jas@proteon.com (John A. Shriver) (02/07/91)
I finished building the Curcio upgrade into my Dynaco Stereo 70 last weekend, and I thought I would offer some preliminary reactions to the sound of it. First, a few technical notes. I bought bare boards from OCSL, and most of the parts from Mouser and MCM. The 0.47uF coupling caps are REL-CAP, the 47K 3W resistors Matsushita, the wire Cardas, the solder Wonder. Driver tubes N.O.S Siemens E88CC/6922. One pair of output tubes Chinese EL34's, the other pair used old stock (>10 years old) Mullards with a little life left in them. Otheriwse, the parts are as listed by Curcio in GA. (If anyone has trouble decoding some of Curcio's now-outdated Mouser part numbers, I can help.) The system is Grado MCX/MCZ (sytlus 2 years old & sounds it, replacement on order), Sumiko Premier MMT with VTA base, Walker CJ55 turntable, Curcio Daniel preamp, Monster Cable Interlink Reference A, Theil CS2 speakers, 11 x 15 room with carpeted wood floor and acoustic tile ceiling. I've only had about 4 hours of listening so far. I've listend to audiophile vinyl warhorses so far. Simply, it does not sound like the stock Stereo 70. This is not a "euphonic" or "liquid" amplifier anymore. The first thing that impressed me was the low end. Curcio has squeezed really excellent, clean, taut, controlled base through those little Dynaco A-470 output transformers. Technically, I presume this is a consequence of the regulated power supplies, the mondo 0.47uF coupling capacitors, and the direct coupling of the cascode stage that eliminates one stage of coupling capacitors. The lows were shown to excellent effect with the Widor Allegro (Dupre, Mercury SR90169), now I can hear what's going on in the lower registers. It makes this record much more interesting than the stock Stereo 70. The lows also sounded good in the Prokofiev Piano Concerto #3 (Frager/Liebowitz, RCA LSC-2465). The drums are clear, even very low-level tympani rolls are clear, the cellos and basses take up palpable sapce. The heartbeats on Dark Side of the Moon (English Harvest SHVL 804) can shake the floor, the stock unit could not do that. On the other hand, the highs are a little "exposed", as noted by the two reviews in Glass Audio. The piano wasn't too harsh on the Prokofiev. However, things got a little touchy in massed strings on the Brahms Violin Concerto (Hefietz/Reiner, RCA LSC-1903). Now, I will admit that my system is merciless in the highs, since the Theil CS2's have flat power response in the highs. This exposes any high-frequency anomalies quickly. The highs did not get harsh with the Arnold English and Scottish Dances (Arnold, Lyrita SRCS-109). Some of the raw highs may well be my ossified stylus suspension. (Joe Grado, where is my new stylus?) One correspondent notes that there is a small rise in the low-power frequency response between 20 HKz and 50 KHz of his Curcio Stereo 70. The suspicion of ringing offered by the listeners in the second Glass Audio review may be right. There is no compensation of any sort in the feedback or inside the loop, this response rise would be a probable sign of ringing. The record I found (so far) whose sound is most "synergistic" with the sound of this system is the Rachmaninov Symphonic Dances (Johannos, Athena ALSW-10001). They are quite a match, cut of the same sonic cloth. The image depth is good. I'm not sure it's quite as deep as the stock unit. The image width is clearly wider than stock, covering the full three feet beyond the speakers to the side walls. [Footnote: If you want to keep the Stereo 70 circuit and sound, Triode Electronics of Chicago seems to have a much better idea than Sound Values. Sound Values offers a new board that uses a 6GN8A, but they have to select the tube for low noise. Triode goes back towards the tube complement of the original Mullard designs for the EL34 tube, and uses a EF86 pentode per channel, and splits one 12AU7 between the channels. These are much more predictable quality low noise tubes, and this seems the best idea. Triode also plans to offer a card for the original Mullard circuit, among others.]