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wigginjs 17th August 2008 07:46 PM

Transformer hum on leach amp, turn on / off thump
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 17th August 2008 08:17 PM

Could be leaky filter caps.

rocco gibralter 17th August 2008 09:26 PM

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 17th August 2008 09:55 PM

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

djk 18th August 2008 12:14 AM

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

jaycee 18th August 2008 03:40 AM

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 18th August 2008 02:37 PM


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 :D

wigginjs 18th August 2008 06:48 PM

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.


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

jaycee 19th August 2008 04:03 AM

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 19th August 2008 01:06 PM


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.

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