We’ve presented less detailed information about this topic in the past but our ongoing dealer and rep conversations lead us to believe it’s still not fully understood by a significant number of installers out there. So we’ve reached out to that unassailable bastion of higher learning, Wikipedia to help us provide more detailed descriptions and links. And we’ve done some further research and found several available solutions for integrating Residential and Pro sound components.
The 30,000 foot overview is that the line output voltage from residential audio gear (that’s from the RCA Line out jacks on CD/DVD players, tuners, tape machines, pre-amplifiers including tape out and zone two outputs from receivers, and just about any other residential audio source) isn’t powerful enough to drive the vast majority of “Pro sound” power amplifiers to their full output. Because of this level mismatch, connecting residential audio equipment directly from their Line out jacks to the Line in jacks on Pro sound gear often results in inadequate speaker volume and reduced signal to noise performance.
Recently we were trolling a home theater/audio forum and came upon a thread concerning the use of Pro sound amplifiers in home systems. Much of the discussion had to do with contributors saying stuff like “It doesn’t seem to play loud enough” and “I have to turn the gain up so high I hear noise”. It took quite a while for someone to chime in concerning the mismatch between Residential and Pro gear voltage levels. And, as is typical on the Internet, a bunch of responders completely ignored/missed this correct explanation and went right on commenting on how using Pro sound amps doesn’t see to work well with home equipment.
In addition to matching levels, many Pro sound units incorporate balanced XLR or Phoenix™ type connectors while Residential gear almost always uses the ubiquitous unbalanced RCA connectors. At the very least, balanced connections offer the advantage of reduced extraneous noise pickup (RF, etc.) and are worthwhile to utilize, if they’re available.
Another aspect to consider for distributed audio systems, particularly but not exclusively for outdoor installs, is using mono signal sources as opposed to stereo. It’s very rare that someone is listening in the stereo “sweet spot” when they’re on the patio, swimming in the pool, cranking up the barbecue or whatever. Mono distributed sound systems mean that everyone hears everything and there’s no concern for left/right channel balance, just for adequate area coverage.
The following indented text is a reasonably concise technical explanation of the Residential/Pro sound level mismatch gleaned from the Wikipedia:
Line Level – line level describes a line’s nominal signal level as a ratio, expressed in decibels, against a standard reference voltage. The nominal level and the reference voltage against which it is expressed depend on the line level being used. While the nominal levels themselves vary, only two reference voltages are common: decibel volts [dBV] for consumer applications, and decibels unloaded [dBu] for professional applications.
The reference voltage for the decibel volt (0 dBV) is 1 VRMS, which is the voltage required to produce 1 milliwatt [mW] of power across a 1 kilohm [kΩ] load. Reference voltage for the decibel unloaded (0 dBu) is the voltage required to produce 1 mW of power across a 600 Ω load (approximately 0.7746 VRMS).
The most common nominal level for consumer audio equipment is −10 dBV, and the most common nominal level for professional equipment is 4 dBu. By convention, nominal levels are always written with an explicit sign symbol. Thus 4 dBu is written as +4 dBu.
Expressed in absolute terms, a signal at −10 dBV is equivalent to a sine wave signal with a peak amplitude of approximately 0.447 volts, or any general signal at 0.316 volts root mean square (VRMS). A signal at +4 dBu is equivalent to a sine wave signal with a peak amplitude of approximately 1.737 volts, or any general signal at approximately 1.228 VRMS.
‘Peak-to-peak’ amplitude (sometimes abbreviated as ‘p-p’) refers to the total voltage swing of a signal, which is double the peak amplitude of the signal. For instance, a signal with a peak amplitude of +/-0.5 V has a p-p amplitude of 1.0 V.
The line level signal is an alternating current signal without a DC offset, meaning that its voltage varies with respect to signal ground from the peak amplitude (for example +2.192 V) to the equivalent negative voltage (-2.192 V).[
For similar reasons, “wye”-cables (or “Y-splitters”) should not be used to combine two line out signals into a single line in. Each line output would be driving the other line output as well as the intended input, again resulting in a much heavier load than designed for. This will result in signal loss and possibly even damage. An active mixer, using for example op-amps, should be used instead.
It is intended by designers that the line out of one device be connected to the line input of another. Line inputs are designed to accept voltage levels in the range provided by line outputs. Impedances, on the other hand, are deliberately not matched from output to input. The impedance of a line input is typically around 10 kΩ. When driven by a line output’s usual low impedance of 100 to 600 ohms, this forms a “bridging” connection in which most of the voltage generated by the source (the output) is dropped across the load (the input), and minimal current flows due to the load’s relatively high impedance.
Although line inputs have a high impedance compared to that of line outputs, they should not be confused with so-called “Hi-Z” inputs (Z being the symbol for impedance) which have an impedance of 470 kΩ to over 1 MΩ. These “Hi-Z” or “instrument” inputs also have much more gain than a line input. They are designed to be used with, for example, electric guitar pickups and “direct-in” boxes. Some of these sources can provide only minimal voltage and current and the high impedance input is designed to not load them excessively.
The bottom line in most cases is in order to optimize the performance of systems using Residential sources and Pro sound power amplifiers there should be active level matching along with stereo to mono conversion and unbalanced to balanced conversion. There are lots of passive, transformer based unbalanced to balanced converters on the market but they don’t address the level matching issue or stereo to mono conversion. And we strongly recommend against using a “Y” connector with Residential stereo sources to get to mono.
We’ve done some research into the various matching/conversion devices out there and have come up with some recommendations. This is by no means a complete listing but we’ve found these units to work well and be affordably priced.
A while back we were suggesting the ART CleanBoxPro™. This handy device does level correcting and unbalanced to balanced but not stereo to mono conversion. Also, although it has two level controls they’re not for Channel 1 and Channel 2. Instead, one controls the unbalanced output levels and the other controls the balanced output levels. This isn’t a big deal if the power amp has channel level controls but should be noted.
Recently we found several devices made by RDL™ (Radio Design Labs™). The ones we’ve tried have excellent specs and sound great. They’re consistent with high fidelity distributed audio installations and at a quality level you can be proud of.
The RU-AFC2 Stereo Audio Format Converter provides balanced to unbalanced to balanced, level settings with built-in LED meters, and stereo as well as mono output capability. Although not inexpensive it’s a superior and flexible device.
The EZ-AFC2 Audio Format Converter ‐ Stereo Balanced ↔ Unbalanced is similar to the ART box above but includes LED metering to provide accurate level matching between unbalanced and balanced. It requires a separately ordered power supply as do all the RDL units.
The FP-UBC2 is similar to the above unit, but in a different physical format and the level controls are intended to simply adjust the gain on each channel as opposed to setting the correct levels using built-in LED meters like on the EZ-AFC2. Neither of these units does stereo to mono conversion. http://rdlnet.com/product.php?page=45
Of all the units we’ve found, the TX-LC2 Unbalanced to Balanced Line Combiner is the best and most practical match for the way Terra packages will almost always be installed. It combines a stereo pair of Residential unbalanced line level inputs to and converts them to a balanced mono output. It corrects the level difference between Residential and Pro and provides a dedicated level control for precise system matching. It’s quiet, has low distortion and sounds excellent. It’s also affordable and very compact, with several factory supplied optional mounting options available as well.
We’ve decided to offer the RDL TX-LC2 and a matching RDL 24 volt power supply as a courtesy to you, our dealers. We are working out the details with RDL and will let you know as soon as we have confirmed prices and inventory here in Maine. Please note: The included photos are not to exact scale and have been enlarged to highlight detail.