RCA to XLR adapter and amp gain

[nvidia_7]

I am trying to get information on amp gain when converting an amps XLR input to RCA by the use of a passive adapter. To clarify, using an RCA out from a preamp, to an XLR input on the amp. Specifically a hypex amp using an nCore module. The manual states an input gain of 28db. However, it does not state if that input gain changes with the use of an unbalanced signal vs a balanced signal.

Here are some scenarios. Lets say you used the amp without converting the XLR input, as in you went XLR -> XLR and the pre-out voltage was 2V (+1 - (-1)). With an input gain of 28db, that would be 50V amp output.

Now lets say you used an RCA to XLR cable. As in, going from RCA at the preout to XLR on the amp side. The RCA preout voltage is also 2V (+2 - 0). Would I still be getting 50V amp output?

I have read many people say you will get 6db less gain, however I assume they are saying that because typically XLR (balanced) cables have twice the voltage as RCA (unbalanced). If however the voltage is the same in both unbalanced and balanced scenarios, would amp output also be the same? Or am I missing something here? Wouldnt 6db less output depend entirely on the voltage of the balanced or unbalanced connection? For reference, Hypex states this in the manual:

The audio input is DC coupled, this means the audio source has to be free of any DC voltage. The audio input is differential. This means that ground is not part of the audio signal. When connecting an unbalanced source, treat pins 1 and 2 as a floating input with pin 2 being the “audio ground”. Pin 4 may be used to attach the shield of a shielded twisted pair cable, but the “audio ground” connection of an unbalanced cable should never connect here.

 

[DVDdoug]

The power amp gain isn't changing. The differential input is always the difference and it doesn't matter if one of the inputs is ground. ...If you had a DC amplifier and you connected a battery to the input, grounding one other the other battery terminal wouldn't make any difference.

Sometimes a balanced/differential output has twice the voltage as the unbalanced, on the same piece of equipment.

and the pre-out voltage was 2V (+1 - (-1))....

...The RCA preout voltage is also 2V (+2 - 0).

Audio is aways AC, swinging positive and negative. But it's either positive & negative relative to ground, or positive & negative relative to the other pin.

A 2V RMS sine wave is about 2.8V peak (both positive & negative peaks) or about 5.6V peak-to-peak.

With an input gain of 28db, that would be 50V amp output.

Can your amp put out 300W? 50V (squared)/8 Ohms = 312W.

 

[nvidia_7]

The power amp gain isn't changing. The differential input is always the difference and it doesn't matter if one of the inputs is ground. ...If you had a DC amplifier and you connected a battery to the input, grounding one other the other battery terminal wouldn't make any difference.

Sometimes a balanced/differential output has twice the voltage as the unbalanced, on the same piece of equipment.


Audio is aways AC, swinging positive and negative. But it's either positive & negative relative to ground, or positive & negative relative to the other pin.

A 2V RMS sine wave is about 2.8V peak (both positive & negative peaks) or about 5.6V peak-to-peak.


Can your amp put out 300W? 50V (squared)/8 Ohms = 312W.

The above numbers are hypothetical to help me understand why many are saying that an RCA to XLR cable is going to cost me 6db of output. To simplify, if an amp with a single input, being balanced XLR, had a gain of 28db, would using an RCA to XLR input have any affect on gain? Or can I still use the stated amp gain to calculate for amp output regardless of using a balanced or unbalanced cable? In this hypothetical scenario the unbalanced and the balanced cable are both carrying 2v RMS. Would the amp output be the same with both of them?

 

[NTK]

The Hypex amplifier amplifies the difference in the voltage between V_IN+ and V_IN- (the definition of differential input), regardless of whether the upstream signal comes from an RCA (ground referenced) or XLR (floating) connection.

If the difference between V_IN+ and V_IN- is 2 Vrms, the output will be 2*(10^(28/20)) = 50.2 Vrms, regardless of whether it is from an RCA or XLR connection.

 

[Dumdum]

I think your asking if the overall output will be less… gain will be the same, but half the voltage out… as balanced for example with 4v input… would make more output voltage vs rca input with 2v input

 

[nvidia_7]

The Hypex amplifier amplifies the difference in the voltage between V_IN+ and V_IN- (the definition of differential input), regardless of whether the upstream signal comes from an RCA (ground referenced) or XLR (floating) connection.

If the difference between V_IN+ and V_IN- is 2 Vrms, the output will be 2*(10^(28/20)) = 50.2 Vrms, regardless of whether it is from an RCA or XLR connection.

This is exactly the answer to my question. So basically, RCA center pin goes to XLR pin 2 which is going to V_IN+. Then the ground goes to XLR pin 1 and 3? If there is any noise on the ground I assume that could potentially lower the amps output as it would be referencing that from V_IN+?

 

[NTK]

This is exactly the answer to my question. So basically, RCA center pin goes to XLR pin 2 which is going to V_IN+. Then the ground goes to XLR pin 1 and 3? If there is any noise on the ground I assume that could potentially lower the amps output as it would be referencing that from V_IN+?

If the ground is noisy, then for a input that is referenced to the noisy ground, the differential voltage at the Hypex inputs will be:
V_IN+ = V_signal
V_IN- = V_noise
Therefore: V_differential = (V_IN+) - (V_IN-) = V_signal - V_noise

Since V_noise is usually not correlated to V_signal, the output is usually not a reduction, but the sum of the amplified V_signal and amplified V_noise.