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Spread Spectrum Technologies Thoebe II preamplifier and Son Of Ampzilla II stereo power amplifier Review

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The push-pull Son Of Ampzilla II

It is worth noting the unique features of the push-pull topology; I hereby paste in some explanations from the Wikipedia page on “Push-Pull Output”:

“A push–pull output is a type of electronic circuit that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load…A push–pull amplifier is more efficient than a single-ended “class-A” amplifier. The output power that can be achieved is higher than the continuous dissipation rating of either transistor or tube used alone and increases the power available for a given supply voltage. Symmetrical construction of the two sides of the amplifier means that even-order harmonics are cancelled, which can reduce distortion.[1] DC current is cancelled in the output, allowing a smaller output transformer to be used than in a single-ended amplifier. However, the push–pull amplifier requires a phase-splitting component that adds complexity and cost to the system…A push–pull amplifier is more efficient than a class-A power amplifier because each output device amplifies only half the output waveform and is cut off during the opposite half. It can be shown that the theoretical full power efficiency (AC power in load compared to DC power consumed) of a push–pull stage is approximately 78.5%. This compares with a class-A amplifier that has efficiency of 25% if directly driving the load and no more than 50% for a transformer coupled output.”

These virtues of the push-pull design were very easily discerned when I began using the amplifier. Knowing that it is rated at 200 Wpc, I was surprised at that it played, within its power capacity that is, more like the big Pass Labs X600.5 monoblocks, or even the Red Dragon S500 Class D amps, which are rated at 500 Wpc. Certainly, I had not used an amp with such small proportions and physical weight among Class A/B designs capable of effortlessly driving the Kingsound King III electrostatic speakers. It made me recall the many gratifying listening sessions from years ago when I used a Threshold T-50 Amplifier to drive Magnepan 1.6 Speakers. The push-pull technology does wonderful things for dipole speakers! Chiefly, it makes them sound cleaner and more impactful, as well as unstrained.

The word that comes to mind is comfortable, the Son of Ampzilla II was silky and slightly warmer than the Pass Labs monoblocks. The first system I put up was with the Son of Ampzilla II only, as I would add the Thoebe II in time to note the changes:

Mac Mini with HQPlayer Software; Silnote Epirus USB Cable; Exogal Comet DAC with Verastarr Grand Illusion II power cable; Teo Cable Liquid Reference MkII IC; Son of Ampzilla II with Silnote Poseidon GS power cable; biwire Teo Standard Liquid Speaker Cables; Kingsound King III speaker with VAC Royal Power Supply fed by Clarity Cable Vortex power cords.

As I listened to tracks such as the Estrada Brothers’ “In Your Own Sweet Way,” and “Liquid Spirit” by Gregory Porter, I noted a rich and full bottom-end, but also a pleasant openness in the midrange that many solid state designs don’t quite capture. The vibes of the Estrada Brothers were plunked by the mallets and plumped fully, making enormous envelopes of resonant air around each note.

“Let Me Roll It” by Lake Street Dive rolled along jauntily, a most leisurely musical stroll. The band members could be heard lingering on their lines and the words of the vocals hung together as a loose, looped chain. The polite, but powerful characteristic of the Son of Ampzilla II emphasized these aspects of the songs.

This is a “brown sugar” amp, while the Red Dragon S500 is a “white sugar” amp. Both exude sweetness not common in their genre of amps, but which excite the ears particularly. It is possible that younger listeners, or ones who play harder music, especially electronic music, might find the Son of Ampzilla II too laid back, leaning too closely to a warm tube amp sound. The SST’s top-end is not overt, and if you crave upper frequency intensity this is not your amp. This is a midrange lover’s solid-state amp, as it does sultry vocals quite well.


Insertion of the Thoebe II

I did not spend much time with Rhapsody streaming audio as heard through Sonos music system via the Thoebe II, as there was much better sound to be had by playing back files up converted by HQ Player software. However, the quality of the streaming audio was as good as I have heard, and I would be pleased to use the Thoebe II for such purposes in casual listening.

Initially, I had some compatibility issues to work through in terms of the USB cable for file playback. Lately, I have been using a gorgeous Silnote Epirus USB Cable, which has lifted my file playback to its best sound. When I tried the Epirus with the Thoebe II there was a light electronic crackling or intermittent buzzing sound which I had never experienced using other DACs such as the iFi iDSD Micro, Eastern Electric Minimax DAC Supreme or Exogal Comet.

Initially, I thought it was due to the settings of the HQ Player software, but as I switched to Amarra and heard the same noise, I determined that the Thoebe II must be involved. Tony Holt at Wyred4Sound was as gentlemanly and helpful as I could expect. He recommended an unexpected solution – trying a toss away USB printer cable! I did just that; I sourced it from my upstairs printer, and it worked! I sat dumbfounded, as I heard no vestiges of imperfections on the same tracks that had noisy elements with the Epirus USB.

Tony admitted that he had encountered this issue once prior, and that replacement of an $800 USB cable with an inexpensive USB cable did the trick. Great! And, bummer! Great, because the Thoebe II would now operate flawlessly. Bummer, because easily heard was the gulf of sound quality difference (despite the noise) between the two USB cables. I tried several more USB cables, some of which worked and some which did not. A prototype of the Clarity Cable Organic USB was inoperative, but a $100 Furutech was functional. This is no condemnation of these upper end USB cables, as they all work fine and sound much better than fine with other DACs. I can only conclude there is a potential incompatibility with the Thoebe II (according to Tony, effecting very few units) which may require trying a couple different USB cables. Weirder things

happen in audio, so SST is not alone in the “inexplicable incompatibility” department.

One visitor to my room suggested that the Thoebe II is highly sensitive to the impedance of the USB cable, which I understand is to be at 70 Ohms. Too far off that mark and the cable is useless. However, I recall Mark Williams of Silnote making the point that his USB cables are dead on at 70 Ohms, so this is only speculation. The pertinent question is, does this niggling issue render the Thoebe II a lesser design? Not in the least! Once the USB cable issue was resolved to my satisfaction there was never an interruption in performance and I forgot completely about it. I suggest that there are many potential areas touching upon performance, the USB cable being one of them, but not wholly determinant of the sound quality. My results show that aftermarket USB cables will work with the Thoebe II, but you may need to put in a bit more effort than simply slapping one into the rig.

Unless, that is, one decides to mate a Wyred4Sound PCOCC Premium USB Cable to the SST set. Recommended by Tony, I found this to be an excellent solution, as the Wyred4Sound USB, while not the ultimate in performance, was certainly above average. Starting at $199 for 1M, it never made the Thoebe II hiccup, nor was it a boring sounding cable. It is well constructed, and has adequate sonic transparency for my taste. I did find that at times it was brighter than I wished, but this could be compensated by use of one or two alternative power cables in the system.

Where does this put me in terms of satisfaction with the Thoebe II? Considering the benefits conferred by the generous contouring controls my enthusiasm is not much dimmed.


Robust sound, but not “limitless” power

Some amps seem to have untapped potential no matter how hard you drive them. One such product was the Pass Labs X600.5 monoblocks. I put them on the Kingsound King III electrostatic speakers and they loafed, seemingly unfazed by the load the King III presented. Similarly, the VAC Phi200 tube amplifier in mono mode easily drove any speaker without limitations.

The SST Son of Ampzilla II cannot make the same claim, though it is a push-pull design. The amp is robust, seems authoritative right through the limit of its power, and never turned brittle or anemic, however it did hit a wall. That wall was introduced by the top output setting of the Thoebe II preamp. I took the King III to the “89” setting on the volume several times, and could have used more, but the pairing simply would not provide it. Was this due to the preamp? Not necessarily; I found that with the Exogal Comet the Son of Ampzilla II also reached its limit, bound by the upper end of the Comet’s volume control. Would the amp continue to deliver clean power at even higher levels? Perhaps, but panel speaker lovers should note that the Son of Ampzilla II when paired with more demanding, that is, less efficient speakers, may not provide for your definition of “live” sound if that means uber-decibel levels.

4 Responses to Spread Spectrum Technologies Thoebe II preamplifier and Son Of Ampzilla II stereo power amplifier Review

  1. Frank Wong says:

    Hi Doug,

    You have listened to many Pass amps. How close in sonic signature is the SOA II to the Pass XA30.8? Is it true that the SST amp sounded c warmer than the Pass X600 monos?

    • James Romeyn says:

      A late reply for reader’s potential edification: in his full review/rave of SST SOAII, Srajan Ebaen @ 6Moons directly compared it favorably vs. Pass Labs’ mono blocks with several times higher SRP. Interestingly, IIRC Srajan echoed Doug’s opinion Re. the II’s superb smoothness and musicality.

  2. Frank,
    God’s Joy to you,

    I have to date worked with three Pass products; two Pass Labs amps and the First Watt J2. Why do you not believe my statement in the article about the Son of Ampzilla II being warmer than the X600? I have told you the truth as I heard it in my system. Some people find it incredible that any amp might be warmer, richer, etc. than a Pass Labs product. Each amplifier manufacturer has his (or her) own sonic signature, and they can vary widely.

    Douglas Schroeder

  3. Frank Wong says:

    Thanks Doug! God bless you!

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