Wednesday, September 18, 2013

Refurbishing a vintage Sony TA-3120A solid-state amplifier


The original Sony TA-3120 is quite the ancient (solid-state-wise) piece of gear.  As far as I can tell, the original TA-3120 amplifier was released in 1966, the start of the decline of vacuum tube technology.   It was this model, along with many others, that was also the beginning of the end for American stereo manufacturers.  Thanks to truly portable radios, the Japanese had a leg up on making solid-state gear that didn't blow up and could provide countless hours of trouble-free operation.

(right click to see larger image - or check out the service manual)
Note: this is the 3120 schematic, the 3120A appears to be slightly different.

The TA-3120 and the latter TA-3120A features very simple circuitry, looking quite valve-like in operation.  A quick look at the schematic reveals a very simple single voltage power supply, a single transistor gain / and single transistor phase-splitter, two pairs of output transistors per channel, and a large capacitor coupling the transistors to the speaker.  My own interest in these types of amplifiers are purely historical.  Though I believe a good tube or a modern circuit will lay waste to these early units, I also have an interest on hearing just how good these things could potentially sound.  Those vintage electrolytic capacitors, especially back then, weren't particularly well-made, not compared to the tightly wound models of today.

These stand-alone "one third" amplifiers seemed to have been rarely used with a preamplifier - instead they were supposed to be mated with integrateds, powering the back channel in early Quad / primitive surround sound systems.  Making a decent - for the time - 50 watts per channel this amplifier will have enough moxy to drive most speakers, provided the they don't dip below 4-ohms impedance.  The capacitor coupling will also save woofers if the output transistors decided to go south.  I also like adjustable gain knobs in back, perfect for matching up with any number of different preamplifiers.  With that in mind, along with the handsome chrome shoebox looks and potted power transformer, I decided to buy one for a paltry $129 and see how it sounded.


Initial listening results through the UREI 813A speakers were unfortunately short-lived.  However, before the problem came to light, I heard an amplifier that sounded pretty good but a tad "confused" with not very good control over imaging and soundstage placement.  There was a "blur" to the sound, along with a recessed midrange and a slightly tizzy treble.  Bass response was half-decent though definitely not approaching the uber-depth and control of my old Threshold S/500 or even my Eico HF-60 tube amplifiers.  But still, I would rate these higher than stock Dynaco ST-80/120s or even many vintage receivers I've heard.  I could tell the fine fellows at Sony had put some serious thought into these amplifiers but were hampered by the technology of the time.

A few minutes of listening and the sound suddenly cut off.  And then mysteriously it came back.  Only to be cut off again.  At first I thought some speaker protection relay - as seen in the TA-3120 schematic - was clicking off and on.  However, an examination of the 3120A innards showed no such circuit.  A quick check of the multimeter showed that the power supply was undulating from 62 to 82Vs roughly every two seconds.  As the voltage dropped, so would the sound.  This pointed to a potential capacitor problem - a device that stores and releases voltage.  To the Digikey site I went - it was time to replace every capacitor on the circuit board.  Like the original amplifier, I went with Elna (mostly Silmic) capacitors where possible.  I also discovered that the TA-1320A is quite different than the original 3120, with different values and some there not at all.  So if you need to do this job, record the capacitors needed from the components on the circuit board, not the schematic.

Yikes!  This was a nightmare.  Even though Sony had thoughtfully made it easy to pop off the three circuit boards in question, I still opted to keep the multiple leads intact.  This left me a few inches between the back of the PCB and the heatsink.  The solder traces are also extremely fragile and appear to be made of very thin copper - use caution!  When I was done, I had a nice pile of used capacitors and a much more modern looking board.


When I finally finished, I hooked up my Pioneer test speakers and took the output directly from the SACD player.   I turned the amplifier on ... and hurrah!... there was music!  I let it play and tested this power supply voltage.  This time it was a rock steady 86 volts, higher than the initial value.  Since I hadn't replaced the main bucket capacitor which I thought was the source of the original problem (and the most expensive fix) I was happy to hear undistorted sound.  The Pioneer speakers, however, aren't the highest in fidelity, but still good enough for some initial thoughts and component break-in time. 

Hmm... good bass response and a bouncy engaging rhythm.  A very pleasant background sound as I help my fellow marines hammer through the defense of Iwo Jima - courtesy of the virtual world of Rising Storm, a Red Orchestra 2 upgrade pack.  However, the old saw "all amplifiers sound the same" really does hold true, but only if the speakers in question can't resolve the differences.  It was time for a true sonic torture test, the venerable UREI 813A monitors with their compression horn tweeters, a speaker that has broken many an amplifier.

Hooking up the Sony amplifier was easy as pie - the back connectors work great with the mid-sized spades of the Cardas Hexlink speaker wire.  Large spades and banana plugs please do not apply.

The first cut was my original U.S. pressing of Tom Waits - Closing Time.  This recording is a tad dark and it showed.  The upper midrange / lower treble grit I normally associate with solid-state gear was pretty suppressed - you had to really listen for it to hear it.  Only a few non-tubed amplifiers truly get rid of it - at least according to my experience.  The next recording was the 180g re-issue of Ray Charles - Genius + Sould = Jazz.  I was taken aback by the sheer musicality of the sound.  I began to feel a tad suspicious, wondering if I was hearing one of the better amplifiers of my life.  Seriously.  The dynamic swings worked well and the spaciousness of the recording was evident; this sort of depth is something I normally associate with tube amps.  Hmmm....

Next up was a first pressing of Donald Fagan - The Nightfly.  Ah, now I could hear the limitations of this early digital recording.  The treble was noticeably splashy and forward.  The very deepest bass was not exactly missing, but not as low as I remember with other, more powerful amplifiers.  The separation of the vocals were not as defined as other amplifiers caused by a slight blurring and loss of detail.

Now it was time for something really dynamic - The Classic Records re-issue of The Who's - Tommy and my original pressing of Frank Sinatra - Sinatra At the Sands.  With these two recordings, everything was there and the music was still emotional as ever, but the lowest bass registers were just slightly underdamped and lacking - perhaps an effect of the music going through such a large electrolytic output capacitor.  The very top could get hard and uncontrolled when listening too loudly, the sign of an amplifier running out of steam - obviously 50Ws can only do so much.

Conclusions?  This is a very nice amplifier, especially with a bevy of new Elna capacitor replacements.  Perhaps it is the simple circuitry or the effects of the (fairly) non-linear output capacitor, but the Sony TA-3120A lacks much of the grit and harshness I associated with solid-state gear.  Sure I've heard better - notably a large number of tube amplifiers, but if I didn't know any better, I could really live with this piece of vintage amplifier.  What it lacks is the ultimate finesse of the best gear I've heard - detail retrieval, bass control, and soundstage depth.  However, it does - much like the B&K ST-140 but not quite as warm - convey the music in an enjoyable way.  Of course much of this may come from my VPI table and Quicksilver preamplifier; the flaws of the Sony diminished by the strength of the other audio gear.  Nonetheless, recommended for vintage junkies who want to try something different.

Review System:
VPI Aries with JMW 10.5i tonearm and SDS Power Supply
Dynavector 10X5
Cardas Cross 1M interconnects
Quicksilver preamplifier with Mullard short-plate 12AX7s, RCA 12FQ7s, Amperex 12AU7s
Cardas Quadlink 5C 1M interconnects
EICO HF-60 monoblocks with Mullard XF2 EL34s & 5AR4s, GEC CV4085s, and GE 6SN7GTAs.
Cardas Hexlink speaker cable
UREI 813A monitor speakers
VTI BL503 equipment rack

Tuesday, September 10, 2013

New Project: Universal Single-Ended Pentode Amplifier

I've recently built two budget orientated single-ended amplifiers, one using the WWII-era 1625, and the other using the 6CB5A connected as triode.  Both were built to a price point - Edcor output transformers and parts sourced from Mouser.  The sonic results for these unusual designs were quite gratifying, considering the sub-$700 price range.

Sonically, I actually preferred the sound of the 1625 amplifier which was used as a pentode with screen regulation.  It had a pleasant forward sound compared to your average triode, and also seemed to develop much more power than expected, along with greater speaker control with the use of (yikes!) negative feedback.  This project made me rethink an older amplifier I built using the EL156 output tube connected in Ultralinear.  I began to wonder how a true pentode assault on the "state of the art" would sound.

With that in mind, I came up with the following concepts to guide me through the design process:
  • Ability to use octal socket output tubes: 6550, EL34, 6L6GC, KT66, etc
  • Fixed Bias - allowing the maximum range of bias 
  • Plug-in Plate cap: for the weird tubes
  • Pentode operation - possibly with a triode switch
  • Tube rectification using damper diodes
  • Screen regulation using gas tube shunt regulation
Perhaps "state of the art" was the wrong term to use since I won't be spending hundreds of dollars on Teflon coupling caps, but I will be using higher-level James output transformers, Rel-Cap polysterene couplers, and some really nice Nichicon electrolytics.  And I have a pair of vintage TungSol 6550s that are just crying out to be used!  Stay tuned for the schematic, build pictures, and listening results.