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Old 28th June 2006, 08:28 PM   #1
ahrsix is offline ahrsix  United Kingdom
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Default Power Supply Query

Iím looking for a power supply
Many years ago I made up a circuit I found in an old TTL Cook Book. I had a heart Bye Pass operation ten years ago and I think they took most of my brains out also, Iím now 77 and I need that circuit again, It consisted of a bridge rectifier, 24v LM7824 voltage regulator, resistor, couple of caps and a LM3055, if I remember right, And from a 35v transformer gave an output of 24v DC at 10amps. Does any one know the circuit as I would like try PNP NPN transistors in a dual supply as I have two 35v dual secondary 500va, trannys and two 50v dual secondary 650vaĎs that I would like to use in some amps (LM4702 do you think?) all I can think of to say here is ďPLEASEĒ. Tarra For Now.
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Old 28th June 2006, 09:45 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the transformers you have would better suit a higher voltage amplifier.

35Vac will give about 50Vdc and 50Vac about 72Vdc.

I assume 4702 are chip amps?
What voltage can they use into your proposed speaker impedance?
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Old 29th June 2006, 09:42 PM   #3
ahrsix is offline ahrsix  United Kingdom
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Default Power Supply Query

Hello Andrew. Thank you for your comments, Iím starting my first amp and it will be the LM4780, I will make the LM4702 next, this will be the first amp I have built so I need to find tone and volume control circuits as well as everything else. I have very little knowledge on amplifiers and any help is very welcome. I am unsure of the safe working voltages at this time. I have built two dual speakered eight-ohm cabs using 200watt speakers so I think they will do. Both the 4702 and 4780 are chip amps, Iíll get stuck into the data sheets and see what they have to say. Thanks again for your speedy reply. Roy.

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Old 30th June 2006, 12:59 AM   #4
lndm is offline lndm  Australia
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There occasionally is confusion over dual secondaries so I migh ask: do you have dual 35VAC secondaries or a centre tapped 35VAC winding. The latter would give dual DC rails of up to 24 and a half volts.
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Old 30th June 2006, 01:50 AM   #5
ahrsix is offline ahrsix  United Kingdom
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Default Power supply query

Yes I do have dual secondary 35v winddings. Could you tell me if the BYW80-200 fast recovery diode is o/k to use in a power supply in full bridge mode for the LM4780 chip, people seem to be using the MUR860 but its about twice the price of the BYW-80-200, how many would I need? I know this request sounds stupid to of you butÖÖÖÖ.
Dad how high is the London Tower? Father, I donít know son,
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Dad you donít mind me asking you questions do you? Of course not son how else will you learn.

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Old 30th June 2006, 07:21 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
I just went through the data sheet. http://cache.national.com/ds/LM/LM4780.pdf
It appears that +-40V when delivering max power to the load is the absolute maximum rail voltage the chip amp can use when on 8r loading. It will take +-42V when lightly loaded and survive +-47V when no input or output signal is present.

If using these highest voltage supplies you will need to carefully follow NS recommendations for heatsink capacity and do not er on the small side.

If you are prepared to settle for a lower supply voltage then the heatsink will be smaller and the chip is going to be more tolerant of abuse and difficult speakers.

I would recommend 2 by 25Vac transformer of about 200VA for a stereo pair of lm4780, if you want maximum reliable power with minimum intrusion from the protection circuits. For difficult speakers 22 to 24Vac might be better.

Can I suggest that, until this first amp is up and running, you build a power amp only and use your existing source controller for volume adjustment and tone control, if any.

It becomes a much bigger project to build an integrated amplifier with all the accessory features you may want.

You can add a control pre-amp later, if you have a mind to.
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Old 30th June 2006, 10:12 AM   #7
ahrsix is offline ahrsix  United Kingdom
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Default Power Supply Query

Hello, youíre up early. Yes my transformers are to powerful that is why Iím trying to find the control circuit in my original post so I can make the outputs variable voltage dc regulated with that circuit! The amps are for my wifeís rock and roll band and they need 100watts +, volume and tone controls are a must have. Thanks again for your valued help.

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Old 1st July 2006, 07:26 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
PA use!
I suggest you keep well clear of a chip amp.
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Old 1st July 2006, 08:48 AM   #9
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I second Andrew wholeheartedly. I also strongly suggest you simply purchase the correct transformer, as it will likely be cheaper than building a regulator to drop the voltage, which will also be big, heavy and hot.
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Old 1st July 2006, 08:48 AM   #10
lndm is offline lndm  Australia
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Here might be an example of what you're looking for (down the page)
http://www.shine7.com/audio/gainclone.htm

I haven't used this so I can't vouch for it. You might consider whether you'd want added pass transistors.

On the other hand, there are probably good examples on this site that I may have missed.

EDIT: Hi richie00boy, overpost. Good point BTW.
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