Spatial Audio Raven Preamp

True. I surmise leaving the input section of the Bruno Putzey modules as they are was a deliberate design decision on Bruno’s part. The modules are an almost-complete power amp, but are incompatible with existing RCA and XLR interfaces, due to the low input impedance and medium-level voltage drive requirements.

@lynn_olson If you simply design an instrumentation amplifier that is balanced and using good opamps, it will work just fine. It seems to be a bit of a testament that so many class D products using Bruno's modules fail at this task thru no fault of the modules!! You don't need much gain either (2 is fine) so you stay well within the requirements of modern opamps, allowing them to be completely neutral. Even then, despite the low distortion of the opamps, they will dominate the distortion character of the finished amp.

The power supply requires special attention as well. Class D amps can go from almost no load for a power supply to quite a heavy load, so the power supply has to be overbuilt if you want the design to be musical!

It is the variables of the input buffer and power supply as to why you read so many disparate experiences that audiophiles have with class D amps.

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