Transformer Out of Spec?

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.
 
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.
 
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":confused:
 
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....:D
 
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?
 
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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?
 
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.
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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.
 
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.
 
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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...
 
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. 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?


Its unlikely the fixed amp was sent out untested so perhaps you need to think a bit laterally ?
Did the amp going wrong fry your speakers and make them go short circuit ?
Is your speaker lead faulty ?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Without cutting in on Nigel's amp. faultfinding, there is one last issue with the preamp; earthing.
On a lot of DIY and commercial amps, the input earth connection is lifted by 10-12 ohms from the star earth. This reduces the influence of earth currents flowing in the signal cable. These can be significant with double insulated (2-pin) appliances such as we buy now.

It could be that the amplifier is actually sensitive to the current for some reason not addressed by the manufacturer or repair agent. Unfortunately, at the very low impedance of these currents, they are very difficult to sense and quantify. A good treatment of this is in earlier editions of D. Self's amplifier manual but now its on his website at Douglas_Self_._com.

This is not helpful directly but it would be worth connecting to another DC amp and checking for tiny 60 Hz content in the output and what happens when you break shield earth between units. I'll guess that the current might be less with the MF preamp. Of course, a proper CRO would be a good start here.
 
A quick history. I have a pair of monos on this Denon preamp (PRA-1500), running fine for a year. I leave them on all the time. One begins blowing one of the four on-board fuses (4 pairs of BJTs). I get music out of it, but its low and distorted. I send amp in for repair. I operate the remaining mono and another I have as a backup on the same setup for the next 3 months without issue. The manufacturer finally gets around to fixing the one sent in, says he cannot and wants the other mono back, he will swap out the boards into the same xformer/caps and case. I get these two back, one immediately has a loud hiss, I ship it out. The other works fine for a few days, then blows the primary fuse. I replace it, then blows one of the four onboard fuses. I go ahead and replace the fuse and install a Musical Fidelity x-pre (powered by two prong transformer) and another set of speakers and has now worked fine for several days, turning off and on, with 4 ohm speakers attached.

The original speaker set measure fine at the cable ends, 4.4 ohms each. The Denon is in fact a two prong device.

The manufacturer is saying the Denon is responsible, mentioned interaction of cable and possible DC coming through. I measured no DC at no load on the preamp and no reaction, even at startup, but I have not measured under load, probably will not bother. The Denon pre has a 10K output imedance, the amps have a 22K input impedance.

So later today I will find out if the unit I sent back is experiencing the hiss at the manufacturer. Anyway, I had not suspected my existing setup, as it did not change, but now I'm discovering these amps are very finicky and the manufacturer, who continues to help in resolving the issue regardless of fault, specifically mentions the Denon design as being problematic, particularly with the mono arrangement, much less or not with the same board configured in stereo.
 
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Even better to have x100 impedance 'mismatch'.

The output impedance of a preamp cannot cause a power amp to blow fuses. Whoever told you that is either an idiot or he thinks you are an idiot.

Some preamps have a voltage spike at the output at or just after switch-on. How big and how long this spike is depends on all sorts of things. A main amp may or may not blow fuses if presented with this spike. Nothing whatsoever to do with the transformer. These spikes or transients can be worse in equipment which foolishly goes down much below 10Hz, in the belief that we need to hear down there. The quick solution is to switch on your main amp only after the preamp has had plenty of time to settle (say, 30 secs delay?). Switch off main amp first, then wait before switching off preamp.

I doubt if the Denon preamp has a 10K output impedance - this would be quite poor! More likely that it has a 10K 'minimum following power amp input impedance' - this means it will happily match anything from 10K up. Your 22K is a bit close but good enough. 100K would be better. Remember, you actually want a serious 'mismatch' here!
 
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Well, I found the service manual which appears to have a different figure than what I had found on other posts, I'm using the RCA connection, but it appears the 10K is not correct, much lower in reality.
 

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