Transformer Out of Spec?

[DreadPirate]

This is a Plitron 400VA audio transformer with rated 40VAC secondaries. I'm reading 42.1VAC. Is it possible there is something wrong with this transformer, I seem to be overloading the board and blowing fuses. The Plitron site says 3% at maximum load and I'm testing at no load. It is possible that it is not blowing the onboard fuses until I attach the speakers and play through it, but I'm kind of getting tired of opening the unit changing fuses, etc to strongly confirm.

[Enzo]

So don't connect a speaker load until you verify there is no DC coming out of your amp.

Transformer ratings are for fully loaded secondaries. And they are specified at a certain mains voltage. For example, if your transformer is 120v primary, and your mains is up to 125v, then that 40v secondary will also be up to 41.7v.

And certainly 2 extra volts on that secondary is not the reason fuses are blowing.

[lanchile]

Originally Posted by DreadPirate

This is a Plitron 400VA audio transformer with rated 40VAC secondaries. I'm reading 42.1VAC. Is it possible there is something wrong with this transformer, I seem to be overloading the board and blowing fuses. The Plitron site says 3% at maximum load and I'm testing at no load. It is possible that it is not blowing the onboard fuses until I attach the speakers and play through it, but I'm kind of getting tired of opening the unit changing fuses, etc to strongly confirm.

Are you using slow-blow fuses or fast acting fuses."usually" in power supply you use slow-blow fuses for the "inrush"

[Tony]

Originally Posted by DreadPirate

This is a Plitron 400VA audio transformer with rated 40VAC secondaries. I'm reading 42.1VAC. Is it possible there is something wrong with this transformer, I seem to be overloading the board and blowing fuses. The Plitron site says 3% at maximum load and I'm testing at no load. It is possible that it is not blowing the onboard fuses until I attach the speakers and play through it, but I'm kind of getting tired of opening the unit changing fuses, etc to strongly confirm.

seems normal to me, secondary voltage depends on your primary voltage if you have a high line that secondary may even go to 45volts.....

you have other problems that is why you are blowing fuses....and it may not have anything to do with your plitron at all....

[DreadPirate]

My line voltage is 118VAC. The fuse on the primary is in fact slo-blow, as are the four ones on the board (four pair of 2SA1216 & 2SC2922). The unit is in warranty, so back it goes. Manufacturer replaced the board already, but did not replace the power supply (xformer, caps- rectifier is onboard so was changed). When I got it back, same problem, blowing one of the four on board fuses, therefore I'm suspecting what was not changed. Manufacturer likes to swap out boards instead of understanding what went wrong and fixing it. Now says its my Denon preamp (PRA-1500), because Denon preamps put out DC. I countered with questioning how that DC got by his input capacitors, a reply he was not expecting from a customer :}. I measured the output from the preamp anyway and, of course, 0VDC.

By the way, the input impedance of the amp is 22K and the output impedance of the preamp is 10K. Does this affect anything? I got the PRA-1500 specs from:

http://www.classic-audio.com/denon-pra1500-p-498.html

I notice that the preamps they normally sell to go with this amp are much lower output impedance (one 2K, the other 600), is it possible that the mismatch in impedance is a factor in the failures?

[DreadPirate]

Well, I have subsequently replaced the blown fuse and installed the amp on a low output impedance tube preamp (MF x-pre) and I no longer get blown fuses! The manufacturer said his monoblock version of the amp was very sensitive to the output impedance of preamps and I'm now believing it. But what is the explanation for this?

[AndrewT]

Dread,
build and use a mains light bulb tester.

It saves fuses and saves incorrectly wired projects.


The open circuit voltage of a transformer is
mains voltage / rated voltage * secondary voltage * [1+regulation]

if regulation is 4.5%, your transformer is 115:40+40vac and mains is 118Vac
then the measured open circuit voltage is 42.89Vac +- manufacturer's tolerance +-DMM Vac tolerance

BTW,
3% regulation for a 400VA transformer seems unbelievably good.

[Ian Finch]

Originally Posted by DreadPirate

Well, I have subsequently replaced the blown fuse and installed the amp on a low output impedance tube preamp (MF x-pre) and I no longer get blown fuses! The manufacturer said his monoblock version of the amp was very sensitive to the output impedance of preamps and I'm now believing it. But what is the explanation for this?

Having found preamp change to solve the problem, It would reasonable to load the output with a 22k load and measure it at power-up with (preferably) an analog meter, so you can see if a high spike at turn-on or other internal problem causes a high output level, DC or AC, for whatever duration.

Then you might even get angry with the guy who sold the Denon instead.

BTW the output 10K impedance rating is most likely nominal, referring to the load as many people get confused, stubborn and refuse to buy things where impedances don't read as similar. Read some fairly recent posts in Bob Cordell's Book thread for confirmation.

[DreadPirate]

Hi Ian. I did measure the output of the preamp at no load during startup and after with a firm 0VDC result. I guess I could attach a 22K load to it and do same, but it appears the preamp was the cause of the problem, as I have run the amp for a full day with another preamp and 4ohm set of speakers with no problems whatsoever. Very strange, but it appears this amp is in fact very sensitive to the preamp attached to it. It is an Odyssey Audio Khartago monoblock, fantastic sounding for the money (based on the Symphonic Line RG1 which sells for $6800US in Europe). A finicky amp to be sure, but well worth the ocassional issue.

[Tony]

By the way, the input impedance of the amp is 22K and the output impedance of the preamp is 10K. Does this affect anything? I got the PRA-1500 specs from:

Denon PRA-1500 - Classic Audio

I notice that the preamps they normally sell to go with this amp are much lower output impedance (one 2K, the other 600), is it possible that the mismatch in impedance is a factor in the failures?

my rule of thumb in this case is that the input impedance of the power amp should be 10x the output impedance of the preamp...