Denon AU-320 MC step up transformer, purchased from an estate. I don't have a MC cartridge to test this, but I tested all the switch positions for continuity (or open as appropriate) and everything tests as expected. Has switch positions for 2 cartridges, and switch for 2 primary impedances or pass through for a MM cartridge. $275/ make offer.
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Interested, sending message.
I've been visiting this section for a couple of weeks and was about to post a SUT wanted ad...
I need it for a low output MC cartridge.
What is likely to fail on a 40+ year old transformer?
I've been visiting this section for a couple of weeks and was about to post a SUT wanted ad...
I need it for a low output MC cartridge.
What is likely to fail on a 40+ year old transformer?
Hi Jeffs,
Not much can go wrong I expect, unless it's abused with high voltage. There was an open switch position, but I opened the unit up and sprayed the switches with DeOxit, and everything's good.
-jgf
Not much can go wrong I expect, unless it's abused with high voltage. There was an open switch position, but I opened the unit up and sprayed the switches with DeOxit, and everything's good.
-jgf
Wrong practice as this is a DC coupled transformer device. A DC voltage causes the transformers to be remanently magnetized. A DMM puts out enough DC to cause this. Seller or buyer needs to demagnetise them. Slowly and very carefully ramp up an AC voltage till the transformers start to saturate and slowly turn down amplitude to 0. Easy peasy.
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Ok, thanks for that. I'll do that as soon as I figure out what sort of voltage range is required. Any particular driving impedance needed? Should the secondaries be loaded?
Yes, exactly my concern.
jean-paul, do you know if a tape head demangnetizer would be useful in this instance (as I have one, wand type, and I do know how to properly use it).
jean-paul, do you know if a tape head demangnetizer would be useful in this instance (as I have one, wand type, and I do know how to properly use it).
You are not serious, are you? What kind of current do you reckon passes through an average dmm when measuring resistance? What will this do to the core?
This is common knowledge on signal transformers with such small cores and that have no air gap (so not DC proof). Just a few mA are enough to cause this and using the continuity setting exactly does that. So yes I am serious, I have some experience with transformer coupling. We all make the same mistake by using a DMM to measure if they're OK and then wisdom comes around the corner. If the core is remanently magnetised it is easy to see what happens on an oscilloscope when a signal is applied.
I just googled some info for those that still have their doubts. For some reason we are over-cautious with matters that are quite easy to show and way less strict on matters that can't even be seen on an oscilloscope (like capacitors sounding different but with equal signal shapes). Funny 😉
http://www.jacmusic.com/lundahl/tech-papers/Measure-Coils.htm
* Demagnetisation: at all times use common sense and first find out maximum voltages or ramp up voltage till saturation very slowly and carefully. Always use a load on the secondaries. I hope one can imagine what will happen if ,let's say, a sudden 30V AC is applied. Warning: getting to know transformers and their theory may involve conversions to units that are unknown to you and might involve formulas you never heard of. Don't underestimate the complex mathematics that are involved with apparently simple devices like transformers.
I just googled some info for those that still have their doubts. For some reason we are over-cautious with matters that are quite easy to show and way less strict on matters that can't even be seen on an oscilloscope (like capacitors sounding different but with equal signal shapes). Funny 😉
http://www.jacmusic.com/lundahl/tech-papers/Measure-Coils.htm
* Demagnetisation: at all times use common sense and first find out maximum voltages or ramp up voltage till saturation very slowly and carefully. Always use a load on the secondaries. I hope one can imagine what will happen if ,let's say, a sudden 30V AC is applied. Warning: getting to know transformers and their theory may involve conversions to units that are unknown to you and might involve formulas you never heard of. Don't underestimate the complex mathematics that are involved with apparently simple devices like transformers.
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While I was slightly joking with those last sentences (still true though) I thought it may be handy for those that are not that experienced with transformers to post a few of the rules what NOT to do with some transformers. I am just learning the theory myself again on high voltage transformers and like before I am amazed at the amount of theory surrounding transformers. These devices are often misunderstood and knowing them well might not be everybody's cup of tea. It may be easier to remember a few general rules what not to do with some types of transformers. Even if you don't grasp why things go wrong just read them a few times, it might save you money and stress in the future:
1. never disconnect loudspeakers from a powered on transformer coupled tube amp (amplifier with output transformer). Remember: "tube amps always need a load"
2. Never disconnect the load from the secondaries of a current transformer when current is passing.
3. never measure small signal transformers with a DMM. Remember:"measuring small signal transformers with a DMM = bad sound".
1 and 2 are destructive and not reversible. 3 is reversible.
1. never disconnect loudspeakers from a powered on transformer coupled tube amp (amplifier with output transformer). Remember: "tube amps always need a load"
2. Never disconnect the load from the secondaries of a current transformer when current is passing.
3. never measure small signal transformers with a DMM. Remember:"measuring small signal transformers with a DMM = bad sound".
1 and 2 are destructive and not reversible. 3 is reversible.
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Jean-Paul, could you please explain what in the transformer becomes magnetised. The core? What measurements can be used to ascertain this? Is distortion performance affected?
And are all cores equally at risk, or just permalloy?
I find it hard to separate the old wives tales from actual facts here. Surely there is some solid data on the subject.
You say it is easy to see this on the scope, but apparently for me it is not. Could you elaborate?
A prototype circuit if mine runs about 0.1mA of dc through the primary of a non-gapped amorphous transformer. On power-on the current routinely rises up to a miliamp or even two. Never been able to measure any ill effects of this.
And are all cores equally at risk, or just permalloy?
I find it hard to separate the old wives tales from actual facts here. Surely there is some solid data on the subject.
You say it is easy to see this on the scope, but apparently for me it is not. Could you elaborate?
A prototype circuit if mine runs about 0.1mA of dc through the primary of a non-gapped amorphous transformer. On power-on the current routinely rises up to a miliamp or even two. Never been able to measure any ill effects of this.
If you find it hard to separate the old wives tales and facts I think I can't tell you anything. It occurs to me that some old facts are buried under a thick layer of ignorious dust. Sure there is hard data but as I found out the web is not the source to find that information, this is old school info found in books 🙂 The more I learn the more I realise that winding transformers is an art becoming extinct. Transformers apparently aren't sexy in 2017.
Of course it is the core that becomes remanently magnetised. Copper wire is not easily magnetised remanently 😉 Check the BH curves of the core material to understand what will happen. The core material is THE parameter here but it is often unknown what is used. When remanent magnetism occurs the signal will distort. I made the mistake myself with studio line transformers and I noticed it by listening and not understanding what was wrong with previously good sounding transformers. The effect of it slowly becomes less when the devices are simply used (music is AC after all) but demagnetising is a better way of doing things. Most irritating is when only one channel has a magnetised transformer.
I think you must go into theory a little deeper and not let my noise affect you. Fact is that non air gapped transformers most often don't like DC. There must be something done to prevent DC current flowing through the windings. What is and what is not tolerable is dependent on the transformer in question and I would not know what to advise as I keep it simple to keep it manageable in my head: no DC allowed. +/- 0V, +/- 0 mA. This is contrary to popular "can I get away with it?" practice.
Of course it is the core that becomes remanently magnetised. Copper wire is not easily magnetised remanently 😉 Check the BH curves of the core material to understand what will happen. The core material is THE parameter here but it is often unknown what is used. When remanent magnetism occurs the signal will distort. I made the mistake myself with studio line transformers and I noticed it by listening and not understanding what was wrong with previously good sounding transformers. The effect of it slowly becomes less when the devices are simply used (music is AC after all) but demagnetising is a better way of doing things. Most irritating is when only one channel has a magnetised transformer.
I think you must go into theory a little deeper and not let my noise affect you. Fact is that non air gapped transformers most often don't like DC. There must be something done to prevent DC current flowing through the windings. What is and what is not tolerable is dependent on the transformer in question and I would not know what to advise as I keep it simple to keep it manageable in my head: no DC allowed. +/- 0V, +/- 0 mA. This is contrary to popular "can I get away with it?" practice.
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The current depends on the DC offset of the DMM and the DC resistance of the winding when I must believe mr. Ohm 😀.
My DMM has a DC offset of some 400 mV but I guess the value depends on the DMM.
When I measure the secondary DC resistance of one of my SUT's the DMM says 83 ohm, so there is some 5 mA of DC current flowing through the winding.
Measuring the primary would provoke some 0.5 A of DC current so better not do that....
I checked the measured SUT on the scope, and don't see any difference with the other one which has not been subject to DC current.
My DMM has a DC offset of some 400 mV but I guess the value depends on the DMM.
When I measure the secondary DC resistance of one of my SUT's the DMM says 83 ohm, so there is some 5 mA of DC current flowing through the winding.
Measuring the primary would provoke some 0.5 A of DC current so better not do that....
I checked the measured SUT on the scope, and don't see any difference with the other one which has not been subject to DC current.
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Forget the post above.
DMM work with a current source to measure DC resistance.
I am not sure how much current went through the secondary winding.
When I compare the measured SUT with an unmeasured one I don't see any difference (bandwidth; square waves).
I guess it depends on the core how sensitive it is to DC current.
DMM work with a current source to measure DC resistance.
I am not sure how much current went through the secondary winding.
When I compare the measured SUT with an unmeasured one I don't see any difference (bandwidth; square waves).
I guess it depends on the core how sensitive it is to DC current.
Yes but core size also is a relevant parameter. What type/size transformer did you measure ? A small signal 1:1 line transformer or a tube amp output transformer ?
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The core of the SUT is a nanocrystalline toroid, but a pretty large one for the application because I like low DC resistances of windings.
The SUT I measured is a 1:15 type.
The SUT I measured is a 1:15 type.
Aha, what is your opinion on the small signal MC transformers as being offered in this thread. Are they easily remanently magnetised or not ?
The AU-320 is used for 0.3 mV output MC cartridges.....
The AU-320 is used for 0.3 mV output MC cartridges.....
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