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Old 9th Dec 2015, 5:16 pm   #101
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Default Re: EL34 power amp project

Is it transformer noise or hum coupled to the output transformers, this has caught me out before.
 
Old 9th Dec 2015, 6:13 pm   #102
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Default Re: EL34 power amp project

They are back in their packaging. God knows how much the postage is on a 10 kilo package............... but they will be sent back tomorrow for the makers to inspect.
Majestic were very concerned that the transformer was mechanically noisy, they explained to me that they expected one of their products to be "silent" both in free air so to speak and when bolted down. I asked about the drop through issue and I was told it shouldn't make the transformers buzz.
I have tried various permutations but the things hum whatever I do. top cap on top cap off. fibre washers underneath and under the bolts on top of the top cap. Different types of isolation material also tried.
I broke out the second transformer and wired the primary and earth wire on, left the rest unconnected. Just as bad and I didn't undo any of the supplied mounting bolts and washers just in case I had done a boo boo.
Unless I have a too high expectation, but I expect as a minimum to have the same freedom from buzz as my Leak stereo20 mains transformer (totally silent) or my World designs kit KT88 amp which I can only just hear the windings buzz if my lug 'ole is against the transformer cap. I know for that amp I have to use a strategically placed blob of "Blu Tac" on the top cap to damp a wee bit of resonance but its very minor noise.
Anyway the matter is back in the hands of the makers. They told me today that end caps and top mounting wouldn't be the cure, that the transformer should be mechanically quiet whatever its mounting method. And were very concerned that one of their products hadnt given total satisfaction. Can't speak fairer than that.
A.
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Old 9th Dec 2015, 6:15 pm   #103
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Default Re: EL34 power amp project

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Originally Posted by merlinmaxwell View Post
Is it transformer noise or hum coupled to the output transformers, this has caught me out before.
Transformer noise. Mechanical vibration.

A.
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Old 9th Dec 2015, 6:24 pm   #104
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Default Re: EL34 power amp project

One sideways thought...

What's the mains waveform like in your location? Badly distorted mains can affect acoustic noise from transformers.

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Old 9th Dec 2015, 6:55 pm   #105
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Default Re: EL34 power amp project

Good question.
I dont know but I suspect its fairly mucky especially at milking time.............
But that would surely affect the existing kit I own with large mains transformers and they aren't noisy.
And I did try different mains sockets as the utility room where i do my stuff is fed by it's own ring main which also does washing machine tumble dryer dishwasher etc. I've even got a single socket spur off the main board in the porch that I have considered running an extension into the lounge for.
Anyway they are going back to majestics for a check over. If they are ok then I know its a mechanical issue.
A.
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Old 9th Dec 2015, 7:04 pm   #106
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Default Re: EL34 power amp project

Heres a thing.
Why is it so hard to buy a mounting kit for a transformer?
Just thinking about it, most manufacturers supply a set of cheapo steel bolts with a handful of star washers and some fibre washers. But some of us would like something a bit nicer cosmetically for starters and things like the top hat insulators that David mentioned in a previous post. And Accoustic isolation materials. Mostly I have been fumbling around in the dark with assistance from the chaps on the forum. I still have a niggling feeling I have done something fundamentally stupid that I just can't see.
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Old 9th Dec 2015, 8:16 pm   #107
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Default Re: EL34 power amp project

Are the transformers at fault I have heard that sometimes the mains supply can have a small dc component that can cause buzzing ? please look into this does the buzz come and go is it worse at different times
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Old 9th Dec 2015, 9:20 pm   #108
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Default Re: EL34 power amp project

read post 105

A.
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Old 9th Dec 2015, 11:31 pm   #109
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Default Re: EL34 power amp project

Understand what you mean about questioning your own standards - i think building amps has made me hear humming in empty rooms with everything turned off!
Mechanical hum has been the bane of my life, i had blamed it on crappy mains mine is normally up over 250v so most modern transformers are over-voltage to start with. But with variac reducing primary volts to 220v, and presumably blocking any DC, they still hum. My anti-vibration mounts work but i do feel it is cheating and would love to understand the problem properly. Hope you get to the bottom of it with Majestic, sounds like they're on side at least.
Glyn
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Old 9th Dec 2015, 11:55 pm   #110
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Default Re: EL34 power amp project

Insulating kits for stud mount SCR's and Triacs are available that should get you out of trouble with insulators.
I use brass dome nuts on top of chassis mounted transformers as a cheap aesthetic trick, which does work well with my fancy brass chassis amps. You can also get dome nuts in chrome. I use studding ( threaded rod) instead of bolts/screws to hold the transformer to the chassis.

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Old 10th Dec 2015, 12:35 am   #111
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Default Re: EL34 power amp project

To give you an idea:
Pic one is a KT88 amp I made a few years ago and used polished brass dome nuts on the output transformers. One way to "trick the eye". Underneath the dome nuts are the aforementioned SCR insulating bushes.
Pic 2 is using socket head bolts to make for a more "industrial" look on the 807 amp I made a lil while ago. Underneath the shiny washers are, again, the insulation bushes for SCR's
Pic 3 is an underside of the 807 amp, showing NO SPECIAL mounting of the output transformers ( I made these myself)

Other pics are general views to show my constructions.

Joe
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Old 10th Dec 2015, 7:16 pm   #112
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Default Re: EL34 power amp project

Nice work Joe. Lovely neat wiring too, makes me feel a bit inadequate!
Yeah I used stud bar and dome nuts on my Kit amp instead of the supplied "el Cheapo" bolts and they are nice and silent with proper base isolators to keep the gap so to speak.
I think I have been unlucky with the transformers and the new ones are back at the makers by tomorrow for a comprehensive check over. I did wonder if the over specifying by myself which meant they had to use a different and possibly thicker laminate stack to allow for a 250VA rating has meant that there's more flux density than usual for one of these things? I should have stuck with a 200mA job maybe, but that said the heater windings arent particularly over specified.
On the other thread I have been interested in how people impregnate transformers, but at my level of poverty and lack of knowledge I neither have the means nor the knowledge to invest in and use equipment to force impregnant in.
Is the wax as used in the Hinchley transformers in old Leak amps a special sort? I know that in Australia ( I assume the hotter parts ) there were issues with the wax melting out in hot weather and I can see evidence of wax leakage on my old Stereo20.
I also suspect that modern "Elf and safety" concerns make it more difficult as most transformers seem to be wound on slackish bobbins with the primary and secondary kept separate, at least for mains transformers.

A.
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Old 10th Dec 2015, 8:34 pm   #113
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Default Re: EL34 power amp project

I'm quietly amused following this thread - there seems to be a discontinuancy between the audio-as-reproduced-through-the-speakers, and incidental 'noises-off' from the amplifier itself [which seemingly includes power-supply hum].

What are the actual audio-levels involved? When your audio-amp is being exercised to its capaciability, can you actually hear the residual sounds of power-transformers vibrating?

Pragmatically, I have more of an issue with my dog's snoring [and he's got his head stuffed into one of my speakers that's playing https://youtu.be/HLCCM-b5bMk ]
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Old 10th Dec 2015, 10:22 pm   #114
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Default Re: EL34 power amp project

Ok, I don't know where you get the idea that I have PSU hum issues, what hum there is on the audio was sorted very early on in the saga and any other refinements are exactly that. Just tweaking it down to inaudible with ears up to the loudspeakers.

I can hear the mechanical buzz from the transformers 12 feet away when the musics not playing or on very quiet passages. My other amps don't do it, ergo theres a problem.
The missus can hear it sat in her armchair about 5 feet away from the electronics.

There's no audible effect on the actual audio, and to be blunt if i bunged the full 37 watts through my speakers i'd be listening at next door but ones sofa. It's that loud.
That wasn't the target, I was aiming for 20 to 25 clean watts which is plenty for any normal Human.

However, and at the risk of repetition, "good enough" just ain't good enough for me. That was the whole point of this exercise, and apart from the annoying thrumming of mains transformers I reckon I have achieved a rather marvellous sounding bit of kit, with the help and encouragement of many of the experienced heads on the forum.

A.
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Old 10th Dec 2015, 11:52 pm   #115
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Default Re: EL34 power amp project

There is one thing we havent discussed ( I think) It could be magnetic coupling from mains to output transformer. Its not difficult to find out if thats so.
All thats required is a piece of steel sheet metal ( doesnt matter what its from) and ideally at least 1mm thick, and larger than the transformers. Two pieces are even better!!! Between the two transformers is a gap, which is where these pieces of steel sheet are inserted. That should short out any flux that is leaking between the two transformers. They dont need be screwed down. A couple of sheets of paper between the sheets helps, and if you fold it over the edge of the steel, you wont damage your flash paintwork.
In severe cases you can actually "feel" the steel vibrating if held gently, and not pressed against the chassis. This is crude "mumetal".

To take this further, if you look back at my pics of the 6BW6 amp, and the pics of the 807 amp, I have mounted all the iron in different positions, with the flux leakage areas pointing AWAY from the adjacent transformers or choke (s), and mounted as far as possible from sensitive valves ( hence the cans on the 12AU7's and 6BR7's) which are as far as practically, away from the iron clump.
With the KT88 amp, I put the mains transformers and chokes into steel welded cases to stop flux leakage.
If you look at photos of older commercial amps, you will see that I havent invented anything, and iron is always oriented to face away from each other.

Another two bobs worth, but may help in sorting the problem.

Regards
Joe
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Old 11th Dec 2015, 12:22 am   #116
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Default Re: EL34 power amp project

Gotcha Joe.
I dont think its the issue.
The transformers vibrate away from the chassis. and I do know enough about it now to know about orienting the transformers so they dont induce into each other.
But worth thinking about for sure.
A.
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Old 11th Dec 2015, 1:12 pm   #117
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Default Re: EL34 power amp project

Quote:
Originally Posted by Radio Wrangler View Post
One sideways thought...

What's the mains waveform like in your location? Badly distorted mains can affect acoustic noise from transformers.

David
See here too: https://www.vintage-radio.net/forum/...d.php?t=121181
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Old 11th Dec 2015, 6:46 pm   #118
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Default Re: EL34 power amp project

I am sure it is not the mains.
NONE of my other transformer supplied kit has this issue, therefore the only conclusion I can draw is that either the transformers are somehow faulty or that my mounting method is flawed and right now i can't see where I might have gone wrong.
All going on the back burner for a week or three as I doubt I'll have time to play with my stuff.
The New transformers are, as I write, back with the manufacturers who are going to give them a good check and if they find a fault they will resolve it.
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Old 11th Dec 2015, 8:55 pm   #119
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Default Re: EL34 power amp project

Take one mains transformer out of the amplifier and run it up with just a mains cable and no loads on the secondaries. Stand it on a cardboard box, upturned plastic bucket or something. No mountings, no steelwork near it. Is it now noisy or quiet?

Another thing worth doing with it off load is to measure the primary magnetising current.

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Old 11th Dec 2015, 10:17 pm   #120
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Default Re: EL34 power amp project

David,
They are back at the manufacturer.
Their problem at the moment.

I must have not made myself clear in previous posts, but I did detect vibration off the chassis.

The manufacturers are confident that their design should not vibrate as a drop through into a steel chassis.

Lets just see what I get back next week.

A.
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