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Old 19th December 2010, 11:47 PM   #11
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Originally Posted by DreadPirate View Post

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 ?
Murton-Pike Systems PCBCAD51 pcb design software.
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Old 20th December 2010, 11:07 AM   #12
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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.
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Old 20th December 2010, 11:47 AM   #13
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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.

Last edited by DreadPirate; 20th December 2010 at 11:50 AM.
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Old 20th December 2010, 01:33 PM   #14
DF96 is offline DF96  England
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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!

Last edited by DF96; 20th December 2010 at 01:37 PM. Reason: add Denon comment
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Old 20th December 2010, 02:08 PM   #15
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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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Old 20th December 2010, 03:09 PM   #16
AndrewT is offline AndrewT  Scotland
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the RCA (unbalanced) output impedance is 10r. This will drive any load >=10k
The balanced output impedance is 600r. This too will drive any balanced load >=10k but better if it were >=500k to ground.
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Old 20th December 2010, 05:18 PM   #17
DF96 is offline DF96  England
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The 10K was probably the right figure; it just didn't mean what you thought it meant.
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