help for PSU

[f.pelosa]

hello,
I salvaged a PSU from an old amp and I would like to build one for a Leach Amp.
I'm wondering whether it's worth to build a regulated pwr supply, or the combination transformer+rectifier+capacitor would be nice enough.

So: is it worth to put $ and time into a regulated psu? Or is it worth to put other capacitors in series to those already there?

Also, knowing that "690W" was written on the back of the salvaged amplifier (a Technics SU VX-700), that the primary fuse is a T3 15A 250V, and the whole PSU's output was 55V,0,-55V, how can I figure out the VA rating of the transf?

thanks all
Ale

[Andy5112405]

The general rule of thumb for a transformer is that they will regulate fairly well within 5% of their rated output voltage up to their rated current output.

A simple test is to measure the AC voltage under increasing load conditions until the output voltage drops to about 95% of the normal load voltage.

You will need a heap of large power resistors.

The normal AC voltage should be in the order of 55/1.414 = 39VAC or 78VAC across the whole secondary.

Load each secondary to about 100VA with 15 Ohms of resistance. Measure the AC Voltage. Add another 15 Ohms and measure AC Voltage again. Keep adding 15 Ohm resistors until the AC Voltage starts to drop. A few volts is normal, you are looking for a drop to around 35V or so on each secondary.

Each 15 Ohm resistor is roughly equivalent to 100VA so if you see the drop with 6 resistors the VA rating is around 600VA.

Bear in mind each 15 Ohm resistor will need to be made up of resistors that can dissipate 100W.

Another way of achieving the same result is by using 100W 110V domestic light bulbs connected across both secondaries in series.

[AndrewT]

Hi,
using the 3.15A fuse rating and the 690W power claim, I'd guess the transformer is somewhere from 500VA to 750VA.

This is big enough to power ~250W to 750W of ClassAB domestic use maximum power output.
It will easily power two channels of Leach. Expect +-55Vdc to give ~150W to 160W into 8r0.

[f.pelosa]

wow thanks a lot for the answers!

I'd be curious to see your opinion on the issue regulated psu vs. standard psu..

[AndrewT]

Don't regulate until you know how to design and debug an amplifier.
See all my previous posts on the subject.