Rectifier options for stereo Daniels LM3875 with huge center tap transformer - diyAudio
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Old 21st May 2010, 11:33 PM   #1
diyAudio Member
Join Date: May 2010
Lightbulb Rectifier options for stereo Daniels LM3875 with huge center tap transformer

I got a great deal on a 230VAC 160VA toroid from Newark, not realizing that the VA rating would drop running on 115 mains. I now understand that . So I picked up one of these beasts:

Audio Power Transformer 550VA, 59V AC output, Great For Amplifier Projects

The manual and many posts here are clear on how to hook up the center tap. Now, my question:

Since I have such an overvalued transformer, is there any benefit to using both rectifier boards? And if so, how would I hook it up? I understand the risk of thermal/current runaway with parallel rectifiers, so I'm wondering if there would be a better way to do it... or maybe the center tap admits some simple solution. What do you think?
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Old 23rd May 2010, 07:15 AM   #2
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Join Date: May 2008
Not much sense in using two boards in parallel. Rather use one per amplifier channel.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 24th May 2010, 12:55 AM   #3
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That's sort of what I mean. The basic setup illustrated for a center tap transformer has one rectifier board for two channels. I'm wondering if it's appropriate to use two rectifier boards in parallel with the same transistor, one for each audio channel.
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Old 24th May 2010, 06:11 AM   #4
parb is offline parb  United States
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Join Date: Dec 2009
Location: Asia and United States
If you use two lm3875 then you can use a separate rectification board for each chip.
Those two rectification circuits can be connected to your transformer in parallel and both utilize the center tap for power ground.
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Old 25th May 2010, 11:03 AM   #5
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
That 59Vac transformer is actually a 59Vac centre tapped transformer. More often specified as 29.5-0-29.5Vac transformer.

The 29.5Vac only happens when the supply is exactly 115Vac and the load is a resistor that draws exactly the rated current (9.32Aac) from the transformer. You may find that this same transformer when idling along on almost silent signal and maximum mains voltage supply is putting out ~+-45Vdc across the smoothing capacitors.

29-0-29Vac is very high for any chipamp. Check the 3875 datasheet for maximum DC supply voltage to suit different speaker (load) impedance.
Check Peter's guidance document.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 25th May 2010 at 11:10 AM.
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