• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

caculating wattage , db gain etc,

Status
Not open for further replies.

MrDEB

Well-Known Member
using an LM386 amp connected as a 200db gain how much wattage (speaker is 8 ohm or the transducer is as well).
output is a 5v 18f1320 PIC
presently using a .2 watt speaker but set up with 20db gain
plan is to power the LM386 either with 5v or 12v (going to be used on an automobile.)
If I can locate a higher wattage speaker that is under 1 1/2 dia or a transducer.
never used a transducer before so not real sure of usage.
needs to be water proof
pui audio is sending some samples but still would like to know the calcs behind db gain vers wattage etc.
 

Attachments

Nigel Goodwin

Super Moderator
Most Helpful Member
Wattage is determined by the supply voltage and the load impedance, gain has relatively little to do with it.

I suggest you study what db's are as you seem to have no clue - or you wouldn't have mentioned something as silly as 200dB.

A lecturer at college always told us "only people who don't know what they are talking about use dB's" :D
 

audioguru

Well-Known Member
Most Helpful Member
The output of an LM386 amplifier is only 0.14W for a sine-wave at clipping into an 8 ohm speaker with a 5V supply. Its power with a 12V supply is 0.66W. The power is double for the square-wave from the micro-controller.

A PUI transducer is very tiny and probably will not be heard unless it is worn on the ear. They do not spec the distance for the sound pressure.
 

unclejed613

Well-Known Member
Most Helpful Member
with a square wave output going into the LM386, use a gain of 20, and adjust the pot for an overall gain of 1 or 2. any more than that and it's not really doing anything, since you'll be clipping the amp anyway. shown is the minimum parts, gain of 20 circuit from the datasheet.
 

Attachments

MrDEB

Well-Known Member
would a transducer be better?

the PIC is randomly going from 2khz to 6khz.
hopefully can fit it all into a 1inch pvc pipe?
 

MrDEB

Well-Known Member
how dos one determine clipping?

am amazed that you guys determined that the amp will clip.
 

unclejed613

Well-Known Member
Most Helpful Member
it's actually very simple..... input voltage (assuming 5V as the supply for the PIC) =5V peak to peak, minimum amp gain is 20. 5*20=100V p-p would be required for "undistorted" output. the LM386 runs off of a 12V supply, so the clipping level is just short of 12V p-p. even if your input were a sine wave, at such a high input level, you would get a 12V p-p square wave out....
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
Look at the datasheet for the LM386 amplifier. With a 12V supply, it cannot provide enough current to an 8 ohm speaker for 12V p-p output. The output at clipping is only 6.5V p-p and nearly half the power supply voltage is wasted by heating the IC.
 

MrDEB

Well-Known Member
any suggestions

a smaller load say an 4ohm speaker or ??
I could also decrease the input resistor which is presently a 10k.
 

Mikebits

Well-Known Member
A lecturer at college always told us "only people who don't know what they are talking about use dB's" :D
I don't understand that comment. People who design RF circuits always use dB's.
 

audioguru

Well-Known Member
Most Helpful Member
a smaller load say an 4ohm speaker or ??
Then the poor little LM386 is straining to provide enough current and the output power is only slightly more than if the load is 8 ohms.

I could also decrease the input resistor which is presently a 10k.
Use a series resistor feeding a trimpot used as a volume control to adjust the level to anything you want.
 

unclejed613

Well-Known Member
Most Helpful Member
i was just stating the "rule of thumb" that you can't swing more than the rail voltage in response to the question of how we could know the amp would clip. and that's regardless of load. without some kind of circuit trick like bootstrapping, you simply cannot go past the rails.
 
Last edited:

Mikebits

Well-Known Member

unclejed613

Well-Known Member
Most Helpful Member
actually the most abused term i've seen is WATTS. especially the "Peak Music Power" kind. take an ordinary 50Wrms/channel 5.1 surround amp, measure the total rail voltage (+/-45V=90V), then measure the short circuit current of the power supply (10A is good, if measured just before the fuse blows). this gives you the Watts PMPO of the amp equalling 900W. then add the sales department's "fudge factor" to get a nice round number of 1000W. for 5 channels, this equals 200W/ch, but the 1000W looks better on the box. db's aren't very useful for sales departments, since the log function limits the size of db measurements to 2 digits, and rarely 3 digits.
 

audioguru

Well-Known Member
Most Helpful Member
Peak Music Power is measured in Whats, not in Watts.
Little computer speakers are powered with a 12V/1A power supply (12W) and are rated for 1000 Whats.
 

MrDEB

Well-Known Member
the term DB is used in sound measurment

what the problem with using the term?
db is used to measure sound preasure n such that 120+ db will cause hearing damage in a human.
then the term comes up 20db gain or 200 db gain?
is this the amount of gain that the amp makes?
and when does an amp clip?
when the current draw is higher than the input voltage or ???\
this is my orginal question
 

audioguru

Well-Known Member
Most Helpful Member
what the problem with using the term?
db is used to measure sound preasure n such that 120+ db will cause hearing damage in a human.
then the term comes up 20db gain or 200 db gain?
dB is a ratio. 120dB is extremely loud when compared to 0dB which is barely heard.

The basic voltage gain of an LM386 little amplifier is 20 times, not 20dB. A voltage gain of 20 times is +26db.
The LM386 can have a voltage gain of 200 times, not 200dB. A voltage gain of 200 times is +46dB.

When does an amp clip?
The output of the amplifier clips (squares-off) when its input level is too high and its output cannot increase any more because of the limited voltage of the power supply.
If an amplifier is severely overloaded by a load resistance that is too low then the output current will try to exceed the max allowed output current and either the amp blows up or it will limit the current safely but cause severe clipping distortion.
 
Last edited:
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top