Preamp - 18 dB Gain too low?

Nguyen787,
A doubling (2x) in voltage is (almost exactly) 6dB. A 10x in voltage is exactly 20dB. So for example if you have a 20x factor in gain then you could say: 20x = 2x * 10x = 6dB + 20dB = 26dB

Note that dB values are always added together (since dB values are really exponents), while their corresponding multiplication factors are multiplied together.

When comparing power: a 2x in power is approximately 3dB, while 10x is exactly 10dB.

The precise formula for converting Voltage ratios to dB is: dB = 20 * log(V1/V2), where V1 and V2 are two different voltages, and the log function is base-10. Note that if you flip V2/V1, you get the same magnitude dB value but with a different sign. For power ratios, it's: dB = 10 * log(P1/P2), where P1 and P2 are two different powers in the same unit (e.g. Watts).

So in my example with the Apollos, you start out by comparing Voltages:
20 * log(4V/1V) = 12.0412 dB

Thus, the 4V amp will require a 12dB higher input signal to achieve its "full output" (the typical definition of amplifier sensitivity), versus the 1V amp. Next we have to compare the difference in "full output" between the amps:

10 * log(250Watts / 25 Watts) = 10.0 dB

So the amp that puts out 250W will be 10 dB above the 25W amp when both are at full output. Add the two together: 12 + 10 = 22dB difference in amp gain.

--
Mike

An input sensitivity for an amp of 1v is quite normal as nearly every amp I've seen on the market has been .5v to 2v for max rated rated output (to clip/distort).
An input sensitivity of 4v is wrong/bad and I would call the amp broken or seriously badly designed.
You have a problem ask the manufacture to give you a fix to make the amp/s 1 to 2 volt input sensitivity, as the standard is.

Cheers George

Nguyen787
Here is a way out there thought on your problem. You say your running the amp on 115v mains? It is possible that this amp/s are set up for 220-240v mains operation, this could? drop your gain/input sensitivity.

Cheers George

Another thought, your Oppo cdp has a remote volume control, is it at full output level? As it may default to 1v or less when powered up and you need to bring it up on the remote to full 2v output.

Cheers George

More on the Oppo's stereo only output, are you getting the siganl from the FL and FR outputs on the back to the Lightspeed?

Also taken from the manual are you setup it the programing for this way.
"4. Output Volume: Allows you to enable or disable the analog volume controls (also known as Variable
Audio). The available options are:
�� Variable – Enables the analog volume control. Use the VOL+/- buttons on the remote to
increase/decrease the analog audio output level (the maximum is 100).
�� Fixed – Disables the analog volume control. The VOL+/- buttons on the remote will no
longer affect the analog audio output, and the volume level is fixed to 100."

And then this also
"For the stereo outputs, if the audio system you are connecting to does not have surround speakers,
set the Down Mix option in the Setup Menu to Stereo. If the audio system has surround speakers
and Dolby Pro Logic or other surround audio decoding capabilities, set the down mix mode to “LT/RT”.
(For more information, refer to “Down Mix Mode” on page 68)."

Cheers George