Project for 1010VA 37.5V transformer?

downhere

Member
2001-10-03 10:24 am
Went to enquire about the clearance transformers, sounded like a good deal for $35 each. Details on another thread. Going to order 2 for surround sound. Any suggestions for projects?

For all those who are interested in the progress of my previous 50W amp, i'm going to the workshop on saturday to drill the holes for my amp. Nice 3RU unit with aluminium faceplate
will take pics when its done.

my calculations are as follows. 37.5VAC gives 53VDC after rectification. Voltage sag should be about 20%, giving 42VDC after rectification. The leach amp should be quite good, but it's VERY complex. and requires a million transistors. Are there other projects out there that can take 42V and above and sound good as well?
 

downhere

Member
2001-10-03 10:24 am
there's no center tap
its 4 leads of 37.5VAC
I suppose I could connect it to a floating ground to get 53V rectified

Thinking about ordering the AKSA amp as a comparison to the 50W model, or even building the Rod Elliot 60W amp. I just don't want to match my own transistors, cos the amount I need to buy, + the time spent testing them would not be worth it.

RodE's amp is cheaper to construct, at $30AUS for board and 20 bux or so to populate, not including matched transistors, which will add say $15-20 (estimated), and its more tolerant of the 45-50V the transformer will produce. But AKSA has a supposedly "tube" sound which may make better comparison with the 50W chip amp. For 35US dollars, I can't find a cheaper transformer anywhere else. At the most I'll just have to unwind a few turns...
any sites on unwinding transformers?
 

downhere

Member
2001-10-03 10:24 am
haha
I suppose they would sound good and look impressive, but it wouldn't be a nice computer amplifier at all. It'll probably cause a trip when I come home and switch everything on at the same time. Not to mention cook me and heat my dinner. It could be useful in those can warming dispensers I've seen in Japan. Heats coffee and makes music... I'm filing a patent : ^)

Ah.. the commercial models and some schematics I've seen of regulated supplies are really unsuitable. 50mA max, 8W max etc etc. and efficiency slightly greater than 50%! Wouldn't it introduce more high power transistors? Some pro's and con's of regulated supplies?
 
You don't need to match the transistors for Rod's 3a amp. I didn't.
But back to power supply: So if you use both transformers, you can get +/- 53 VDC, which you figure will sag to +/- 42 VDC under a load. And you can pull 20 Amps (!) off this easily. Hmmm...
You can use a combination of Rod's capacitance multiplier (project 15) and the resistor-Zener combination you pointed out to reduce the voltage: place a zener diode across the 12K resistor. This will reduce the voltage at the base of the transistor, which will be about 1.4 V less at its emitter. The reduced voltage will appear across the transistor, which will have to dissipate a lot of power to drop this voltage at whatever current its supplying.
There's some design work that would need to be done; like choosing transistors, parts values, etc. The zeners should have their cathode at the base of the transistor.
Rod also uses this technique with a voltage regulator instead of a Zener in his project 77. In this one, he uses multiple transistors to share the load.
I suggest build the power supply in a separate box, with both unregulated and regulated (lower voltage) outputs available separately. Start with a small amp to get something going (you could even power your 50W amp from this), then later look for a more ambitious design that will use the full capability of the supply. They are out there.
Man, you could probably run like 3 Class A amplifiers from this supply - how about triamping?
 

downhere

Member
2001-10-03 10:24 am
get your own then and tell me what happens : ^)
It won't arrive for 6-8 weeks. Surface mail.
But yeah, I'm hoping for a biamped or surround system. Have a set of old kenwood speakers just sitting around. Sound pretty decent too. Still wondering if I should get 2.
not sure if the circuit breaker can take the current inrush.

2 would be reasonably cheap, 70US + 40US(shipping) to where I am. Gonna see how the 50W sounds before shelling more money & time.

Still, building a rectified power supply sounds like an interesting project for a few months.
 

downhere

Member
2001-10-03 10:24 am
I just canceled mine.

Rod Elliot sed that he wasn't sure any DIY amp uses such high rails. I also found that transistors don't usually take such high voltages without producing large amounts of heat.

The transistor problems are also largely resolved. I'm just using devices from an older amp.
 

downhere

Member
2001-10-03 10:24 am
I wouldn't mind taking a look at your schematics.
Aussieamplifiers have designs that are wayyyy out of my budget. I'm a cheapo person myself. + I always believe I need no more power than 100W. More gets me in trouble with the law.

I suppose if I really wanted to build one I'll just go get MORE transformers... something thats 3:2 would probably work well, giving 35V rails...
Anybody experienced with more stepdown transformers? I'm way out of my league in this one.
 

cp642

Member
2001-08-17 6:00 pm
Downhere,
You can check out IR's website for an application note AN-948A which happens to be a 60W/4ohm AB amp. I'm not too sure if it's there..my last visit to IR's website happens to be a couple of months back. Mail me if you want to have the schematics for the 100W class B amp... suits the transformers that you've just cancelled...:(