Sony TA-4650 VFET Amplifier Service & Reliability Enhancement

I’m reluctant to use the word ‘sexy’ when it comes to hi-fi gear because, well – it just feels weird. If I were to use it though, it would be about this baby Sony TA-4650 VFET amplifier.

What can one say after working one of these stunning Sony TA-4650 VFET integrated amplifiers? I really fell for this little beauty and came to understand why so many people treasure these amps.

Having said that, there are a few critically important things to be aware of, if you own one of these or are planning on buying one. You definitely don’t want to take one of these to just anyone for service. I’ll explain why as we go.

Low power output, two output devices per channel, airy sound and great switches and controls, plus heavy build for a 30 watt per channel amp. Modern 100 watt amps don’t weight 12+ kg like this does. This is called build quality, something you no longer get, unless you have thousands to spend.

Before we get into the details, the HiFi Engine has a great page on the TA-4650, with manuals and specs. You should also check out my video covering the Sony TA-4650, and maybe a few others on my YouTube channel while you’re there.


Power output: 30 watts per channel into 8Ω (stereo)
Frequency response: 10Hz to 100kHz
Total harmonic distortion: 0.05%
Damping factor: 45
Input sensitivity: 2.5mV (MM), 150mV (line)
Signal to noise ratio: 70dB (MM), 90dB (line)
Output: 150mV (line), 17mV (DIN), 1V (Pre out)
Speaker load impedance: 4Ω (minimum)
Semiconductors: 6 x VFET, 4 x FET, 36 x transistors, 26 x diodes
Dimensions: 460 x 168 x 323mm
Weight: 12.4kg


So, VFETs, or vertical field effect transistors are a long since obsolete output device that promised and delivered great sound. This is great of course if you own one of these little 30 watt per channel beauties, with original output devices. If you blow them however, you are s$%t out of luck. There is NOTHING that can replace these very unusual output devices.

VFETs – unique, fabulous sounding, no longer made, impossible to find new and impossible to substitute. These are as they came in the TA-4650, before service and reliability enhancement.

Because they are FETs, these devices are sensitive to static discharge and mishandling. You can kill these just by handling them the wrong way. And because of how Sony implemented them in these amplifiers, they have little protection in the event of  something going wrong. Because of all of this, many owners stockpile VFET amps and the VFETs themselves, whenever they find them.

The legendary amplifier designer Nelson Pass, famously pissed off the entire worldwide Sony VFET community when he purchased the very last stocks of VFETs, directly from Sony, for use in his modern VFET amplifier. Lots of unhappy Sony VFET owners…

Reliability Enhancement

Sony used an varactor dual-diode package in the bias circuitry of these amps. I’ve studied the schematic and can verify that, if/when these diode packages fail, they will take out the VFETs. This apparently happens quite often, too often to leave them in place.

If you follow what I do, you’ll know that I aim to improve reliability of hi-fi gear wherever possible. This approach means we keep great equipment like this running well into the future, maybe longer than it’s even been running already. I suggest that, rather than scrounge around for VFETs, you should have your VFET amp properly serviced, to achieve the lowest possibility that the VFETs will fail in the future.

Don’t forget to check out my video covering the Sony TA-4650, featured on my YouTube channel.

Servicing the TA-4650

As usual, we’ll start by looking at the amplifier layout and then break it down and look at the details of the reliability enhancement process I applied to this little beauty.

With the lid off the TA-4650, we see the nicely modular layout. Power transformer to the left, power supply and regulator board immediately to the right of that. There’s an adjustment here that sets the rail voltages that critically important to get right. bottom of the image are the tone control and switching boards. Centre right, we have the output amplifier module.
Here’s that module in close-up. Note that Sony numbered every connector and that the board is extremely well laid out. This whole module comes out very easily. Thanks Sony.
Here’s that module, removed from the chassis. Most of the reliability enhancement work I’ll do on this amplifier is on this board. The lovely die-cast aluminium heatsink should be obvious. No sharp edges here.
Here’s the underside of that assembly. Note the original Sony VFETs, on original mica thermal pads. I’ll replace all of these mica washers with silicone thermal pads as parts of the reliability enhancement process.
VFETs!! It’s like the cry of “VTEC” from Honda fanboys, except in electronics, and about Sony.
While we are here, note the little black package, with sleeved legs and blue dots on it indicating the positions of the two cathodes. These are the VD-1221 twin diode packages that should be replaced to further enhance reliability of this amplifier.
I start by removing the VFETs, thoroughly cleaning them and the heatsink, and then placing them in a static-proof bag for safe-keeping while I get on with other work.
Note the clean and nicely machined heat sink mating surface. Sony weren’t messing around when they designed this.
Here are the other elements of the amplifier assembly. At the top there’s a metal shield, underneath that is the board itself and the die-cast heat sink sits below that.
Sony issues several service bulletins for this amplifier. One of them asks for two very important changes to be made. The first is for the replacement of four resistors with updated values. The second is a revision of the bias current applied to the VFETs. Both of these changes are critically important to improving the reliability of this amplifier.
Here I’ve removed the old resistors…
And here you can see the brand new Philips/Vishay precision resistors I’ve installed in their place, of the updated value of course.
A view of the other two. Note the component dress I’ve applied to these parts. The parts are of a higher power dissipation and better temperature coefficient than the factory parts. I’ve also installed them off the board, to allow for the greatest thermal dissipation.
In this next stage of the reliability enhancement, I’ve removed the unreliable twin diode packages, and shown them next to the twin diode replacement packages I’ve made to take their place. Note that I’m re-using the sleeves that prevent the diode leads from shorting against any nearby components.
A close-up of one of the replacement diode packages I made, installed on the board. Again, note lead-dress, everything I’ve done here is with long-term reliability and quality of work in mind.
Another view of one of the new diode packages. Note that it is pressed up to the adjacent transistor. This is on purpose, they are thermally coupled and I applied thermal paste between the diode package and transistor before reinstallation in the chassis.
Here are the VFETs, now sitting on premium silicone thermal pads, fasteners correctly torqued and ready to go back into the amplifier.
You’ll perhaps note that they are all marked J55. This is Sony binning nomenclature from the factory where they were made and graded after production. This matching is also critically important. If any of these parts were to fail, they can only be replaced by VFETs that match this ‘J’ grade.
Just so that there is NO CHANCE of me supplying the wrong line voltage to this irreplaceable TA-4650! Everything is now back together and ready for testing. In this critical stage of the work, I set the power supply rail voltage and VFET bias current.
Testing and adjustment went perfectly to plan, so here are a just a couple of last images of this wonderful piece of Sony amplifier history.

And that’s it, the work on this Sony TA-4650 is complete. Her owner reports that she sounds absolutely fantastic after the service she just received and he is very happy with added peace of mind he now has when running his TA-4650.

If you would like me to perform similar work on your Sony VFET amplifier, I’d be happy to. Just get in touch via my contact form.

Thanks for reading, leave a comment and let me know what you think!