Radial Engineering Ltd., which is the parent company that owns both the Hafler and Dynaco nameplates, is no stranger to audio. Have been keeping an eye on them for quite some time within the professional audio mags i read. They have been producing pro audio products for quite some time and is now (finally) concentrating their efforts to such classics as the Dynaco Stereo 70 vacuum tube amplifier. Giving you a world premiere hands-on review of the Dynaco ST-1 and ST-70 prototype is no easy feat. While the ST-1 was on static display, the ST-70 in prototype form brought back memories of my love for the original many decades ago.
Dynaco ST-1 ($1500) stereo vacuum tube preamplifier includes a headphone amplifier has MM/MC phonostage plus four line inputs, tape loop and RCA outputs. Tube compliment are four 12AX7 triode. It has analog volume, bass, treble (defeatable) mute, low cut filter and two headphone jacks. This unit is their very first showing and in prototype form and looks quite well thought out. DIP witches on the rear of the unit provides MM/MC adjustments, with RCA jacks being of the high quality gold-plated variety. On the front are push buttons for tape loop, mute and a high-pass filter. A variable loudness control is an interesting addition. Production units are expected to be shipped during Q4 2016.
World Premiere! Dynaco
ST-70 MK3 Hands-On Review
Parent company Radial Engineering Ltd. new they needed to be excellent parents to the Dynaco brand and as such the ST-70 Series 3 draws upon the proven design philosophies of the original 70. The low open-loop gain is maintained, thereby minimizing transient intermodulation distortion (TIM). In fact the pentode-triode gain circuitry is virtually the same as the original ST-70(!) and they have kept the signal path as short as practical. Naturally the output tubes are the same type, being of the EL34 / 6CA7 variety.
Furthermore, David Hafler's original output transformer designs have been retained, yet have been modernized and updated with the latest knowledge. Transformer winding has come a long way over the decades, and these improvements include a faster transient response plus allows for both 4 Ohm and 8 Ohm operation using a simple selector switch. Other modern upgrades include the use of precision metal-film resistors, poly-composition capacitors and miniaturized high-capacity power supply electrolytics, which were completely unavailable at the time of the original design. By re-designing the original with conservatively rated parts, you can expect many years of trouble-free use.
Keeping things safe, dual primary power transformer with improved line regulation is achieved with added iron in its core. This also provides operation for both 120 and 220 volts. One bit i am sure will cause a bit of contention is the elimination of the original 5AR4 tube for rectification, which is now replaced with a solid-state circuit. Dynaco's new owners feel that by removing the 5AR4's inherent losses and reduced regulation, it benefits the new design by eliminating a "failure-prone out-of-production tube in the process" says the company. Personally, wish they kept the 5AR4 as it is widely available, plus we have NOS versions including the CV378 if you have deep pockets. Also changed out is the troublesome selenium rectifier used in the bias circuitry, which has also been replaced by a solid-state rectifier with additional supply capacitance. The filter inductor has been reduced in value to complement the increase in filter capacitance with a resulting reduction of DC resistance. This provides more voltage for use by the outputs and somewhat higher peak power output. The takeaway here is that they have paid quite a bit of attention to the power supply section, thus eliminating any long-known weak points.
Dynaco has installed an input frequency limiting filter network to limit bandwidth to 20 Hz to 20 kHz, which some feel is regarded as the human hearing range. This filter allows the removal of the tailoring components within the active circuitry and permits the active gain circuitry to be flatter across the frequency spectrum. The audible benefit, says Dynaco, "is a great improvement in distortion components at the frequency extremes while still maintaining adequate bandwidth. This network requires the drive impedance from the preamplifier to be less than 2kOhms for proper top end response. Even tube types frequently achieve less than 2K using cathode follower output drivers. This also minimizes the effect of cable capacitance between the preamplifier and the power amp". A high-pass, three-position filter switch is provided with bypass and two different frequencies of gentle 6dB per octave roll off.
Seen above is the bias adjustments, volume controls and take note by the Dynaco logo are the three settings for curve.
More design changes include that the original 7199 vacuum tube is a bit of a rare bird and so a pair of EF86 pentodes are used for the input and a 12AU7 dual triode to drive the output tubes. Keep in mind that this combo is tried and true within our audiophile community. Of course the internal circuit board has been updated, since the original ST70 was exposed economical phenolic, which all ST70 guys know deteriorate with age. The Series 3 unit uses military-grade, double-sided epoxy fiberglass PC material and shields the recessed PCB with a metal cover plate. Keep in mind that nearly two-thirds of the original's point-to-point wiring has been eliminated via circuit board. Bias adjustment is now done with indicator LEDs as seen within my photo. All that is required is to set a pair of LEDs to equal brightness. To accommodate unequal aging between initially matched pairs of output tubes, separate adjustments are provided for both tubes of each pair.
Upon seeing the new ST-70 i was in awe of the very solid build quality. Much care has been taken to provide a long and trouble-free life. Sure we could wish they kept the 5AR4 rectifier, yet only time will tell if someone designs a mod to reinstate this prized tube into the circuit. i predict that once the ST-70 makes it into the hands of modders, we may see many interesting developments. In stock form it could be a great sounding unit and excellent value for the money. There is no doubt that my brief hands-on time with the unit proves we could have another winner here to add to the legacy of this legendary stereo vacuum tube amplifier.