Ideal input voltage?

[deigm]

Hi! Question if I may..

So I notice the two selectable gain settings on the 262 amplifier (26.5db/20.5db) are said to be ideal for 2v and 4v inputs respectively..

If my DAC outputs 4 volts over XLR into a preamp with 18db (7x) gain (XLR), then full output would be about 28 volts (if i understand correctly).

Even with the 262 on the low setting of 20db, that's about 280-volts, or 19,600 watts over a 4 ohm load.

Now, I know that can't be right, but I don't know what I'm missing.
Can anyone help explain how preamps with 4-volt XLR inputs are supposed to work with typical 20db gain power amplifiers?

 

[Alan March]

Hi @deigm

Good question.

The gain of the P262 is set so that full power output will be reached with either a 2 volt input (26.5dB) or 4volt (20.5dB). In fact with the 262 you wont quite need these levels to hit full power out.

Whatever gain your preamp has (18dB is unnecessarily high), remember that its output voltage is set by the volume control. So even if your dac is outputting its max 4 volt rms, you wont necessarily be getting that (or more) out of the preamp. You could even turn the volume control down to zero and get zero volts out!

Can you tell me what pre amp it is? 18dB is very high gain and will only serve to increase noise. What its doing is amplifying the signal and then having to attenuate it, which is not ideal. Hopefully it has a setting to reduce the overall gain.

So, for a standard 4 volt XLR source, a pre-amp doesnt need to have any further gain, although a few dB, maybe 3dB to 6dB is a good idea to give a bit of latitude with the volume control.

hope that helps

Alan

 

[deigm]

Hi @deigm

Can you tell me what pre amp it is? 18dB is very high gain and will only serve to increase noise. What its doing is amplifying the signal and then having to attenuate it, which is not ideal. Hopefully it has a setting to reduce the overall gain.

Thanks Alan.

I wasnt aware that 18db of gain was unusually high for a preamp. Having just had a bit of a look around, it seems 12db of gain is fairly common.

Take the PSAudio GainCell Preamp for example. It has 12db of gain, but even in this case, a 4volt input over XLR would be amplified to a 16v output, then if that is run through a 20db power amplifier we get 6,400 watts. So again, way too much.

I'm having trouble understanding what use case XLR inputs/outputs have if their standard 4volts causes too much amplification..
Is it just normal to only use the first 20 percent of the volume dial?
It seems to me these preamp are expecting a 1-2 volt input, but I've not seen a DAC that outputs that low over XLR.

Aside from, as you suggest, finding a preamp with only 6db of gain, would a power amplifier with 15db of gain or lower rectify this issue also?
Or would it's inputs clip?

 

[Alan March]

Thanks Alan.

I wasnt aware that 18db of gain was unusually high for a preamp. Having just had a bit of a look around, it seems 12db of gain is fairly common.

Take the PSAudio GainCell Preamp for example. It has 12db of gain, but even in this case, a 4volt input over XLR would be amplified to a 16v output, then if that is run through a 20db power amplifier we get 6,400 watts. So again, way too much.

I'm having trouble understanding what use case XLR inputs/outputs have if their standard 4volts causes too much amplification..
Is it just normal to only use the first 20 percent of the volume dial?
It seems to me these preamp are expecting a 1-2 volt input, but I've not seen a DAC that outputs that low over XLR.

Aside from, as you suggest, finding a preamp with only 6db of gain, would a power amplifier with 15db of gain or lower rectify this issue also?
Or would it's inputs clip?

I do think most pre amp designs are somewhat stuck in the past where sources were lower voltage RCA at 1 to 2 volts. To be fair were are only recently seeing a definite turn towards XLR in domestic audio. This is very welcome as RCA is fundamentally flawed.

Power amplifiers have for a very long time been typically around 26 to 30dB gain. Again this is more geared towards a source level of 2 volts (RCA) and not ideal for 4 volt (XLR). However the majority of domestic kit is still RCA which is why we provide selectable gain options of 20.5dB and 26.5dB.

There are a small number of power amps out there that have lower gain in the region of 15dB, but this doesnt necessarily solve your problem. Your source would need to provide 10 to 12 volts rms output without distortion. A pre amp with a high gain wont necessarily do that. High gain doesnt equal high voltage clean output.

Dont get too het up about this. Excessive gain is simply sub optimal. In certain circumstances it may cause noise issues, especially with high sensitivity speakers. Its just something to be aware of.

 

[Alan March]

I'm having trouble understanding what use case XLR inputs/outputs have if their standard 4volts causes too much amplification..
Is it just normal to only use the first 20 percent of the volume dial?

This isnt an XLR issue. This is a pre amp design issue.

XLR is a far superior connection system. RCA has a fundamental issue of ground loops and will always be more noisy in real world use. This is the reason XLR is used in *all* professional applications, including the recording of all the music in your collection

 

[deigm]

Yes, I just meant that because the XLR outputs are commonly about 4 volts rather than the 2 volts or so on RCA, that the higher voltage being sent into preamplifiers seems to cause issues. I understand that isn't the fault of the XLR standard itself. But practically speaking, it does make it's use problematic, seemingly.

 

[Alan March]

Yes, I just meant that because the XLR outputs are commonly about 4 volts rather than the 2 volts or so on RCA, that the higher voltage being sent into preamplifiers seems to cause issues. I understand that isn't the fault of the XLR standard itself. But practically speaking, it does make it's use problematic, seemingly.

Its probably less of an issue than you may think. More of a case of "sub optimal" in terms of SNR.