transformer question

[sakis]

i have a very nice altair amplifier ...its the MF16...that is a seriously big amplifier and also was a very good construction ...

unfortunatelly this amp is burned beyond repair ....or even though someone decides to repair device like that it will probably not be cost effective ....boards are burned from thunder ....halls here and there all mosfets are gone and so on and on ....


i am thinking on bulding a brother of qausi inside there but voltage is too high .... expect 85+85 volts with no load conditions

can i wind some more winds in the primary to low the voltage on the secondary ???? any other sugestion ???

[analog_sa]

Originally posted by sakis

can i wind some more winds in the primary to low the voltage on the secondary ???? any other sugestion ???

You probably can if there is lots of room. Would it not be more practical to rewind the secondary?

[AndrewT]

If you are very confident on the quality of your workmanship, then you could add extra primary windings. You will need lots of turns to make a significant difference. The enameled wire must be at least the same diameter as the original primary.
The safest place for the primary is UNDER the secondary, but that is a lot of work. Take care!

This will reduce the VA of the transformer, but in return you will increase the off load and on load efficiencies and you will reduce the regulation. The transformer will as a result run cooler and should perform better upto it's original current rating.

[Geoff H]

Rather than rewinding the transformer you could try a smaller transformer connected as an auto transformer to reduce the input and thus the no load voltage.

As an example a 240 / 30 volt transformer will reduce the voltage by about 12.5%. The VA of the autotransformer only needs to be a proportion of the main transformer.

Regards

[wg_ski]

Originally posted by analog_sa


You probably can if there is lots of room. Would it not be more practical to rewind the secondary?

Probably. To lower the voltage by more than 10% or so, it's better to just take some turns off the secondary. For a small output voltage adjust, the adding a few primary turns may be easier and you shouldn't have to take the trafo apart. For toroids, you can wind a few turns of solid-core THHN and add it to the primary in phase. The more turns you add in this manner however, the worse the coupling becomes and the more leakage reactance and poorer regulation. I've even done this with EI's where there was a little room between the bobbin and core.

I would not recommend rewinding a primary from scratch unless you have experience doing it and the proper winding equipment. It's way too easy to scrape insulation off or to get too much voltage between turns. On lower-voltage secondaries mistakes are more easily forgiven.

[bear]

Two other possibilities:

- regulate the whole B+ down to what you need
- regulate the driver board's B+ down to the spec'd voltage, BUT put in an output stage that can handle the full B+ voltage!

Now, what happens? The driver board can never drive the output into clipping against the rail. So the clipping characteristic of the amp will be determined by the driver's clipping only. Heresy, but no reason it won't work fine.



_-_-bear

Third possibility <edit>: redesign the amp in question to handle the extra voltage... the same idea applies, you need the voltage gain to be able to swing the requisite drive to run the outputs to the rails (the usual method) so then you can use higher voltage supply... you might be able to get away with keeping the more difficult to adjust front end at a lower voltage (regulated, vdropped, or separate supply), as long as you have enough gain in subsequent stages available.

[sakis]

thanks everybody ....i will consider re winding the trafo to lower voltage ....

it seems to me that either adding from the primary side and/or removing from the secondary will also effect the power rating of the trafo .... so in this case will be easier to rewind the trafo and keep the charactirestics if not upgrade them ...

will see
still i will make this a quasi since semis i get are so cheap and the rest of the parts is available ...

regards sakis

[ostripper]

Why not port the BO quasi to handle 85V? I use same OPS / input
with +- 80v all day !

Make all rail caps 100V , Change T2/3 to 2sa992(120v) and T4 to another 2sc1845(120v). use mje15032 / 33's for drivers(250v) , make r8 240- 270R.
A lot easier than rewinding the trafo. With 27K Rfb you will never clip , but be aware , at the higher rails you will be approaching SOA limits.
OS

[wg_ski]

Third possibility <edit>: redesign the amp in question to handle the extra voltage...

+1. +/-85V makes a really nice 250W/ch amplifier. Didn't you say you could get MJW21194's cheap? Put 10 in parallel and they'll handle it.

[megajocke]

Agree. +-85V isn't that high. Actually, less than 10 pairs should be ok. How many will fit?

As a comparison, the Crest CA4 has +-90V rails and just 4 pairs of 2SC3281/2SA1302 in series-parallell (to get better SOA at high voltage). MJW21194 has better high-voltage SOA so series-parallell won't be necessary. Also, these are rated 200W instead of 150W like 2SC3281.