Transformer hum on leach amp, turn on / off thump

[wigginjs]

What causes transformer hum when I turn on an amp? Also turn on / off thump? This amp hasn't always done this, but it's been sitting for a while. Any ideas?

[DigitalJunkie]

Could be leaky filter caps.

[rocco gibralter]

Could be a grounding issue, try cleaning up the chassis earth connection or re'seat it, maybe turn the transformer a few degrees can help>>>> if its Toroidal!

[AndrewT]

do we want this moved to the permanent "Leach amp problem" thread?

[djk]

Doesn't seem like a design specific complaint to me, it seems rather generic in nature.

[jaycee]

Is it a self build, or a Leach that was pre-built and sold?
If it's a pre-built chances are the smoothing caps have had it, and could be going short hence causing the toroid hum.

It could also be DC on your mains but i'd guess youd notice other devices buzzing too.

[h_a]

What causes transformer hum

DC - either on primary or secondary side. On secondary easily avoided by using 2 bridges (if symmetrical rails are used), on primary there exist DC-filters.

The probably annoying noise aside it shouldn't do any harm.

Have fun, Hannes

PS: the mechanism behind turn on/off thumbs is also still unclear to me; I sometimes hear that different impedances on the balanced inputs can cause this (blame your preamp) or that the caps for both rails need different times to charge up. Maybe someday I have a satisfying reply to this

[wigginjs]

Actually, the transformer doesn't continue to hum. There is a hum when I initially turn it on that fades out within 3 or 4 seconds. Then it is quiet. At the same time there is an initial "thump" that forces my woofer and mid-range drivers to peak excursion in my passively crossed speakers.

rocco:

Could you please elaborate on "grounding issue"? What should I look for?

[jaycee]

Ah, this sort of turn on thump is often normal. The buzzing is simply the transformer under stress as it tries to charge empty capacitors.

It's possible your amp had some kind of power-on delay circuit which has stopped working, and this is why you get the thump.

[wigginjs]

jaycee:

I didn't add any additional circuitry to the amp and the leach amp doesn't have any sort of integrated "soft start" circuit. I did, however, add some additional line-level active filters at one point (a linkwitz transform and additional EQ), but I have since removed them. As I recall, the thump and buzzing started to occur when I removed those circuits, but all they were really attached to is the RCA inputs and I've restored the original connections just as they were.