Well, as the title suggests I have the usual problem that also occurred with a previous amplifier I made... I suppose it can be the magnetic coupling between the power transformer(s) and the OT's.
Or rather it is more evident in the speaker coming from the central OT (closer to the toroidal tunnel) than in the more external one, and therefore further away, where the hum is barely perceptible to the speaker by placing an ear upon the cone.
I had told my carpenter friend to make me a wider frame, but he was wrong with the measurements and I had to settle with that...
Now my question, to avoid running into the same problem in the future is there an optimal method to minimize this coupling? I mean orientation between transf and (possible) power choke plus suitable spacing between these elements...
The support plate is made of aluminum as is the toroidal cover.
Thanks in advance
Or rather it is more evident in the speaker coming from the central OT (closer to the toroidal tunnel) than in the more external one, and therefore further away, where the hum is barely perceptible to the speaker by placing an ear upon the cone.
I had told my carpenter friend to make me a wider frame, but he was wrong with the measurements and I had to settle with that...
Now my question, to avoid running into the same problem in the future is there an optimal method to minimize this coupling? I mean orientation between transf and (possible) power choke plus suitable spacing between these elements...
The support plate is made of aluminum as is the toroidal cover.
Thanks in advance
Attachments
As a test, you might swap the wires to the OPTs and see if the hum follows the wiring or stays with the transformer. Once you've determined that, you could try rotating the inboard OPT 90° and see if that quiets things down. It looks like you could slide the inboard OPT over quite a bit away from the PT. Just musing here though. I usually energize the PT and with some earbuds hooked to the OPT determine which way is quietist. I do the same with separate filament transformers as well.
It's kind of hard to tell, but if the power supply choke is underneath the output transformer, its magnetic field may affect the transformer as well. Try moving the choke.
Air Gapped Single Ended Output Transformers, and Magnetic Steel Chassis . . . causes Hum.
(well not if you use a separate chassis for the power supply; and keep those chassis apart).
Chokes are commonly air gapped, and a magnetic chassis will pass the magnetic fields to the air gapped single ended output transformers.
I have been known to use aluminum standoffs and non magnetic screws, to elevate the air gapped transformers from the magnetic chassis.
I would also argue that magnetic steel chassis can reduce the separation of L and R channels, if you use Air Gapped output transformers.
Once you have addressed all those potential causes of Hum . . .
It is time to fix any and all Hum Ground Loops.
That is another subject, with other hints and kinks.
(well not if you use a separate chassis for the power supply; and keep those chassis apart).
Chokes are commonly air gapped, and a magnetic chassis will pass the magnetic fields to the air gapped single ended output transformers.
I have been known to use aluminum standoffs and non magnetic screws, to elevate the air gapped transformers from the magnetic chassis.
I would also argue that magnetic steel chassis can reduce the separation of L and R channels, if you use Air Gapped output transformers.
Once you have addressed all those potential causes of Hum . . .
It is time to fix any and all Hum Ground Loops.
That is another subject, with other hints and kinks.
The cover of the PT is aluminium.
I tried to rotate the toroidal inside keeping the points where the wires come out away from OT. In a first time it seemed to have a smaller hum in the right channel, but it's tricky to find the right position (the magnetic filed is complex by itself).
The suggestion to swap the wires beetween the OTs is good. I didn't tried yet and is a simple test...
6A3sUMMER, the meral plate where thransformers lay on s aluminium too. BTW now I orderd a mu-metal sheet and I mean to wrap it internally inside the PT cover.
I tried to rotate of 90 deg. the choke but nothing change. May I have just to change the location inside the cassis (physically away from OTs)?
Supposed magnetic coupling with OTs
You have a toroidal power transformer... Despite what is usually said about them, this kind of transformer is far from being free from parasitic radiations of all kinds : hum (magnetic), buzz, spikes... I speak about my personal experience with toroidal power transformers, which gave me unpleasant surprises !This one below - despite being very tiny and having its transformers quite close each other - doesn't show any hum due to magnetic coupling. This comes from the orientation of the main magnetic axis radiation of their cores...
Power transformer is left-right, choke is vertical, output transformers are front-rear. All are EI transformers, with their magnetic axis at 90° each other. Conversely, the radiation of a toroidal transformer is rather omnidirectional, and so more difficult to deal with - that's what I noticed... Unfortunately :
Many years ago I built a dual-channel guitar amp using a toroid power transformer :
The amp was so full ridden with hum, buzz, adverses noises that it was unusable as is 😡... I changed for another toroid, and installed it in a shielding :
The improvement was poor, and in no way acceptable 🙁... So after numerous tests, I shifted to a classic EI transformer, suitably orientated, and fitted with magnetic "wing" screens, made from 3mm thick aliminium :
All hum, buzz and adverse noises were eradicated. End of the story... Merci, au revoir ! 😎
Needless to say that now I am suspicious about toroidal transformer's "reputed quietness properties"... 😕
T
Low-voltage/high-amperage wiring is a source of magnetic field radiation that may prevail stray field of toroidal mains transformers.
You have to be quite accurate with the twisted filament wiring.
You have to be quite accurate with the twisted filament wiring.
Supposed magnetic coupling with OTs
You have a toroidal power transformer... Despite what is usually said about them, this kind of transformer is far from being free from parasitic radiations of all kinds : hum (magnetic), buzz, spikes... I speak about my personal experience with toroidal power transformers, which gave me unpleasant surprises !
This one below - despite being very tiny and having its transformers quite close each other - doesn't show any hum due to magnetic coupling. This comes from the orientation of the main magnetic axis radiation of their cores...
Power transformer is left-right, choke is vertical, output transformers are front-rear. All are EI transformers, with their magnetic axis at 90° each other. Conversely, the radiation of a toroidal transformer is rather omnidirectional, and so more difficult to deal with - that's what I noticed... Unfortunately :
Many years ago I built a dual-channel guitar amp using a toroid power transformer :
View attachment 1338414
The amp was so full ridden with hum, buzz, adverses noises that it was unusable as is 😡... I changed for another toroid, and installed it in a shielding :
View attachment 1338416
The improvement was poor, and in no way acceptable 🙁... So after numerous tests, I shifted to a classic EI transformer, suitably orientated, and fitted with magnetic "wing" screens, made from 3mm thick aliminium :
View attachment 1338417
All hum, buzz and adverse noises were eradicated. End of the story... Merci, au revoir ! 😎
Needless to say that now I am suspicious about toroidal transformer's "reputed quietness properties"... 😕
T
You brought a quite a good explanation about the behaviour of toiroidak transformers. They can be joy or pain.
A part of strong magnetic flux and the difficult of orientation/placement couldn't be depending on manufacturer?
I remember some time ago I made "Lo Scherzo" with a toroidal one I employed placing it inside the chassis (always wooden frame) and it was silent, no hum at all.
I think it was of quality, the actual PT I arranged here is made in China. This can be a problem, I think.
Another thing, I'm using at the limit of the manifacturing data of current (120mA - 350V versus 115mA I'm using). It's well known that they are often undersized.
So, I can do all the trials you want but if the trouble is on the "root" it's a waste of time maybe.
I'm taking my considerations on what is appropriate to do...
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It is the main problem
I always ask to my supplier a toro vernished and coocked
The the calculation of the winding is normally done at 8000 gauss that is around the 50% lest than normal + a screen between primari and secondary
And an external magnetic coil
In addition the hole is with resin and a little socket to fix it
In the preamp there is a metal round box where the trafo is potted
This is the way to get the right results
Yes, I figured it could depend on that, as was quite obvious... now you can confirm that it is even basic.
Unfortunately the transformers are the most expensive but also the most important part, skimping on those does not bring good results: I always delude myself, no matter how good the layout and other components are, it is not enough).
We have to take note of it...
Unfortunately the transformers are the most expensive but also the most important part, skimping on those does not bring good results: I always delude myself, no matter how good the layout and other components are, it is not enough).
We have to take note of it...
Yes - toroids have an even poorer reaction to loading than a EI lamination in terms of radiations. In your case, it possibly increases the magnetic / electrostatic hum issue you front...
T
If I were you - and according to my personal experience - I would discard the toroid and go for an EI transformer of suitable power and size... 🙂
Since you are in Italy, check for PIEMME Elektra transformers : made in in Italy, good products at correct prices... 😉
https://shop.piemme-elektra.it/Catalog
T
Perhaps it is worth trying the flux belt around the mains transformer (as suggested, it is best to use an EI core):
http://richardschurmann.com.au/Other/Electronic_Burrow/?p=248
In case of a toroid mains transformer, the flux belt principle could be tried on the output transformers, to minimize stay magnetic field pickup. It works both way.
Shielding a toroid is not easy, if possible at all. The best solution is to place it as far from the OT as possible.
http://richardschurmann.com.au/Other/Electronic_Burrow/?p=248
In case of a toroid mains transformer, the flux belt principle could be tried on the output transformers, to minimize stay magnetic field pickup. It works both way.
Shielding a toroid is not easy, if possible at all. The best solution is to place it as far from the OT as possible.
Work to get the least hum that is caused by magnetic coupling.
If you replace a toroid with a traditional EI laminations transformer, consider a higher voltage secondary so that you can use Choke input B+ filtering.
The transient currents of capacitor input B+ filers can be 10 times more peak current than choke input B+ filters.
And capacitor input filter transient current is very fast rise, lots of upper harmonics all the way to the ears most sensitive frequency range,
versus choke input filters much faster high frequency roll off.
Think: Bandwidth = 0.35/rise time (very approximate, the cap input filter and associated parts does not comply with simple Gaussian single low pass filter; but the Gaussian factor of 0.35 is a good starting place.
I use choke input filters fror B+, but I have sometimes used 1, 2, 3, or 4uF cap before the choke to get the exact B+ voltage I want.
In spite of the small input cap up to 4uF (not the 10, 47, or 60uF of common cap input filtering), I have been getting less than 100uV of hum to the speaker (not good enough for headphones, unless you use an output attenuator, which is needed anyway to protect your ears, they are irreplaceable..
. . . Note: that low amplitude of hum is also dependant on solving all the Hum Ground Loops.
There is more than 1 way to get less than 100uV hum; it was difficult for me at first, but I found a way that worked for me.
Have Fun!
If you replace a toroid with a traditional EI laminations transformer, consider a higher voltage secondary so that you can use Choke input B+ filtering.
The transient currents of capacitor input B+ filers can be 10 times more peak current than choke input B+ filters.
And capacitor input filter transient current is very fast rise, lots of upper harmonics all the way to the ears most sensitive frequency range,
versus choke input filters much faster high frequency roll off.
Think: Bandwidth = 0.35/rise time (very approximate, the cap input filter and associated parts does not comply with simple Gaussian single low pass filter; but the Gaussian factor of 0.35 is a good starting place.
I use choke input filters fror B+, but I have sometimes used 1, 2, 3, or 4uF cap before the choke to get the exact B+ voltage I want.
In spite of the small input cap up to 4uF (not the 10, 47, or 60uF of common cap input filtering), I have been getting less than 100uV of hum to the speaker (not good enough for headphones, unless you use an output attenuator, which is needed anyway to protect your ears, they are irreplaceable..
. . . Note: that low amplitude of hum is also dependant on solving all the Hum Ground Loops.
There is more than 1 way to get less than 100uV hum; it was difficult for me at first, but I found a way that worked for me.
Have Fun!
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Here again, you can see the orientation of the EI transformers :
Direction 1 = Core axis maximum radiation.
Direction 2 = Laminations plane intermediate radiation.
Direction 3 = Minimum axis radiation.
As you can note, the directions 3 are "coincident" for minimum radiation. On such a compact unit, a Toroidal transformer would have been a disaster !
All transformers are EI magnetic circuits. The power transformer was intended to a Marshall 50W 2xEL34/6CA7 in class AB fixed bias push-pull. It is spec'ed for 690V CT / 350mA, while the need of the circuit is circa 110mA class A cathode bias operation.
A+!
Direction 1 = Core axis maximum radiation.
Direction 2 = Laminations plane intermediate radiation.
Direction 3 = Minimum axis radiation.
As you can note, the directions 3 are "coincident" for minimum radiation. On such a compact unit, a Toroidal transformer would have been a disaster !
All transformers are EI magnetic circuits. The power transformer was intended to a Marshall 50W 2xEL34/6CA7 in class AB fixed bias push-pull. It is spec'ed for 690V CT / 350mA, while the need of the circuit is circa 110mA class A cathode bias operation.
A+!
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