Hello, starting my first build with OPTs today, along with a power transformer I wound myself just yesterday but as all of the transformers are unshielded and I have some mumetal around I was wondering if it would be worth the time to construct some boxes around them to keep any interference away?
Maybe just wasted time and definitely something I could add later but thought I'd ask 🙂
Maybe just wasted time and definitely something I could add later but thought I'd ask 🙂
If you bend the mumetal it will lose its magnetic properties. I would create a rectangular box only if there is hum due to stray magnetic field. Try first without shielding.
You can also use an external copper band like this:
You can also use an external copper band like this:
Alright good to know! Don't think I have any copper tape around but that can't be too hard to find. Will add it if I have hum issues then 🙂
Try a steel partition between the transformers and the audio circuits.
But first mount the transformers as far away as possible from the circuitry.
But first mount the transformers as far away as possible from the circuitry.
There is an aluminium partition at the moment, could switch that to steel it I have an issue. It just unscrews so it's not too bad to do 🙂
Alright but it should be alright to do on the power transformer at least?
No harm done as long as outer winding is properly isolated
You wound your own power transformer? Wow, seriously, I have great respect already! Only once did I wind a transformer (how hard can it be, says I) for an old electric fan. Pretty cool, yes, but Had to be careful as it went along.
Ok, so about building some kind of shield for transformer hum.... I have done it successfully in some cases, and just couldn't get it right in other cases. I have tried MU-metal, copper foil and all kinds of experimental stuff, but in the end there was a case that I just used a separate chassis and shazaam, hum gone. Please keep us informed as to your success/failure. Most of the time shielding and or distance works out.
Ok, so about building some kind of shield for transformer hum.... I have done it successfully in some cases, and just couldn't get it right in other cases. I have tried MU-metal, copper foil and all kinds of experimental stuff, but in the end there was a case that I just used a separate chassis and shazaam, hum gone. Please keep us informed as to your success/failure. Most of the time shielding and or distance works out.
Sure did! It only ended up being about 1500 turns, honestly not too bad if a little boring and time consuming.
Had a spool of wire set up at an angle to get the right amount of tention and then rotated the transformer in sets of 10 and kept track. Took a couple of hours, but with some good music in good headphones it didn't feel too bad 🙂
Will do! Or rather already done so nice that I don't have to rotate it! 😁
Also ended up boxing in the OPTs together in an aluminium box just by necessity of the physical layout of my case, not sure how well aluminium does at shielding magnetic fields but it's probably better than nothing 🙂
Attached a picture of how it's looking from the front atm 🙂
Power light, volume slider, left and right chanel VU meters and heater and HT is the plan for the small round meters and the tubes are a pair of ECC88s!
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I'll grab a picture tomorrow, here's a sketch:
It's pretty late so sorry for the poor quality 😅 the ones without full outlines are bellow the aluminium sheet the tube sockets mount too.
This box felt huge when I grabbed it yesterday, today it don't feel so big 😂
Here is the interior: OPTs will be mounted bellow the aluminium piece the sockets are mounted too, between the tube sockets and the divider wall in the "x-axis"
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I wind my own power toroid tfmr's and have used aluminum foil when I had no copper foil to reduce hum on a SE amp I built where the power tfmr was inside the chassis. I taped the foil in place using kapton tape and earthed it by stripping about 3/4" of good multistrand copper wire, fanning this out, then secured with tape: it worked.
Andy.
Andy.
Alright, sounds very similar to the copper tape technique. Good to hear that it's been used with success using aluminium foil. I do have aluminium tape around I could use if I need some hum reduction so maybe I'll try that. Gona finish the amp first though, hopefully I'll be done today, pretty much half way through the PSU at the moment so if everything flows on well I could be ready for a first flick of the power switch tonight 😊
Remember to check your work, then check it again, go and have a cup of your favourite beverage, then re-check. That said valves are pretty forgiving not like three legged fuses but every amp I've built I usually make a few daft mistakes.
Good luck, Andy.
Good luck, Andy.
I tried steel partition as you desribed, an it had zero effect. Steel shielding caps on the tubes had zero effect too.
Mumetal partition had some effect, though. But don't expect miracles.
Well, I got it fired up and playing. First had the volume pot wired wrong but that was quickly fixed.
Now it plays, but it sounds really awful. There is no bass at all and it's not doing much of the amplification part as it's quieter at max than just my headphones from the source 😅
Something is up with the OPT wiring for one channel at least as for a while I only got output with the scope hooked up and then that fixed itself...
Also need to adjust the PSU as I don't think ECC88s should have 160-170 volts over them.. Could probably just swap taps on my power transformer because I was already suspecting this might happen and made a lower turns tap 😁
Anyway for what's relevant to the topic, I might put copper tape on the power transformer as there is a small hum with the input grounded, but this was also with half of the case missing as I was probing a bunch of stuff so maybe that will go away. Will work on the other issues first.
Now it plays, but it sounds really awful. There is no bass at all and it's not doing much of the amplification part as it's quieter at max than just my headphones from the source 😅
Something is up with the OPT wiring for one channel at least as for a while I only got output with the scope hooked up and then that fixed itself...
Also need to adjust the PSU as I don't think ECC88s should have 160-170 volts over them.. Could probably just swap taps on my power transformer because I was already suspecting this might happen and made a lower turns tap 😁
Anyway for what's relevant to the topic, I might put copper tape on the power transformer as there is a small hum with the input grounded, but this was also with half of the case missing as I was probing a bunch of stuff so maybe that will go away. Will work on the other issues first.