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Old 7th October 2009, 01:39 AM   #1
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Default When transformer has too much voltage

Hi, I am building a 3886 stereo amp just for fun and to compare with a D-class. After rectification my transformer has 6 Volts or so too many. I know I can buy another one but how can we loose voltage in an elegant manner. 10 diodes in a string will do it but it is a bit messy. I could loose 10V and still be happy.

Also am I correct in assuming that derating the power supply volts for 4 Ohm operation is only necessary if the amp is asked to deliver near full power.

Thanks for the help guys.
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Old 7th October 2009, 01:50 AM   #2
ttan98 is offline ttan98  Australia
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Originally Posted by pheonix358 View Post
Hi, I am building a 3886 stereo amp just for fun and to compare with a D-class. After rectification my transformer has 6 Volts or so too many. I know I can buy another one but how can we loose voltage in an elegant manner. 10 diodes in a string will do it but it is a bit messy. I could loose 10V and still be happy.

Also am I correct in assuming that derating the power supply volts for 4 Ohm operation is only necessary if the amp is asked to deliver near full power.

Thanks for the help guys.
You didn't tell us your DC voltage after rectification. My DC volt after rect. for my LM3886 is 34Vdc. It is ok at this voltage, a little higher(2-4V) does no harm, the chip would get a little warmer, use adequate heatsink.
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Old 7th October 2009, 01:53 AM   #3
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You didn't tell us your DC voltage after rectification. My DC volt after rect. for my LM3886 is 34Vdc. It is ok at this voltage, a little higher(2-4V) does no harm, the chip would get a little warmer, use adequate heatsink.
The transformer puts out 33VAC unloaded. Bit high! Should be around 46VDC after rectification and that is 4VDC above the chips max.

Thanks for the help.

Terry
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Old 7th October 2009, 06:52 AM   #4
ratza is offline ratza  Romania
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Not elegant at all, but you can unwind turns from secondary until the voltage suits your needs.
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Old 7th October 2009, 07:10 AM   #5
simon5 is offline simon5  Canada
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You could use a Zener diode rated at 10V, that can take all the power you want it to take.
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Old 7th October 2009, 08:28 AM   #6
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Originally Posted by pheonix358 View Post
Hi, I am building a 3886 stereo amp just for fun and to compare with a D-class. After rectification my transformer has 6 Volts or so too many. I know I can buy another one but how can we loose voltage in an elegant manner. 10 diodes in a string will do it but it is a bit messy. I could loose 10V and still be happy.

Also am I correct in assuming that derating the power supply volts for 4 Ohm operation is only necessary if the amp is asked to deliver near full power.

Thanks for the help guys.

An apparently cumbersome metod to "loose" 10V is to stabilize the voltage.

It is an elegant way from both an electronic and audio point of view:

You get several adavantages:

- you can set the power supply output at the voltage you prefer/need (ie you can match the voltage for an 8 or 4 ohms load)

- your amplifier works from a power supply free of ripple (it seems beneficial for the sound)

- the work, the parts and the effort you need are well worth the result you get.

Be aware to use a stabilized audio friendly power supply.

I suggest you a Krell like power supply http://electronics-diy.com/electroni...tic.php?id=600 look at the middle of the page the hand drawn schematic.

Despite his (apparent) simplicity I feel that it is more audio friendly than most feedback stabilized power supply

Regards
diy_audio_fo
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Old 7th October 2009, 08:35 AM   #7
Shaun is offline Shaun  South Africa
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You can add on turns and put it in series with the primary, subtractively. sometimes it requires a couple of tens of turns to get sufficient voltage on the new windings. Just ensure you add some insulation tape over the new turns.
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Old 7th October 2009, 09:13 AM   #8
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Thank you all, The transformer is tory and fully encapsulated in steel and then potted. Playing with the windings is not possible.

I was thinking of regulating it to loose the excess. Would I need something that ah, robust for a chip amp. I know the 1A series would not handle it but surely it doesn't need all that mucg regulation. I do like the idea of being able to have a switch on the back to switch voltages for 8 or 4 Ohm. Thanks guys.
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Old 7th October 2009, 09:29 AM   #9
Shaun is offline Shaun  South Africa
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You could also use a second small transformer with its secondary in series with the main transformer's primary.
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