# Amplifers

#### pebe

Joined Oct 11, 2004
626
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)

so 28+28=56dB

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.
[post=7607]Quoted post[/post]​
Yes, you add gains in dB to get overall gain.

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.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

There's a bit of confusion here. The voltage gain of the power amp is not really related to the output level. Remember that there is a transformation from electrical power into mechanical power at the speakers. The Decibel scale is not equivalent for voltage gain and sound pressure level measurements.

The absolute voltage gain of the amp is not a major issue. It's always going to be pretty high, and will be some fixed amount like 60 dB. The preamp feeding the power amp should have a volume control to regulate the level of the input, and thus control the output level of the amp. Otherwise, it's a bit like trying to drive a car with an engine set to run at full power all the time - not too practical.

The amp's voltage gain does not relate to its output power until the speaker load is known. Then you can figure the power from measuring the voltage across and the current through the speaker coils.

The conversion of electrical power to mechanical by the speakers is always fairly inefficient. At one time, the most popular Acoustic research speakers were rated at .9% efficiency. Took a lot of power to make then get loud. Then you measure them as having a sound pressure level - also a log scale in dB units. I seem to recall the a quiet conversation runs about 50 dB, while a military jet taking off is about 130 dB.

But with the volume control, you have the means to prevent hearing damage. Your amp may be capable of 500 watts out per side, but can be played at a comfortable level with a low volume setting.

#### Overclocked2300

Joined Apr 24, 2005
124
Originally posted by beenthere@May 13 2005, 12:22 PM
Hi,

There's a bit of confusion here. The voltage gain of the power amp is not really related to the output level. Remember that there is a transformation from electrical power into mechanical power at the speakers. The Decibel scale is not equivalent for voltage gain and sound pressure level measurements.

The absolute voltage gain of the amp is not a major issue. It's always going to be pretty high, and will be some fixed amount like 60 dB. The preamp feeding the power amp should have a volume control to regulate the level of the input, and thus control the output level of the amp. Otherwise, it's a bit like trying to drive a car with an engine set to run at full power all the time - not too practical.

The amp's voltage gain does not relate to its output power until the speaker load is known. Then you can figure the power from measuring the voltage across and the current through the speaker coils.

The conversion of electrical power to mechanical by the speakers is always fairly inefficient. At one time, the most popular Acoustic research speakers were rated at .9% efficiency. Took a lot of power to make then get loud. Then you measure them as having a sound pressure level - also a log scale in dB units. I seem to recall the a quiet conversation runs about 50 dB, while a military jet taking off is about 130 dB.

But with the volume control, you have the means to prevent hearing damage. Your amp may be capable of 500 watts out per side, but can be played at a comfortable level with a low volume setting.
[post=7671]Quoted post[/post]​
Well the speaker is 4 ohms..

here the other data on the speaker:
Freq: 80Hz to 22Khz (its a 3 way)
SPL:90dB
Power 120W

Now spl:abbreviation for "sound pressure level," a means of expressing sound levels. SPL is frequently used as a comparative measure of speaker efficiency or maximum system output.

so this thing is 90% efficant? Or the max output is 90dB?

So what your saying is, no matter how much I amplify the signal It wont be loud? Im not understanding something here..

EDIT: so no matter how much power I have too..it still wont be loud?

#### Firestorm

Joined Jan 24, 2005
353
Originally posted by Overclocked2300@May 13 2005, 08:18 PM
Well the speaker is 4 ohms..

here the other data on the speaker:
Freq: 80Hz to 22Khz (its a 3 way)
SPL:90dB
Power 120W

Now spl:abbreviation for "sound pressure level," a means of expressing sound levels. SPL is frequently used as a comparative measure of speaker efficiency or maximum system output.

so this thing is 90% efficant? Or the max output is 90dB?

So what your saying is, no matter how much I amplify the signal It wont be loud? Im not understanding something here..

EDIT: so no matter how much power I have too..it still wont be loud?
[post=7677]Quoted post[/post]​
It doesn't measure the efficiency. Bascially your power(watts) can produce loud music depending on the volume of the speaker(in your case 4Ω)
hope this helps.

-fire