LM4780 Capable of driving Martin Logan Ascent i's??HELP??

[Dominick22]

For this, I really need some help and I am hoping that someone might be willing/able to assist me as I am still very much an amateur.

The plan is to wire two stereo LM4780 chips in parallel which would create 120 watts per chip. Then, I want to bridge those chips to make them 240 watts into one channel. Of course I would repeat this to create my second channel.

So far, I have been told to purchase the DRV164 from burr-brown. The DIYAUDIO folks say that it will be able to do the bridging. But they make everything sound so easy!lol

Ill buy them though and try.

Here is where I think someone could really help me. I have been e-mailing Peter Daniel (who sells the kits) and I have been upgrading various parts through him to make sure that this amp will be stable under the load of the ascent I’s. The problem is that he is still shy to say that it would work for sure which I understand because he doesn't want to be liable.

So I will share what I have going on so far and will remain open to any suggestions/comments you are willing to provide.

First, I am starting with this kit

http://www.audiosector.com/lm4780.shtml

This is part for part what I am running now.

And these are the upgraded parts and how he suggested I use them…

"So this is how it looks:

8 Nichion (finegold) 4700/63 =$48

1 Carbon Stereo Pot =$10

1 LM4780 chip...I blew one! =$9

Vishay 0.01u caps to place on rectifier board 8 x $0.80= $6.40

The big Nichicon should be mounted on rectifiers board. The Vishay

> 0.01 caps should be mounted there too (in place of 10u Panasonics).

> 1500/50 Panasonics should stay on amps board (directly beside the chips).
Peter"

So, I guess what I am hoping to hear from someone is either, yes, this will support the ML’s with ease…or, I would add ,such and such, to make it even more stable.

Anyway, I appreciate everyones help thus far and want to say thanks in advance for taking the time out to read this.

Dominick in New Jersey

[lgreen]

You are making a BP Chipamp, which stands for "bridge parallel." If you do a search you can find some posts on this. Not sure how many people have successfully done this so if you complete it please post your results. I am going to attempt something similar 2 amps from the one I'm working on now.

I think the ascents have a low impedance, your amp may be ok for low and mid volumes but not for high volumes. I had a 200 WPC Adcom 555 connected to martin logan sequel ii speakers and it could not play super loudly without getting super hot.

I can tell you that if it were me, I'd go beyond the 2 parallel amps that you have. I would be putting 4 in parallel and then briding the 4 with another 4. I'm not sure this has been done before, but I'm sure it will be a little tricky and generate a lot of heat.

You are going to need a big transformer and things will start getting expensive.

[Bazukaz]

I have had some problems with bridged LM3886 while driving ESL loadspeakers.
I think that the problem comes from the step-up transformer , which has very low DC resistance.Even a low DC offset can make an amp draw lots of current, as another chip tries to compensate.
I am not really sure if BPA-200 is really suitable for such a speakers.

Stability can be improved by adding output resistors.Feedback can be taken after them , so there is no voltage drop at output.
Resistors should be non-inductive.

[tiltedhalo]

A bridge parallel solution will probably not work with the LM4780, the Martin Logans are way to hard of a load to drive. At low volumes they will work for a period of time but high-output levels and long term they will probably go POOF!
I loved the sound of these chips and went on a quest for a high power version that can drive a low ohm load, ITS NOT EASY the pic below uses 2 pairs of 4 parellel LM3875t's bridged, they have tied feedback loops, propriatary DC decoupling, .1% percision resistors and some stuff I dont want to share with the general public yet. The amp below actually held up to a ML's at 90% power for about 20 minutes before it got to thermal /with fan cooling.

[tiltedhalo]

Close up pic of one parellel module to show the thermal managment aspects......

[sangram]

I don't know about MLs but my BPA works fine.

It's four 4780s, one chip's amps in parallel and two chips bridged. 440 VA transformer, rest of PS is per audiosector.com kits. I added 10,000 uF caps for 40,000 per speaker, but am removing them. They muddy up the sound too much.

Speakers are 8 ohm which are not too easy a load, dips to 2.8 ohms at times.

Source is Delta 66 balanced out. It has 20mV offset so the offset at speaker terminals is about 120 mV. A little high but manageable.

Sounds fine, runs hot (minimal heatsinking - 3 mm Extruded Al profile with lots of fins, must be about 1.5 c/w at best) but no shutdowns due to heat. Actually at higher volumes runs cooler than at idle or low levels.

[juma]

Hi tiltedhalo,
I like the way you did it, the alum. plate on the face of the chips gives you additional heatsinking surface on the source of the heat. It would be good to couple it stronger thermally with a main heatsink. Also, copper is of course better thermal conductor than alum., but you use it as a interstage between chip and main alum. heatsink, so in the end it is another stage of thermal resistance - you'd be better off without it (although it would be a minor improvement).
Big thing is that main heatsink's fins are set against the air flow (as the picture shows it).
You should:
a) adjust the main heatsink (rotate it by 90 degrees), or
b) adjust the air flow - connect two heatsinks back to back, in order to create a "tunnel" and attach the fan(s) on the one or both end(s) of the "tunnel".
That would help a lot.

Cheers

[jackinnj]

Originally posted by juma


Big thing is that main heatsink's fins are set against the air flow (as the picture shows it).
You should:
a) adjust the main heatsink (rotate it by 90 degrees), or
b) adjust the air flow - connect two heatsinks back to back, in order to create a "tunnel" and attach the fan(s) on the one or both end(s) of the "tunnel".
That would help a lot.

Cheers

like this:

[tiltedhalo]

Everybodys getting a little of toppic, IMHO these chip in any form will not without a great deal of serious engineering hold up to a close to DC resistive short, or the capacitive upper frequency loading ES speakers present.
Lets face facts these are not 250W TO3PA transistors. the LM4780 looking closely at fact sheet has a dissipation of 120W very misleading its safe area at full output is much lower, but lets say 120/2=60w whats the chips output??
The amp pics I posted previosly are one channel the two parellel boards fit into a channel that is fan cooled, the copper was used because of its quick thermal rise time to equalise the chips. I used gallium,Indium,silver at 8W/mk as a conduction medium.
This amps idle current is less then the sum of all chips, due to tied global feedback loops and feed forward compensation. The amp dosnt even get warm at idle and low power levels.
438 watts at .3% THD enough to melt most voice coils, not enough for the DC resistance of ES speakers long term I tried. The amp will succesfully drive at low to mid power levels, for how long I dont know.
If anyone manages to achieve this I would love to see it........

[jackinnj]

Originally posted by tiltedhalo
Everybodys getting a little of toppic, IMHO these chip in any form will not without a great deal of serious engineering hold up to a close to DC resistive short, or the capacitive upper frequency loading ES speakers present.

I have done extensive testing of these chips -- National Semi is not exagerating their performance capabilities -- I used an in-cal Boonton 1120 THD analyzer (floor 0.0007% THD) to produce these graphs -- and have run the devices hot for hours. the "engineering" magic is just heat sinking them properly and paying careful attention to dressing the leads: