|
Studio Monitor amp transformer mystery |
Post Reply 
|
Page <1234> |
| Author | |
Tedski 
Registered User 
Joined: 15 December 2006 Location: Netherlands Status: Offline Points: 453 |
 Post Options
 Thanks(0)
 Quote Reply
 Posted: 10 January 2026 at 4:24pm |
Ok. Between the washer and the transformer and on the base of the transformer are pretty thick rubbery insulating discs, so it's not metal to metal.
|
|
 |
|
|
fatfreddiescat
Young Croc 
Joined: 15 October 2010 Location: N.E.Wales Status: Offline Points: 1128 |
Post Options
Thanks(0)
Quote Reply
Posted: 10 January 2026 at 5:17pm |
That earth wire could be creating a shorted turn, a loop around the transformer core if there is metal to metal connection with the top plate, bolt and amp chassis.
|
|
|
|
|
Tedski
Registered User
Joined: 15 December 2006 Location: Netherlands Status: Offline Points: 453 |
Post Options
Thanks(0)
Quote Reply
Posted: 10 January 2026 at 5:21pm |
|
I'll open them up asap and remove, thanks all.
|
|
|
|
|
Tedski
Registered User
Joined: 15 December 2006 Location: Netherlands Status: Offline Points: 453 |
Post Options
Thanks(0)
Quote Reply
Posted: 27 January 2026 at 4:11pm |
|
I removed the pointless wire, and replaced the big capacitors (same spec, nice brand). I'm afraid the slight hum is still present. What I noticed while checking the voltage on the capacitors prior to soldering them off the board is that one of them read a negative voltage. Only slightly, 0.6V. But the 'good' amp didn't have that. Could that be a clue as to why that one is humming? Is the bridge rectifier dying? I have no experience with this, that's why I'm asking. It has become a bit of a mission to get these b@stards 100% silent. Thanks for any help.  Edited by Tedski - 27 January 2026 at 4:14pm |
|
|
|
|
kedwardsleisure
Old Croc 
Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 5157 |
Post Options
Thanks(0)
Quote Reply
Posted: 27 January 2026 at 4:34pm |
|
it's possible its partially open, eg one of the 4 diodes is open. But the increased ripple on one of the transformer tappings might lead to an audible buzz from the actual windings.
|
|
|
Kevin
North Staffordshire |
|
|
|
|
Tedski
Registered User
Joined: 15 December 2006 Location: Netherlands Status: Offline Points: 453 |
Post Options
Thanks(0)
Quote Reply
Posted: 27 January 2026 at 4:55pm |
|
It is 100% coming from the 8 inch driver, not the newly replaced transformer. No input present or even connected. The potmeter has no effect on hum level. I just measured between the earth on the power connector and a radiator on the central heating, 0.6V AC. Just recap everything? Also I was searching for a new bridge rectifier, and I could find similar components, but they were labeled PC and not CP 604. Does that matter? Thanks.
Edited by Tedski - 27 January 2026 at 4:59pm |
|
|
|
|
kedwardsleisure
Old Croc
Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 5157 |
Post Options
Thanks(0)
Quote Reply
Posted: 27 January 2026 at 8:05pm |
|
no its fine, any 6A rectifier would do, 604 means 5a 400v but I doubt it needs to be that high voltage...200V would do.
If you have a multimeter you can test it on diode check, it's just 4 diodes in one package. |
|
|
Kevin
North Staffordshire |
|
|
|
|
Earplug
Old Croc
Joined: 03 January 2012 Location: Europe Status: Offline Points: 7752 |
Post Options
Thanks(0)
Quote Reply
Posted: 28 January 2026 at 6:54am |
|
Getting rid of random hum like that can be a PITA. Try put a couple of
100nF caps across the large filter caps. See if that helps. Or it may be
one of the LM3668's. Odd, but you never know. Is it definitely the woofer & not the tweeter - or both? That would be a clue.
|
|
|
Earplugs Are For Wimps!
|
|
|
|
|
kedwardsleisure
Old Croc
Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 5157 |
Post Options
Thanks(0)
Quote Reply
Posted: 28 January 2026 at 8:38am |
|
I meant to type 'six amp' BTW, late night typo.
|
|
|
Kevin
North Staffordshire |
|
|
|
|
Tedski
Registered User
Joined: 15 December 2006 Location: Netherlands Status: Offline Points: 453 |
Post Options
Thanks(0)
Quote Reply
Posted: 28 January 2026 at 4:11pm |
|
While researching this subject I stumbled upon a few forum threads of people building their own amps and I learned a few things: Contrary to al lot of information(wikipedia), the jury is still out on whether torroidal transformers emit more or less interference that a iron core one. That got me thinking, maybe I assumed the hum was present before I changed the transformers. Maybe the transformer is actually causing the humming. So I cracked open the cabs again. Another thing I read that seemed to be backed up by measurements, is that even the orientation of the torroidal transformer can make a difference in interference, so I started with rotating the transformer and powering up the amp. No difference. So then I grabbed the shielding that was around the old transformers and modified them to fit the new ones. Seems to be galvanized sheet steel. I wrapped it in tape to insulate. And presto, the speakers are quite a bit quieter. Not completely silent, but the hum is pretty much inaudible at listening distance. Maybe I'll bend up a chunky piece of metal as a cage around the pcb to get it even quieter. Steel or aluminium do you think? Connected to ground or no? What a faf, but I learned a lot, thanks again for all the advise! Edited by Tedski - 28 January 2026 at 4:29pm |
|
|
|
|
kedwardsleisure
Old Croc
Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 5157 |
Post Options
Thanks(0)
Quote Reply
Posted: 28 January 2026 at 4:40pm |
|
toroidals emit much less stray magnetic fields than EI lam transformers, it's inherent in their design.
The stuff wrapped around the old transformer is a piece of mu-metal sheet (google it). Did you check the rectifier? If you changed both transformers, why does one hum and not the other? Asymmetric ripple from a dodgy rectifier will cause increased electromagnetic radiation from the windings, so it's worth checking just in case. Some rectifier faults in old colour TVs were betrayed by a swirling pattern on the CRT caused by the transformer affecting the colour purity as its magnetic emission increased. |
|
|
Kevin
North Staffordshire |
|
|
|
|
Tedski
Registered User
Joined: 15 December 2006 Location: Netherlands Status: Offline Points: 453 |
Post Options
Thanks(0)
Quote Reply
Posted: 28 January 2026 at 4:50pm |
|
Ahaaa, good info, thanks. I'll order new rectifiers and report back.
|
|
|
|
|
Post Reply
|
Page <1234> |
| Tweet |
| Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum |
Topic Options
kedwardsleisure wrote: