Spatial Audio Raven Preamp

@jc4659  Glad it arrived safely and fired right up.  Thanks for your initial impressions!  It will become more 3D, with more sound stage depth and pinpoint imaging as it runs in for another 50-100 hours.  Tone colours will become more vivid as well.  I am sure the Ayre is a very nice preamp, but it should sound a little flat and two dimensional compared to the Raven, once it has run in.  That eery sense of space will develop on the Raven over the next week or two.  We call it the "trippy" sound:)

@donsachs Soundstage depth, image depth and pinpoint imaging is already better than the Ayre which itself was an improvement over the K-5xeMP in these qualities. 

@jc4659  I know Spatial runs the tubes at least 20 hours before they send things out, just to weed out any failures.   A 6SN7 is pretty well run in by 20 hours.  Yes, there are subtle changes in them out to 50-100 hours, but most of the sound is there in 20 or so.  What you will hear is the cathode bypass caps running in.   They are very large film caps and they are idling along at literally about 2% of their voltage rating.  So they take a while to settle down.  When they do you get that last 10-20%.   Where the real magic lives if the rest of your system can present it.

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.

lynn_olson

Unfortunately, opamps are limited in not being able to dissipate much heat due to the small package size.

That's why so many good balanced components use discrete circuits for the operational amplifiers.