TeamTalk

TeamTalk (http://www.mastercraft.com/teamtalk/index.php)
-   Audio/Stereo (http://www.mastercraft.com/teamtalk/forumdisplay.php?f=53)
-   -   Will splitting RCA's drop the HU voltage output? (http://www.mastercraft.com/teamtalk/showthread.php?t=58994)

craig3972 02-10-2014 03:09 PM

Will splitting RCA's drop the HU voltage output?
 
I saw this on another forum.... and wondered if its true. I have a Alpine CDA-118 that is 10,000 ohm output impeadance at 2V, but i cannot find the input impedance anywhere for my JL amp MD750/1.



Lets take a typical Alpine HU. Lets pick this one.

CDA-9856
Preout: 2 volts / 10,000 ohm output impedance..

Connect this HU to an Alpine amp with 10,000 ohm input impedance,
the voltage is now 1 volt, cut in half.


Instead of buying an Alpine amp, you found a good amp with a 47,000 ohm
input impedance, the loading effect is minimal, you get 1.95v, much better
than 1.0v when you interfaced an Alpine HU to an Alpine amp.

I was shocked years ago when I found this.



Connect two Alpine amps with splitter, you now have 0.66v.

Hint: A good HU will have a 50 ohm - 200 ohm output impedance. Even 1000 ohm is ok,
but 10,000 ohm is stupid circuit design.

Moral of story: Always check equipment specs.




One HU & one amp;

HU output voltage * (amplifier input impedance / (amplifier input impedance + HU output impedance)).

Example.

HU preout voltage: 5 volts
HU output impedance: 200 ohms
Amplifier input impedance: 20,000 ohms.

5 * (20,000 / (20,000 + 200)) = 4.95 volts.

One HU & two or more amps

Step 1. Find the total parallel impedance of all your amps since the same input signal will be driving all the amp inputs.

1 / (1 / (amp#1 input impedance) + 1 / (amp#2 input impedance) + 1/ (amp#3 input impedance ... etc))

Example:

Amp #1 input impedance: 10,000 ohms
Amp #2 input impedance: 20,000 ohms
Amp #4 input impednace: 47,000 ohms

1 / (1 / (10,000) + 1 / (20,000) + 1/ (47,000)) = 5838 ohms

Step 2. Put that 5838 ohms back into the voltage divider formula.

HU output voltage * (amplifier input impedance / (amplifier input impedance + HU output impedance)).

Example.

HU preout voltage: 5 volts
HU output impedance: 200 ohms
Amplifier #1, #2, #3 parallel input impedance: 5838 ohms.

5 * (5838 / (5838 + 200)) = 4.83 volts.

David Analog 02-10-2014 04:39 PM

That's a waste of your time and mental energy.
The Alpine HU voltage is decent. They make good preamps. You can often hear the difference.
All the Alpine HU preouts are discrete, unlike a cheap EQ where the outputs are divided via a fader into front and rear.
Preout OP amps are not like high current amplifier outputs that keep increasing power as you lower the load by adding more terminations (speakers). You don't want to unnecessarily use 'Y' adapters to divide the preamp output, especially with a 2V unit, because the voltage to each amplifier input will drop. Instead, use a higher voltage line amp to facilitate the division.
The JL Audio amplifier has both a high voltage (lower sensitivity) and low voltage (higher sensitivity) input selection plus a variable sensitivity control. It's going to be hard to find a mismatch unless you start dividing RCAs without boosting the voltage.

David Analog 02-10-2014 04:45 PM

One more thing to consider. Much of the above is relevant when using home audio or pro audio amplifier components that only have fixed inputs. Therefore, gain matching can be a problem. Not an issue with most mobile electronics because they all have variable inputs. Just make sure you have enough voltage to begin with. Splitting a preout three ways for example won't cut it even with the ability to level match. You just won't have enough voltage to maintain a low noise floor and high dynamic range.

Cloaked 02-10-2014 05:20 PM

Quote:

Originally Posted by David Analog (Post 1013101)
That's a waste of your time and mental energy.....
.

:D
:D
:D

craig3972 02-10-2014 07:17 PM

1 Attachment(s)
I have one of these but never connected it? I have six amps running off the CDA-118
Attachment 106014

David Analog 02-10-2014 08:26 PM

Yes, the six times 13 volts peak completely solves the RCA division issue plus a little bonus voltage.
Place the line driver as close to the source unit as possible rather than close to the amplifiers.

craig3972 02-11-2014 01:42 AM

What about the pre-out signal that passes thru an amplifier? the JL's have a pre-out RCA jack to send to the next amplifier down the line. Is there signal/voltage loss as it passes thru the amp?

JimN 02-11-2014 05:14 AM

Quote:

Originally Posted by craig3972 (Post 1013242)
What about the pre-out signal that passes thru an amplifier? the JL's have a pre-out RCA jack to send to the next amplifier down the line. Is there signal/voltage loss as it passes thru the amp?

That's probably buffered and therefore, isolates the signal from the incoming device from the last amp. Hopefully, the outgoing signal will be unity gain, or slightly higher. Also, it doesn't actually pass through the first amp, it just comes into the input, to the buffer and out to the next amp.

jonrogers 02-11-2014 08:00 AM

Im currently finishing install on (2) mhd 750s to (2) sets of rev 10s. I had plans to split rca cables to have R and L amp, or should I use pass thru amp to amp and not worry with R/L connection?

David Analog 02-11-2014 10:54 AM

I would use the pass-thru if I was using two amplifiers in different bandwidths or different zones. Just as Jim said, these are buffered outputs on the pass-thrus so there is no loss of voltage because they have their own output OP amp.
However, if using two identical amplifiers to drive symmetrical tower speakers or two identical subwoofers, then I would externally divide the RCAs and not use the pass-thrus. Because the pass-thru is buffered, the gain setting on my two amplifiers would be different. Nothing technically wrong with that other than it makes me uncomfortable.


All times are GMT -4. The time now is 03:14 PM.