Decibels

[Wilder Amplification]

Oh, and BTW -3dB means that your output voltage is 0.7071 times the input voltage.

Hope this helps

Best regards

Bob

Interesting you say this because -3dB in terms of POWER = 1/2 power, yet -3dB in terms of voltage means 70.7% of the voltage.

This is where some people think that RMS power = 70.7% power, but power doesn't work like that. They hear "RMS Power" and don't realize that this actually means 1/2 the peak power, which is actually AVERAGE power and there's really no such thing as "RMS Power".

I think they get the two confused because RMS Voltage and RMS Current are used to figure out Average power.

[Robert M. Martinelli]

Jon,
that's exactly the reason I wanted to clear any confusion and referred to voltage in my last post, because I saw he mistakenly thought that -3dB was 1/2 the signal (voltage).

To deal with power, it's enough to remember that, if we reduce VOLTAGE to 0.7071 (-3dB), CURRENT gets reduced by the same amount too ( keeping the load the same ), so POWER gets halved ( 0.7071...*0.7071.... = 0.5 ).

Cheers

Bob

[lowell]

I'd prefer he do some research on his own, and then post to me what he believes the answer is.


-g

What does this mean? The guy who asked me to build the preamp is who RG is referring to. If your (-g) post is directed at me, I did research online and couldn't for the life of me find any concrete description that made it clear and concise for me. Thus I posted on ampage.org AFTER doing research, in the hopes that the "helpful" posters on here would be able to lay this out in a fashion that I could really grasp. AND in my initial post I DID put what I thought the answer was. Thanks OTM and WA for those answers. I think I get it now.

[Steve Conner]

Decibels measure power ratios.

One bel is a 10:1 change in power, but it's too big a unit for practical use, so it got split into 10 "decibels".

When you have a voltage ratio, you have to square it to get the corresponding power ratio. Squaring something multiplies its logarithm by 2. That's why "voltage dBs" are 20*log whatever, and "power" ones are 10*log whatever.

But technically it's wrong to think of "voltage" and "power" decibels. Just remember that decibels are units of power ratio, not voltage or anything else. When you express voltage in dB, you're expressing the power that that voltage would dissipate in some test load of a constant resistance. 50 ohms in RF work, 600 ohms in audio, but it doesn't really matter because it's a ratio.

[Enzo]

So one could say something is 30db louder than something else. But one could not say something is 30db loud, not without specifying a "than."

SO an amp might have a gain of 26db, but not an output of 26db. Unless specifying an output of 26db over something.

The old jet plane is Xdb loud always implies the threshold of hearing as 0db.

[Robert M. Martinelli]

We all "solder fume breathers" ( thanks Enzo ) normally deal with voltage and ( electrical ) power gain ( or attenuation ), or with SPL, for which the use of deciBels to express the numeric relationship ( ratio ) between two numbers is fast and convenient. The original question was about voltage gain and I think that the thread just followed accordingly.

Being deciBels logarithm-related, and being a non-dimensional entity, they could be used to express all kind of ratios ( between distance, weight etc. ), but in everydays life that would be sort of an hindrance rather than a convenience ( if John buys 1 kg of bread and I want to buy 10 kg of bread right after him, I just can't think about myself asking the baker "20 dB of bread more than that, please" ).

Cheers

Bob

[Steve Conner]

Don't forget dB per decade, very useful for filter responses and house prices

For prices, use 10*log because money is power...

[Robert M. Martinelli]

Don't forget dB per decade, very useful for filter responses and house prices

For prices, use 10*log because money is power...

[lowell]

Right on thank you guys. This has been a very enlightening thread. I figure if I'm an amp tech I should know this sh$t like the back of my chassis right? Duh.

[Enzo]

Actually, in my repair work, I very rarely need to use "decibels." But you definitely should understand it.

[wyomingrocks]

I don't know how this post slipped past me but I found it very enlightening and wanted to say thank you everyone.. Some real good examples.