decibals and project

[duffy]

Nigel's exactly right. It's used by second-rate hacks the same way a squid uses ink. By the time you figure out which db term they mean, where it might apply, and gotten out your calculator to do a log conversion - only to discover they don't know what they're talking about - they have escaped the room. That's the #1 most popular use of the db.

 

[Hero999]

Then it isn't the dB's fault, it's some of the idiots that misuse them.

 

[PhillDubya]

Then it isn't the dB's fault, it's some of the idiots that misuse them.

Agreed! This is ludicrous, dB's are used specifically in analog filter design, there are extremely useful to describe attenuation, roll-off, slope and gain on a log scale. They are not terribly hard to understand either.

dBm = power
dB = ratio

Not exactly rocket science there....

 

[audioguru]

You cannot get 6W from an LM48511 because its max output at clipping into an 8 ohm speaker is only 3W and its max output into a 4 ohm speaker is 5.4W.
The circuit in the datasheet has a voltage gain of only 2 (probably to keep its distortion low) so a preamp is needed.

 

[steveB]

Nigel's exactly right. It's used by second-rate hacks the same way a squid uses ink. By the time you figure out which db term they mean, where it might apply, and gotten out your calculator to do a log conversion - only to discover they don't know what they're talking about - they have escaped the room. That's the #1 most popular use of the db.

You should be able to do the log conversions instantly in your head to reasonable accuracy (within a few tenths of a dB). If you can't, then you don't know dBs well enough, which would explain your dislike for them. If you get mislead by specifications, or can't figure out the meaning before someone leaves the room, that's your fault.

 

[Bob Scott]

I have been given a specification for an audio signal but I'm a little confused about it. The specification was that it was 0dBm into 600 ohms.

From what i have read on-line (we are not being taught about the dB scale in the module) this means that there is 1mW through the resistor? Now using p=(V^2)/r i calculate it to be 1.2*10^-3 volts?

You have the right equation but you made a mistake.

P=V²/R
V²=PR (not P/R)
V²=.001×600
V²=0.6
V=√0.6
V=0.77459666924148337703585307995648
V≈0.775

 

[Pepsiiuk]

You cannot get 6W from an LM48511 because its max output at clipping into an 8 ohm speaker is only 3W and its max output into a 4 ohm speaker is 5.4W.
The circuit in the datasheet has a voltage gain of only 2 (probably to keep its distortion low) so a preamp is needed.

I was planning on using a 4 ohm speaker and 5.4W is close enough to 6W for me.

Thanks bob scott, I didn’t notice that. I was wondering why it was a little low.

OK, so what I have so far is that the input signal is 1mW and 0.775 volts across 600 ohm. And that if I use the LM48511 I will need a pre amp with a gain of 3000 (if u do the maths according to audioguru and Externet).

Have any of you got any suggestions to what i can do instead of using the LM48511? Sound quality isn’t really an issue so an amp made op amps maybe?

Thanks again

 

[audioguru]

5.4W into 4 ohms is a signal level of 4.65V.
If your input signal level is 0.775V then simple math shows the gain of the amp must be 6, not 3000.
The LM48511 amp might oscillate if its gain is as low as 6 so a volume control is needed to attenuate the input level.

You are confusing Watts with Volts.