LME49810 and Leach output stage.

[DigitalJunkie]

Hey guys,

I've been slowly assembling parts for a large power amplifier,based on the LME49810 chip. The amp will be mono,for subwoofer duty.
I'm looking for input,and suggestions..Any pointers are welcomed!
My current plan is to marry the '49810 chip to the Leach output stage.

LME49810 Datasheet

Leach amp schematic

So far,here's what I've got:
-8pairs of MJ15011 and MJ15012 on a heatsink.
I'm thinking I'll use 7 pairs for the output,and one pair for the drivers. (Q16,Q17 in the Leach amp schematic.)
MJ15011/12 datasheet

-The emitter resistors are 0.698 ohm 1% 10W.
(maybe a tad large,but thats okay for now. I can change them later if everything seems well balanced/matched.)

-For Q14 and Q15 on the Leach schematic,I was thinking about using some 2SD845/2SB755 transistors (or similar) that I have in the parts bin. (got a few to choose from)

-Rail voltage will be ~+/-70 (no load/idle) I'm assuming it will drop to ~+/-60V under a decent load.. Eventually I'll find a bigger power transformer,but for now this one is okay.

-Power trans is kind of 'unknown'.It came from a junked SS amp.
Primary (120Vac) is fused at 8A. Each 50Vac tap had a 5A fuse.(There are 25Vac taps also,which were fused at 10A each.)
I think it was some kind of dual-rail design (class H?)..it had big rects,and filter caps for each set of rails. (~ +/-35V and 70V)
I won't be using the 25Vac taps.

-Main filtering is currently 8200ufx2 and 680ufx2,But I will be adding (alot) more,plus some at the output transistors.
The PSU is snubberized (1ohm and 0.047uf?),and bypassed with 0.33uf film caps,along with a 0.22uf cap across the +/- rails.
I've used a large 35A 600V bridge rectifier,with 0.1uf snubber caps across the pins.
So far in testing the noise and ripple is quite low under a moderate load. (scope shows <5mV sawtooth at idle)

I wanted to over-build the output stage by a fair amount.
It will probably only ever be used with 4 and 8 ohm loads.
Assuming that the rails hold up,thats a good 400+W into 8ohms-That's Plenty,and should be easy for 7pairs of transistors.

-I need suggestions for the Vbe circuit in general.
I'm thinking of a transistor in a TO220/TO126 package,so I can screw it to the main heatsink easily.
I've got quite a few types to choose from in the bin,and can always order/buy something if I need to.
What transistors have others used here?
Any suggestion as to what works well with the LME48910 chip?

Any other suggestions or tips are appreciated!

[boricuaso]

hi, im happy to see another LME amplifier being built.
im a be looking forward to your progress, since ill be needing a subwoofer amplifier soon.
im currently working on version 3 of my LME amp project, my amp is no where near as large as your 8 pair output stage, mine uses 2 pairs and the other version uses 3 pairs, hopefully ill learn something from your build so i can upgrade mines. lol.

one suggestion is to implement a transistor protection into your pcb in case of a output shorting. since the LME doesn't protect for shorts.
i didnt implement it on my pcb, so im going to have the protection circuit added on laters, but its really a must.

looking forward to you build.
laters

[boricuaso]

if you haven't already seen these.
they're commercially available LME49810 amps, here is a link where they include the schematics, the board, silk and measurements for tree different LME amps.

ax1300

ax1400

ax1600

at the bottom of the page they have the links to the schematics PCB etc...

he also has power supply schematics for them.

just though it may help.

laters

[ttan98]

Originally posted by boricuaso
if you haven't already seen these.
they're commercially available LME49810 amps, here is a link where they include the schematics, the board, silk and measurements for tree different LME amps.

ax1300

ax1400

ax1600

at the bottom of the page they have the links to the schematics PCB etc...

he also has power supply schematics for them.

just though it may help.

laters

well done, enjoy....

[DigitalJunkie]

It will have protection circuitry for sure.
I have a big relay that I'm going to use for the output. Some say it won't do much good on a high power/voltage design,but I figure it's better than nothing at all!

Fuses,DC protection,thermal,overload,etc. circuitry will also be used. (Based on the circuits on ESP,Project #33,etc.)

I have a couple fans mounted in a piece of plexi that I'm going to use for cooling.They even have the open-collector "third wire" for tacho,or whatever. Maybe I'll make some kind of swanky 'smart' fan controller later on,but for now it's gonna be simple.

I probably WILL use the lower rail voltage,for initial testing,and perhaps when experimenting with lower impedance loads.


The soft-start,and Aux-PSU are done and tested as well. I've got ~15.5V raw,which is regulated down to +12V and +5V for the LME49810,and whatnot.

The soft-start and output relay are operated from the raw +15V supply.
The soft-start is just two BIG 33ohm resistors in parallel,bypassed by a relay after an R/C delay. (150ohms and 1000uf on the relay coil) It works nicely,and the resistors don't even warm up at all. We'll see if adding more capacitance changes that by a significant amount.

[DigitalJunkie]

One other quick question, How hot does the LME49810 chip run?
I suppose it depends on rail voltage,and load..but is a small heatsink enough?

I might try to screw it down to the main heatsink,if I can.

[peranders]

Originally posted by DigitalJunkie
I might try to screw it down to the main heatsink,if I can.

That would be possible.

[ttan98]

Originally posted by DigitalJunkie
One other quick question, How hot does the LME49810 chip run?
I suppose it depends on rail voltage,and load..but is a small heatsink enough?

I might try to screw it down to the main heatsink,if I can.

I have built amp based on LME49810 chipset. It has a sink attached to it, it feels cool, I put thermal paste between them. I think a small and medium size heat sink is suffcient, you need to put a thermal paste between the heatsink and chipset to ensure good contact.

[DigitalJunkie]

Ok,a small heatsink is no problem.
The layout might get a bit 'funky' if I tried to mount the chip to the main heatsink. (I'm building most of this on perfboard!)

[boricuaso]

Originally posted by DigitalJunkie
One other quick question, How hot does the LME49810 chip run?
I suppose it depends on rail voltage,and load..but is a small heatsink enough?

I might try to screw it down to the main heatsink,if I can.

my LME has gotten too hot to touch before, now i use a PC Memory Heatsinks "Ramsinks".



you can use the double sided tape that comes with it, or you can remove it and use thermal paste, drill two holes and attach it to the LME chip, i have one of these but in aluminum and it works great.
i think the one on the picture above may work better then mine, since it has longer fins.
if your driver stage draws the full current that the LME has to offer it will get pretty hot. but as long as it has a heatsink it'll be fine.