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#76974 - 01/23/06 04:20 PM Voltage gain which is correct
ford1215 Offline
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Registered: 01/03/03
Posts: 10
Loc: Decatur Al
Voltage gain: XLR 34dB, RCA 28dB This is for the 7700.

Voltage gain: XLR 28dB, RCA 34dB This is for the 7500.

Which one is correct? I would think the Voltage gain on the 7700 was posted correctly. What is the advantage of having a larger voltage gain?

Fletcher

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#76975 - 01/23/06 09:46 PM Re: Voltage gain which is correct
gonk Offline
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Registered: 03/21/01
Posts: 14054
Loc: Memphis, TN USA
The manual (which applies to both of them) is 34dB (XLR) and 28dB (RCA), so I would expect that to be the correct one.
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#76976 - 01/23/06 09:56 PM Re: Voltage gain which is correct
Doug917 Offline
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Registered: 08/10/05
Posts: 238
Loc: Shawnee, KS
There is a standard in which balanced connections are supposed to be 6dB above RCA connections. Gonk is correct.
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#76977 - 01/23/06 09:57 PM Re: Voltage gain which is correct
Ritz Offline
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Registered: 07/03/05
Posts: 547
Loc: NJ/Beijing
And more importantly, how are they able to extract 6dB more gain from the balanced connection? That's an astonishing difference. Perhaps I just don't understand the inner workings of amp design (I'm slow that way). Can someone shed some light on this?

Cheers,
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#76978 - 01/24/06 07:48 PM Re: Voltage gain which is correct
ratpack Offline
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Registered: 05/26/05
Posts: 110
Loc: Alabama
I'd also like to know where the extra 6 dB comes from. I just don't think that it is from the difference in the connections. Maybe there is a difference in the amps, somehow.

It is my understanding that the balanced inputs only make a difference with long cable runs where noise pickup may become a factor with the RCA type cables.

If I am mistaken, I would like to know the differences.
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#76979 - 01/27/06 02:37 AM Re: Voltage gain which is correct
Kevin C Brown Offline
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Registered: 12/11/01
Posts: 1054
Loc: Santa Clara, CA
For a balanced connection there are two legs sent not just one. The 2 legs are 180 deg out of phase wrt each other. They are then subtracted from each other, so:

+S - -S = +2S

So the signal strength is doubled, which gives you the 6 dB gain. Also, when you do that subtraction, voila, noise present on both legs is subtracted out.
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#76980 - 01/27/06 01:58 PM Re: Voltage gain which is correct
gband Offline
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Registered: 10/19/05
Posts: 65
Loc: Central NJ
Just to add to Kevin's elegant explanation, a doubling of voltage is a 6db gain [20log(2)], and yes its not just the connections, its the differential design overall.

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#76981 - 01/29/06 09:53 PM Re: Voltage gain which is correct
ratpack Offline
Gunslinger

Registered: 05/26/05
Posts: 110
Loc: Alabama
Thanks guys! I obviously forgot how the balanced input works.
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#76982 - 02/05/06 04:26 AM Re: Voltage gain which is correct
Kevin C Brown Offline
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Registered: 12/11/01
Posts: 1054
Loc: Santa Clara, CA
It's a very elegant solution to noise in interconnects. Simple, but very effective. smile
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#96019 - 09/12/16 07:46 PM Re: Voltage gain which is correct [Re: ford1215]
LAH Offline
Deputy Gunslinger

Registered: 08/28/16
Posts: 4
I own the Outlaw Audio 7700 amplifier which I used to power my 3-way active open baffle loudspeaker pair, but I was planning on building another 4-way active system. Of course, this necessitated buying an additional amplifier, hopefully with the same voltage gain. I ran across this thread, along with a couple of others from different forums, where the 7500/7700 voltage gain was in question. I noticed on the ATI site on their specification sheet for different amplifiers, the ATI AT2000, which closely resembled the Outlaw 7500/7700, that for RCA inputs the voltage gain was listed as 34 dB, whereas the Outlaw Audio manual stated a 28 dB gain for RCA. Also, the XLR input gains for the ATI AT2000 were stated as 28 dB, but the Outlaw Audio with XLR inputs were stated as 34 dB. I began to suspect one or the other was wrong, with the Outlaw Audio manual being highly suspect.

Then it occurred to me that I could simply measure the voltage input and output and calculate the voltage gain of the amplifier. Note that this is all a moot point since I have purchased a used 7500, so I can be assured of an identical voltage gain regardless of whether the stated gains are reversed or not. But I figured I would post this here for future reference, just to set the facts straight.

I used a 400 Hz sine wave output generated by TrueRTA to a USB sound card to test all 12 channels of both amps with no loudspeaker load. I measured the output from the sound card (the amp input) and each amplifier output with a Fluke 87-V, a true RMS multimeter with a wide bandwidth. If you have an inexpensive multimeter, you may use 60 Hz as the sine wave output since most meters are accurate at that frequency. The measured sound card output was 248.2 mV (.2482 V) with RCA outputs and between pins 2 & 3 on XLR outputs. I then averaged all 12 channel outputs, which turned out to be 12.59V with an RCA input and 6.485V with an XLR input. Notice right off the bat the roughly 6V difference in channel output between the two. It sort of lets you know what's going on here and what the calculations are going to reveal, and why the voltage gain is 6 dB lower for XLR. Remember, by its differential design, the input voltage will be double that of RCA.

Gain in dB = 20 * LOG10 ( Gain Factor )

RCA Gain in dB = 20 * LOG10 (12.59 / .2482) = 34.10 dB
XLR Gain in dB = 20 * LOG10 (6.485V / .2482) = 28.34 dB

So they have been reversed in the Outlaw Audio manual. But one thing that is still a mystery to me is that the Outlaw Audio 7900 and ATI AT6000 and AT4000 specifications state the voltage gain is identical for either XLR or RCA inputs at 28 dB. Hmmm...

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