Monday, January 14, 2013

Killer sound from the factory head unit / why BLT's are killing you or at least your wallet.

  I was going through my files and I found one that Brian Smith had published in Car Sound & Performance Magazine that I thought is a great tool for those dealing with OEM systems. (Almost everyone)

  I love the factory radios now.  I love the look, the feel and simplicity that the OEM has gone through all that hard work to put in front of me.  New cars even have application based radios and having an AUX input and the ability of playing MP3 from a disc have been in cars for years.

  I often find myself telling clients what they're not used to hearing.  "Keep the factory head unit."
Aftermarket decks are expensive and are often an unnecessary habit that car audio fans keep shelling out for year after year, car after car.

  BLT's are killing you or at least your wallet. Balanced Line Transmitters, (BLT's), are a waste of money.  Even if you just look from the standpoint that your cable distance.  You're not 20'! At such a short distance it's overkill to spend money on such a thing.  (We'll get more into that later, also why do you need 8v pre-outs when you can run your amp into clipping with as little as 3/4 of a volt???)  Many great amps of the past were waiting for you to employ this technology but you didn't even know it.  Take the PPI PowerClass amps.  They use a differential input and even put an optional -12db filter button there for you.

Check out this article by Brian Smith, (NOT ME!). (Brian nor Auto Media Mag could be reached for this post, though last time I checked AMM was out of business.)

Output Alternative
Learn how to make the most out of a head unit's balanced, high voltage, low impedance output section.  By Brian Smith

     In a recent issue of Car Sound & Performance, I made an offhand suggestion about using the speaker outputs of one of those high-powered head units as high voltage pre-amps.  I received several questions via e-mail on the subject and it appears that I wasn't the only one.  Occasionally, our editorial staff will send a topic for these monthly columns, especially if the subject matter seems to be of widespread interest.  This month the question was: What type of input stage does an amplifier need in order to accept input, (in an OEM system), without an LOC? (LOC = Line Output Converter)

     Ooof! The answer is unlikely to make the LOC people, (manufacturers), happy, so I guess that I'll try to soften the blow by stepping on the toes of the line driver folks as well.  Here goes... You don't need any particular type of input on the amplifier, you just have to know how to do the correct interface --- and, almost all head units available today, including OEM and even super-cheap aftermarket units, include a balanced, high voltage, low impedance output section.

     Let's begin with the head unit.  This is going to take a little math and a few diagrams, but hand in there.  It's really not that complicated and there are some definite advantages to be gained.  The block diagram shown fig.1 represents a single channel of the amplifier section in the head unit, way back in the day, when car audio was something relatively new.  This arrangement was capable of a few watts into typical speaker loads, (for this example, we're going to say 4 volts across 4-ohms for an output of 4 watts because it makes the math easy and it's not far from reality) but consumers wanted more power.  One way to increase output would be to add a step-up transformer to the output of this small amplifier.  However, good transformers generally cost more than the rest of the head unit.  Another approach, and the one that was ultimately taken, was to bridge a pair of these small amps (see fig. 2).  This configuration provides twice the voltage, (8 volts), and therefore, 4 times as much power (16 watts) into the same 4-ohm load.

     Now, toss that 4-ohm speaker and consider and consider driving the input of an amplifier with this output.  Yes, you can drive an amplifier with another amplifier.  Typical input impedance on a modern amp is usually in the 20kohm range.  Plug 20,000 into Ohm's Law in place of that 4-ohm speaker; the 16-watt amp will only output about 0.003 watts, so we're not talking about burning anything up here.  That being said, what we're left with is an output section that swings +4 volts RMS between transistor "A" and ground and -4 volts RMS between transistor "B" and ground, or, a total of 8 volts RMS.  The term 'balanced' comes from the fact that the positive and negative potentials are equal relative to ground.

     Balanced inputs make use of a special design called a differential amplifier.  This device accepts two signals, inverts one signal, and then combines the two.  If you send the signal that you want to one input and mirror image of that signal to the other input, the device's output will be exactly what you sent to the first input.  While that sounds like a silly way to go about getting what you already have, the side benefit is the cancellation of any noise that may be induced into the signal cables between units.

     Most modern automotive amps have balanced or "differential" inputs, inputs that are designed to accept exactly the type of signal that comes from the internal amps of a head unit.  Yes, I'm saying that you should be able to solder RCA plugs to the speaker out wires and drive most amps.  So, why is there even a market for LOC's and balanced line drivers?  Probably because someone tried this trick once and the resulting system was noisy --- remember, I said "should" and "most."  If the amp in question has a single ended input (i.e., non-balanced), connecting the head unit in this manner essentially shorts device "B" (shown if fig.2) to ground.  This condition is unlikely to produce either noise-free performance or long life expectancy for the output device.  There is also the consideration that some amplifiers with "balanced inputs" prefer to have the low side of the differential input tied to ground.  This is how the signal chain happens with the single-ended preamp outs on most head units (see fig. 1 again).  So, some manufacturers base other aspects of the overall amp design on the assumption that this will always be the case.  I've certainly heard the "low side to ground" mantra on a frequent basis, but I've only seen a couple of cases where it appeared to make any real difference.  In most cases, tying the low side of the input to ground caused a slight drop in low-level distortion.  I'll trade a couple of hundredths of a percent of distortion for some ground isolation any old day, especially in a car.

     The main gist here is that you need to determine what type of input you're dealing with.  Balanced inputs are usually mentioned in the feature listing if the amp has them, but here's a quick test that will tell you which type of inputs you have.  With a DMM, measure the resistance between each of the RCA's outside contacts and the amp's power supply ground connection.  Also measure between the left and right RCA's outside contacts.  If you see anything below a few ohms, then you've got a single-ended input and you're going to need a single-ended output like the one shown in fig. 1 to properly drive the amp.  To achieve this, you'll need to take the output signal from half of the bridged pair shown in fig. 2 (see fig. 3).  This can be done by connecting the center pin of an RCA connector to the + speaker output and the ground sleeve of the RCA to the chassis of the head unit; no connection is made to the --- side of the speaker output.  Of course, this configuration provides only half of the available voltage but that would still be a 4-volt preamp with our middle-of-the-road example of a 16-watt amp.  There are many more powerful models available, and when you stop to consider that the source impedance of the amp section could easily be 100 times lower than a good preamp section, this approach begins to look awfully attractive, even in an unbalanced configuration.

Again, this article originally appeared in the Tech Briefs section of CAR SOUND & PERFORMANCE, year 2001, (or so), by Auto Media and was authored by Brian Smith, NOT ME.

Patrick Chandler
Co-Founder Robot Underground


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