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Old 29th October 2008, 04:10 AM   #1
cuibono is offline cuibono  United States
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Default 6 channel LME49830 power amp

Due to generosity of the one and only Nelson Pass at previous Burning Amp festivals, I now own 5 very large, very heavy (about 50lbs) toroidal transformers. So I need something to do with them...

Backing up: over the past little while, I've been developing some active speakers, using whatever amps I had on hand. They will soon be threeway, and I was looking at building more gainclones, but then realized that the only way I could easily using the transformers I got from Nelson was to use one of the LME chips. So here we are.

Its pretty straightforward, straight out of the National App sheets:

Here is the pdf for the power amp. Pretty nice, pretty easy. Gonna use the board layouts in the pdf, and get matched IRFP FETs from tech-diy.com

Here is the pdf for the power supply.

One of the nice things about this module is its scalability and bias flexibility. The bass drivers will be powered by one module, capable of 335W into 4 ohms, using only four mosfets, while the mid driver will be scaled down, using only two. The tweeter will also be scaled down, but will be biased much higher than the other sections - National put a nice little graph of the distortion at different bias levels - check out Fig. 8 in the power amp pdf. Not shabby.

My only questions so far mostly regard the power supply.

The toroid I plan on using is labeled 40-30-0-30-40 Vac, 15A, for a total of 1200VA. I plan on using the 40Vac taps, resulting in +/-54Vdc rectified, with about 11amps available. Now, my first question - is this transformer large enough? Might seem like a dumb question, but it will be powering all six modules. I estimate that I'll have at least 700W of actual amp power available, which should be plenty.

The next question is the capacitance - I plan on building a single power supply for all six modules. National uses 80000uF for a single channel of amplification! If I did this for six channels, it would cost about $600! That's not going to happen. Would someone please suggest a reasonable amount of capacitance? I happen to have four 25,000uF caps, rated at 75Vdc, and would like to use them. Am I pushing the voltage rating to close to the rails? Any thoughts would be appreciated, as I'd like to place my parts order as soon as possible.

The last thing is just a sanity check: I plan on using two 1' sections of heatsink from this guy. Rated at .375C/W, I will attach three modules per 1', along with two silent fans per section. That should keep the temp down fine, I hope. National recommends a .6C/W heatsink per module.

Anyway, I'm really excited about this! I think it is going to be a nice amp!
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Old 29th October 2008, 04:36 AM   #2
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Hi,
I'm building a similar project starting here. My amp modules are all the same, ~150 watts/8ohms. I'll be using a 40-0-40 750VA transformer so I envy your 1200VA!.
I'm using 2 power supply with this single transformer, one for each "channel" (bass, mid, high) The total capacitance I'm using is 5 x 4700uf or 23500uF total per rail, so I think you will easily make do with 25K. 75VDC rated caps are plenty good for 56VDC rails.
I used his heatsinks for a couple of other projects include one of those sections for a 1000 watt sub amp. I think that 2 of those will be ok.
Good luck with your project!
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Old 29th October 2008, 05:12 AM   #3
cuibono is offline cuibono  United States
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Ha! Cool!

Thanks for the post, very helpful. Your chassis looks better than mine will, for sure. I hate how the chassis is the single most expensive part.

I bet NP has more of those transformers...
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Old 29th October 2008, 12:59 PM   #4
AndrewT is online now AndrewT  Scotland
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What's the maximum mains supply voltage?
What will be the maximum open circuit voltage from your transformer if the supply is at maximum?
You will probably find that 40Vac can be used with 63Vdc capacitors for all worst conditions through your 1200VA toroid.

My 230:40Vac 1000VA transformer produce 58Vdc at the amplifier and with 4% regulation and +6% (254Vac) on the mains comes to just under 63Vdc in worst case conditions (PSU fuses blown with 254Vac on the transformer).
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Old 29th October 2008, 03:41 PM   #5
cuibono is offline cuibono  United States
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Good questions - I was little worried about the voltage on the secondaries with no load, but I don't currently have the transformer in my hands to check it. I can check the mains though. I suspect there may be some variation in it.
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Old 29th October 2008, 05:41 PM   #6
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by cuibono
I can check the mains though. I suspect there may be some variation in it.
check with your supplier.
In the UK the spec is 216Vac to 254Vac. It used to be 240Vac +-6% until the Europeans forced us to widen the tolerance.
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Old 29th October 2008, 06:39 PM   #7
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Quote:
Originally posted by AndrewT
check with your supplier.
In the UK the spec is 216Vac to 254Vac. It used to be 240Vac +-6% until the Europeans forced us to widen the tolerance.
Not the Europeans. On the British continent people are just as much buying and using an increasing amount of electric and electronic devices demanding ever more power, while at the same time protesting against any new power plants for varying reasons. On that conditions energy suppliers cannot keep up the old standards, so they embraced the opportunity to turn to the less strict IEC standards. Rumour has it that the actual tolerances (115 or 230 V +10 %/-15%) are going to be widened further, because some countries have difficulties to even stick within those limits.
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Old 29th October 2008, 06:52 PM   #8
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by pacificblue

Not the Europeans. On the British continent people are just as much buying and using an increasing amount of electric and electronic devices demanding ever more power, while at the same time protesting against any new power plants for varying reasons. On that conditions energy suppliers cannot keep up the old standards, so they embraced the opportunity to turn to the less strict IEC standards. Rumour has it that the actual tolerances (115 or 230 V +10 %/-15%) are going to be widened further, because some countries have difficulties to even stick within those limits.
I stand by my statement.
Harmonisation of the 220/240Vac is the only reason the UK had to lower the previous 226Vac limit to 216Vac.
Our systems are properly designed, properly built and well run, even after privatisation.
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Old 29th October 2008, 06:56 PM   #9
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Does it matter? His caps are 75V.
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Old 29th October 2008, 08:15 PM   #10
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by MJL21193
Does it matter? His caps are 75V.
That's why I told him how to check and what worked for me and my supply system.
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