VFET Redux, phono musings

[cjfrbw]

Recently acquired a Sony TA 4650 VFET integrated to use as a phono stage in my vintage Santa Cruz system. Also acquired a Yamaaha C70 preamp to try, both cost a total with shipping about 400 for both units.

I used a Sony TA 5650 VFET in my main system for about ten years from about 1978 to 1989, until it got scratchy and Sony told me it couldn't be fixed. No internet in those days for info. The amp section blew, then the preamp, so without any repair means I chucked it after hauling the carcass around for a few years.

VFET is the only transistor ever created that a died-in-the-wool toob-head could ever abide.

Some Japanese high end designers are re-inventing VFET in new build SIT amps (static induction transistor=VFET) and Nelson Pass has a couple of stereo 10 watt single ended SIT amps for 5k/10k(monos) respectively.

I looked at the circuit diagram of the TA 4650, and it uses a small signal VFET in the phono stage, connected to a single FET flat amp then to the VFET push pull output amplifier stage. The push pull VFET amp stage is connected directly to the headphone jack by an impedance matching array of resistors.

Using a headphone adaptor to RCA jack, this turns the TA 4650 into a volume controlled VFET phono amp. This would make it the only VFET phono amp I am aware of, with the exception of its big brother the Sony TA 5650 with the same circuits, and possibly the Yamaha C1 vintage monster.

Using the integrated in this way from phono input to headphone output gives about a 65 db gain, no need for a head amp or input coils. Resistor loads could be added at the input if desired,or the tone controls could be engaged to add another single stage FET amp for tone contouring. The TA 4650 is extremely quiet as well, no hiss.

It sounds a lot better than the Yamaha C70 phono head amp, with the Yamaha C70 itself is one of the best vintage solid state phono stages ever produced.

Sony claimed that the VFET had the output characteristics of the 2A3 vacuum tube with a triode like response. I have to admit, they did not exaggerate much.

The VFET unit as a phono and headphone amp has uncanny transient response, vast 3-D sound staging both laterally and depth, and a vivid, natural sounding tonal character. It is non-fatiguing to listen to.

Bettering the outstanding Yamaha C70 phono preamp is no mean feat.

My unit was mildly refurbished already, but I replaced the notorious VFET "death diodes" myself and used the amp bias setting at 60mV rather than the recommended 75 mV, which allows for warm but not hot performance. I can't tell any difference in sound quality.

Best phono amp I have heard short of the uber expensive LCR tubed types, and as stated, when used this way, one of the few VFET phono amplifier in existence that I know of.

Using it with Sony PS X70 vintage DD turntable/arm and Allnic Verito cartridge.

Cool.

 

[cjfrbw]

The Sony TA 4650 uses two complementary pairs of VFET in class AB (biased) push pull output, not single ended. The Nelson Pass SIT designs are 10 watt single ended.

FET's of various stripes tend to have the pentode characteristic. VFETS are voltage controlled, not current controlled devices. They have construction and pinch off curves that are very triode like. Nelson Pass showed some of the curves in his literature with the SIT single ended amps he has constructed.

VFETs tend to produce mostly even order harmonics. I can only state that the old sony integrated TA 4650 sounds unlike any other transistor type. It does have an open dynamic character similar to big bottle triodes, with incredible imaging prowess and natural tone. As phono stage as described, it sounds pretty special.

That being said, with headphones, I still tend to prefer my 26/300b based amp, but the VFET's are more direct sounding.

 

[cjfrbw]

Dynamic headphones just hook up to the headphone jack, of course. The Stax headphones are mediated through he Stax transformer adapters and require a standard amplifier output. The transformer adapters are hooked up to any kind of amp speaker output, whether tubed or solid state, and act as voltage multipliers by a factor or about 25. Stax are electrostats that like high voltage, but don't need a lot of current, and need to be balanced, which is also done by the transformer.

At my main home for the Stax I have a headphone adapter hooked up to a Manley 300b neo preamp, which feeds the voltage multiplier transformer, and it works great that way because the headphone outputs of the Manley are already transformer coupled and have relatively high voltage and power output. With dynamic headphone, of course, the headphone just plugs directly into the headphone jack.

In Santa Cruz currently, I have the voltage multiplier transformer hooked up to the Sony VFET and/or a hogan 6sn7/26/300b tube amplifier. Dynamic headphones I can hook up directly to the headphone jack of the VFET integrated, or to a headphone adapter connected to a tube rectified Euridice preamp.

Stax electrostats also have many types of dedicated tubed and solid state amplifiers that do not require the transformer adapter, but are used for Stax alone, not any other kind of dynamic headphone.