Sun Audio 2A3 tube integrated amplifier

Here is the story about my beloved, tiny Hercules – Sun Audio 2A3 integrated amplifier.

I bought this unit in 2013 when I was driving home from Black Sabbath concert in Czech Prague, previous owner liked it a lot, but he didn’t owned appropriate speakers to make a good match with relatively low power output of 3.5W that this amplifier can deliver.

For the first and the last time in my life I have seen the full factory assembled Sun Audio,  most of the 2nd hand units available on the market were purchased as a DIY kit. If you google for the pictures of the SV-2A3 internals you will see that there are no two same looking units, each is different.

General schematic is quite simple. 2A3 power tube in a single ended design and classic RCA operating point. Two 6SN7 drivers, while second due to the operating point is mainly a phase inverter, but still – it amplifies the signal a lot. 5U4G full wave rectifier followed by CLC (capacitor – choke – capacitor) filter for the power tubes, and additional RC circuit for the driver tubes.

On the original schematic there is 47uf cap with 5H/150mA choke followed by 47uf, however my particular unit had 100+100uf Black Gate WKZ cap, IMO – too much for a 5U4G to handle for a long period of time.

All the transformers were made by Japanese Tamura – which is well known for the audiophiles worldwide as a manufacturer of absolutely premium iron, equally good as Tango or better, depending on preferences.

Power transformer model SPT-150 or SPT-150B with the difference in primary winding only – see following specs:

Primary: 0(White)-100V(Yellow)
Secondary: 290V(Gray)-0(Black)-290V(Gray) DC150mA,
6.3V(Orange)-2.5V(Red)-0(Brown) 3A,
6.3V(Blue)-2.5V(Green)-0(Yellow) 3A,
5V(White: two wires) 3A,
6.3V(Purple: two wires) 2A

Primary: 0(White)-230V(Blue)-240V(Yellow)
Secondary: 290V(Gray)-0(Black)-290V(Gray) DC150mA,
6.3V(Orange)-2.5V(Red)-0(Brown) 3A,
6.3V(Blue)-2.5V(Green)-0(Yellow) 3A,
5V(White: two wires) 3A,
6.3V(Purple: two wires) 2A

The same power transformer is used for Sun Audio SV-2A3 as well as SV-300B model, although in my opinion 3.5W from 300B in Sun Audio application is not the best choice.

Two years ago I have converted SV-2A3 into SV-300B, by doing the following:

1. 1000R cathode resistor instead of 750R

2. 3.5kohm instead of 2.5kohm

3. 5V heater voltage with CRC filter instead of 2.5VAC

I had KR Audio 300B baloon on hand as well as Schuguang 300B-S and TJ 300B for comparison. While I am not a fan of chinese tubes I must admit that cheap Schuguang performed the best, at least in my system. The sound of this tube was very similar to RCA 45 type tube, with more power and low frequency extension, however without the magical midrange 45 type tube can offer.

Still, it was not in the same league as the original Sun Audio I was used to.

Please have a look on the internal parts – can anyone ask for more?

Black Gate WKZ 100+100uf/500V as primary PSU capacitor, Black Gate STD on 2a3 cathodes, Black Gate N and FK on 6SN7 cathodes, NOS Allen Bradley carbon composite resistors, Jensen Copper foil signal capacitors, Japanese Tocos Cosmos 100k potentiometers.

When I bought this amplifier I told myself that I will never ever touch it with the soldering iron. It was so perfect to me in every sense, that I couldn’t imagine anything better. From the time perspective I see that I was right and wrong in the same time.

Stock amplifier sounded extremely dynamic, bass was the best I have ever heard, and I owned many great tube amplifiers based on 6C33C or even GM70 power tubes.  Overall sound balance was perfectly right. Low, mid and high frequencies were on the same level, equally dynamic and full, very forward sounding which was perfect for hard rock music. Simply sound to die for.

After some time I started to feel some discomfort due to very sensitive Tocos Cosmos potentiometers.  Close look on their specs shown that they linear potentiometers rather than commonly used logarithmic. Please have a look on the chart to understand the main difference. Logarithmic potentiometer slowly decrease its resistance providing large headroom for low up to medium volume listening level, while logarithmic decrease its resistance in a constant, proportional manner.

The amp was silent when potentiometers were 8:00o’clock and it was screaming loud when they were 10:00 o’clock. Considering that all of my DAC’s have at least 3.5VRMS output level, and my primary speakers are 105dB sensitive.. I didn’t had any headroom to find a volume sweet spot. This was the first moment when I warmed up my soldering station and started to look for even better sound.

To make long story short – here are my 4 year observations:

  1. 100uf/100uf capacitor was too large for 5U4G to handle for constant operation. Power transformer was getting hot and rectifier lifetime was short. I have replaced it with 47uf+47uf Black Gate WKZ and checked the performance. Power transformer is now 9*C cooler and the sound is even more dynamic then before. Too large capacitance made it quite dull on bass in comparison.
  2. Two cascaded direct DC coupled 6SN7 drivers provided more than 200x times amplification which is at least 4 times too much if we would like to drive 2A3 to full power with 1V RMS signal. This kind of approach combined with usage of two linear mono potentiometers makes daily operation a nightmare. Only slight touch (0.1mm) made a night/day difference in terms of channel balance. This must be changed!
  1. SOT-525 output transformers were very good quality for 2A3, but they really sounded bad with 300B (using 3.5k tap). I have converted this amp to use with 300B, using original SV-300B schematic, but never was pleased with its performance. Sound was dull and slow, no engaging at all. I think that this amplifier concept was created for 2A3 and due to customer requests – it was adjusted by Sun Audio to use 300B, with poor results.

In the meanwhile I made several changes to push this amplifier even further. Original transformers were replaced by Tamura F-7002 with permalloy core. These beauties costed almost twice more than whole amplifier, but the sound of “permalloy” core is outstanding. It is my favorite core materials and I have tested and listened to Tamura F-2000 series as well as few custom amorphous core transformers. Amorphous core are the most detailed and analytical sounding core I have ever heard, their burn in time is extremely long. Silicon core (i.e Tamura F-2000 series) are very well balanced transformer, simply very good at all manner but no extraordinary in any aspect. All around great performer. Permalloy cores provide exceptional tonal balance, great timbre, musicality and real color of the instruments, however bass note does not go so deep as with F-2000 series, that might be due to lower inductance of the F-7000 series.

To be continued..