Small amps, output transformer hum

[hearforever]

Is there anything you can do about output transformers picking up AC hum from the power transformer in a small amplifier? The problem is that the chassis is so small that the output transformer sits only a couple of inches away from the power transformer. Any tips?

[mooreamps]

I've actually been giving this one some thought myself. I am aware a few of the other builders who use the toroidal power transformers instead of the convientional EI core devices to abate this problem. I am also aware of the little headphone trick to listen to the hum through headphones, and then adjust the O/T on the chassis to minumize the radiated coupling. I am considering taking some copper foil, and making a small box to encapsulate the O/T, acting something like a farady shield. Just a thought.

-g

[Enzo]

Transformers work by magnetic field, and when those fields interact, transformers couple.

The main thing to do is mount the transformers so the laminations don't align. Notice on like a Fender Twin the power transformer has the laminations horizontal, parallel to the chassis, but the output tranformer is standing up, laminations perpendicular. Even on an amp where both transformers are standing up, turn one facing front-rear, while the other faces side to side.

If the laminations are oriented 90 degrees apart, then coupling is reduced. It might be as simple as turning one of your transformers 90 degrees.

[Paul P]

Hi-fi amps are good examples of what you can get away with.
Take a look at this amp :

http://www.dynakitparts.com/store/im...s/zoom/51C.gif

Hard to believe the thing doesn't hum.

Paul P

[Enzo]

Yep. Good example of the output transformers set perpendicular to the power transformer.

[jrfrond]

I've actually been giving this one some thought myself. I am aware a few of the other builders who use the toroidal power transformers instead of the convientional EI core devices to abate this problem. I am also aware of the little headphone trick to listen to the hum through headphones, and then adjust the O/T on the chassis to minumize the radiated coupling. I am considering taking some copper foil, and making a small box to encapsulate the O/T, acting something like a farady shield. Just a thought.

-g

A copper Faraday shield will only guard against electrostatic fields, not electromagnetic. For this, you would need Mu-Metal, the same stuff used to shield AV speakers, and even THAT might not work, because part of the problem might be chassis-borne induction. Some transformers have laminations wrapped with Faraday bands, but this is only for electrostatic interference as well.

When breadboarding a new design, transformer orientation is paramount, right up there with the grounding scheme. Enzo cited the Fender core-mount power transformers, but then vintage Marshall's all used vertical-mount. Then there is a third variable. There are some brands of transformers, most notably Hammond, that seem prone to hum with ANY orientation.

Position is everything in life, including here. You're distance is not as much of an issue as the orientation.

[mooreamps]

I actually do mount the two transformers at perpendicular angles, with respect to them both. But then again, those are the Hammond parts.

-g

[Synaptic Amps]

I wonder if your real hum problem is not transformer interaction at all but plain old DC ripple on your OT supply node. In the effort to cram an amp into a smallish chassis (and undersized budget) the choke and power supply capacitors are often amongst the first things to get downsized or nixed. The result can be a nasty almost sawtoothed DC ripple on the power supply point running the OT. If you're tapping the OT off the first filter stage in a SE design you need to make sure to give a sufficiently large reservoir to minimize the effect of the charge / discharge cycle and smooth out the ripple. Alternatively, try adding another stage of filtering in front of your OT supply node.

[Enzo]

That is real simple to determine. Is there hum in the speakers in standby? No standby? Pull th power tubes. If it still hums, it is most likely transformer coupling.

[mooreamps]

I wonder if your real hum problem is not transformer interaction at all but plain old DC ripple on your OT supply node. In the effort to cram an amp into a smallish chassis (and undersized budget) the choke and power supply capacitors are often amongst the first things to get downsized or nixed. The result can be a nasty almost sawtoothed DC ripple on the power supply point running the OT. If you're tapping the OT off the first filter stage in a SE design you need to make sure to give a sufficiently large reservoir to minimize the effect of the charge / discharge cycle and smooth out the ripple. Alternatively, try adding another stage of filtering in front of your OT supply node.

That is a very good point. However, seems to me the plates of the power tubes can tolerate a certain amount of ripple, even when running Single Ended. I measure about a volt and a half of ac ripple on the plates. The caps on the first tap of this HV supply is 100uF total capacitance, and also taking into account of the inductance of the primary winding of the SE output transformer winding, which is spec'ed at 9 Henrys. [ Yes, 9 Henrys ]..

However, it's the screen supply that which has more of an effect on plate current than the plate supply which is choke filtered, with a 120 cycle resonant bypass cap across the choke. The a-c ripple at this node is well under 50 millivolts rms. So, it's either shielding the output transformer, or it's coming in from the front end from the guitar.


-g