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Old 22nd January 2008, 03:27 PM   #1
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Default Reg and Pass V Potential Divider

Hi all, I'm making a Power supply to power a car amp. Unfortunately by time my Transformer is Rectified and smoothed, its 24vac jumps to 36vdc. My amp requires 22vdc. So I need to lose some volts. Whats better, a LM317T and Pass Transistor or high power Resistors in Potential Divider format?

Both will give off heat and probably need sinking, but I'm just curious which would be better. The amp will drive a sub, so high frequency reproduction isn't a concern.

Thoughts please.

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Old 22nd January 2008, 05:10 PM   #2
Bobken is offline Bobken  United Kingdom
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Hi,

Firstly, is the suggested 22V correct here and not a 'typo', as most car amps would normally be happier with a nominal 12V DC supply?

Assuming that it is correct (and you are entirely replacing internal electronics inside the amp) my advice would be to replace the transformer with a more suitable secondary voltage type.

I wouldn't have thought that reducing such a high voltage here down to what you require, is a very satisfactory way of dealing with this, or at least it is not what I would suggest unless there are some other unknown factors influencing this decision.

Depending on the current drawn, and I guess for a sub-amp this might be quite high, there will be a lot of heat to deal with, as you suggest.

Regards,
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Old 22nd January 2008, 06:40 PM   #3
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Default Hi Bobken

I naturally assumed people would have seen my car audio post, sorry.
I need to power it, bypassing the internal switcher. Which then it requires +/- 22v. I have a Toroid I've had for years and it puts out 2 x 24vac 2x 2.5Amps. More than adequate for now, as in I don't need tonnes of SPL. I'm missing my sub and can't go spending out just now because I have to buy a new car this year, Baby on the way

So I just need a quick fix. Had been using an ATX at 12v but it died and the other spare I got won't turn on!

http://www.powerdesigners.com/InfoWe...converter.shtm

This site has a simple circuit, but the math makes me blind

So I'm looking for something easier

Thankyou

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Old 22nd January 2008, 07:04 PM   #4
Bobken is offline Bobken  United Kingdom
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Hi,

I see, and I wondered if there was some special reason for this, as, of course, it is not the ideal way of achieving what you want here.

I have done plenty of 'compromising' over the years for all kinds of reasons, so I now understand what you wish to do.

If it was me doing this, I wouldn't even entertain the idea of simple fixed resistors, and I don't think for one minute that this would be sonically preferable to using a reasonable regulator. You will also end up with some DC voltage variations with simple resistors when your mains voltage varies, which always happens in my experience.
Also, unlike when using a battery which is relatively 'quiet' electrically, in a case like this when using regulators, there should be some sonic benefits which will not be the case with resistors, due to the regulator's reduction in mains 'noise' etc.

You will need to take account of the current drawn when choosing your regulator circuits (one for each polarity), but there is a wealth of information available on this Forum (and elsewhere) which should help you with this. The heat will be a nuisance as you are dropping around 14 volts DC, but with some suitable heatsinks, this is not insurmountable.

I hope that it works out well for you.

Regards,
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Old 22nd January 2008, 07:24 PM   #5
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Default Hi Bobken

I'm glad you understand, and havn't gone off on one saying I should buy this and that and do this or that.

I think the heat issue was on my mind, knowing the amp case is a lump of heatsink, I was gonna bolt to this to dissipate the waste. The case always feels cold to touch, so assume it adequate. I'm tempted to drag a car battery indoors, WAF comes into that one though, she's highly volatile at the mo , ( I'd get beaten if she read this ), sorry Honey. Whats the chances of it leaking on the carpet. The only issue I see is ventilation whilst charging

Hmmm...................

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Old 22nd January 2008, 09:44 PM   #6
Bobken is offline Bobken  United Kingdom
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Hi,

Again this is a personal thing, but car batteries indoors are not what I consider to be a very good move.

If they are of the sealed variety, there shouldn't be any leaking of electrolyte, but many such lead/acid batteries do give off flammable gases as they charge up, so do take care if you try this.

Good luck with your endeavours.
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Old 22nd January 2008, 10:29 PM   #7
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Default Hi Bobken

I have a lowpass and Linkwitz Transform to power as well, so I I have to consider them. The ATX I was using fed those as well. I've got the car battery on charge and I have a simple inverter, which I built from http://sound.westhost.com/project95.htm a 555 jobby.
As you say, its not sealed, so I got to be careful. When the sub hit low the speaker distorted, so I'm curious if the battery's current ability, would improve this???
Then, theres this
http://www.users.qwest.net/~ptaylor/...MOTPSpage.html

I've heard its dangerous playing with microwave trannies, I've experienced HT supplies, and had many a tickle at 240 I can respect it, insulation of the cut HT lead would be a priority, did he say he started his chevvy on it?
Quote:
(Yes,I've jumpstarted my old chevy V-8 with it!)

I had some grief with my Vauxhall Corsa Diesel, I wonder if it would start my Diesel?LOL

I dunno...

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Old 17th February 2008, 10:00 AM   #8
walkura is offline walkura  Poland
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Maybe i post something you already seen .
But if not this might be the design for you .
http://sound.westhost.com/project89.htm
Relative simple and easy to adjust for as far as that counts in smps's .
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