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How to measure decibels

 
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mandarcy



Joined: 31 Jul 2007
Posts: 3

PostPosted: Tue Jul 31, 2007 9:05 am    Post subject: How to measure decibels Reply with quote

Hello,

I'm an enthusiast trying to get deeper into electroncics and as a personal project I've been restoring an old keyboard that wasn't working. I now have sound and everything seems okay. Now it just needs calibrating. I have the service manual and I'm okay with all the steps apart from the last one.

It says I need to measure and trim the output of a particalur op-amp until it is at -23db. What I'm confused about is how do I measure decibels in this way? And, what with?

Thanks in advance for any advice.

Regards,
Dave.
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Tue Jul 31, 2007 4:46 pm    Post subject: Reply with quote

In general db measurement is used as a logaritmic unit in describing the ratio between in/out signals. In other words -23db is meant to show the relative gain of that stage--(actually a loss in this case) Most often, db is used in calibration along with some sort of a reference signal (ie signal generator) for measuremant purposes & can get rather confusing for the untrained. Suggest doing a search & read about it.

However, in real world there are "cheat sheats" used all the time. Look at the db chart on the bottom of this page as a reference where you can use the voltage ratios for relative measurements

http://en.wikipedia.org/wiki/Decibel

Also, I will tell you that in "repair world", going exactly by the book is not always necessary or time efficient.
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mandarcy



Joined: 31 Jul 2007
Posts: 3

PostPosted: Wed Aug 01, 2007 8:16 am    Post subject: Reply with quote

Hi vtech

thanks for the reply. It makes more sense to me now! I'd only ever thought of decibels as a measure of sound before now which is why I was confused about it. This is the first time I've come across it as a way for measuring voltages. (I'm new to tronics Wink )

Thanks again,
Kind Regards,
Dave.
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