Gain matching question between differnt amps

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Gary Weller

Sophomore
Mar 11, 2011
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I've got a gig tomorrow that I have to substitute a QSC GX7 for my usual Carver PM1200 on two mon mixes. Carver has issue on one channel.

I want to match the gain structure approx during the swap out.

The Carver is 32db gain and 1.5v input sensitivity, 450 watts @ 8 ohms

The GX7 is 36.1db gain and 1.2v input sensitivity, 725 watts @ 8 ohms

What's the formula for this?

Thanks,
 
Re: Gain matching question between differnt amps

Rather than use a formula, I'd just match gain using a dB meter and a noise source.
If you don't have a dB meter, use a microphone and your console meters.
If you don't have a noise source, tune a radio to static or download a pink noise file.
 
Re: Gain matching question between differnt amps

Art Welter said:
Rather than use a formula, I'd just match gain using a dB meter and a noise source.
If you don't have a dB meter, use a microphone and your console meters.
If you don't have a noise source, tune a radio to static or download a pink noise file.
Click to expand...

I've got pink on my desk (GL2400). The gain should be turned down on the QSC approx 4 db correct as a starting point?
 
Re: Gain matching question between differnt amps

I would use a sine wave and a voltmeter.

Use 60hz or close.

Noise will give you fluctuating meter movements-while a sine wave will be nice and steady and much easier to get right.

It should take less than a minute
 
Re: Gain matching question between differnt amps

Bennett Prescott said:
This is why I hate amplifiers that have varying sensitivity throughout their lines, so the more powerful amps also have more gain. If you can't do it with 26dB (x20) you need to go home!
Click to expand...

If you bridge amps like the Powersoft K20 or Lab FP14000, can you really run them into limit with 26dB of gain and a common drive source (looks like 24dBu is a common clip point)?
 
Rob Timmerman said:
If you bridge amps like the Powersoft K20 or Lab FP14000, can you really run them into limit with 26dB of gain and a common drive source (looks like 24dBu is a common clip point)?
Click to expand...

When you bridge them their sensitivity goes up 6dB.

Loaded at 8 ohms the Powersoft K20 takes a maximum input of 27dBu. You have my permission to use 32dB (x40) gain in that circumstance.
 
Re: Gain matching question between differnt amps

Bennett Prescott said:
When you bridge them their sensitivity goes up 6dB.

Loaded at 8 ohms the Powersoft K20 takes a maximum input of 27dBu. You have my permission to use 32dB (x40) gain in that circumstance.
Click to expand...

The Lab FP14000 has amplifier gain adjustments of 23, 26, 29, 32, 35, 38, 41, and 44 dB, and an automatic -6 dB gain compensation when bridging channels.
 
Re: Gain matching question between differnt amps

Rob Timmerman said:
If you bridge amps like the Powersoft K20 or Lab FP14000, can you really run them into limit with 26dB of gain and a common drive source (looks like 24dBu is a common clip point)?
Click to expand...
You are confusing the amount of gain a device has (in this case 26dB) with an absolute level (in this case 24dB).

A mixing console has a lot of gain-but is limited by the absolute max output.

Yes they are both "dB", but are basically not related.
 
Re: Gain matching question between differnt amps

Gary Weller said:
I've got pink on my desk (GL2400). The gain should be turned down on the QSC approx 4 db correct as a starting point?
Click to expand...

Measure wedge on existing amp with dB meter. Bring GX7 input all the way down and apply the same signal as you just sent to the existing mon amp. You could do this from console or just move the XLR I/O. Bring up GX7 input sensitivity until it closely matches the reading you had on the existing amp. Close enough. Do show.

Or honestly what I'd do... I'd send a song (CD/MP3/Noise) thru the existing amp and wedge. Get a quick reference level by ear. Swap over the same signal to the new amp. Increase input gain on the amp until they sound close and roll with it for the gig. That should keep your mix out masters the same as normal, and your channel strip will mostly be dictated on what the artist wants anyway. And you'll now have the amp close enough in the ballpark that your channel strip will look and react reasonably normal to you as far as between what is asked for, what you hear, and where the knob is positioned compared to where you'd expect it to be.
 
Re: Gain matching question between differnt amps

Ivan Beaver said:
You are confusing the amount of gain a device has (in this case 26dB) with an absolute level (in this case 24dB).

A mixing console has a lot of gain-but is limited by the absolute max output.

Yes they are both "dB", but are basically not related.
Click to expand...

Yes, I am well aware that while decibels are a ratio, that the inclusion of a reference makes them into absolute quantities.

My question was if 26dB of gain on top of a 24dBu signal was enough to drive something like a bridged Powersoft K20 into clip. Not sure where the confusion showed up.
 
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