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westers  
#1 Posted : Wednesday, December 16, 2015 6:49:49 PM(UTC)
westers

Rank: Member

Groups: Member
Joined: 11/29/2015(UTC)
Posts: 7
United Kingdom
Location: Nottingham

My boards should be with me in the next couple of days so I've turned my attention to whether I need an active/passive preamp with the amp. My source will be a raspberry pi (acting as the music server), and a PiDac++ from Qaudio. Qadio have confirmed the output of PiDac+ is 2.0Vrms, assuming an input load of 10K and an output impedance of 480 ohms.

My conclusion from the above is that I don't need any extra gain from an active amp - the PiDac should be enough to drive the Sympatico all the way to full rated 100W into 8 ohms output. My calculations are as follows:

Power=Voltage^2/load resistance => sqrt (100W*8) = Voltage => 28.28 Volts, or 28.3 Volts rounded up. So for my 8 ohm speakers operating at full 100W power, the amp is delivering 28.3 Volts at full power.

Using the formula Voltage Gain (Av) = 20 * Log (Vout/Vin), plugging in the known values and then rearranging the formula, I get 10^(28.3/20) = Vout/Vin = > 26.001. So the amp is boosting the input signal by a factor of 26.

From all of the above I now know that my source needs to deliver Vrms no less than 28.3/26 to drive the amp to full power. For me, the minimum Vrms for the source is 1.08 Vrms. So given the Pidac+ delivers 2 Vrms, it should have no problem driving the amp, and thus I don't need any additional gain from a preamp.

I know I read that the Sympatico has an input impedance of 100K (can't find the thread so it would be good to double check this), hence a 10K pot would be fine for this.

Anything wrong with my calculations above? I'll be running it in SE mode, hence my gain will be the 14.8db that you posted in another thread.

Thanks
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