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Author Topic: op amp gain stage?  (Read 2434 times)

absolutkj

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op amp gain stage?
« on: November 08, 2005, 11:16:54 PM »

Let's talk gain in op amps.  If we have an input of -10dBu and an output of +4dBu, what is the gain in dBu?  I'm familiar with the equation, 20log(Vout/Vin) but i'm probably just being careless and not thinking of something else, now that we are working in dBu rather than Volts.  Any ideas?  
 
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Brian Roth

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Re: op amp gain stage?
« Reply #1 on: November 09, 2005, 02:24:31 AM »

In your example, the gain would be 14 dB (not dBu).

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bobkatz

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Re: op amp gain stage?
« Reply #2 on: November 09, 2005, 07:24:47 AM »

absolutkj wrote on Tue, 08 November 2005 23:16

Let's talk gain in op amps.  If we have an input of -10dBu and an output of +4dBu, what is the gain in dBu?  I'm familiar with the equation, 20log(Vout/Vin) but i'm probably just being careless and not thinking of something else, now that we are working in dBu rather than Volts.  Any ideas?  
 



A dB is a dB is a dB. Decibels are relative. Decibels with a suffix are absolute and refer to absolute volts.

Therefore, gain should never be expressed in "dBu", but rather in straight dB. People used to talk about "voltage gain" and that's a sticky wicket, but for your purposes I would say the simple answer is +4 minus -10 = 14 dB gain. As long as you measure the dBu by the same method on either side of the equation; if it was power matched, then old timers would argue that the impedances involved were important and could change the decibel calculation.

BK
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absolutkj

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Re: op amp gain stage?
« Reply #3 on: November 09, 2005, 08:06:53 AM »

thanks for clearing this up!
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absolutkj

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Re: op amp gain stage?
« Reply #4 on: November 09, 2005, 08:42:36 AM »

but what if we had a summing amp, meaning a few different channel voltages summing together with an added gain from the summing op amp as well.  Here's an example:

on your input you have signals of -10, -20, -30, and -10dBu.  The op amp itself adds +10dB of gain.  What would the total output be? which by the way is unbalanced.
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bobkatz

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Re: op amp gain stage?
« Reply #5 on: November 09, 2005, 10:16:03 AM »

absolutkj wrote on Wed, 09 November 2005 08:42

but what if we had a summing amp, meaning a few different channel voltages summing together with an added gain from the summing op amp as well.  Here's an example:

on your input you have signals of -10, -20, -30, and -10dBu.  The op amp itself adds +10dB of gain.  What would the total output be? which by the way is unbalanced.



Are the incoming signals in phase and correlated?

If you are mixing music, the sum will be higher than the individual components. That's about all you can say for sure, even if you do an average calculation assuming random, uncorrelated pink noise on each incoming channel (port).

BK
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absolutkj

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Re: op amp gain stage?
« Reply #6 on: November 09, 2005, 10:42:02 AM »

in theory the signals would be in phase.  Is there any way to calculate an actual total for this?
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Geoff_T

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Re: op amp gain stage?
« Reply #7 on: November 09, 2005, 10:55:40 AM »

absolutkj wrote on Tue, 08 November 2005 20:16

Let's talk gain in op amps.  If we have an input of -10dBu and an output of +4dBu, what is the gain in dBu?  I'm familiar with the equation, 20log(Vout/Vin) but i'm probably just being careless and not thinking of something else, now that we are working in dBu rather than Volts.  Any ideas?  
 


Hi

Actually, there's another factor you need to consider.

The -10dB you mention is usually -10dBV (ie reference 1 volt, normally a video standard) which is a smidgen hotter than -10dBm which is referenced to 0.775V (1mW into 600ohms).

We went over to dBu at Neve in the early 1970's because our levels were at many different impedances, not just 600 ohms.

Smile
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bobkatz

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Re: op amp gain stage?
« Reply #8 on: November 09, 2005, 01:09:42 PM »

absolutkj wrote on Wed, 09 November 2005 10:42

in theory the signals would be in phase.  Is there any way to calculate an actual total for this?



In theory? If one is from the bass player and the other is from the vocalist, what is their phase relationship?

There is probably a less complicated formula, but I would add the RMS values and compute the RMS sums. This would handle the correlated or the uncorrelated case. Some more math-minded person will tell you the direct way, but I'm the lazy type and won't figure out the log relationship, so I'll just convert each of the dBu's to volts, square the volts, add them, and then take the square root and then convert back to dBu.

BK
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