Quick doubt about PSU for parallel LM4780 - diyAudio
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Old 5th July 2013, 10:21 PM   #1
tent is offline tent  Germany
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Red face Quick doubt about PSU for parallel LM4780

Hello,
I've built the PSU made of two cases each with a 500VA dual 22V toroid, 8 MURs and 2 22000uF caps. It seems to be ok and giving me 30V DC without problems (tested also with a 40W test bulb between PSU and mains).

When I connect it to the AMP which is an LM4780 in parallel configuration for each PSU in another case I read that the output goes down to 18V DC slowly rising to 19V DC maximum. DC offset at the amp is 11mV and AC with no input sound is 0.20mV so everything seems ok. I also tried it with some test 8ohm speakers and seem to work fine. My doubt comes also because the same 40W test bulb between mains and PSU is now glowing dimly orange with the AMP.

So wanted to be sure that such lower PSU Voltage DC and the glow in the bulb is not anything abnormal..

Thanks,
tent:wq
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Old 6th July 2013, 09:49 AM   #2
AndrewT is offline AndrewT  Scotland
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The bulb is a mains bulb tester.
It is used to help prevent you blowing up your project due to wiring it up incorrectly.

It is NOT used to operate a working amplifier.

It IS used to TEST a project after it has been modified.
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Old 6th July 2013, 10:48 AM   #3
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To expand on Andrew's point, the bulb itself may be causing the voltage drop in your test. It is a safety device for the first switch-on to make sure nothing blows.

You should have fuses too, so go ahead and power it up without the test bulb. Measure your PS with no load (not connected to amp) and load (connected to amp). Quickly, just to be extra safe ...

With your beefy 500VA trans you should be seeing in the range of ( 22 Vac x 1.4 = ) 30 Vdc when connected.

Report back and we'll go from there!

Cheers,
Jeff

Last edited by AudioLapDance; 6th July 2013 at 10:58 AM.
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Old 7th July 2013, 10:45 AM   #4
tent is offline tent  Germany
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Hi,
actually I AM in the TEST phase.. my doubt comes since on other ones I've built there is never even dimly orange light.. so can I really dare to connect it without since you think it's safe?
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Old 7th July 2013, 05:47 PM   #5
tent is offline tent  Germany
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? I quite do not follow you Andrew.. I'm in the process assembling my four cases and all.. so measuring PSU out voltage, DC offset of AMP, and ok as well hook up some test speakers, etc.. but anyway..

what I'm asking is, since I understood the mains bulb tester should "detect" shorts, and since ONLY with one AMP it glows dimly and not with the other (I've just tired this today, and voltage is 25V in this case compared to the 19V of the other that glows the bulb) and also it does not glow at all if I do not connect the amp at all of course (30V in this case).

Also when I used this mains bulb tester with other PSUs and AMPs in the past I managed to have it completely "off" and without light.. if of course all is wired correctly.. now I am not able to spot differences, evident, between this two amps..

now my reasoning is: since only with one of the two amps connected, the same psu dims glowly, is it correct I assume there is something that if not really a "short" is maybe some "leak" somewere?

but could you suggest also in general if it is safe I do further measurements without the bulb now in the end?

tent:wq

Last edited by tent; 7th July 2013 at 05:50 PM.
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Old 8th July 2013, 08:12 AM   #6
AndrewT is offline AndrewT  Scotland
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If you test the transformer , then the transformer+PSU, then transformer+PSU+amp. You will find that a low wattage test bulb is OK to prove you have the correct working voltages around the amp circuit.

As you add extra amplifier channels, you draw more quiescent current. This will make the filament warmer and increase it's resistance, making for excessive voltage drop across the test bulb.
At this stage you should be increasing the test bulb wattage slightly to compensate for the increased current draw. This is to prevent damage as each extra channel is added on.

Only once all your modifications are complete and all your voltage have been measured and shown to be correct do you remove the test bulb. Then a final recheck of all voltages from a direct online power up, BEFORE connecting a cheap test (throw away) speaker.
You DO NOT connect a speaker as a test load.
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Old 8th July 2013, 12:13 PM   #7
tent is offline tent  Germany
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ok, I've tested the amp now also without test bulb and without speakers. Voltage draw from one AMP is now more or less 1V less than the other "good" AMP: do you think there is no issue at all and it is normal? What could I check in the AMP to see if there is some issue?

I.e.:
PSU alone 30V DC
PSU+AMP1 30V DC
PSU+AMP2 29V DC
Other measurements like DC offset or AC at the output binding posts of the AMPs do not seem to change at all without the bulb (c.a. 22mV DC on AMP1 and 11mV on AMP2, 0.20mV AC both).

tent:wq
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Old 8th July 2013, 02:42 PM   #8
AndrewT is offline AndrewT  Scotland
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check the output noise (mVac) and the output offset (mVdc) after 10mins and 20mins and 30mins, with the input shorted and connected to your Source, with the Source both OFF and ON.
When you are happy all is OK, then connect a test (throw away) speaker and listen to some audio signals.
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Old 8th July 2013, 10:19 PM   #9
tent is offline tent  Germany
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ok, but is there some scientific explanation then about the two AMPs to behave so differently? (one does "short" a bit and the other no) Additionally one drives the PSU to 30.3V and the other to 29.8V, is this also rather common?
About output noise and ofset what do you expect? They become better after an hour? But still with difference between them then?

tent:wq

output noise is 0.24mVac on the AMP1, on 0.23mVac on the AMP2
output offset is 22mVdc on AMP1, 11.3mVdc on AMP2
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Old 9th July 2013, 07:08 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by tent View Post
output noise is 0.24mVac on the AMP1, on 0.23mVac on the AMP2
output offset is 22mVdc on AMP1, 11.3mVdc on AMP2
these numbers are acceptable. But all 4 can be improved.

Can you insert a 1r0 into each power feed and measure the quiescent current drawn on each rail by each amp separately and when both amps are connected at the same time?
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