Explain Preamp Gain to an Idiot

[kevin1969]

Just to clarify I'm the idiot.

Based on the great work of Amir and the vendors that contribute to this site I just purchased a new March Audio P252 amp. The specs state it has an input sensitivity of 26dB.

Being the idiot could someone explain how you figure out how that translates into how the gain of different preamps will work with the amp (or what would be the most cost beneficial).

I'm not even sure what pre-amp specs one would look at to even determine this.

For example how would these preamps differ in respect to to the amps 26dB input gain.

Schitt Saga
Schitt Freya
Gustard P26

 

[noobie1]

Some amps have multiple input sensitivities. I believe Benchmark AHB2 is one such amp probably because it finds use in pro settings. These amps might be connected to pro audio interfaces which can have higher outputs thus a lower input sensitivity might be needed to have more play in volume knob. You probably won’t see that much difference between the preamps you’ve listed except maybe when you go from balanced to single ended.

 

[solderdude]

The specs state it has an input sensitivity of 26dB.

It states Voltage Gain 26dB. The dB is a ratio. This works out as 20x gain. (20log of voltage ratio)
150W into 8 Ohm = 34.6V output voltage.
The amplifier thus has 20x (+26dB) gain so the input voltage to achieve the 150W in 8 Ohm = 20x lower.
So input sensitivity = 1.73V and means that when you apply 1.73V the amp will deliver 20x higher output voltage which is 34.6V

To make it confusing again... 1.73V = +7dBu = +4.8dBV

When your pre-amp cannot reach 1.73V (+7dBu, or +4.8dBV) then you cannot reach max. output power.
When your pre-amp can reach above 1.73V (+7dBu, or +4.8dBV) you can take full advantage of the max output power.
The higher the output voltage of the pre-amp is (and its gain) the lower the volume control can be to reach max. power levels.

This means any DAC or pre-amp that can deliver 2V will be able to drive the amp beyond clipping levels even using passive pre's.

 

[Willem]

And if I may suggest an alternative preamp for this great power amp: the RME ADI-2 DAC. In fact, you do not need a traditional preamp at all, only an external volume control. The RME ADI-2 gives you that, plus a very good DAC with tone and balance controls, dynamic loudness, filters, etc etc. And no tubes that only detract from the sound quality.

 

[peng]

Just to clarify I'm the idiot.

Based on the great work of Amir and the vendors that contribute to this site I just purchased a new March Audio P252 amp. The specs state it has an input sensitivity of 26dB.

You obviously know the 26 dB is gain, but with that given you can calculate input sensitivity that is typically defined as the input voltage required for the amplifier to output its rated value. Sometimes it is specified as the input voltage for the amplifier to output a given value, such as the 28.28 V that Parasound used for that spec on their website.

Being the idiot could someone explain how you figure out how that translates into how the gain of different preamps will work with the amp (or what would be the most cost beneficial).

To avoid complications requiring/involving the gain of the preamp you have in mind, it may be easier to just figure out the require output voltage the preamp is rated for to see if it is high enough to drive your power amp to its rated output.

To figure that out you basically can use the two formula below:
1) Po = Vo^2/R
2) Vi = Vo/10^(Gain in dB/20)

- the ^ sign represents the superscript 2, so V^2 means the square of V, or V X V, R is the value of the load resistor in ohms.
- Vi is the input voltage to the power amp
- Vo is the output voltage of the power amp
- Po is the power output of the power amp into the resistor load, manufacturers typically based their input sensitivity on an 8 ohm resistor load.

Note: As we all know, speaker loads are not resistors, but the purpose of input sensitivity calculations it doesn't matter.

I'm not even sure what pre-amp specs one would look at to even determine this.

To make sure the preamp has high enough output to drive your power amp, you would look for their specified rated output voltage and sometime they also specify their "maximum" output voltage.

For example how would these preamps differ in respect to to the amps 26dB input gain.

Schitt Saga
Schitt Freya
Gustard P26

First of all, you can use the formula above to find out the preamp output voltage required to drive the P252 to its rated output.

Using formula 1) above, Vo = square root (PoXR), so for 8 ohms Vo = sqrt(150X8) = 34.64 V
Using formula 2) above, Vi = Vo/10^(26/20) = 1.7362 V

So you can see that any of the preamp you listed should have no problem driving the March P252 to its limit.

 

[kevin1969]

You obviously know the 26 dB is gain, but with that given you can calculate input sensitivity that is typically defined as the input voltage required for the amplifier to output its rated value. Sometimes it is specified as the input voltage for the amplifier to output a given value, such as the 28.28 V that Parasound used for that spec on their website.



To avoid complications requiring/involving the gain of the preamp you have in mind, it may be easier to just figure out the require output voltage the preamp is rated for to see if it is high enough to drive your power amp to its rated output.

To figure that out you basically can use the two formula below:
1) Po = Vo^2/R
2) Vi = Vo/10^(Gain in dB/20)

- the ^ sign represents the superscript 2, so V^2 means the square of V, or V X V, R is the value of the load resistor in ohms.
- Vi is the input voltage to the power amp
- Vo is the output voltage of the power amp
- Po is the power output of the power amp into the resistor load, manufacturers typically based their input sensitivity on an 8 ohm resistor load.

Note: As we all know, speaker loads are not resistors, but the purpose of input sensitivity calculations it doesn't matter.



To make sure the preamp has high enough output to drive your power amp, you would look for their specified rated output voltage and sometime they also specify their "maximum" output voltage.



First of all, you can use the formula above to find out the preamp output voltage required to drive the P252 to its rated output.

Using formula 1) above, Vo = square root (PoXR), so for 8 ohms Vo = sqrt(150X8) = 34.64 V
Using formula 2) above, Vi = Vo/10^(26/20) = 1.7362 V

So you can see that any of the preamp you listed should have no problem driving the March P252 to its limit.

Wow thank you so much for this amazing explanation !!!

 

[izeek]

It states Voltage Gain 26dB. The dB is a ratio. This works out as 20x gain. (20log of voltage ratio)
150W into 8 Ohm = 34.6V output voltage.
The amplifier thus has 20x (+26dB) gain so the input voltage to achieve the 150W in 8 Ohm = 20x lower.
So input sensitivity = 1.73V and means that when you apply 1.73V the amp will deliver 20x higher output voltage which is 34.6V

To make it confusing again... 1.73V = +7dBu = +4.8dBV

When your pre-amp cannot reach 1.73V (+7dBu, or +4.8dBV) then you cannot reach max. output power.
When your pre-amp can reach above 1.73V (+7dBu, or +4.8dBV) you can take full advantage of the max output power.
The higher the output voltage of the pre-amp is (and its gain) the lower the volume control can be to reach max. power levels.

This means any DAC or pre-amp that can deliver 2V will be able to drive the amp beyond clipping levels even using passive pre's.

All those numbers made my head hurt but I generally understand.
I'm audiophile-illiterate.
Please answer this for me.
I have a JBL SDP3 I use as my pre-amp.
In the input gain section, there's an adjustment for gain of 0- + 12db. I have it set at +7 for the nonce. I truly have no idea how correct this is. But I figured out I needed at least that. I'm sure it is not optimal. I'm sure I'm not getting the max out of my amps.
I'm not entirely clear on how to use the internal test noise nor the external.
The op manual was vague at best.
I don't have test equipment though I do have a minidsp I haven't worked in yet.
Thanks for any advice.

 

[solderdude]

perhaps just set the gain to: auto
I would assume it would set itself so it does not recognize clipping.

 

[izeek]

perhaps just set the gain to: auto
I would assume it would set itself so it does not recognize clipping.

Thanks. I'll certainly give a go.

 

[tiagobdonoso]

I have a similar issue.
I use a built in phono preamp with my turntable, AT Lp120x. I use little bear, just a passive knob (reviewed here in ASR), and connect it to my audiophonics nc252mp.
Is there a problem, except for the fact that I get a low volume (which is enough for me)? I have 8ohms speakers, Polk RT55i, but I'm moving to the JBL Arena 120, 100W into 8 ohms (if that matters).
I'm not hearing any hiss or clip, but all that made me worry. The Audiotechnica manual says that line out is 240 mV nominal at 1 kHz, 5 cm/sec (and 36dB gain).
If I need 1.7v, that is way low and might clip my amp, right?
The manual where I got that information from: https://www.audio-technica.com/medi...T-LP120XUSB_UM_162501380_V1_EN_web_190312.pdf
The Gustard X16 is to be delivered, and as I read here I won't have a problem.