Pushing the limit of supply voltage

[enzedone]

I have a tranny that'll give me 42.4VDC (+/-) unloaded. I think this is right on the limit for a LM3785?

But I got the thing free out of an old amp.

 

[Sherman]

The spec sheet says 84 volts max so you are .8 V over the limit.

However since the 42.4 is unloaded you can count on a few volts dropping when you connect it to your amp. You will be under the limit.

Build it!

 

[enzedone]

Build and check

Yeah, I'll build the power supply up and then check again, just to make sure it's not too far out.

 

[pinkmouse]

You will need big heatsinks though...

 

[enzedone]

Heatsink

Well, I have a 3 metre length of heatsink sitting in the garage, so I think that should cover it




Brett

 

[pwan]

The spec sheet says 84 volts max so you are .8 V over the limit.

However since the 42.4 is unloaded you can count on a few volts dropping when you connect it to your amp. You will be under the limit.

Build it!

just wonder if using resistor ( one or more "1 Ohm 10 W" per rial) in series will help more voltage dropping and a bit lower ripple?

 

[DigitalJunkie]

Or perhaps some chokes with a bit of DCR?

 

[AndrewT]

Hi,
using the higher supply voltages has the consequence of demanding you use the higher load impedances as well.

Effectively 4 to 8ohm and 6ohm speakers are out the window.
Some demanding 8ohm may also overstress the chips.

12ohm or 16ohm speakers become your ideal match. Headphones?

 

[macboy]

The LM3886 will tolerate extreme supply voltages better than the LM3875. The LM3875 doesn't have a lot of wiggle room (its safe operating area is more limited than the LM3886).

If you parallel two LM3875s and drive no less than 8 ohms, you may get decent results.

You could also try lowering the voltage using a capacitor multiplier or a regulator. Either one will help clean up the power supply in addition to lowering the voltage to a more ideal point.

 

[rabbitz]

Throwing a larger heatsink at it will work to a certain limit and you can forget about it for a TF package but the TA package is on the limit with a 6R load.

Reducing the voltage is a better way to go and have a look around for the Overture Design Guide spreadsheet at National Semi Conductor. That will give you a guide at what you can and can't do.

 

[enzedone]

strange

Checked the tranny today. secondaries are 30Vac. But after hook up to bridge rectifier I get 29.2Vdc. Should have around 42Vdc
if I'm right.

If this ranny is stuffed I'll use my others from my Lm4780 project. They are 300VA 25-0-25, so they'll do just fine.

Brett.

 

[macboy]

Checked the tranny today. secondaries are 30Vac. But after hook up to bridge rectifier I get 29.2Vdc. Should have around 42Vdc
if I'm right.

Yeah, you're right. But you forget to connect some filter caps to the outputs of the bridge right? You need those to get the proper reading. Otherwise the meter will see a rectified sine wave which peaks at around 42 V but with an RMS voltage around 30 V minus the diode drop, which is exactly what you measured. After adding the filter caps, this changes to be a steady 42 VDC.

 

[AndrewT]

Hi,
the 25-0-25 transformer seems a better match.
The 300VA will easily drive two channels, or even three channels. It might struggle with four channels.

Use a separate rectifier and smoothing bank to each channel.

 

[cpemma]

Throwing a larger heatsink at it will work to a certain limit and you can forget about it for a TF package but the TA package is on the limit with a 6R load.

Reducing the voltage is a better way to go and have a look around for the Overture Design Guide spreadsheet at National Semi Conductor. That will give you a guide at what you can and can't do.

The Overture's TF/TA heat sink figures are based on grease only, no insulating washer (not needed with the TF); an average-quality washer will also put the TA out of bounds, a top-quality one you'd need to do the maths very carefully and/or watch the volume control.

However, with a Class B amp in a normal domestic situation, we may make too big a fuss over heatsinking to cater for the maximum design output. Just because it's there, we don't have to use it. And with most music source, we don't.