Jensen or Mundorf coupling capacitors?

[W9TR]

Tim,
Also I forgot to mention - put RCA shorting plugs in the phono inputs if you are not using them. The phono section has very high gain and is potentially susceptible to who-knows-what.

All points lead to the pre at this point as it is the thing that has changed.

Tom

[timm]

Thanks. I have the flu bug. But hopefully this weekend I will go out and find some measurement gizmos. I have a high end brick and mortar not too far from me as well. I'm going to try and limit the variables as much as possible. But once we figure this out - I'll get the caps

[W9TR]

Get well sir! I checked and Amazon has Kill-a-Watts for less than $20.

[jadedavid]

It has been stated that you should not have to use a cheater plug on any well designed gear.
I have found that to be untrue.
The issue is AC leakage from the transformer. An unavoidable situation.
This can be minimized. First, using a cheater plug, measure the voltage on the ground to ground pin. Then reverse the cheater plug to the wall receptacle and re-measure the ground to ground. The lowest reading will be the best connection.
This may require the transformer in the unit to be rewired (inverting the two input wires) to the best polarity (phase).
Greg Weaver from The Absolute Sound has written an article on this in the February 2017 issue.
Also additional info can be found online.
Once all of your equipment is properly AC phased many realize a much quieter, more detailed presentation.

[W9TR]

Since I'm the one that stated it - I'll respond.

Greg's article in TAS covers the subject of AC leakage from the transformer core to the chassis and how to figure out which connection has the least amount of leakage to the chassis. Everyone should do what he suggests. So far so good.

But it does not go far enough and suggests you do something that is inherently unsafe.


After you are done, if the 'flipped' connection turns out to be the quietest, you'll need to connect the cheater plug ground to your outlet ground.


If you don't do this and the transformer fails or something else fails that puts high voltage on the chassis the resulting fault current will go through your equipment and maybe you, causing lots of damage and the potential for electrocution.

Now on to the 'well designed' part.

If the designer has not determined the correct polarity for the transformer primary winding then they are not doing their job. Shame on them. It. Doesn't change part to part, it is inherent in the design of the transformer.

I'll reiterate. If you have to use cheaters then the equipment is not properly designed. This problem has been solved years ago by manufacturers that know what they're doing. These savvy manufacturers use low leakage r-core or similar transformers, provide primary to secondary electrostatic shields, float inputs and outputs off chassis ground, and do everything in their power to prevent AC leakage currents from flowing through the chassis in the first place.

Tom

[timm]

So I have moved it to a secondary system - different amp/room- no inputs - only amp connected. I want to see if it blows independent of the amp.

My dac direct to my amp does swimmingly well. This pre had been online for over five years with this amp - so something went south - I just want to see if it is specific to the pre.

[jadedavid]

W9tR. Not doubting what you are saying. All I know is that on a well known $6K preamp and on a well known pair of $8K mono blocks the transformers needed to be reversed. It was so bad that the amps could not be used in balanced mode due to the transformers being out of phase one to the other.
Once I determine the correct polarity, I correct it at the transformer itself by reversing the leads.
YMMV

[timm]

Well - put the pre on my secondary system. Different everything -amp/room/circuit. Connected different amp - let it run. Fuse blown in less than 24 hr. This has to be an issue w pre - not external to it. It's blown on two diff amps ... different rooms ... two different cords .. 3 different sets of interconnects ... thru a power conditioner... straight to the wall ...different circuits ...in a house I have had zero electrical issues w any component.

This only occurs when an amp is connected.
Anyone disagree with this mindset ?

[jadedavid]

Sure sounds like the preamp. Do you have or know anyone with a good tube tester? A tube can start up OK but develop an interelemental short when it gets hot.
Or if you have another set of tubes to replace these it would be worth a try.

[timm]

Interesting you mention the tubes. I also question the tubes - but the mfg doesn't seem to be concerned w them. I want new tubes - but I do have concerns because if I pour more money into it - I need to know it is something they can get their arms around. The unit went in for this repair - and they thought it was just a blown fuse - nothing else. Maybe when I tell them my further findings they will get more interested in that. They said they tested the tubes btw- and I was supposed to get the measurements (not that I would know what I was looking at ).

Let me ask - if I am not running the phono - could those tubes impact this at all?

And one more question. If hooking an amp up causes the fuse to blow over a 24 hour period - but nothing blows when an amp is not connected - that is the curious part to me. What is it about having the load present (not playing) that would create an issue with a fuse in the power supply.

Do we think tubes would run hotter when resistance like the amp was present?