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Old 7th January 2009, 02:43 PM   #1
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Default 4A +24v -24vdc regulated psu help for GC

hi guys

i am wondering if i could make a regulated psu that output 4A of +24vdc 0v and -24vdc?

schematic really helps

thx in adv Erwin
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Old 7th January 2009, 03:00 PM   #2
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A LM338 will work only if you have two separate raw voltages with no common ground.
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Old 7th January 2009, 03:12 PM   #3
AndrewT is offline AndrewT  Scotland
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consider what peak transient current that the speaker may demand.

An amp fed from 24Vdc could maybe achieve 20Vpk into a load.
If the speaker is 8ohm the max current could be around 20/8/0.35 ~=7Apk.
Where will this peak current come from?

If the speaker is 4ohm the output current can be around 14Apk.
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Old 7th January 2009, 04:02 PM   #4
Eva is offline Eva  Spain
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Quote:
Originally posted by AndrewT
consider what peak transient current that the speaker may demand.

An amp fed from 24Vdc could maybe achieve 20Vpk into a load.
If the speaker is 8ohm the max current could be around 20/8/0.35 ~=7Apk.
Where will this peak current come from?

If the speaker is 4ohm the output current can be around 14Apk.
Where is the 0.35 factor coming from?
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Old 7th January 2009, 04:21 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi Eva,
that appears repeatedly in the Forum from expert designers. The GEM in particular is designed to be tolerant of that type of effective impedance variation. They seem to have agreed that fast stopping and fast starting (audio) transients deliver large currents to the crossover/speaker combination.

Just a couple of days ago there was a further reference that the 0.35 factor is a bit light for severe reactance speakers and that the factor could be around 0.2 to 0.25 for severe speaker loading.
Even the relatively gentle impedance curve of the JBL example used in the test demanded very large current peaks on a few excepts of music signals. It seems that 0.35 may be a closer estimate for moderate or moderate to severe speaker loadings.

I find that using 0.35 demanding enough when taking gain droop back through the stages to the VAS to determine the peak transient currents in the outputs.

If this is a valid way of designing for peak currents, it puts into context the frequently quoted peak currents available from amplifiers when manufacturers deem it worth bragging about.
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Old 7th January 2009, 04:35 PM   #6
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hi guys

thanks for the reply. but your answer is beyond my understanding. i am still newbie that build few from schematic and do not really quite understand all the terms

please bare with my questions:
peranders, do you mean using two transformer or dual secondary? can i achieve this with ct transformer as i have on CT PT at hand?

AndrewT, this psu is intended for lm1785. does it still apply to your answer?

thx in adv
erwin
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Old 7th January 2009, 05:43 PM   #7
Eva is offline Eva  Spain
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Quote:
Originally posted by AndrewT
Hi Eva,
that appears repeatedly in the Forum from expert designers.
This is a myth to sell "high current" amplifiers. I have been repeatedly unable to get such a result.

You can design a passive crossover to exhibir reactive low-impedance dips at certain frequencies, but this is neither common practice nor a good design criteria. Voice coil resistance always limits current and its value increases due to self heating when the speaker is fed some power.

One of the systems that I have used for years is a 4-channel chip-amp setup. It's fed +/-40V from LM338K regulators. Output capacitance is 10.000uF per rail. With music it was just not possible to fold back the regulators even when driving 4 ohms on all channels above clipping. Crossover was active for bass/mid and passive for mid/high (2+2 channels).
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Old 8th January 2009, 02:01 AM   #8
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Quote:
Originally posted by Eva

One of the systems that I have used for years is a 4-channel chip-amp setup. It's fed +/-40V from LM338K regulators. Output capacitance is 10.000uF per rail. With music it was just not possible to fold back the regulators even when driving 4 ohms on all channels above clipping. Crossover was active for bass/mid and passive for mid/high (2+2 channels).
Hi Eva

do you mind share the schematic for lm338K regulators circuits?? i am interested in building one. does the PT use dual secondary or CT? thx
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Old 8th January 2009, 04:13 AM   #9
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You only have to download a datasheet
http://www.national.com/mpf/LM/LM338.html
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Old 8th January 2009, 04:19 AM   #10
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how to do the negative rail? just as connected the two batteries? (+-)(+-) and take the negative rail from the negative? sorry for newbie question. as i did some reg psu with pcb. but for this new configuration i cant find any suitable one. thx for the help
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