Pushing the limit of supply voltage

enzedone · 15th August 2006, 11:08 PM

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 · 15th August 2006, 11:13 PM

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 · 16th August 2006, 12:00 AM

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

pinkmouse · 16th August 2006, 12:07 AM

You will need big heatsinks though...

enzedone · 16th August 2006, 12:13 AM

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

Brett

pwan · 16th August 2006, 05:27 AM

Quote:

Originally posted by 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!

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 · 16th August 2006, 05:53 AM

Or perhaps some chokes with a bit of DCR?

AndrewT · 16th August 2006, 08:02 AM

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 · 16th August 2006, 01:16 PM

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 · 16th August 2006, 04:17 PM

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.

15th August 2006, 11:08 PM	#1
enzedone diyAudio Member Join Date: Nov 2004 Location: New Zealand	Pushing the limit of supply voltage 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. __________________ I realized that a major part of my job was to figure out how to use technology control to create economic force, or leverage, such that money and business flowed in Microsoft's direction” — Alex St.John, father of DirectX.

15th August 2006, 11:13 PM	#2
Sherman diyAudio Member Join Date: Feb 2004 Location: Chicago area	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! __________________ --Sherman

16th August 2006, 12:00 AM	#3
enzedone diyAudio Member Join Date: Nov 2004 Location: New Zealand	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. __________________ I realized that a major part of my job was to figure out how to use technology control to create economic force, or leverage, such that money and business flowed in Microsoft's direction” — Alex St.John, father of DirectX.

16th August 2006, 12:07 AM	#4
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	You will need big heatsinks though... __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

16th August 2006, 12:13 AM	#5
enzedone diyAudio Member Join Date: Nov 2004 Location: New Zealand	Heatsink Well, I have a 3 metre length of heatsink sitting in the garage, so I think that should cover it Brett __________________ I realized that a major part of my job was to figure out how to use technology control to create economic force, or leverage, such that money and business flowed in Microsoft's direction” — Alex St.John, father of DirectX.


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

16th August 2006, 05:53 AM	#7
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	Or perhaps some chokes with a bit of DCR?

16th August 2006, 08:02 AM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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? __________________ regards Andrew T.

16th August 2006, 01:16 PM	#9
macboy diyAudio Member Join Date: Oct 2003 Location: Ottawa, Canada	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.

16th August 2006, 04:17 PM	#10
rabbitz diyAudio Member Join Date: Jul 2003 Location: Newcastle, Australia	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. __________________ No longer DIY active

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Power supply current limit for testing	kubeek	Solid State	1	12th February 2009 09:57 AM
Preamp output voltage limit	ashlar	Car Audio	2	2nd December 2008 11:41 AM
Pushing OnSemi transistors to the limit	EWorkshop1708	Solid State	23	15th March 2008 12:44 AM
Variable 6922 heater to cathode voltage limit...?	ashok	Tubes / Valves	2	13th April 2007 08:16 AM
How to make current limit for PC supply?	luka	Power Supplies	10	27th February 2007 03:31 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read