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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jun 2019 at 6:50pm
I started to measure magnetising current (off-load current) of all the transformers in the "grave yard" but on the 4th transformer the inrush current was too much for the small fuse I had fitted in my quick-connect. I therefore went to replace it but on pulling out one of the banana plugs, rather than pressing the connector down as one should, the entire connector snapped and flew apart rendering the quick-connect useless. It was 45 years ago the first time I used a quick-connect, and I was still using the same one 35 years ago when I left that particular employment. Not once did it break!

I will continue testing but will comment on the AnTek transformer. Either I'd got a leaky connection or it only draws 16mA!!!

I am unable to repeat the test right now because of the quick-connect breaking, but if it's to be believed, then it stands a good chance of working well in this design. Its spec says 23mA but that's on both primaries in parallel on 120V at 60Hz. Here I was testing primaries in series on 240V at 50Hz.

Perhaps it is believable. I think I ought to give it a try in the amp.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jun 2019 at 8:58pm
The AS3450 transformer (http://www.antekinc.com/content/AS-3450.pdf) is now powering the amplifier. Bruce's choice was the AS4450, but I didn't feel it needed 400VA, and so chose to fit the 300VA version. I bought both types anyway, so I can fit the 400VA if necessary, but unless its duties are for disco parties, then 300VA is perfectly adequate.

How will the amp sound? Well, we'll all have to wait at least 72 hours. Incidentally 72 hours is the on-time specified for a transformer in IEC60950 testing, so maybe burn-in isn't so daft after-all...Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jun 2019 at 9:48pm
question graham - if you burn in a transformer then drop another in and then revert back to the first will it just need part burn in again?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jun 2019 at 10:10pm
or could you burn them in outside of the amp?
The Source turntable, Audiomods V micrometer, Dynavector XX2-2, Accession M, Elevator, Leema Antila 2Seco CD player, TDL studio 1.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jun 2019 at 10:31pm
Originally posted by Richardl60 Richardl60 wrote:

question graham - if you burn in a transformer then drop another in and then revert back to the first will it just need part burn in again?


More than likely. You see, the components most sensitive to change are capacitors, especially electrolytic capacitors, and they're unavoidable in any power supply. Even after being properly discharged before changes are made, voltage can creep back up, and on switch-on again, there can be a striking voltage, even if it doesn't result in 'operator alarm'.

If the transformer interwinding capacitance allows sufficient mains distortion 'through', then the capacitors are exposed to frequencies they're not terribly good at. There will be degradation in their performance which is not immediately noticeable.

Although the off-load current is much smaller than the laminated transformers just replaced, the new toroidal might still be designed for greater saturation on-load, and the distortions that causes will take some time to affect capacitor performance.

Best to read an apllication note such as this to get a better idea of how things can affect how an electrolytic works: http://www.nichicon.co.jp/english/products/pdf/aluminum.pdf
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jun 2019 at 3:31pm
Originally posted by Graham Slee Graham Slee wrote:

The AS3450 transformer (http://www.antekinc.com/content/AS-3450.pdf) is now powering the amplifier. Bruce's choice was the AS4450, but I didn't feel it needed 400VA, and so chose to fit the 300VA version. I bought both types anyway, so I can fit the 400VA if necessary, but unless its duties are for disco parties, then 300VA is perfectly adequate.

How will the amp sound? Well, we'll all have to wait at least 72 hours. Incidentally 72 hours is the on-time specified for a transformer in IEC60950 testing, so maybe burn-in isn't so daft after-all...Wink
 You may find the 300VA (AS3450) able to pump 1.5 times 300VA very easily... and still fill the disco hall.Smile
Bruce
AT-14SA, Pickering XV-15/625, Technics SL-1600MK2, Reflex M, Lautus, Technics SH-8066, Dynaco ST120a, Eminence Beta 8A in custom cabs;; Using Majestic DAC
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Jun 2019 at 7:01pm
I still have hopes of turning the findings of this topic into a hi-fi amplifier kit or part-build so that others may have some fun with it. I would not dare accept payment for something I know won't impress you - it has to impress me first - and it has to do that over some considerable time.

It still has at least 24 hours to go using the latest (AnTek) transformer, but so far it's not indicating any particularly bad vibes.

My thoughts turned back to the RS frame transformers, and although they're a bit on the small (VA) side, the sound was pretty good. If I was offering the amplifier as parts then somebody might wish to fit these transformers on grounds of cost, or that they didn't have any other choice.

So what is it they have going against them? Well, for a start, they're run with a bit too much magnetising current: 70mA as measured, which suggests they're cheapened up. If the laminations were of better steel they'd run at lower no-load current.

With 70mA they're going to buzz a little, but it's not that intrusive, and I can only hear the case-buzz when everything else in the workshop is switched-off, and with all doors shut it is very quiet in here.

There is some hum just audible with ear to grille cloth. Otherwise the amp sounds quite good.

So how could they be DIY improved? Now, here I must state I don't have the education in transformers to be sure whether my suggestions will be any use, but here goes anyway.

The mounting frames (not the laminations!) are obviously part of the magnetic circuitry, but being cheap mild steel, are they making no-load current worse? Better? Or no different? Only one way to find out, and I will be doing.

If the frames are removed it would allow a flux band to be wrapped around them as shown on Rod Elliott's website http://sound.whsites.net/xfmr2.htm#s13

This would require the transformers to be mounted slightly differently, but probably easier, on long bolts with spacers.

The flux band might be made from copper "slug-tape" (also sold for EMC control). But care must be taken not to get too close to the solder-tags, especially the primary mains side! 6.35 mm should be allowed for sufficient creapage and clearance!

With sufficient turns and the end seam soldered it might reduce induced noise sufficiently. I say might, as this is yet to be tried.

But it does lend the design a bit more DIY interest.

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