What we’re listening to these days, and why.
TAI has added three pieces of the excellent K&K gear to our inventory, a triode-converted amplifier, a tube headphone amp / control front end, and a tube phono stage. K&K tube components use current sources (CS) and regulation, which flatten tube curves, slash distortion, and increase drive. Of all available single-stage amplification, triodes are already lowest in distortion but supporting them with the CS drops this already low level through the floor. The sound is powerful, dynamic, and decidedly un-tubelike. A nice addition for the work we’ve been doing lately on a half dozen new speaker models, and more than resolving enough to reveal what they’re doing during design. (The main The Audio Insider site is here, where we feature Swan, Dana, and other brands.)
The K&K ST70-based amplifier converted to ~15w triode output.
The K&K amplifier starts life as a common Dyna ST-70, the ubiquitous 35w/ch EL34 amp from the Sixties (be sure to read K&K’s take on this design). Ours kept only a pair of mint condition output transformers – designed by David Hafler, expertly wound on grain-oriented steel, and very good, as it turns out – which were rebuilt with Teflon-and-silver wire leads, new powder-coated end bells, and stainless hardware.
New chassis, rebuild output transformers, and new power transformer.
A new 300ma high-current PA060 power transformer from Ned at Triode Electronics supplies current. A new nickel-plated chassis houses the parts. The real star of the show is the K&K differential input stage and driver, a very modern circuit using special Lundahl amorphous input transformers (I opted to use K&K’s top Premium series for all three new components, and all use the upgraded amorphous Lundahl transformers.)
The K&K Premium input / driver board with tube CS and Lundahl amorphous transformers.
I also converted the K&K ST70 from the amp’s original EL34 pentodes to 6B4 triodes. Since the EL34 runs at over 400v in the original design, a few modifications were needed to adapt the circuit to the 6B4′s, which want a B+ rail of 250v. I retained the original tube rectification, plugged in a new Mullard GZ33 rectifier, and I also converted the power supply, using the voltage drop of individual inductors per channel (there are four altogether) to get the Sovtek output tubes down to 350v. While still 100v high, the Sovteks are actually closer to the WE 300b than the classic 6B4 and run fine at the higher B+. Quiescent current is set through the CS to 50ma per tube, which through a CLC filter makes even the big power transformer run warm.
New-old-stock Mullard GZ34 rectifier.
Heavy-duty Sovtek 6B4 output triodes.
Russian ex-military dual triode signal tubes, one per channel.
Input stage adjustable Current Sources (CS).
While the amp is customized, The K&K phono stage is closer to its original circuit, having been modified only in the power supply. There I installed two pieces of vintage NOS iron, the first being a potted ITC power transformer, and the second being a potted UTC choke to filter the B+ supply. The supply is thus L-C, which offers the best passive regulation. With such a large input L, the power supply’s turn-on time constant (K&K use a relay to energize the active circuit when the PS is up to speed) has moved up to about 30 seconds. A NOS Sylvania 6C4 does the rectification.
The K&K Premium Phono Preamplifier.
One phono channel showing Russian dual triode, CS, and on the PCB behind them, adjustable MC input loading and Lundahl amorphous step-up transformer location.
Phono preamp back panel. Adjustable MC loading and optional balanced outputs.
Phono preamp’s custom power supply, finished in a vintage-look green hammertone.
The final piece is the K&K Differential Line Stage board, left stock but populated with PIO output caps bypassed with Teflon-and-foil caps, and joining signal switching, attenuation, and system power from a modified new Quicksilver chassis (thanks to Mike Sanders.) I use this amplifier to drive dynamic headphones like the Sennheiser 600, which serve as one of my references. The power supply for this component uses a NOS Chicago Standard power transformer fed by a potted isolation transformer, and the back-panel switched outlet I plug the phono preamplifier into is fed by another isolation transformer.
K&K line preamp / headphone amp built on Quicksilver preamp chassis. Solid copper ground planes and subchassis, dual K&K shunt attenuators, and point-to-point OFHC copper and solid silver wiring throughout.
The use of a very highly modified ST70 as one of our test amps for speaker development was a calculated move – more on the just what can be done when both the electronics and speakers are bona-fide ultra-resolution when we reveal the new Dana models later in 2009. Watch this space for a series of three articles on the new Danas…and how they sound driven by the K&K superstars.
While typically a 15w amplifier would be doubted in such a setting, especially when development moves beyond 2-way monitors, the K&K strategy paid off. This amplifier can energize a large room through 88dB speakers, and in the nearfield we’ve even gotten near physical levels out of the ~85dB Swan D2.1se. The advantage such an amplifier has is near-instantaneous overload recovery, something big consumer amps lack.
One of the system’s eight Lundahl amorphous-core transformers.
The use of current sources and Lundahl amorphous transformers has revolutionized this very basic amplifier and turned it into a reference tool that sounds just as wide band and powerful as any good 50w transistor amp I’ve used over the past 25 years. This assortment of equipment has removed the veils of conventional transistor amps and yet has the sheer speed, extension, micro- and macro-dynamics that not only belie that power comes from a moderately-sized tube amplifier, but turns on its head any doubts about transformer-coupled tube amplifiers.
The Lundahl amorphous core transformers used throughout are a revelation. This is ultra-fi resolution and a deeply engaging, spacious, vivid sounding amplifier, as are all the K&K pieces. The sound is tremendously detailed and “wideband”, yet organic and faithful to the recordings. For a little less risk of transistor artificiality, I prefer tube rectification where possible, and while the push-pull circuit’s lack of 2nd order harmonic distortion removes the usual tube sound, the complete lack of grain says that this isn’t solid state we’re listening to. The system actually sounds “faster” than that, owing to that grain-free presentation and such rapid recovery from big passages. The system is challenging, to be sure, as it is quite ruthless to recordings, but it’s this quality that makes it such a valuable tool, at least until more power is called for.
Thanks are due to Kevin Carter at K&K. This year we’re turning out a number of what I think will be seen as affordable reference-class speaker designs and we couldn’t have done it without Kevin and K&K.