Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems 557 Amplifier with Noisy Toroid

Shaun,

PM sent.


Cheers,


Steve

Gomez,

To change from the 240v primary winding to the 220v primary winding, you should desolder the blue wire coming from the toroidal transformer and then solder the red wire wire to the pin that the blue wire was removed from. The red wire should be presently isolated with it folded and secured with a black tie. After you remove the blue wire, you should isolate and secure it in a similar manner. You do not have to touch the brown wire as it is the common active wire for a 240v or a 220v setup.

The wires we are talking about are from the transformer itself and not the blue and brown wires that can be seen in your photos coming from the back of the plug going up the to circuit board on top of the transformer cover plate.

Cheers,
Steve

Hi Steve,

Are you sure...?

This is contrary to the details in the tech note TN16 listed in 557 section on Duncan's site?
Which says Blue is unchanged and always connected (Neutral)
Then use one of the two live feeds:
Red is 220V
Brown is 240V

I must say that seems more 'logical' with UK mains colouring

Rick

Yes be careful. Change between brown and red. Blue stays put

C

Thanks Rick,

Yea your right I tried the way it's explained on the tech note i.e. replacing the Brown with the Red, but it popped the fuse. If you look at the photo's I posted they are in-fact different to the wiring in the tech note.

I have just tried Steve's way round, removing the Blue wire from transformer at the PCB with the relays above the transformer and put the Red to that terminal and it now works.

Well that's the good news! The bad news is The Bugger still hums in-fact it maybe louder but then again I maybe imagining that.

I also still get a Transient about 20 seconds or so after power of as the voltage rails collapse, according to my meter it's about -3.2v as they finally collapse.

I must say a massive "THANK YOU" to Steve for taking the time and effort to help with this Amp.

Regards Paul

Hi Rick & Chris,

I was aware of what the Tech note said but I based my instructions above on the transformer label that the link to the photos above showed for this specific amp. For both the 220v and 240v options the brown wire was labelled as "L" which I assumed as representing "Live". In the 220v option, red was labelled as "N" which I assumed as "Neutral" and the Blue was labelled "N/C" which I assumed stood for "Not Connected". For the 240v option the labelling on the Red and Blue wires is reversed, hence my the reason for my instruction above, i.e. there was method in my madness.

If someone goes to the trouble of creating a legend on a label and then sticks it on the inside of an electrical component, then I would lean towards following the label legend rather than conventional wisdom or the Tech Note.

Fortunately the fuse did what it was meant to do anyway before any damage was done.

Cheers,
Steve M

Hello all!

This thread is ancient so there might be no replies to this post.

I just want to write and sincerely THANK YOU - especially SteveM for the description of reconfiguring the voltage on the 557.

My pair of 557's have been humming along since I got them - way back. Not too worried about the humming cycle but at night it could definitely be heard. On one of them the humming was louder to the point of disturbing at times.

A few months back I decided to switch out parts of my system and packed away the 502 preamp.
The new preamp has some creature comforts that the missus and I appreciate. But I got ground noise on top of the humming and after some consideration I ran a ground wire from the preamp phono ground to a chassis screw on the back of the 557 which completely sorted the ground noise.

Since I only use one of my two 557's I chose the quieter of the two for this setup. I figured there has got to be a way to do at least something about the transformer noise. I have had them for fifteen years give or take so if not now - when?

Searched around. Found this thread. Dwelled on it a month. Opened up the noisy one.
Tightened the transformer center bolt (others were solid) and did the cable switch (Blue switched out to Red) and... ...quiet. Not just quiet-er. Quiet.
Almost thought I messed up and broke it. Zero humming noise.

Did the other one in about fifteen minutes from start to finish - also quiet. Dead quiet. I am amazed. And extremely grateful - this simple procedure has given new life to my old power amps!

Funny thing is that the Red cable was in the "wrong position" and ran from the Transformer core to the top and a Brown wire was isolated and tied off at the same spot as the Red one was on the first unit I fixed. Thanks to this thread and the pictures linked in the thread I opened them up with confidence and after tracing the Brown/Red wires on the second one I was sure I was not going to blow a fuse.

Long rant, but you guys really have really done me a solid favor!

I have two 502's stowed away I no longer use, and a the fixed 557 as a spare. Hopefully I can get some use out of them further down the line.

I have a noisy 557 also, but only when using the XLR (with using Oehlbach endcaps). I tried also different xlr cables

https://oehlbach.com/en/protection-...RAlH7sFqBD8yDeD2TLqjUquABwDPNEr1Kc37RaOj

But when using the RCA connections there is almost no noise. Is this normal?
Should I try the above mentioned solution, or is this not applicable because RCA seems to be working fine.

Those Oehlbach endcaps don't short the RCA inputs, so you are not using the amplifier properly, which may cause the hum.

You need to use something like this (Link)

All the best,

Steve

Aha super thanks for reply.
Will try this solution.

As I see here in NL packages of 16 or more pieces.
Will it be an advantage to use this in not used inputs/outputs on G65? (Or should this not be done at all?)

(Is there a difference using these for input vs output?)

The shorting plugs can be used on the unused inputs; it will ensure that there are no signals on the unused inputs that could be picked up from EMI / RFI sources that then could crosstalk onto the used inputs and or to other circuits.

They shouldn’t be used on outputs as you’d be shorting the output circuit, the non shorting caps could be used there to protect from dust ingress.

All the best,

Steve

Great, the 557 is complete silent at the moment when using XLR.

But when I use Marantz sr7015 pre out to G65 now there is a hum. ???? (That for dolby atmos).
This wasn’t the case before. Any ideas?