SVC24

SVC24

We have to attenuate the DAC output signal one way or the other. What would be better than connecting the DAC output directly to the power amp (minimum losses) and simply use digital (software) volume control? This approach always feeds full DAC output noise into the power amplifier, regardless of volume setting (noise is not attenuated together with the signal). This translates to poor SN ratio, something we want to avoid in order to get maximum resolution. Software volume control needs to change the original audio file data, so output is no longer bit-perfect. The audio file dither pattern (required for maximum SN ratio) is permanently destroyed in the process. In order to get lossless 60dB attenuation we need to sacrifice up to 10 bits. If we use 24 bit resolution we could end up with only 14 bits. But it gets worse, there are practical limits to resolution caused by thermal noise and imperfect circuits, so the theoretical 24 bits resolution cannot possibly be maintained in any audio set. So we always end up with a lossy volume control with worst SN ratio. Now how do we obtain a lossless volume control? Well we have to connect the DAC output directly to the amplifier input, that's for sure. So the only remaining option now is a shunt volume control in parallel with the Fractal DAC output, using the 375 OHM Fractal DAC fixed output impedance as series resistor. This will attenuate both, signal and noise providing best SN ratio. With given 375 OHms output impedance we need pretty low shunt resistor values down to 0.38 OHms for -60dB. We also want to avoid any possible noise injection or distortion from active circuits and power supplies. This leaves little other option than a manually controlled rotary switch that puts only one switch contact and one resistor in the shunt signal path at any given volume setting. The related low resistor values help to maintain lowest noise levels and highest resolution. The SVC24 is a 24 position, manually controlled lossless shunt attenuator. The attenuation per step is carefully calculated to get the best overall attenuation response with the given 24 steps. Additional advantage of the SVC24 is that it can be used to control the volume with 250 OHm studio headphones connected directly to the Fractal DAC output (SVC24 in parallel with the headphones).