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Old 21st February 2008, 10:33 AM   #1
zdr is offline zdr  Belgium
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Default Toroid output voltage for 4780 parallel?

I am starting new clone with one 4780 per channel in parallel. I kinda have problems to find 22V RMS voltage output toroids here in Benelux. Conrad has only 18 and 30V for 400VA and 500VA toroids. for 100W RMS per channel, calculation shows 22V at 4 ohm. Should I go for 18V, 30V or something (somewhere) else?
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Old 21st February 2008, 10:52 AM   #2
AndrewT is offline AndrewT  Scotland
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What is your target output power and intended load impedance for your paralleled stereo amplifier (4chipamps)?
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Old 21st February 2008, 11:04 AM   #3
zdr is offline zdr  Belgium
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4780 in parallel is capable of 100W RMS per channel, and load should be standard, 4-8 ohm.
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Old 21st February 2008, 11:38 AM   #4
zdr is offline zdr  Belgium
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After rethinking it a bit, I guess 18V is out of the question. It seems that both versions have very similar output current values, so voltage ceiling might be hit easily with 18V one. On the 30V side, 6.6 Amps RMS current capacity of 500VA 30V trafo should run down to 2.3 ohms (approx) just fine at 100W peak (not sure what are the losses exactly inside those transformers, but that should affect mostly RMS voltage drop, and 30V is lot more to drop from than 18V).

Need to check 4780 output specs too.
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Old 21st February 2008, 12:38 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
if you want 100W into 4r0 from a parallel pair, then each chip thinks it is driving 50W into 8r0. That is equivalent to 28.3Vpk and 3.5Apk into the resistive load.
Allowing about 4.1V loss through the amplifier indicates that 28.3+4.1=+-33.5Vdc is required from the PSU when delivering full power.
Now make a decision.
Do you design for continuous (say for 1 or 2 seconds) maximum power into this load or will you accept a short term power peak into that load.
For continuous power you must allow for sag in the PSU voltage as the smoothing capacitors discharge to the fully loaded condition. This tends to give more controlled and deeper bass.
Alternatively, the peak power rating can be met from the fully charged capacitors but as the duration of the signal is increased the PSU drops to pass less continuous power to the load. This reportedly tends to favour midrange and treble frequencies.

The sag could vary between 2V for a very high capacity PSU to 6V for a cheap low capacity PSU.

I would suggest you design for +-[33.5+2V] for your first guess at quiescent load. After allowing for rectifier loss and Vdrop supplying quiescent current and for 5% transformer regulation this 35.5Vdc can be supplied by a 23+23Vac, 300VA toroid for powering 100+100W. Use at least +-20mF for the two chipamps if you want deep bass, or twin PSU each with >=+-10mF/chipamp. Personally I would fit +-15 to 20mF for every chipamp.
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Old 21st February 2008, 12:45 PM   #6
BWRX is offline BWRX  United States
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You could get the transformer with 30VAC secondaries and unwind some turns to lower the voltage a bit.
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Old 21st February 2008, 04:58 PM   #7
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Or you could parallel some LM1875's with the 18v. Sure that's a bit warm, but it'll rock the house quite nicely. They're about 2db behind the big chip, but with firmer driver command, they'll knock the grills off the speakers a bit more quickly.
Sort of a "personal taste" difference between dynamic power or average power.
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Old 21st February 2008, 07:12 PM   #8
zdr is offline zdr  Belgium
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Quote:
Originally posted by BWRX
You could get the transformer with 30VAC secondaries and unwind some turns to lower the voltage a bit.
Why would I want to wind down the output voltage, unless I would want to use a bit cheaper, 40V caps? If 22V is good, 30V is probably only better. Current capacity will stay the same anyway, so I don't see any other technical reason for this.
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Old 21st February 2008, 08:47 PM   #9
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30+30vAC (60vct rated transformer) = 42+42vdc and that's too hot. This will set off the spike protection.

22+22vAC (44vct rated transformer) = 32+32vdc and that's pretty good (somewhere near optimal).

18+18vAC (36vct rated transformer) = 27+27vdc and that's going to "less hot," but make less amplifier power.

In decibels, the end result is probably little to none noticable difference between any of the three options. Remember, it takes 2x the amplifier power to make each +3db to the speakers, and it takes 10x the amplifier power for every 2x difference to the ear.

Bring in speakers and ears, then the 18v secondaries look good.
Hey don't worry, it'll still get the heatsinks plenty hot, especially if its one of the "TF" chips.
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Old 21st February 2008, 10:11 PM   #10
zdr is offline zdr  Belgium
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Yep, you are right. Not only that, I just noticed that |V-|+|V+|<= 84V, which is right on the edge with 2x42Vdc.
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