Something about this SOTA keeps "drawing" me to it, so I have spent more time delving into it, making some tests and the result is a more (yes even more!) detailed description, my thoughts and yes the asking price has increased slightly - surely you don't think I am going to spent a lot of time on these descriptions for nada zero/zilch return? Time is money ... I have a LOT of equipment to work through and I really don't want to spend more time on the SOTA but it does keep "drawing" me to it!
WARNING!
I am passionate about "some" gear and tend to talk and talk and talk in my descriptions .... this is a LONG one, so brew a coffee, grab a snack and read on!
As a very famous person once said "I have a dream ...."
A long time ago, in the mists of time, I planned to turn this SOTA Head Amplifier into a very special "tube", including a Silver or Gold plated body so it looked as good as it sounds and something that fellow audiophiles would be in awe of .... well, as with so many of my dreams, this one never came to fruition!
HISTORY:
If you move in the world of the audiophile, moving coil cartridges and general turntable equipment that occupies rarified space, the SOTA equipment and specifically this head amplifier will be very familiar.
Never heard of a "head amplifier"? No problem, basically this allows your nice vintage amplifier to correctly receive the audio signals from a moving COIL cartridge. These MC cartridges provide a substantially lower signal level than "normal" MM cartridges but a far superior sound quality. So the head amplifier's task in life is to substantially lift the level of the audio from the cartridge while at the same time not introduce any unwanted/additional audio nor alter the original audio signal characteristics when it is presented to your amplifier input.
Even though this is an old design and based on dated technologies, this head amplifier is still extremely well considered and sought after by the audiophile (commonly being offered at various audio outlets from US$1000 and upwards!) an alternative to a transformer based solution.
Did I say "dated"? Sure is, this equipment is from the 1980's, uses a simple double sided PCB and discrete through-hole components - no SMD and absolutely no integrated circuits to be seen!
The secret of this head amplifier's success is in the design, by the American audio engineer, John Curl. To read more about the design philosophy used by John in creating this Head Amplifier, go HERE
YOUR TASK, WHAT LIES AHEAD ...
Lets be frank about this, someone (a prior owner) had taken this lovely old SOTA head amplifier and turned it into an abomination!
I rescued this equipment from an estate sale a long time ago and my intention was to bring it back to it's former glory, I already had a couple of very nice MC transformers/pre-amps in my professional capacity but this SOTA represents something very special from the past and was considered a worthwhile restoration task.
Problem is, I didn't take into account the time I would need and the "project" sat in its own special box waiting for me to look at it. I never did until yesterday (30 years waiting)!
The basic original SOTA head amplifier is "as original" meaning the casing, the PCB and the components are originals but heaven forbid, someone has decided to add their own pre-amplifier INSIDE (like a daughter board) and attached this extra board to the original SOTA printed circuit board using wire links - reminded me of the ALIEN series of movies where the alien lives inside the human host, a true horror story!
I almost cried when I first saw what the original owner had done! The "alien" daughter PCB is not even anything great, just an integrated circuit (LM833) used as a low noise pre-amplifer, a design from an Australian electronics magazine - sacrilege!
*** Update note ***
I now know why this SOTA was "infested" with the foreign board .... I give full details below but I have now discovered that ONE transistor must have failed, it has been substituted (badly) and also two resistors have been pulled off the board, for reasons unknown. So I believe the original SOTA failed and an attempt was made to repair it. Apparently it didn't work out too well so the previous owner decided to introduce a completely different preamp (very low grade) into the body to solve their problem! Sort of like the situation where your Mercedes engine blows up and you decide to fit a Suzuki engine inside instead - OMG!
Restoring this SOTA is going to consume some time and research/testing or experimentation with that one transistor which needs replacing. The cosmetic look of the "barrel" is not great, but has the potential to be made "better than it ever was".
Some soldering will be required, this is an ordinary double sided printed circuit board and not difficult to work on - NO SMD at all!
Of course it is assumed the new owner has the technical skills to diagnose the transistor characteristics of the replacement transistor and possibly other fault finding if something should "show up"
If you are expecting to put in a couple of batteries and "away she goes" then this project is NOT for you ... move on
If however you are like me and enjoy (and have a passion for good restoration work) restoring both the appearance and electronics of "special" equipment then you could end up with a VERY SPECIAL vintage SOTA head amplifier that you can be proud of AND these do seem to still command a pretty decent price these days (looking around online, these SOTA Head Amplifiers are pretty scarce but when offered for sale, they typically are asking A$1000+)
THE FOLLOWING INFORMATION RELATES TO THE ORIGINAL SOTA
SOTA INDUSTRIES - Head Amplifier
Designed and manufactured in USA
Circa l980's
Serial # 731
Low Z input preamplifier for use with MC cartridges
To avoid any possibility of introduced AC hum when using an external power source, the unit is entirely powered by 2x D size (big fat ones!) 1.5V Alkaline batteries which are mounted INSIDE the unit. This amplifier is running in Class A which means that even the quiescent current is relatively high compared to other amplifiers and the batteries need to have a high capacity. Fortunately, changing the batteries is a relatively easy task.
REAR PANEL PORTS & CONTROLS
2x Gold plated Input Phono sockets
2x Gold plated output Phono sockets
Large central knurled nut for signal ground connections
Input loading is switch adjustable, 5 Ohm, 30 Ohm or 100 Ohm using sub-miniature 3 position C&K toggle switch
Gain has two external settings, HIGH or LOW and set using another sub-miniature 2 position C&K toggle switch
DIMENSIONS: 175mm length x 100mm Height x 90mm Width
WEIGHT:1.2Kg (Without an internal power source)
THE INFORMATION BELOW IS ALL RELATED TO THE ACTUAL SOTA I AM OFFERING
Now down to the "business end" of things ... from here on I am describing the ACTUAL equipment being offered.
I am describing the SOTA head amplifier AS IT WILL BE SUPPLIED
BODY/END DISCS/BASE:
Also refer to Photographs 4,5,6,7
Very simple design, basically a section of tubular iron which is GLUED to a wooden base.
The iron tube adds substantially to the overall weight, coming in at 800g just for this tube.
TUBULAR BODY MEASUREMENTS: 158mm Length x 89mm Diameter x 3mm (Yes 3mm!) thick Iron
Each end of the Iron tube has a large, one piece circular disc that fits snuggly into each end of the iron tube.
Material used is Ferrous (Iron based) but has some sort of treatment so it has a "Gold Like" colouring, this is not electroplating though, I am not sure what it is! It cannot be Brass as it is ferrous, perhaps some sort of anodising was used.
From a cosmetic perspective, these discs may provide the greatest challenge to restore. The base metal shows through the Black top coating revealing the text, SOTA logo etc. The Black coating has lost it's "visual appeal" (you can see small pit marks and a general dullness) and will definitely need some sort of restoration.
One disc has all the rear input/output connections, ground post and two miniature toggle switches. Due to the changes made by the person who attacked this SOTA head amp some time ago, two of the Gold plated Phono connectors are "missing in action".
These will need to be replaced and in fact I was seriously thinking about purchasing a complete set of four but haven't gotten around to it yet! A nice gold plated panel mount NYS367 Phono socket from Neutrik/REAN would be perfect (insulated from the panel) is HERE
Shame (but if often happens) that SOTA used what are pretty "el-cheapo" Phono connectors (evidenced by the Gold plate rubbing off!) I see this often, great well designed equipment but it's sometimes the cosmetic detail that can let the whole thing down.
The switches are fine, operating properly and look in great physical shape and the same goes for the binding post (Ground point)
The remaining disc has only one control, the very antiquated (but nice) self indicating switch. These were popular in the 1980's as they provide a visual indication of the switch position without using any power. When pushed in (turned ON) this switch changes to Orange in the centre.
The only other items of interest on this disc are the two bolt holes. I have used a temporary substitute bolt to keep the PCB safely secure inside the tube but again the intention was to upgrade these bolts to something more visually attractive (Gold plated bolts or similar)
These two bolts mate with two threaded L shaped brackets on the PCB and when the PCB is inserted into the tube, tightening these bolts will hold the PCB firmly in place within the tube. Their function is important but they should look "pretty" too!
END DISC MEASUREMENTS: 86mm Diameter x 1.25mm Thick
The wooden base is "nice", being REAL timber (type of timber is unknown) with a nice longitudinal distinctive grain. On the underside of the base are 4x felt feet to provide surface grip. The original serial number paper label is also on the underside of this base.
WOODEN BASE MEASUREMENTS: 130mm Length x 90mm Width x 25mm Height
GENERAL COMMENTS (Relating to the above sections):
Obviously the Iron tube needs cleaning up, the iron oxide (rust) is "dust like" but has not eaten deep into the tube surface. The original coating looks to have been a simple dark coloured (almost Black) laquer to protect against rust formation but it has "failed" so would it be worth doing it again? At one time I toyed with the idea of Silver plating this tube but decided against playing around with the Cyanides involved in electroplating - too dangerous for my liking. Yes, you can electroplate Iron with Silver - there is a patent online about a person who did this way back in 1948. My other concern about electroplating is the fact that the Iron tube is glued to the wooden base, it is certainly securely attached! Trying to separate the two could prove disastrous!
Restoring the end discs is probably going to present the greatest challenge because of the graphic design methods used originally. It would be great to bring the disc metal back to a Golden "bright and shiny" finish but then the challenge is how to update the Black overall coating AND have the graphics/text showing through the Black.
The base would be greatly improved with a matt or gloss laquer to enhance the wood grain and with the addition of a nice Brass plaque, suitably inscribed on one side of the wooden base, it would look very cool.
PRINTED CIRCUIT BOARD:
Also refer to photographs 8,9
Now we get down to the "nitty gritty" (not the dirt band) ... the board.
Double sided, all tracks are undamaged and those that have been reworked are not damaged.
NO new holes for components have been created by the previous owner - all original.
MOST components have not been moved or changed, these look to be ALMOST ALL original and the solder pads are untouched.
*** Update note ***
Two resistors (one in each channel) have been removed. The resistors are all DALE RN550 series metal film type. DALE is no more, having been swallowed up by the behemoth Vishay but you can fairly easily get low wattage MF resistors - in fact I have some although they are German made and have a different coloured body.
Comparing to an original SOTA Head amplifier, it appears these are 1.3K resistors (printed on the body - no colour coding) and in fact I have some and if you wish to look at those, Search my website using the keyword 1K3 ....
Next, it is obvious that there is NO 2x D cell battery holder or contact springs fitted. The previous owner used an external power source for their "creation" and removed the battery contact springs to make way for the alien board. The board clearly indicates where the positive and negative connections are on the tracks (3V DC). A few solutions to this battery container issue, perhaps just a twin D cell holder or two separate single cell holders or even a board mounted contact spring set - I have seen all of these offered at RS Electronics and Element 14.
You may be thinking, why use such large capacity batteries? Remember this is a Class A amplifier which has the best audio signal handling characteristics but the downside is that the current drawn by the amplifier is much higher than other types of amplifiers. To change the battery to AA or even AAA size would not be a great idea!
Now to the most important part ... the transistors .... THREE are originals, one is a "ring-in" and should not be there, in fact I have removed it!
The transistors are NPN/PNP pairs, biased in Class A. The type of transistor is unknown and SOTA has been very careful to remove all identification (common practice in those days)
I have made some basic measurements of the transistors .... in order to properly test these transistors, I actually needed to remove every one of them and test "out of circuit"
Left Channel: PNP hFE 125 NPN hFE 185
Right Channel: PNP hFE 155 NPN PULLED Why? Because I discovered that the NPN transistor of this pair had been substituted with a BC546B (measuring 268 current gain for what it's worth!)
I also established the pinout of these transistors, it is not your normal configuration for the BC5xx series for example and that is why you can see in one of the photographs that the BC546B is actually facing the reverse direction. The two pairs of transistors normally face in the same direction, see the photograph(s) of an original SOTA head amplifier board (not mine)
Here is the pinout diagram for these transistors ...
Fortunately, although the PCB does not have much by way of identifying components, it does have the collector/base/emitter holes identified - yeah for small mercies!
These TO-92 package transistors only have the three colour bands on the "back" and all other identification has been removed. I seem to recall that the colour bands related to current gain grading and not a type number. RCA used this method of identification as I'm sure other manufacturers also did. I strongly suspect these transistors were USA manufactured, very unlikely that SOTA would have used Asian product.
HERE is some background information to the SOTA Head Amplifier (from SOTA) and you will note that it mentions the design around the "special" transistors but does not indicate what type they are. I seriously doubt they had special transistors made specifically for their product but more likely that specially selected transistors were used based on current capabilities, gain, very low noise and high speed switching. They are definitely Silicon bipolar and not FET's or other technologies.
One thing for sure, the humble BC546B should not be "living" on this board! I have removed and discarded it (much like a tumor!) leaving the solder pads ready for something more suitable.
Actually my thought at the time of looking into this area of the board was to fit a nice Gold pin transistor socket and then it is easy to have a "play around" with various transistors that might be suitable for the SOTA. If a transistor is subsequently settled upon, then remove the socket and fit the transistor permanently.
Next is the fixed resistors, all of which are DALE (USA) metal film, RN550 series. I have not measured these, usually the MF resistors are pretty stable BUT you will need to fit 2x 1K3 resistors which someone has removed and not fitted back onto the board ... no idea why.
I did measure the ERO (Germany) KT1800 series metal foil capacitors, nominally 0.33uF and they measured 0.347uF and 0.381uF. Bit surprised at the latter capacitor going high in value. I also pondered the design philosophy, with a supply rail of 3V and yet these capacitors are 63V!
The Nichicon electrolytics were also tested, some in-circuit and some I had to lift one leg as I could not get a "proper" reading in-circuit - nominal value should be 2200uF and ALL measured in the region of 2275uF ~ 2400uF and the ESR's ranged from 0.8 ~ 1 Ohm
All components are "through-hole" and only high quality components were used, until the "alien board" started living inside!
PCB DIMENSIONS/IDENTIFICATION: 154mm Length x 83mm Width (Fibreglass substrate)
PCB is marked as ...
sota (One Side)
8.14.80
R A HERMAN
(Other Side)
Component on board identification is next to none, with just the transistor pads being identified and the positive/negative supply rail points but not much else, including the input and output points. This is not hard to figure out and referring to the photograph of an original SOTA definitely is helpful too. Those two Black wires (used for Ground/Earth) are very heavy duty (no need for that) and should be changed for a much "nicer" wiring, as used in the original SOTA.
So there is my story THUS FAR .... if someone wishes to purchase this SOTA EXACTLY as described above, it is available. If no real interest in the SOTA is evident, then I will continue working on it myself and possibly offer it at some later stage once I have fitted the new phono's and sorted out the transistor situation plus run a "live" test of the unit.
Keep in mind however, the asking price will keep INCREASING as I spend more time on this equipment - I have plenty of other things to look at (some nice SANSUI gear is coming up) and at this price, this is one VERY low cost SOTA Head Amplifier even if the current condition is taken into account
Please read the FULL description above, evaluate if it is a restoration task that may be of interest .... I don't really mind if no-one is interested, I am imaging this nice Gold plated SOTA with a Brass side label might one day be "sitting on my audio shelf", connected to the HANA SL cartridge that is in one of the boxes around here "somewhere"!
