Using LM4780 with +/- 60V supply

[zepple]

I'm looking to use one transformer for each rail to give +/- 60V supply for a LM4780. The transformers are 21-0-21, which I would use without the center tap, giving 42*1.414 ~ 60V after rectification. Any considerations running supply significantly higher than the usual +/- 35V? I plan to use 8 ohm speakers and a single LM4780 in stereo. I've also attached the transformer specs. Thanks in advance!

[zepple]

Correction: Just realized the datasheet says |V+| + |V-| = 84V, so obviously +/-60 is out of the question unless I want a IC cookoff. So I will use the other center tapped transformer output to give me around +/-25V instead. How will 8 ohm speakers fair?

[geraldfryjr]

If you are using the transformer in the data sheet that you have presented your LM4780 will be greatly under powered and will not produce the 60watts per channel into 8 ohms that it is rated for.

You will need at least a 25v-0-25v transformer to do this if you are planing on a paralleled version you should consider a 28v-0-28v transformer.

Also the plitron in the data sheet is rated at 93VA (watts) you will need Double this to properly supply that chip enough current at its fully rated output at the least.

The output power that an amp can produce is determined from the voltage that the trans former can produce at its rated loading (rms before the rectifier stage or 70.7% of the rectified voltage).
In this case 18.5v therefore 18.5*18.5v/8=42.78 watts of output power per channel also the windings are only rated for 1.18 amps roughley 1/4 of what you will need to power that chip even at 18.5v.
Double that if you plan on using it at 4 ohms.

I hope this helps you.

jer

[zepple]

Thanks Jer. I was hoping to get away with two of those transformers, they were only 10 bucks at a surplus store. Guess I'll have to find something with a bit more power, or look at a lower powered chip.

[geraldfryjr]

hmmmm .... $10 is still a good deal!
if the 18.5v windings are of a thicker gauge than what they are rated for you could easily add a few more turns to get the higher voltages you need.
That is what I love about torriod transformers.

But they could support a chipamp of lower power easily as there are several to choose from in the 10 watt to 40 watt range.
It is definitely big enough to support those but the 1.18 amp rating is what bothers me the the most.

I have a 200watt one that I got cheap and it has a 35v winding that is fairley heavy ,but the 16v-0-16v winding that I needed was wound with 26 gauge or something and would burn up if I tried pulling 2amps continuosly from it.

But,as I mentioned $10 for a 93watt core with a 110v primary winding then adding your own winding is somthing that works very well with these things.
usaly the secondary windings are on the top of the primary and these can be removed very easily if the space is needed.
On mine only 10 turns is needed to get like 5v to 6v and with some heavy gauge wire I can pull some major amperage from it with plenty of room for more turns.

Just a few idea's! he,he

jer

[AndrewT]

Hi,
if you can open up the secondary insulation and locate the junction pin6 (labeled 8) you can easily modify this transformer.

Split the 8.3+8.3Vac centre tapped winding in to two windings.
Bring all 4 ends out when you re-wrap the insulation.
Now, you can add the 8.3V to the 18.5Vac to give a centre tapped 26.8+26.8Vac transformer. That winding is rated at ~63VA. A bit light for a power amp but it will work with 8ohm speaker, or higher.

While you have the insulation open, you can also split the other centre tap connection and bring those 4 ends out.
This gives you much more flexibility in how you can use the transformer for a variety of projects.

One of the big advantages of dual secondary windings is that that can be used alone or used in parallel, or used in series, or added to other windings.

Use the 1.2Aac windings in parallel. Use the 1.18Aac windings in parallel. You now have a 26.8Vac 63VA single supply. Do the same for the other transformer. Series connect these two supplies and you have a +-38Vdc PSU rated at ~120VA. Perfect for the higher voltage chipamps. Note you will have to use 50Vdc or higher rated capacitors in the PSU and anywhere else that they see the full PSU voltage, eg. decoupling.

You will be working with a mains transformer. If there is anything you are unsure about, then ask. Don't take risks.
Use a bulb tester in the mains primary circuit to protect the transformer from damage if you mis-wire it. It will also protect the mains fuse !

[danielwritesbac]

Originally Posted by AndrewT

. . .
Use the 1.2Aac windings in parallel. Use the 1.18Aac windings in parallel. You now have a 26.8Vac 63VA single supply. Do the same for the other transformer. Series connect these two supplies and you have a +-38Vdc PSU rated at ~120VA. Perfect for the higher voltage chipamps. Note you will have to use 50Vdc or higher rated capacitors in the PSU and anywhere else that they see the full PSU voltage, eg. decoupling.

Instead of series connecting the two transformers to make a center tap, would you use twin rectifiers for a fraction less voltage and enhanced durability?

[AndrewT]

One cannot use twin rectifiers on a centre tapped transformer to get enhanced durability.

[danielwritesbac]

Originally Posted by AndrewT

One cannot use twin rectifiers on a centre tapped transformer to get enhanced durability.

Excellent news!

Hey, I had said "instead of" creating a center tap.

Why not use the dual transformers at one rectifier per each. . . pretty much exactly like you'd do with an ordinary dual secondaries transformer--specifically with one rectifier per each secondary winding?

[AndrewT]

Daniel,
read post one and look at the pdf he attached.