Preamp suggestions for Atma-Sphere Class D Monoblocks

I own a CarySLP 05 with ultimate upgrade. Bought it used a few years ago in pristine condition for $4300. It has a decent headphone amp. I’ve had great fun tube rolling and my current setup uses Raytheon VT231’s in the gain stages and CBS Hytron 5692’s in the buffer positions. I have a variety of VT231 and 5692 tubes to play with and it is fun to optimize certain characteristics (for example, Ken-Rad VT-231’s seem to have the most solid bass while still being creamy in the highs, while Sylvania seem to have a bit of sparkle in the highs, with this being heard from VT-231’s to NOS 6SN7 GTB’s from the 1960’s. 
 

Tube rolling doesn’t have to be expensive- I really like the NOS Sylvania GTB’s from the 1960’s that I bought at $35 matched each from Brent Jessee, enjoying them far more New Sensor Electroharmonix tubes.

 I have really enjoyed this preamp. Tremendous imaging with well-resolved holographic instrument placement, very quiet with black backgrounds, tremendous sense of dynamics and inner detail. Make sure to use a high quality rectifier tube!

It is definitely on my short list, as long as remote capability is available.

@srama The remote is an option (no charge); scroll down and you'll see it.

http://www.atma-sphere.com/en/mp-3.html

Decades ago when we developed the MP-1, part of that was figuring out how to do a balanced direct-coupled output which had to support the balanced line standard. We did that and have obtained two patents on the process. Before that, there were only two ways to support the standard; one is to use and output transformer and the other is a solid state circuit that can float and sink current at the same time (today there are semiconductor companies that make chips for this purpose). We patented a 3rd method.

In a traditional tube preamp, the biggest impediment to transparency and bandwidth (which includes the ability to cast a proper soundstage) is often the output coupling capacitor, which has to be large enough in value to allow the preamp to play bass when driving the lower input impedances of a solid state amp. That all goes away when you direct-couple and its the sort of thing that's immediately apparent.

@atmasphere, thank you for clarifying regarding the remote as well as the additional insight into the design.

I understand if you would rather not do so in a public forum, but it would be great to hear your engineer’s perspective on the Spatial Raven and the Backert Labs Rhumba, and how the MP3 differs from these designs (or provide some links if you have already discussed this).

Srama,

Between the LS-100 and the MP-3 I thought soundstage and realism were roughly equal, but I definitely preferred the tonality of the MP-3, which is likely due to the reasons Ralph outlines above. One other variable is that the LS-100 is tube rectified and the rectifier has a definite effect on the sonics. I ended up liking the 5U4GB much more than the stock 5AR4. The MP-3 is solid state rectified and eliminates that variable.

I failed to mention before that I also owned a Don Sachs DS2, sort of the forerunner to the Raven. It was terrific in its own right but more romantic than the LS100. I owned them at the same time and slightly preferred the LS-100, but if the Raven is as much better than the DS2 as most (including Don Sachs himself, who should know) are saying, that probably changes the equation.

I understand if you would rather not do so in a public forum, but it would be great to hear your engineer’s perspective on the Spatial Raven and the Backert Labs Rhumba, and how the MP3 differs from these designs (or provide some links if you have already discussed this).

@srama There are two main differences. The first you already know about- the MP-1 and MP-3 employ a balanced direct-coupled tube output, which allows it to be very transparent. Usually in most preamps the output coupling cap defines the low frequency cutoff and also causes the output impedance to increase as the frequency goes down. When you direct-couple, the low frequency cutoff is defined by coupling caps prior to the output and so are much smaller in value (typically by 2 orders of magnitude) which allows them to be more transparent and the output impedance with respect to frequency is a ruler flat line across the audio band, something you can't do with coupling caps and very hard to do with output transformers.

To my understanding the Spatial uses an output transformer. Personally I think if you can eliminate the transformer the circuit will be lower distortion and wider bandwidth. Transformers unavoidably introduce distortion due to the hysteresis loop that is always present (although minimized in good designs).

As best I can make out, the other difference is that in addition to being fully balanced, the MP-1 and MP-3 are also fully differential. This allows a lower parts count, lower noise (in theory up to 6dB per stage of gain) and lower distortion although the distortion difference as opposed to a non-differential balanced circuit is fairly small.

Not having heard the Spatial or Rhumba I can’t comment on the sound (which by all accounts is excellent), but what I would expect (as this has played out in other comparisons over the last 35 years) is the use of the direct coupled output is immediately obvious in terms of transparency and bandwidth (the latter best heard in the bass region- the deeper your speakers go, the more obvious this becomes).

Not having an output transformer also means the preamp is less critical of the interconnect cable used between it and the amplifier. Transformers are inductors and cables have capacitance, put the two together and there will be a resonance at some (usually ultrasonic) frequency. Not having the numbers available (such as the inductance of the transformer presented to the cable) its hard to know what frequency, but I noticed that the designer recommends avoiding higher capacitance interconnect, so I’m sure he’s aware of this issue.