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: As the affordable stereo power amplifier modeling the flagship VM60 monos at half their price, the VS70 utilizes the same patented Dual Balanced Feedback Topology (DBFT) which claims to deliver improved linearity for high- current amplification. The proprietary output transformer's primary caters to ultra-linear mode, the secondary winding to fully balanced configuration. Two KT77 per channel yield no less than 35 watts into 4 or 8 ohms. Equipped with the same balanced shunt-type volume control as the VL10.1 and VL20, the VS70 can be operated as an integrated amplifier. Although designed to accept balanced or unbalanced inputs, only one type should be connected with the corresponding setting of the BAL/UNBAL toggle. (Likewise the VL20. Each input accepts either BAL or UNBAL and under no circumstances are both to be connected to the same line input.) A wrong setting of the toggle will still produce sound but may harm the source component. More on that in a later SideBar.
Dual Balanced Feedback Topology according to JE Audio: "Fully balanced operation is achieved by employing the DBFT as shown below. The +/- signals are first fed to the input stage formed by 2 x 12AX7/ECC83 double triodes. The first stage thus consists of four small signal triodes in two bottles. They are configured such that the first stage amplifies the input signals as well as two pairs of feedback loops.

"The second stage is formed by 1 x 6DJ8/ECC88 to amplify the first-stage signals and provide low output impedance to the 2xKT77 output stage and balanced output transformer. The feedback signals are taken in pairs to maintain accurate balance.
The first feedback loop is taken from the outputs of the second stage. A small amount of feedback is applied to improve the bandwidth, distortion and output impedance of the combined first two stages and set gain. A second feedback loop with low applied feedback is taken from the transformer outputs to again improve bandwidth, distortion and output impedance of the overall amplifier.”

The power supply and filtering circuits are loaded with audiophile-grade components including a low-noise/high-efficiency Japanese R-core power transformer, a C-core filter choke, premium filterr and copper-foil/oil signal coupling caps. The proprietary output transformer is made with US-imported M6 grain-oriented steel.

Specs include THD of below 0.2% at 10 watts, less than 1% at full output. Input sensitivity is 1.1V, input impedance 47/94K on RCA/XLR respectively. S/N ration is better than 85dB on the balanced input, power consumption 250 watts.

JE Audio does not encourage swapping their small signal tubes. Those were factory tested and matched to offer best performance in the positions delivered. "For the 12AX7, we recommend only our own replacement because we direct-couple them to minimize phase shift and enhance LF performance. The first 12AX7 is critical to setting up the operating voltages for the second 12AX7 and 6DJ8. We select the best tubes after at least 50 hours of burn-in and extensive testing." There should be no issues with NOS 12AX7 and 6DJ8/ECC88 tubes. (Late-breaking update: JE Audio has a newly patented circuit to render most tubes compatible in their balanced circuit.) As for the VL20, "it is not recommended to swap any 6H30. The two 6H30 in each channel have been tested such that they work best in their current positions in terms of noise level and operating voltages."

The only really swappable tubes on the VS70 are the power bottles. In addition to KT77, you may use EL34 or 6CA7. Since they operate in fixed bias, adjustments are recommended every 3 to 4 months. The necessary pots and indicator LED are located at the bottom. Stand the amp on its side (level with a wedge or paper stack), turn down the volume all the way, connect the speakers but no source, warm up for 20 minutes. Switch to select the appropriate tubes and use a screw driver to adjust the bias current until the red LED lights up.

The stock power tubes are Gold Lion Genalex KT77. This Russian reissue has also gained the trust of other tube amp manufacturers. When Jadis and Rogue Audio fit Genalex in their current line-ups—the Jadis JA80 with KT88, Rogue Audio's Stereo 90 Super Magnum with KT88 and Atlas Magnum with KT7—you know the Russians did something right. My personal experiences were jaw-dropping good when I fitted their KT77 and KT88 to the JohnBlue LT66 monos. Just to confirm our mutual taste in power tubes, I asked John what had him opt for Genalex. "I have tried EL34 from SED, Electro Harmonic, reissued Mullard, reissued Tung Sol and reissued Gold Lion KT77. Generally speaking, the GL KT77 sounds a little warmer and has better bass than the EL34. We like the overall performance of our VS70 with the GL KT77. All JE stock tubes are burned in for at least 50 hours before being tested for electrical characteristics and good matching."

Run in and dig in:
It seemed to take the JE Audio duo quite some time to get on song. At first I had them operating side by side with my Symphonic Line system. Whether I had them on the Apogee Centaur Minor or Mark & Daniel Diamond, the duo sounded comparatively thin and bright. Worse, the Symphonic Line delivered more tube warmth and euphonic charisma which is exactly what made the Germans famous. When my friend brought over his fresh out-of-the-box Opera Callas SP floorstanders, we were very disappointed by the lack of synergy or more precisely the lack of bass and musical substance. Apparently both speakers and electronics needed more time. As usual, I had the amps in the basement to mature, playing a few CDs every day while driving the not so amp-friendly Dynaudio Facette. Meanwhile I took the opportunity to dig deeper into the JE design philosophy.

The more I scrolled through the site, the more I was impressed by John’s rational disciplined mind. I particularly enjoyed his White Paper on Vacuum Tube Balanced Power Amplifiers. The well-articulated document enriched with simple block diagrams takes us through the theories behind single-ended i/o power amplifiers, single-ended input/push-pull output power amps and eventually JE Audio’s patented fully balanced circuit. It's a most succinctly written paper that explains the pros and cons of various circuits plus the merits of receiving versus transmitting type power triodes, beam power tetrodes and pentodes. The critical argument is the part covering push-pull amplifiers with phase inverters and why and how JE Audio's own differential amplifier with dual balanced feedback is believed to be the best solution.

Once a tube power amp sacrifices the simplicity of a single-ended circuit and goes after the higher output of push-pull, the designer must implement a perfect phase splitter. To summarize John’s argument, it is much easier to build an accurate differential amplifier than accurate phase splitter. Feedback improves the linearity, noise and distortion performance while bandwidth is extended. The patented DBFT arrangements runs two pairs of feedback loops. Two are better than one because the first pair connects between the input and second stage, the second pair to the output transformer for a small amount of global feedback. Each loop only applies a small amount of feedback to easily control the amp's overall stability.

It’s no secret that die-hard single-ended triode fans do not subscribe to the idea of a balanced amplifier, never mind twin NFB loops. Many friends shrug off true XLR connections by saying they are for studio applications with long wiring. For home audio they believe them to be mere gimmicks. Being a scientific man with a passion for audio, John never hid his appreciation for SETs. "Since the triodes in the input and output stages are operated in class A, they amplify the complete positive and negative signal cycles and are characterized by the dominant second order harmonic. Balanced operation effectively cancels electromagnetic noise picked up by cables. With the additional enhanced linearity of our wideband balanced topology, the sonic result is improved sound quality with three-dimensional images in a natural acoustic landscape. Single-ended input signal also benefits from balanced operation since all incoming signal is automatically converted and processed dual differential."

As I planned to audition the VL20 and VS70 both balanced and unbalanced—mix and match—I requested a private tutorial from John Lam. To avoid digressing, the summary of my questions and John’s answers has been moved into a separate SideBar.

The Silver Dream comes true
: John stressed more than once how important good XLR interconnects are to the ultimate performance of his amps. I simply don’t have many in my stable. Apart from the low-priced Audioquest Ruby x3 HyperLitz and Digiflex Tourflex N3-XX, the best of the lot is the Goertz Triode Quartz TQ2 Silver of which I only have one 0.5m pair. I noticed our NewsRoom Artisan Silver Cables post and asked for a loan of two 1-meter pairs of XLR cables to facilitate my review. Alister Staniland generously accommodated me. He could have declined on the basis of Chris Redmond’s then pending report. While a second opinion isn't intended, I’m in total agreement with Chris that "...from top to bottom, everything improves compared to copper conductors without drawbacks… moving from copper to silver is like upgrading a camera lens to one with a finer-grade polished glass. You don't simply see more detail, you see more light, more contrast and shades between light and dark and you get more sense of depth."

The Goertz TQ2 Silver gave me that feeling when I upgraded from Audioquest Ruby connecting the Restek Sector preamp and Sim Audio Celeste W-4070 power amp, a transistor system. I had better silver-cable results matching tube amps, never with the balanced variety however. For many years the reference interconnect for my tube gear has been the Canadian Luscombe LBR35 RCA handcrafted by violin maker Lief Luscombe. They are unshielded unbraided solid-core silver with three strands for +, five for –, with each individually insulated by a Teflon/air dielectric. These are very thin cables whose three + strands add up to merely 20 gauge. Silver content is 99.9%, termination by Eichmann Silver Bullet. Compared to my more expensive silver-plated copper leads like Clearaudio Silverline, Symphonic Line Reference NF and Deltec Black Slink—all shielded by the way—the Luscombe LBR-35 exhibits the same if not better holographic imaging and airy spaciousness. EMI/RFI noise is non-existent. Tonal balance is as good as with the other cables. I get natural treble and midrange without sacrificing the bottom octave. So much for thin silver cables not delivering bass. I'd been using them since 2005 but Mr. Luscombe is now busy making violins and has no more time for audio cables.