Post by Owen Y on Dec 14, 2017 17:27:07 GMT 12
This is a Japanese amplifier that I purchased, in kit form secondhand from Trade Me, quite a few years ago. Sun Audio interested me because (i) the late Harvey Rosenberg had raved about a Sun Audio 300BPP amp (ii) Japanese build quality (iii) I had always wanted to have an amp with the reputed Tamura or Tango transformers. This is a 2A3 single-ended amplifier, with only 3.5Wpc power output - but I was using lspkrs of 96dB/W sensitivity at the time, so I figured that it could work. Here's what the kit looked like (Sun Audio picture): 
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Post by jon on Dec 14, 2017 18:16:00 GMT 12
not cheap tho'!
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Post by colinf on Dec 15, 2017 4:46:05 GMT 12
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Pundit
Post by tonyd on Dec 15, 2017 6:36:14 GMT 12
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Post by Owen Y on Dec 15, 2017 15:09:05 GMT 12
Hi jon - yes, I see that the kit new price is now JPY258,000 (NZD3400) - I paid one-third of that (s/h), around 10 years ago. tonyd - that looks exactly same as mine (below) - how does it sound to you?    
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Post by Owen Y on Dec 22, 2017 17:05:01 GMT 12
Time to look inside....  (1.6mm steel chassis  )  (Colour-coded wiring provided, nice.) (The coiled wires are unused OPT & PTX connections.)
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Post by Owen Y on Dec 25, 2017 8:33:40 GMT 12
One of the handy construction diagrams provided by Sun Audio: 
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Post by Owen Y on Dec 25, 2017 8:36:43 GMT 12
The only change I made was to swap in some shielded Cardas 21awg for the supplied input signal cable (blue) from RCA sockets to volume pot & first tube. 
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Post by Owen Y on Dec 25, 2017 8:49:59 GMT 12
The schematic of this amp seems quite conventional - 2 x 6SN7 stages with 2A3 single-ended output, for 3.5 watts. The PSU is also unsurprising - 5U4G tube rectifier with CLC choke smoothed high voltage supply & AC-heated tubes. The tubes supplied are all Russian Sovtek. It all belies how good this amp sounds (more later). However we can note that the first 2 stages are direct-coupled (no coupling cap) & thus there is only one coupling cap in the amp per channel. And of course, another key factor may be the quality of the Tamura iron. 
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Post by Owen Y on Dec 29, 2017 12:52:48 GMT 12
Japanese electrical devices typically have un-earthed chasses. As this does not comply with our electrical rules, I changed the following: - Connected the steel chassis to mains Earth. - Connected the Circuit Ground to the chassis Ground post via a 10 ohm resistor / 0.1uF cap bypass. (Often called a 'Ground loop break', this partial Ground lift is often recommended to minimise Ground loop currents, whilst maintaining RF shielding.) 
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Post by Owen Y on Dec 29, 2017 17:49:02 GMT 12
OK, firing up an amp that has been sitting idle for 5+ years, maybe 10 yrs.... Capacitors unused for a long period, especially high voltage caps, technically should be "re-formed", a process of re-forming oxide dielectric onto the foil electrodes. This is to avoid arcing & damaging the electrode foil (or worse with real old caps/amps.) Modern caps are less critical arguably & a convenient way of re-forming is to use a Variac (if you have one) & ramp up the mains voltage slowly, over an hour or more. (Low leakage current is the key parameter.) ( LED AC meter + variac in use....) 
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Post by michaelw on Dec 29, 2017 18:27:11 GMT 12
looks tidy under the hood variac ftw 
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Post by Owen Y on Dec 30, 2017 13:04:04 GMT 12
Typical Japanese hard-wired style, as anyone who has studied MJ Magazine will be familiar with.
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Post by michaelw on Dec 30, 2017 14:15:14 GMT 12
was the kit pre-wired ?
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Post by Owen Y on Dec 30, 2017 16:33:20 GMT 12
Nope, but the main hardware was pre-installed on the chassis - ie. transformers, the tube sockets, binding posts, the big green power stage resistors. (As in the first pics above.) It can be purchased pre-built - at extra cost.
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Post by Owen Y on Jan 1, 2018 10:38:03 GMT 12
The amp sounds very good, surprisingly good considering the unsurprising circuit schematic. As said, the first 2 stages are direct coupled, which is always good, IME. Of course, the Tamura iron, the Output Transformers & PSU choke, would be of high quality, excellent possibly & with single-ended transformers, achieving wide bandwidth together with adequate low end power delivery is almost 'black art'. The sound is lively, 'fresh' & open, lively and dynamic. Plenty of 'resonance' and ambient 'decay'. Bass is strong, deep and rhythmic, but not loose. Treble leaves no complaints - as any tube-rectified, single-ended triode amplifier should. Plenty of bandwidth and attractive tone.
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Post by Owen Y on Jan 4, 2018 11:07:02 GMT 12
I've found that in summer weather (26deg C ambient in the attic today, damp outside), the power transformer case gets to 50+deg C (almost too hot to touch) after a couple of hours. I've had to break out a couple of old Pabst computer fans (series-wired to slow them down) as a precaution in hot weather. Case temp drops to 33deg C. Good for the valves too. My old Audio Note Kit One 300B SE amp was similar as I recall.   Dang, this amp sounds good.
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