Spatial Audio Raven Preamp

@lynn_olson 

Or a well-known American manufacturer that omitted, or forgot, the required 10~100 uF decoupling/stabilization capacitors for the multiple 3-pin voltage regulators, so they sang happily in the 10 MHz range.

I've seen preamps and amps where the parts were held in place with hot glue. All that had to happen was have it dropped in shipment and you have a shock or fire hazard. I've encountered oscillating regulators in some American built stuff so I know what you're talking about. Tends to make the 'regulated' circuit a bit noisy...

Sheesh. If our stuff had been damaging speakers we'd not have been in business very long. Odd how a solid state manufacturer could get away with that!

That’s the real difference between pro gear and audiophile equipment. The pro stuff is reliable, because it has to be.

@lynn_olson If you're going to stay in business for the long haul producing high end audio products they have to be reliable too. I can't imagine how anyone thought that a solid state amp without the ability to protect itself from something like this was a good idea. At the very least if the amp smoked it should have blown a fuse...

The most dangerous of all are DC-coupled preamps and power amps. If the DC servo system in a preamp fails, the failure can propagate all the way to the loudspeaker. I saw this happen when I was visiting the home of a reviewer.

@lynn_olson The highlighted statement above is false.

I don’t contest what you saw.

We’ve been making preamps with a direct-coupled output (which we patented) and its been one of the most reliable design aspects of the preamps over the last 35 years. If it were to fail, our preamps wouldn’t make a high Voltage anyway, even as it warms up. As a result this is something that we’ve never seen.

If a tube amplifier was used, DC at the input would not have caused damage.

If a solid state amplifier was used, it should have gone into protect mode as any competent solid state amp will have a proper protection circuit!

So IMO if this anecdote is true, the designers of both the amp and preamp didn’t know what they were doing. Equipment should always be fail safe!!

Was the preamp of this story a tube preamp?

Do the tariffs affect the used markets as well? I'm thinking yes but not sure.

@dz13    Yes. Think of it in terms of shortages. This will push the used market up as new prices go up as well. Its a manufactured inflation. 

We make our boards here in the US, but I expect materials to make those boards, plus parts that go on the boards (for example, tube sockets, resistors, semiconductors, capacitors) will likely go up even if they are made in the US.

@donsachs +1

@lynn_olson +1 The tariffs will be doing no-one any good at all; just shortages, inflation and ill-will.

@yyzsantabarbara The Raven employs an output transformer so should support AES48. There is a correct loading for the output transformer that will prevent it ringing- you might inquire how that is handled.

Traditionally if the output transformer is designed to drive a low impedance balanced line, it will be designed to drive 600 Ohms. On older gear if the load wasn't present on the line there would often be a switch allowing a 600 Ohm resistor to be placed across the output of the transformer to prevent ringing. A that point a much higher impedance load (10K-100K) could be placed across the output with no worries about being able to drive it.

Perhaps @donsachs will respond with more information.

@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.

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.

The Bruno Putzey Class D modules are designed so they need they need about 10V drive and an input impedance of 6300 ohms

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.

Personally, I find the tonality and subjective realism of transformer-coupled vacuum tubes to be unmatched by other technologies, but that’s just me.

There are class D amps now that easily rival the best tube amps for every quality mentioned in the above post. Every bit as smooth in the mids and highs, depth properly portrayed, good low level detail and so on.

Instead of a 1/4" thick sculpted aluminum faceplate (marketing!), we put the money into custom parts that are in the audio path.

FWIW Dept.: A machined front panel can be used to reduce chassis resonance (by having a different resonant frequency as opposed to the chassis, the two rob energy from each other), making for a lower vibration environment for the tubes to do their work. If a circuit board is used, it can be isomerically isolated from the chassis, further reducing microphonic interaction. If no circuit board is used, the sockets themselves can be isomerically isolated from the chassis with similar results.  Especially if the preamp is used in the same room as the loudspeakers, this has both measurable and audible improvements- bass is better, the mids and highs smoother with greater resolution. Damping materials can be added to most chassis, further assisting to reduce microphonics (distortion).

The Raven has split windings on the output transformer so both RCA and XLR’s can be used at the same time (the RCA output uses one-half of the secondary winding). So feel free to use to use the RCA output to power the active subs.

@lynn_olsen This suggests that there is a grounded center tap of the output transformer. Normally a balanced line output (if it supports AES48) is floating (this practice is to prevent ground loops). Did you provide a switch to break the center tap's ground connection if the RCA is not used? I don't see it in rear panel photos.

was just thinking sub(s) and your post said it better. I'm not sure how to attach a sub(s) to the Raven Pre? Must be an easy way? 

@wsrrsw It appears there is a single-ended output in parallel with the balanced output. You'll have to check to see how this works but it might be quite simple- the main amps on the balanced outputs and the subs connected to the RCA outputs. I would expect to see a switch that grounds pin 3 to pin 1 so the RCA connections will work properly but they may not be connected to the output transformer driving the XLRs, negating the need for a switch.

@wsrrsw You have an ideal situation for using subwoofers. You might look into something called a Distributed Bass Array which is the use of 4 subs placed asymmetrically in the room so as to eliminate bass nulls and peaks in the room. This is often particularly noticeable at the listening position if the room has regular dimensions.

My main speakers are flat to 20Hz yet I have no deep bass at the listening position in my room. So I use a pair of subs to break up the standing waves that cause this problem. One is to my left and the other slightly to the right and behind me. Both are as close to the wall as I can get them.

In your case since your speakers don't go much below about 45-50Hz, you are an ideal candidate for all 4 subs. The Swarm from Audiokinesis is an ideal candidate for this since they are meant to be placed directly against a wall so as to be out of the way as possible. The subs are designed to roll of at 3dB per octave starting at 100Hz going down. The room boundary effect provides a +3dB/octave boost, so they are flat to 20Hz. They are 1 foot square and 2 feet high. I have the 10" drivers actually facing the wall so as to maximize the room boundary effect.

The idea here is to run them no higher than about 45-50 Hz, at which point its very easy to get them to blend seamlessly with your Klipsch speakers since below 80Hz in most rooms the bass is already entirely reverberant before your ears can sort out the bass notes being played. As long as the are not allowed to go too high they won't attract attention to themselves- the main speakers will convince you the bass is coming from them via the harmonics of the bass instruments.

It will be easy to get this setup to integrate with the Spatial amps and preamp. Then you will hear how much bass is really there and not only that, you may find that the mids and highs get more relaxed and detailed due to how our ears alter the tonality we perceive when part of the spectrum is missing. So getting the bass right calms down the mids and highs.

Good cores are essential for low distortion.

Capacitive coupling exists between windings regardless of the quality of the transformer. For this reason alone its important for the transformer to be loaded correctly so it expresses its winding ratio without inter-winding capacitance as a parallel element. Otherwise the frequency response won't be flat.

@donsachs Is this meant to answer my question?

Unfortunately, opamps are limited in not being able to dissipate much heat due to the small package size. Very few opamps are designed to be used with heat sinks. So the only way to keep heat emission low is efficient Class AB output stages, relying on feedback to linearize the crossover region (opamps typically have very high feedback). Higher powered transistor and tube amps also use Class AB to keep heat emission to acceptable levels, at the expense of higher distortion in the Class AB transition region.

AB amplifiers have no problem with the crossover region and do not rely on feedback to sort this problem out. Its a matter of the output devices being properly biased rather than anything to do with feedback. Of course, feedback is helpful (if applied properly) to improve the overall linearity of an A or AB amplifier. As Norman Crowhurst indirectly pointed out, most of the time feedback is incorrectly applied.

@lynn_olson Since you are using an output transformer, there is an ideal load for which the transformer is optimized. As you know, transformers transform impedance and  this goes both ways, so if the load at the output is higher, the load on the output tubes is higher too. But transformers can ring if the load is too high and conversely, roll off if the load is too low. Traditionally, balanced line transformers are designed for low impedance operation (in the old days 600 Ohms, hence the dBm rating). Ampex provided a switch with a 600 Ohm resistor on the back of their tube tape electronics so if the electronics driving something with a high impedance input, the load could be provided to prevent the transformer ringing.

So I'm very curious how you handled this issue- did you design the transformer for a low impedance and simply installed a loading resistor?

I saw a comment from you about no servos in the context of noise which I assume was a typo since as you know, servos do not introduce noise. You might consider one, since your circuit is direct coupled from input to output before the transformer. Offsets (possibly at the input) can result in an sizable imbalance which introduces distortion, even if you run matched tubes. A servo can be easily used to correct this issue, allowing the output transformer do have its lowest distortion and they are inexpensive to implement.