Monoblock hum- curable?

[mhuk]

I have two monoblocks - Monarchy Audio SM70.

One is silent, the other hums. Is this likely to be curable (ideally DIY and for free)?
Anything to prod or things to tighten under the hood?

It hums with no cables attached and with the power cable from the silent SM70 it still hums.

Ideas and suggestions please.

And if you remember my pre-amp question: a passive pre-amp may have been a better choice than the EC4.5 as the volume goes from quiet to loud with a 1 degree turn of the power knob.

[mrwippy]

mhuk wrote:

I have two monoblocks - Monarchy Audio SM70.

One is silent, the other hums. Is this likely to be curable (ideally DIY and for free)?
Anything to prod or things to tighten under the hood?

It hums with no cables attached and with the power cable from the silent SM70 it still hums.

Ideas and suggestions please.

And if you remember my pre-amp question: a passive pre-amp may have been a better choice than the EC4.5 as the volume goes from quiet to loud with a 1 degree turn of the power knob.

I take it this is a physical hum (vibration)as you say it does it with no cables attached and not hum through the speakers? If so I would check the mounting of the transformer inside as a starting point.

[mhuk]

There's no hum through the speakers. If I put my hand on top of the unit and lean on it the hum lessens slightly.

Cheers for the suggestion, I'll have a look. Any components to avoid touching inside?

[mrwippy]

mhuk wrote:

There's no hum through the speakers. If I put my hand on top of the unit and lean on it the hum lessens slightly.

Cheers for the suggestion, I'll have a look. Any components to avoid touching inside?

Sounds like the transformer, steer clear of the capacitor terminalsas they can hold a charge for a long time.

Actually, if you don't make a habit of delving inside this stuff perhaps I should say everything! I assume you will unplug before play commences?

[mhuk]

Tightening the bolt through the centre of the transformer has reduced the hum a bit but I'm wary of over tightening it...

[mrwippy]

mhuk wrote:

Tightening the bolt through the centre of the transformer has reduced the hum a bit but I'm wary of over tightening it...

Can't advise really on how tight is enough or indeed if it should be looser! There is the factor that if it has just started doing this, what has caused the change? I'm not technical enough to know what would make a transformer produce additional mechanical noise - it could be symptomatic of something else. How tight is the transformer in the other unit? Do they make the same noise when running in stereo mode?

[mhuk]

Stereo or mono makes no difference. It's new to me so it may have always hummed.

I'll get the top off the other one and see how tight the transformer bolt is.

[Biscuit]

the funky mains in my house makes the transformer buzz on my Pioneer amp (audible from a couple of metres away when no music playing) - one cheapo filtered mainsblock later - hum greatly reduced (only audible with ear to the amp)

[mhuk]

I've got a filter, haven't tried that.

I emailed Monarchy Audio and the main man (CC Poon) suggested losening and turning the transformer slightly. He also suggested using a mains lead without an earth wire.

Just seeems odd that one unit is silent and the other hums, when they both use the same mains lead plugged into the same power socket.

Cheers

[hifinut]

Another thing to check the fuse's a full 13 amp jobbie in both amps and cables clean/replace them also the power cable itself make sure it's wired the correct way Iv had cables that where not get a simple plug tester about £4 delivered via ebay worth testing sockets and check the cables with a multi-meter + to + etc.

I have had my amps apart for mods inside each one has an earth jumper both amps where set different and the set in one hummed so set it as the otherout and basicly no noise now

[Effem]

mhuk wrote:

And if you remember my pre-amp question: a passive pre-amp may have been a better choice than the EC4.5 as the volume goes from quiet to loud with a 1 degree turn of the power knob.

You need an inline attenuator to allow for more volume control movement. Cheap as chips on ebay

EBAY LINK

[mhuk]

How do I use them with XLR connections? ;)

[Effem]

mhuk wrote:

How do I use them with XLR connections? ;)

Why didn't you say that before?

[mrwippy]

Effem wrote:

mhuk wrote:

How do I use them with XLR connections? ;)

Why didn't you say that before?

Rothwell XLR'sare £69 a pair or you could try running them unbridged and Bi-amping if you speakers allow, orjust use one in stereo mode, or try one of the other 4 methods of connection -havehave you been through all the various permutations. these can be seriously confusing amps to use!

Hows the hum - what was the best result bolt loose, tight with or without slight rotation?

[Kev]

Physical hum can only be vibration in the unit, leaning on it causes it to hum less then the unit must be physically damped to cure it.
The most likely cause is, as has been suggested, the transformer buzzing. Transformer buzz is usually caused by either

it becoming overloaded through oscillation; does the amp get hot?

It can be caused by the windings not being wound tight enough, tightening the fixing bolt can cure this but it may not. If the transformer is a potted toroidal i.e. the centre hole is filled with a hard compound you can tighten the nut very tight. If the trfansformer is held down with a shaped metal plate at the top then it can still be tightened quite tight but be carefull as you are tightening the windings onto each other, they must be tight but not crushed.

Buzzing can also be caused by DC on the mains, commonly described as dirty mains, you could build a dc trap on the transformer but that involves a re-wire of the input to the transformer, a mains conditioner won't cure this unless it uses an isolation transformer. One amp could be more susceptible than the other so don't discard this idea. Does the buzz go quieter at night?

Hope this helps, good luck and be carefull around capacitors they can hurt lots!!!

Kev



quick edit

if there is a centre bolt through the transformer then both ends of the bolt MUST NOT under any circumstances be connected together either through the case or by allowing a spanner to toch the case while tightening the nut, there isn't an earth lead on the top nut is there?

[mhuk]

There's no earth lead on the top nut.

The transformer is held down with a shaped metal plate at the top with a big rubber washer underneath.

My plan is to make an earthless power cable and see what it does but haven't got around to it yet.

I'll plug it in tonight and see if it's quieter.

Cheers for all the suggestions.