Transformer Help

[robbobnbill]

Hi im looking to build a chip amp based on Mark Hennessy's micro amp, i just got a 225va transformer but its got 2 x 40v secondaries, is this too much voltage for an amp based on the LM4780?

or is there a way round this?

[leadbelly]

Need to know more about the transformer: does it have single or dual primaries? Are the secondary windings center-tapped?

[djk]

Absolute maximum voltage for the LM4780 is ±42V.

Your 40-0-40 transformer will have a no-load voltage of about 58V~59V (depends on the regulation factor of the transformer).

"is there a way round this?"

New transformer.

[AndrewT]

build a Leach Lo Tim.
He has optimised the design for an 80Vac Centre tapped transformer.

The 225VA is just about perfect for a 150W into 8ohm monoblock.

[robbobnbill]

Hi leadbelly thanks for your quick reply, it has 2 x 110v primaries and the secondaries are 40 - 0 - 40v

the transformer hasnt yet been delivered there is a picture here:

http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=140323887232

thanks in advanced

[robbobnbill]

that'll teach me to buy something thats cheap rather than what i actually need!! im sure ill use it for something in the future

[Artie]

Now I'm confused by others answers. Won't there be a VR, somewhere between the secondaries and the amp? Why not just regulate to whatever voltage you need? Having a higher voltage relieves you of worrying about using a low-dropout reg.

[AndrewT]

Originally posted by Artie
Won't there be a VR,

why add more complication when we are trying to understand just what AC to DC voltages mean?

[peranders]

Originally posted by Artie
Now I'm confused by others answers. Won't there be a VR, somewhere between the secondaries and the amp? Why not just regulate to whatever voltage you need? Having a higher voltage relieves you of worrying about using a low-dropout reg.

We are talking about a PSU for power amps and it's rather unusual to have stabilized supply voltage. If you still want to have a voltage regulator you must design the PSU a little but more carefully. Some engineering skills must be used.

[kscharf]

The pictures on ebay show this is a 40 / 40 volt not really a 40-0-40, though it COULD be connected that way.
The good news is that since you have two separate 40volt windings you can regulate the output using two positive regulator chips. 40v AC to the rectifier will give you about 53-58 volts under load which is too much for the popular LM338 regulator chip. There are other voltage regulator chips that can be used, but dropping the 56 volt output down to 35 volts will generate lots of heat (regulator will need a big heat sink.)

Another trick would be to reduce the voltage to the primary of the transformer but since this one is potted you can't add additional turns to the primary. You can connect the output of a smaller transformer in series with the primary to reduce the input voltage though. Since this transformer has a dual primary if you connect the windings in series and run the transformer off 120v you will end up with a 20-0-20 volt secondary (half) which will give you 28 vdc. This is perfect for 68 watts into 4 ohms or 30w into 8 ohms. Also 120w into 8 ohms with two amps bridged.