LME49811 Questions

[db!]

Hey guys,

I am planning a LME49811 build since I already have the IC and a few 120VCT toroids that I'd like to put to good use. These transformers should yield approximately 180VDC or +/- 90VDC; this is very close to the limit of LME at 200V. Since I have no experience in designing an output stage, I've been looking around the web and I stumbled upon this:

300/500W Subwoofer Power Amplifier

The diagram I am referring to is the second one down. Now for the big question; will it work properly without any extreme modifications/experimentation?

I plan to use a much higher voltage (+/- 90VDC) instead of the +/- 56v in the original circuit; are there going to be any problems? By running it at my intended voltage, would I be noticeably reducing the life of the transistors?

In case circuit above is not suitable, are there any other designs that are more ideally suited? I would prefer something that doesn't use Japanese/exotic transistors; it's hard to find genuine parts and they tend to go obsolete rather fast.

I apologize my questions seem trivial/stupid; this is quite a step up from my previous gainclone project (LM3875).

Any advice/info would be greatly appreciated!

[Sebastiaan]

From Rod's site

"Do not even think about using supplies over ±70V, and don't bother asking me if it is ok - it isn't!"

So don't! What you can consider though to reach higher power levels is to build two of those amplifiers and bridge them. But then you need a even lower power supply voltage.

With kind regards,
Bas

[AndrewT]

6pair of mjl4281/301 will just about do 400W into 8ohm from +-87Vdc.
8pair will do about 750W into 4ohm if you can keep them cool enough.

I assumed +-75mF, 4% regulation, Re=0r27, 60degree phase angle, Tc<45degC

6pair of MJ21193/4 will do 4ohms, Tc<60degC

[db!]

Originally Posted by Sebastiaan

From Rod's site

"Do not even think about using supplies over ±70V, and don't bother asking me if it is ok - it isn't!"

So don't! What you can consider though to reach higher power levels is to build two of those amplifiers and bridge them. But then you need a even lower power supply voltage.

Oh okay, I was thinking that since the transistors are rated for 350V 250W dissipation, the extra voltage shouldn't hurt them.

AndrewT - Can I wire the transistors the same way as seen in the schematic?

[bobodioulasso]

There is one way to use your transformers which is paralleling the secondaries in order to supply bridged amplifiers using a floating ground. This halves the voltage between the two rails...
in your case +/- 45v rails.

[tomchr]

Originally Posted by db!

I am planning a LME49811 build since I already have the IC and a few 120VCT toroids that I'd like to put to good use. These transformers should yield approximately 180VDC or +/- 90VDC; this is very close to the limit of LME at 200V.

So you're messing with 180 V DC. That's lethal voltage. Notice Rod's warning in red - DO NOT ATTEMPT THIS AS A FIRST PROJECT. At 180 V a good toroid will deliver enough power to make power transistors EXPLODE. Also an accidental slip by an oscilloscope probe can result in a short circuit which at the voltage, power levels you're dealing with will result in vaporized metal flying into places it isn't normally welcome - such as your eyes... If this is one of your first projects, I would highly recommend starting out at a lower voltage, say, +/-25 V from a current limited lab power supply. In fact, I start out all my amplifier projects that way to limit the risk of catastrophic failure. You can always step up the voltage once you've verified that the circuit works. You have been warned!

That said, as long as you can guarantee that your supply won't get above 200 V under the worst case conditions, there should not be any issue using the LME49811 IC for your amp. There will be some margin on the specified 200 V abs max for the LME49811. Look at the abs max number as the absolute maximum continuous operating voltage ever reached in your design under worst case conditions (including power line fluctuations).

Originally Posted by db!

I plan to use a much higher voltage (+/- 90VDC) instead of the +/- 56v in the original circuit; are there going to be any problems? By running it at my intended voltage, would I be noticeably reducing the life of the transistors?

It's not just the voltage rating of the output transistors that determine the max operating voltage. It's also the ratings of the small signal transistors. If you raise the supply voltage, the different stages may run at higher current, thus, dissipate more power. This will increase device temperature which will impact device lifetime -- and in worst case flat out fry the devices.

The topology for Rod's amplifier is covered stage-by-stage in many college textbooks. My favorite is Sedra/Smith. Given the voltage levels that you throw out, I would strongly recommend that you fully understand the circuit - and preferably run some simulations of it at the voltages you mention - before building and testing it. If Sedra/Smith is above your head, then I'd recommend building a kit such as the ones offered by Panson Audio. Panson is pretty active in this forum as well. I have no personal experience with his kits but judging from his contributions on the forum, I have no reservations about recommending them.

Good luck!

~Tom

[db!]

There is one way to use your transformers which is paralleling the secondaries in order to supply bridged amplifiers using a floating ground. This halves the voltage between the two rails...
in your case +/- 45v rails.

I would prefer not do deal with bridging at this point, this is only my second solid state amp and I don't really want the extra complications of phase matching, etc that is involved with bridging.

I would like to drive the LME49811 close to it's maximum rating to get maximum output.The only problem I have is designing/finding an output stage that can put out 500W.

When you mention paralleling the secondaries of the transformer, do you mean paralleling the 2 transformers? The toroids I have only have a single 120VCT secondary...

Tomchr - I am aware of the the voltages I intend to operate the amp at are dangerous. I've been in the HV scene for a while so I am fully aware of all the safety precautions. I'll probably use a variac to slowly increase the voltage before hitting it with the full 180V. As for Rod's circuit, I am only interested in using the output stage for the LME.

I appreciate all the info guys!

[tomchr]

Originally Posted by db!

I would prefer not do deal with bridging at this point, this is only my second solid state amp and I don't really want the extra complications of phase matching, etc that is involved with bridging.

All you really need is a phase inversion. If you design the LME amp with differential input all you need to do is to swap the two inputs between the two channels to get the phase inversion.

Originally Posted by db!

The only problem I have is designing/finding an output stage that can put out 500W.

I'd look at the ThermalTrak devices. There's a thread going elsewhere. I think it's in the first pages of the "comparing LME49810, 49811, 49830" thread. There are some schematics you can look at, though you'll need to use many devices in parallel. I think B&D Enterprises have them, though, they're kinda spendy.

Originally Posted by db!

I am aware of the the voltages I intend to operate the amp at are dangerous. I've been in the HV scene for a while so I am fully aware of all the safety precautions.

That makes me feel better. What you'll be looking at is quite a challenge. One challenge will be to make sure the Vbe multiplier tracks the temperature of the output transistors to prevent thermal run-away. Hence, I suggest looking at the ThermalTrak devices (STD03N, STD03P for example). Another challenge will be the thermal management and making sure the output devices don't fry at max output power. Then, of course, there's the challenge of designing the circuit...

~Tom

[bobodioulasso]

The toroids I have only have a single 120VCT secondary...

Cant you access to two 60v secondaries? It is generally not impossible with toroids.

[bobodioulasso]

Is your project intended for PA or HIFI?
If HIFI you can google for Rotel RB1090 and get the schematic. I have copied this output stage designed for 90 volts rails and i use a LME49810 as driver. I must say i have been running it at 63v rails only for now.