Yes, you add gains in dB to get overall gain.Originally posted by Overclocked2300@May 11 2005, 02:58 AM
For the input I was just gonna use an op amp such as the TLO82 as a preamp.
The circuit shown gives abut 28dB Gain, and the max the speaker can take is 90. Human hearing is damaged at 85 I think. Im going to aim for 50 to be safe. So IF I design the preamp for another 28dB gain and dB add (correct me if I am wrong)
its gains that multiply correct?
The reason using an op amp for a pre amp because it has good rejection of noise, and it is also simple.
Don't get mixed up between gain and output power. The 'dB' is a measure of power *ratios* - not power levels. 6dB is a ratio of 4. So if you have 1mW input and a 4mW output you have a gain of 6dB. The same would be true if you had 250W input and 1KW output.
Strictly speaking, by definition dB can *only* refer to a power ratio. But it can also refer to a voltage gain if the input and output impedances are the same. Over the years people who should have known better started to refer to voltage gains in dB even when the in/out impedances were dissimilar - a typical case being the open loop gain given for op amps. So we now have a situation where any voltage ratios are refered to in dB. It's quite wrong - but such is progress!
Your reference to human hearing being damaged at 85dB is different. This is where the dB refers to an *absolute* power level. In these cases the level always has a suffix to show it is an absolute level. Audio measurements are usually in dBA, though I seem to remember that high noise levels are in dBC, because the response curve is different to allow for changes in the human ear at those levels. So the figure should be 85dBA or 85dBC. The levels are referenced to 0dB which has a specific level.
Other refences are dBW, dBmW, dBV etc.
I hope that may clarify things.