Don,
It is loud but not screamingly loud. I just wanted too make sure I did not have a problem. My speakers have a measured sensitivity of 91.5. It is more than loud enough for me. Thanks for the response.
Richard
Spatial Audio Raven Preamp
Spatial is supposed to be shipping the first "wave" from pre orders of this preamplifier in May, does anyone have one on order? Was hoping to hear about it from AXPONA but I guess they were not there. It's on my list for future possibilities. It seems to check all my boxes if I need a preamp.
209 responses Add your response
Any info on your power amps? Input sensitivity (volts RMS to clip the amp) and input impedance should tell the tale. Transistor amps (with feedback) are typically 1~2 volts to clip amp, and input impedance is typically 10K to 22K. The Bruno Putzey Class D modules are designed so they need they need about 10V drive and an input impedance of 6300 ohms. This non-standard input is intentional; it gives the third-party amp designer the freedom to design an input stage that has the tone color they want: op-amp, solid-state discrete, or vacuum tube. |
Actually if you want to be safe you should expect to drive 2000 Ohms rather than 6K or higher. Typically an input buffer will provide roughly 12dB of gain and allow an input impedance of more like 47KOhms. That would put the total gain in the region of 22-25dB which is plenty for most speakers. |
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. The OEM is then free to add as much or as little sonic flavor as they like. If they are catering to the ASR crowd, there are superb op-amps these days with truly exceptional measurements (they also sound good). If the OEM is up to a challenge, they can design a discrete transistor circuit, but it is very unlikely it will match the specs of the best modern op-amps. The days of the evil 741 and the mediocre 301 are long gone. The 5532/5532 is very old, dating back to 1979, but is still seen in pro gear. And if the OEM wants to earn the contempt of the ASR folks, they can use one or two vacuum tube stages, which will increase the distortion of the Putzey module a hundred or a thousand-fold. |
@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.
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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_olson Just to set the record straight, I don't think any class D designer or those planning to use and existing class D module would ever consider using a wall wart as a power supply for the input buffer! The switching noise is far lower than you suggest! In fact so low that many tube amps radiate more noise (due to their rectifiers). As a result, its quite practical to put the input buffer opamps on the same board with the class D section as a complete module and still have it so quiet that you'd struggle to hear its noise floor on a horn system. You really do want the switching noise quite low because if it radiates it can mess with digital devices. Noise is really a matter of good layout. Typically its nice to have the radiated noise about 60dB below the level required to meet EU standards to obtain the CE mark. We used an input transformer on our prototypes. It worked quite well. But they are impractical due to reflected impedances- how well they drive the load has a lot to do with the output impedance of the source driving the transformer. Since the preamp is what we're talking about, the result would be variable; in some cases the transformer would drive the input of the comparator circuit quite well and in other cases, not so much, just because of the preamp driving the transformer.
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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. |
@lynn_olson DACs are quite different from actual class D amps- the latter draw considerably more power. DACs OTOH do not- and have internal regulation. onboard with the DAC. So different kettle of fish. There are inexpensive (less than $200 or even less than $75.00 using Texas Instruments chips) class D amps that do use wall warts but I've yet to hear one that you can take seriously. |
I was thinking of the oddball use case of a wall-wart powered DAC driving a Purifi Class D amp module directly. Stupid, I know, but I bet some folks have tried it. I imagine it would (just barely) work. It certainly wouldn’t sound good, though. Measurements would probably be fine, though, if the DAC module had a decent op-amp output buffer. |