RJM B-board vs. 47 Labs 0247

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# RJM B-board vs. 47 Labs 0247

Posted 5th May 2012 at 12:29 PM by rjm
Updated 6th May 2012 at 06:16 AM by rjm

B-board vs. 0247.

Comparison of the noise baselines, measured at the circuit output using a NI USB-6215 DAQ. Unloaded for the preamps, and with a 6 ohm load for the 0347 amplifier.
• B-board: -139 [300Hz-100kHz] 0 dB gain (-139 - 0 = -139 dB, 110 nV sqrtHz input referred). The actual B-board output noise is below this measurement threshold.
• 0247: -124 dB [300Hz-100kHz] 14 dB gain (-124 - 14 = -139 dB, 110 nV sqrtHz input referred).
• 0347: -109 dB [300Hz-100kHz] 31 dB gain (-109 - 31 = -140 dB, 100 nV sqrtHz input referred).

See the attached plot for the FFT data. Note the peak at 28 Hz is an artifact of the measurement apparatus.

By way of comparison, a typical audio opamp has an input referred voltage noise figure of 3-8 nV sqrtHz (-170 ~ -160 dB) and can be expected to return this datasheet specification in most well-designed circuits. In other words the output noise is going to be about -160 dB + the circuit gain.

The 0247 has some additional power line harmonics 100-500 Hz compared to the B-board, as a result of, I assume, the unregulated power supply. The input referred noise of the 0247 is the same as the 0347 amplifier, a reasonable result considering they share the same basic circuit and power supply design.

Total output noise for the B-board + 0347 combination is -109 dB. The B-board makes no contribution to the total output noise.

Total output noise for the 0247+0347 combination is -94 dB. (-139 + 14 + 31) This is 14 dB above the amplifier output noise.

In practice what this means for me is the following:

The 0347 amplifier by itself is quiet. I have to press my ear against the speaker grill to hear any hiss/rustle. Using the B-board as the preamplifier, there is no additional noise. Using the 0247, however, the hiss/rustle/sputtering noise becomes much louder, easily audible from the listening position when the room is quiet.

The high noise is a combination of the high gain and the high input-referred noise of the the 0247. It's a nice headphone amplifier, but not so well suited as a preamplifier.

I may re-wire the line outputs to take the signal directly from the volume control rather than the amplifier stage. The 0247 becomes a passive preamplifier, but I think that's the better option here.
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