Spatial Audio Raven Preamp

Yes, the requirements for the input/buffer stage are actually quite modest, not even a headphone amp, really. But the current fad for floating 12 or 15V supplies from a switching wall wart limits the output swing and current available. Barely enough for an op-amp (+/- 6 volts), plus losses from local sub-regulation.

It makes sense for the op-amp (or discrete circuit) to be fully isolated from the Class D switching module. The Class D module generates program-modulated switch noise ... it's effectively a low-frequency AM transmitter contained within the chassis. That's where the efficiency comes from, after all ... when there's no program material, switching is still going on at 200~500 kHz, but no power is going through the output devices, and very little is drawn from the support circuits. There's no residual Class A idling as there is with Class AB amplifiers. The output devices are either on or off, with only extremely brief switch transitions.

As program material level increases, more power and switch noise is created by the output switcher, and filtration demands on the speaker output and AC power supply increase. It is not trivial to silence a 200-watt AM transmitter in a can ... that energy is going to escape any way it can. Through the speaker wires (which make a great antenna), through the AC power cord, and even through the input jacks if it can find a way. Or leaks in the metal can itself. The adjacent linear audio equipment will have varying levels of tolerance for nearby RF emitters, which not usually tested in most test scenarios.

Oddly enough, this is an argument for input filtration using transformers to prevent RF emission on nearby equipment. I doubt many will do this, though, since designers that use Class D modules also like the very low distortion of those modules. In the Class D world, distortion specs (and the respect of the ASR crowd) make a difference,

So I doubt few, if any, designers of Class D amplifiers will use input transformers. The vast majority will use their favorite op-amp, or maybe try discrete op-amps designed for studio consoles. Boutique vendors that have a trademark "house sound" will design discrete transistor circuits that create the house sound.

Lynn,

Thanks for your attention to my issue.  What I can find on the quicksilver site is the input sensitivity is 1.5 volts, its  impedance is 100k ohms.  Power output is 170 watts into 4 or 8 ohms.  Peak power is 180 watts at 1 Khs.  

I hope that is what you are looking for.

Richard Vince

Hi Richard, that amplifier input load sounds very easy to drive. 1.5 volts to full clipping at 170 watts is very sensitive, and would require a quiet preamp. Any preamp, including ours, should drive it to ear-shattering levels.

As for Ralph’s point about wall-warts, lots of DACs are powered by wall-warts these days. And lots of people use the DAC as the system volume control, since that’s a common feature of many DAC chips, such as the ubiquitous ESS Sabre chipset found in entry-level and uber-expensive flagship DACs.

In an all-digital system, it’s up to the user if they want to use a separate preamp or not. In principle, a direct connection to the power amp from the DAC would have the cleanest sound. But in practice, it doesn’t always work out that way, and a dedicated linestage between the two sounds better. In that setup, the user disables the DACs volume control, so it runs at 100% output, and the preamp handles volume and signal selection.