LM4780 ESL Amp

Dominick22 · 27th January 2006, 07:11 PM

Is there a way to create a "chokepopint" so that the amp only sees a max of "2" ohm impedance at most?

What is the negative to this?

Also, What is the max transformer that the LM4780 can handle?

Thanks,

Dominick

I_Forgot · 27th January 2006, 11:23 PM

Sure, put a 2 Ohm resistor in parallel with the speaker.

But.... why?

The "negative" is that the impedance will go below 2 Ohms, and few amps handle that without getting hot or even commiting thermal suicide.

I_F

pinkmouse · 27th January 2006, 11:27 PM

Seconded, why?

As for the traffo, I'm running on a 30-0-30, about as high as you can go. This needs good heatsinking.

Dominick22 · 28th January 2006, 12:03 AM

Well,

I guess i posed my question wrong.

What I am trying to do is create an ohm minimum chokepoint.

Something that will PREVENT the Impedance load from going Below 2 ohms.

I havent settled on this value yet, but I am just wondering if there is a way to do it.

WHY?????
The Martin Logan speakers the amp will be driving can go down to .1 ohm at high frequencies.

Thanks,
Dominick

Dominick22 · 28th January 2006, 12:05 AM

pinkmouse,

What would the VA rating be with the 30-0-30. Could I push it to 500 VA....or father as long as I remain at 30-0-30???

Thanks

mwmkravchenko · 28th January 2006, 12:56 AM

A proper transformer will keep the impedance at an acceptable level. The transformer used for stepping up the voltage off of the amp that is.

MArk

tiltedhalo · 28th January 2006, 01:02 AM

Back down the supply voltage to +/- 22VDC, that will give you a little cushion for resistive dissipation in the amp. If you are useing the lm4780 in parellel, and I imagine you are, increase the output resistors to 1ohm 5W 1% non- inductive resistors, the amp will double its output current and stay within its dissipation limits, use a zobel of 2.7 3W and 1NF, and a 1MH coil that can dissipate peak current of the amp in rarellel with a 8ohm 3W resistor to unload capacitance. You wont drive them hard but the amps will work. PS lots of heat sink........

Dominick22 · 28th January 2006, 02:20 AM

tiltedhalo,

What an answer...thanks a ton.

Everyone else has suggested maxing out the transformer values..I wonder why?

Your right, I am going to be running 2 paralled LM4780 chips per channel.

Here is where you lost me though...

"and a 1MH coil that can dissipate peak current of the amp in rarellel with a 8ohm 3W resistor to unload capacitance."

I will do everything you mentioned, but what is a 1MH coil and where on the board am I placing the 8ohm 3w resister????

Also, just so you know, I upgraded the 50V 1500uf caps to the Nichion 63V 4700uf caps thanks to Peter at audiosector.com.

Thanks for the great response,
Dominick

tiltedhalo · 28th January 2006, 04:33 AM

This might help, and if you need more voltage to load you could bridge two parellel chips together with a DRV134

Peter Daniel · 28th January 2006, 04:49 AM

Bridging is not recommended here, as the speakers impedance is too low.

2 x 22V transformer should be fine, and that's what I'm using with all my amps (though those are mostly LM3875)

You could still use higher voltage (especially if chips are run in parallel), but because of your "difficult " speakers it's probably better to keep it low.

27th January 2006, 07:11 PM	#1
Dominick22 diyAudio Member Join Date: Aug 2005	LM4780 ESL Amp Is there a way to create a "chokepopint" so that the amp only sees a max of "2" ohm impedance at most? What is the negative to this? Also, What is the max transformer that the LM4780 can handle? Thanks, Dominick __________________ A real amateur enthusiast!

27th January 2006, 11:27 PM	#3
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Seconded, why? As for the traffo, I'm running on a 30-0-30, about as high as you can go. This needs good heatsinking. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

28th January 2006, 12:03 AM	#4
Dominick22 diyAudio Member Join Date: Aug 2005	Well, I guess i posed my question wrong. What I am trying to do is create an ohm minimum chokepoint. Something that will PREVENT the Impedance load from going Below 2 ohms. I havent settled on this value yet, but I am just wondering if there is a way to do it. WHY????? The Martin Logan speakers the amp will be driving can go down to .1 ohm at high frequencies. Thanks, Dominick __________________ A real amateur enthusiast!

28th January 2006, 12:05 AM	#5
Dominick22 diyAudio Member Join Date: Aug 2005	pinkmouse, What would the VA rating be with the 30-0-30. Could I push it to 500 VA....or father as long as I remain at 30-0-30??? Thanks __________________ A real amateur enthusiast!

28th January 2006, 12:56 AM	#6
mwmkravchenko diyAudio Member Join Date: Jan 2002 Location: Perth Canada	Transformer ! A proper transformer will keep the impedance at an acceptable level. The transformer used for stepping up the voltage off of the amp that is. MArk __________________ Mark


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27th January 2006, 11:23 PM	#2
I_Forgot diyAudio Member Join Date: Jan 2005 Location: Phoenix, Az.	Sure, put a 2 Ohm resistor in parallel with the speaker. But.... why? The "negative" is that the impedance will go below 2 Ohms, and few amps handle that without getting hot or even commiting thermal suicide. I_F

28th January 2006, 01:02 AM	#7
tiltedhalo diyAudio Member Join Date: Sep 2005 Location: Melbourne Florida	ESL's and the lm4780 Back down the supply voltage to +/- 22VDC, that will give you a little cushion for resistive dissipation in the amp. If you are useing the lm4780 in parellel, and I imagine you are, increase the output resistors to 1ohm 5W 1% non- inductive resistors, the amp will double its output current and stay within its dissipation limits, use a zobel of 2.7 3W and 1NF, and a 1MH coil that can dissipate peak current of the amp in rarellel with a 8ohm 3W resistor to unload capacitance. You wont drive them hard but the amps will work. PS lots of heat sink........

28th January 2006, 02:20 AM	#8
Dominick22 diyAudio Member Join Date: Aug 2005	tiltedhalo, What an answer...thanks a ton. Everyone else has suggested maxing out the transformer values..I wonder why? Your right, I am going to be running 2 paralled LM4780 chips per channel. Here is where you lost me though... "and a 1MH coil that can dissipate peak current of the amp in rarellel with a 8ohm 3W resistor to unload capacitance." I will do everything you mentioned, but what is a 1MH coil and where on the board am I placing the 8ohm 3w resister???? Also, just so you know, I upgraded the 50V 1500uf caps to the Nichion 63V 4700uf caps thanks to Peter at audiosector.com. Thanks for the great response, Dominick __________________ A real amateur enthusiast!

28th January 2006, 04:33 AM	#9
tiltedhalo diyAudio Member Join Date: Sep 2005 Location: Melbourne Florida	LM4780 ESL amp This might help, and if you need more voltage to load you could bridge two parellel chips together with a DRV134

28th January 2006, 04:49 AM	#10
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	Re: LM4780 ESL Amp Bridging is not recommended here, as the speakers impedance is too low. 2 x 22V transformer should be fine, and that's what I'm using with all my amps (though those are mostly LM3875) You could still use higher voltage (especially if chips are run in parallel), but because of your "difficult " speakers it's probably better to keep it low. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

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