+/-30v tranny too big for LMxxxx? - diyAudio
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Old 31st October 2004, 07:18 AM   #1
Riku-78 is offline Riku-78  Finland
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Default +/-30v tranny too big for LMxxxx?

I am positive that there are around 1 billion threads about the subject, but didn't find what I was looking for. I did try to search, I swear.

So here is a quick question. We all know that National chip amps run optimally with +/-40v, which means a +/-28 (40 / sqrt(2) = ~28.28) volt tranny is needed to get the maximum output. 28 volt transformers just aren't available anywhere. Using a +/-30v transformer doesn't ever appear to be recommended, because they produce +/-42.43 volts after full rectification.

Now, LM3886 and LM4780 datasheets state that maximum supply voltage with no input signal is 84 volts, i.e. +/- 42. Isn't it fair to assume that as soon as the amp starts working and draws some current, i.e. when there IS an input signal, the voltage in the supply rails drops below the 84 volts (+/- 42 volts) maximum? Just a 0.86 volt drop would be enough.

However, if I use a +/-30 volt transformer in my active speaker project, I'll face another problem. The active crossover is going to power its opamps using the same toroid as the amplifiers and a 7815/7915 or a 7812/7912 pair. These regulators don't accept over 40 volt input voltages. Can I just put a voltage divider before the regulators? The voltages on the inputs of the regulators will drift with the load, but I guess it would be rather easy to keep them in the acceptable range.
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Old 31st October 2004, 07:37 AM   #2
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Why don't you just use a transformer with 18 volt secondaries?

If you want to use a higher voltage, use a transformer with 25 volt secondaries.

If you must use a transformer with 28 volt secondaries, get one custom wound.

IMHO - it is far better to power pre amps/buffers/crossovers from their own transformer. Remember, if you don't, you will need some way of delaying the power amp or you will get a very nasty noise when you power up and may even damage your speakers (particularly as there will be no crossover components between amp and drivers).
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Old 31st October 2004, 08:11 AM   #3
Riku-78 is offline Riku-78  Finland
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True, true. There are 2 reasons to use the same power supply for both the power amps and the opamps:
1) Power suplies are expensive.
2) I'm going to build the amp in the speaker cabinet, so I don't have much room to waste.

I think I need to protect the drivers somehow. There is a circuit on the ESP site to protect the drivers from both DC and the power-on. What do you think, does that suffice to protect my drivers?

However, running the amps with +/- 35 volts is probably ok. Guess I'll be using two bridged LM3886s for the tweeter, and a parallel LM4780 for the woofer, so I should have enough power to not to fry my drivers. Even with the lower voltage.
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Old 31st October 2004, 08:22 AM   #4
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Ther is no problem in using a 30 V AC transformer, but you need good cooling of the LMxxx.

Use the 317/337 regulator to get the +/-42 V down til something like +/- 15 V. They only have a input/output dif of 37 V, but no low max. input.

(the 7815/7915 will work with a higer than 37 V input most of th e time)

Regards

Thomas
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Old 31st October 2004, 08:42 AM   #5
Riku-78 is offline Riku-78  Finland
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Thanks for the info! Just what I wanted to hear.

It appears that 317 regulators aren't available at the local electronic components store. Any idea where to get them? Or any good alternatives? Or can I couple a 337, which is available, with a 217?
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Old 31st October 2004, 09:16 AM   #6
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STOP

Don't do anything until you have read through the posts on this forum and through all this .
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Old 31st October 2004, 10:04 AM   #7
Riku-78 is offline Riku-78  Finland
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I've read your (?) site and a lot of stuff on both this forum and the ESP site.

I'm trying hard to stick with readily available designs (and even better, PCBs) as much as possible.
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Old 31st October 2004, 10:14 AM   #8
Nuuk is offline Nuuk  United Kingdom
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Quote:
I've read your (?) site and a lot of stuff on both this forum and the ESP site.
But why not stick with an 18 volt transformer then?

And you will have seen the stuff about driver protection/switch on delay.

If you want a ready-made active design that goes down to 40 Hz (easily OK for a single sub), there is a design on the site including details of the active crossover. You only have to substitute the amps for some chip amps and you are there.

I appreciate that you are trying to save money although that doesn't seem to be an issue with your search for drivers, but I would still recommend a separate transformer for the crossovers (you only need a small one).

Keep your amps close to the speakers, but not in the box. Put your crossovers in a case with a selector switch and volume control and have that close to your CDP.
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Old 31st October 2004, 10:18 AM   #9
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Why do you need to bridge two 3866s for the tweeter? A single chip should be more than enough unless you have a very low crossover.

I would recommend the use of a 25V or lower traffo, as Nuuk does. Yes, going for a 30-0-30 theoretically gives you more power, but in practise you won't even notice the difference in perceived loudness. It also gives you more margin for error, rather than pushing every component to the extreme, and the whole thing blowing up if your mains voltage fluctuates.
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Old 31st October 2004, 10:59 AM   #10
Riku-78 is offline Riku-78  Finland
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A sufficient transformer for the opamps doesn't cost that much. But they're all relatively big physically. I guess I just have to try to find a very small transformer.
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