenario; but, do what you want.

BTW, you can think of the outputs from the head unit as balanced when referenced to 12V ground. Some caps for AC coupling and some resistors for voltage dividers will get you what you want.

[dogsbark26]

Originally Posted by theAnonymous1

You want to take the output of a power amplifier and feed in into another power amplifier.

Um... Yes. That is what I'd be doing anyway with a single ended connection. I'm just trying to figure out how to take advantage of the balanced circuitry in the ESX.

Originally Posted by theAnonymous1

I think you can toss any advantage of using a balanced connection between the two right out the window.

I see. Why is that please? My understanding is that using a single ended connection, I will get an input signal of about 0.5*Vsupply where with the balanced connection that signal would be doubled simply by matching the impedance of the two legs of the first amplifier's outputs (well, and using the alternate input circuitry in the ESX unit).

Originally Posted by theAnonymous1

A single ended connection is more than sufficient in this scenario; but, do what you want.

I do appreciate your input and am not disagreeing with you. I am simply trying to learn and investigate the possibilities before making any decisions.

Originally Posted by theAnonymous1

BTW, you can think of the outputs from the head unit as balanced when referenced to 12V ground. Some caps for AC coupling and some resistors for voltage dividers will get you what you want.

Interesting, thank you. I will now go off to research what AC coupling and voltage division are.

Tom

[BigE]

The advantages of a balancing are at the signal level, *before* the signal goes through the output transistors.

Since your signal level gear itself is not balanced, you get no advantage from forcing it through a power amp into a balanced connection.

You can't retrieve a noise free signal by using a balanced connection once it has already become polluted.

Full marks for originality, but sorry, it's not going to sound anywhere near as good as having a fully balanced head end.

[dogsbark26]

Originally Posted by BigE

The advantages of a balancing are at the signal level, *before* the signal goes through the output transistors.

Well, yes. But in this case it is before the output transistors of the ESX unit that I intend to use a balanced line to transfer signal from the CD43 to the ESX.

Originally Posted by BigE

Since your signal level gear itself is not balanced, you get no advantage from forcing it through a power amp into a balanced connection.

You can't retrieve a noise free signal by using a balanced connection once it has already become polluted.

The advantage I expect from using a balanced connection is more from the increased amplitude of the incoming signal at the ESX than the most common advantage of common mode noise rejection (CMRR).

Originally Posted by BigE

Full marks for originality, but sorry, it's not going to sound anywhere near as good as having a fully balanced head end.

Yes. I do understand. But can’t I expect better results than just using one leg of the CD43s output on a single ended interconnect?

Thank you for your input. I value it highly.

Tom

[theAnonymous1]

Originally Posted by dogsbark26

The advantage I expect from using a balanced connection is more from the increased amplitude of the incoming signal at the ESX than the most common advantage of common mode noise rejection (CMRR)

If you took the signal directly from the HU amplifier outputs and referenced them to 12V ground, the max signal level would be something like +16dBu.

How much amplitude do you think you need and how much can the amp even handle before damaging it's input circuitry?