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Sony TA-4650 VFET Amplifier Service & Reliability Enhancement

I’m reluctant to use the word ‘sexy’ when it comes to hi-fi gear but if I were to, it would apply to this baby Sony TA-4650 VFET amplifier!

What can one say after working on one of these stunning Sony TA-4650 VFET integrated amplifiers? I fell for this little beauty and came to understand why so many people treasure these amps. Having said that, there are a few really important things to be aware of, if you own one of these or are planning on buying one. I’ll explain why as we go.

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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 as 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.

Sony TA-4650 Specifications

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 if you own one of these little 30 watts 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. I can still obtain some of them, certain grades, from Japan. They all need to be matched though so it often means a completely new set and at up to $100 each, if you can find them, it’s expensive.

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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 by handling them the wrong way. Because of how Sony implemented them in these amplifiers, they have little protection in the event of something going wrong. For these reasons, many owners stockpile VFET amps and the VFETs themselves.

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 a 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 happens quite often, too often to leave them in place.

If you follow what I do, you’ll know that I aim to improve the reliability of hi-fi gear wherever possible. This approach keeps 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 probability 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.

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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 are critically important to get right. Bottom of the image are the tone control and switching boards. Centre-right, we have the output amplifier module.
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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.
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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.
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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.
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VFETs!! It’s like the cry of “VTEC” from Honda fanboys, except in electronics, and about Sony.
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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 the reliability of this amplifier.
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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.
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Note the clean and nicely machined heat sink mating surface. Sony wasn’t messing around when they designed this.
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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.
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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.
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Here I’ve removed the old resistors…
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And here you can see the brand new Philips/Vishay precision resistors I’ve installed in their place, of the updated value of course.
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A view of the other two. Note the component dress I’ve applied to these parts. The parts are of 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.
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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.
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A close-up of one of the replacement diode packages I made, installed on the board, to ensure correct placement, prior to application of thermal paste. Again, note lead-dress, everything I’ve done here is with long-term reliability and quality of work in mind.
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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. These days I also heat-shrink the parts together for even better thermal coupling.
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Here are the VFETs, now sitting on premium silicone thermal pads, fasteners correctly torqued and ready to go back into the amplifier.
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You’ll perhaps note that they are all marked J55. This is Sony grading 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.
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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.
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Testing and adjustment went perfectly to plan, so here are just a couple more images of this wonderful piece of Sony amplifier history.
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Other than this, I always clean and service switches, controls, trimmers, connectors and so on and this adds to the general improvement in reliability and performance.

With everything done and the amp back with her owner, he reports that she sounds absolutely fantastic 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.

8 thoughts on “Sony TA-4650 VFET Amplifier Service & Reliability Enhancement”

  1. Hey great article i really enjoyed reading this, i have been searching around the internet for years now looking for new articles and info on the TA-4650 it was refreshing to find yours.
    Here is a funny story i actually picked up one of these off of the side of the road someone had dumped it for verge collection so i scooped it up to find that it worked perfectly. it had the original vfets in, so i dismantled it and gave it a good clean also gave it a recap, lucky it had already had the death diodes done, i proceeded to reset the power supply voltage and bias and now it sits used as my main computer stereo and it sounds wonderful 😀

  2. My VFET is a TA 5650. Nickel plated heat sinks. Much different than yours on the inside. Had it since it was new. Along with the epicure 30s.
    43 years plus.

    1. Hi Rebecca, thanks for sharing. The 4650 you refer to belongs to a customer of mine but yes there were variations across the range and your 5650 is much more like the TA-8650 I’m restoring, the big daddy of these other VFET amps. Sony VFET amps are very well-made and can be reliable, but you must attend to the diode packages (if this hasn’t been done already) because they destroy the VFETs when they die and VFETs are very hard to come by now. The 8650 needs a complete new set of VFETs, no doubt due to failure of those diodes…

  3. Thank you for the jerney through this beautiful amp…I have 2 off these fine pieces and want them to use as a mono amp for each channel What are your thoughts about this

    1. Hi Wim, Thanks for commenting. Lovely amplifiers but they were not designed to be bridged so they would need some modification to run this way. I wouldn’t risk it.

  4. Tank you for your reply and sorry, but i don’t intent to bridge the channels, just playing 1 channel, and take out 1 channels vfets, also unconnect the preamp electrical as well. Could that bring improvement of the sound quality evenmore?

    1. Hi Wim, removing FETs from one channel will not make any difference sonically or technically and I cannot see any reason or advantage in doing so. Taking the preamp out of the circuit may improve things, as long as you have everything buffered correctly and are feeding the signal from another preamp of higher quality than the Sony or are keeping things the same and just removing one unwanted gain stage, ie the 4650 preamp.

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