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How Much Power?

Rottmannash

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I have the D90 SE>XLR Schitt Frey+ (passive buffer) >XLR VTV Pufifi>Revel F208's. Today I wanted to see how loud the system could play at unity gain so put on the hi rez version of Comfortably Numb and let's just say it was a quasi-religous experience. Is there a way to calculate how much power the Purifi was putting out at unity gain? I assume the DAC was outputting 4V on loud passages, as I found the 5V setting to overdrive the Freya+ and introduce distortion. It was so loud it was kinda scary and after the song was over my ears were ringing. The Freya+ was at full volume which I assume is unity gain. As an aside the Purifi amp case wasn't even warm. Crazy efficient.
 

rdenney

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It appears to me that the input buffer is at unity gain, while the tube amp provides 12 dB of gain. But it also appears that you can run it 1.) passive (neither input buffer nor tube amp), 2.) with just the input buffer, and 3.) with the tube amp. Based on that understanding, I don't think you can be sure of "unity gain" unless you select one of the first two of those options, for both of which the volume control is at zero attenuation when turned up all the way.

So, a 4-volt source in the first two options would produce 4 volts at the outputs.

The 1ET400A has 12.8 dB of gain, and doesn't reach 1% THD until the input voltage reaches 9.6 volts RMS.

4 volts plus 12.8 dB of gain equals 17.6 volts. (Voltage doubles every 6 dB.)

The F208 nominal impedance is 8 ohms, but much of it is less than that. We'll go with that. 17.6 volts into 8 ohms is 39 watts. The F208's are 89-dB/W/m efficient at one meter, so a signal at that voltage level will produce about 105 dB SPL at one meter. (Power doubles every 3 dB.) Two speakers will add 3 dB. That will make my ears ring if the average level of a song is close to that peak. Even if the song's peaks are 15 dB above the average, though, it will still be quite loud.

The Purifi module is good for 180 watts at less than 1% distortion into 8 ohms, though that is at the brink of clipping protection. Running it at full power would increase loudness by 7 or 8 dB--bring your earplugs if what you were listening to was loud. That would take 38 volts at the speakers, or just about 8.7 volts at the amp's inputs. Again, 7 or 8 dB of gain needed by the preamp to achieve full power.

Rick "calculating in haste" Denney
 
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Rottmannash

Rottmannash

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It appears to me that the input buffer is at unity gain, while the tube amp provides 12 dB of gain. But it also appears that you can run it 1.) passive (neither input buffer nor tube amp), 2.) with just the input buffer, and 3.) with the tube amp. Based on that understanding, I don't think you can be sure of "unity gain" unless you select one of the first two of those options, for both of which the volume control is at zero attenuation when turned up all the way.

So, a 4-volt source in the first two options would produce 4 volts at the outputs.

The 1ET400A has 12.8 dB of gain, and doesn't reach 1% THD until the input voltage reaches 9.6 volts RMS.

4 volts plus 12.8 dB of gain equals 17.6 volts. (Voltage doubles every 6 dB.)

The F208 nominal impedance is 8 ohms, but much of it is less than that. We'll go with that. 17.6 volts into 8 ohms is 39 watts. The F208's are 89-dB/W/m efficient at one meter, so a signal at that voltage level will produce about 105 dB SPL at one meter. (Power doubles every 3 dB.) Two speakers will add 3 dB. That will make my ears ring if the average level of a song is close to that peak. Even if the song's peaks are 15 dB above the average, though, it will still be quite loud.

The Purifi module is good for 180 watts at less than 1% distortion into 8 ohms, though that is at the brink of clipping protection. Running it at full power would increase loudness by 7 or 8 dB--bring your earplugs if what you were listening to was loud. That would take 38 volts at the speakers, or just about 8.7 volts at the amp's inputs. Again, 7 or 8 dB of gain needed by the preamp to achieve full power.

Rick "calculating in haste" Denney
Wow- only 39 watts?! I was guessing near 75. It was deafening. Yes, I was using the passive input. Thanks for the calculation. It truly is shocking how little power we use. On a related note, why would the D90SE on 5V mode cause distortion, even at moderate volumes? Is it overdriving the Freya's input stage? I tried it on 5V with all 3 inputs and they were all distorted.
 

phoenixdogfan

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Wow- only 39 watts?! I was guessing near 75. It was deafening. Yes, I was using the passive input. Thanks for the calculation. It truly is shocking how little power we use. On a related note, why would the D90SE on 5V mode cause distortion, even at moderate volumes? Is it overdriving the Freya's input stage? I tried it on 5V with all 3 inputs and they were all distorted.
Are you using the Eval 1 buffer or did you set it to bypass? Isn't the efficiency of the F208's something like 90 db at 2.83v input?
 

JRS

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I have the D90 SE>XLR Schitt Frey+ (passive buffer) >XLR VTV Pufifi>Revel F208's. Today I wanted to see how loud the system could play at unity gain so put on the hi rez version of Comfortably Numb and let's just say it was a quasi-religous experience.
What religion was that? That's some fine scripture.
 
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DVDdoug

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Wow- only 39 watts?! I was guessing near 75. It was deafening.
It's pretty hard to guess wattage.

And loudness is "complicated"... We are usually interested in peak power (because of potential clipping) but loudness is more correlated to average power, and frequency content, and speaker sensitivity, and room acoustics, and how far you are from the speakers...

And 75W is only 2.8dB louder than 39W.
 
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