Humming noice from Z5500 Subwoofer

[struthio]

hello,

I have a humming noise coming of sub-woofer (for sure caused by me), maybe someone can help me how to get rid of it.

At beginning I would like to give a bit of background to my issue.

I live in EU and I have Z5500 Logitech Speakers imported from USA (this means working on 110V). For many years I was working in setup where I had one transformer converting 220V->110V and then connecting to it Z5500 - and it was working fine (except of few inconveniences of such setup). This Year I decided to switch transformer in my Z5500 so I can power it directly from main power without any additional converters.

I ordered transformer dedicated for Audio devices with proper output levels and switched it in my Z5500.

After switch everything works fine except of one issue. From sub-woofer there is hum coming out (and it seems that this only affects sub-woofer speaker) - hum is very silent and constant (changing volume does not affect hum volume).

At first I thought about some noise from main power so I've added filter on incoming main power line to transformer - didn't changed anything in terms of hum.
Then I thought that transformer inducts power in speaker (since transformer is close to speaker), so I taken transformer out of Z5500 and started speakers with transformer being external, but this also didn't fix the problem.

What might generate this noise and how to get rid of it ?

 

[johnmath]

Where did you get the replacement transformer? The original has two isolated secondaries 26.4-0-26.4 & 14.5-0. I would have thought that impossible to replace with anything other than a Logitech part.

 

[struthio]

Unfortunately no replacement can be found in shops, so I ordered custom made transformer from company that makes transformers for audio equipment.
I just made a mistake in specification and ordered transformer with outputs 26.4-0, 26.4-0, 14.5-0 (so I had to connect two 26.4-0 together to get 26-0-26).

 

[johnmath]

Connecting two seperate windngs to create a centre tapped winding is perfectly OK provided the correct ends of the two winding are joined together.

1. Are you confident that you have connected the end of one winding with the start of the next?

2. Have yup measured the power supply voltages after rectification? You should have ~ -40-0-40 volts and ~20 volts. A volt or two either way isn't important.

 

[struthio]

1. Are you confident that you have connected the end of one winding with the start of the next?

Yes, assuming connection (1) 26<->0<->26(2), measuring voltage between (1) and (2) gives me a way over 40 volts.

2. Have yup measured the power supply voltages after rectification? You should have ~ -40-0-40 volts and ~20 volts. A volt or two either way isn't important.

Good suggestion. I will measure that tomorrow.

 

[johnmath]

From end (1) to end (2) you should measure 2 x 26.4 = 52.8 Vac (plus a few % because the transformer is not loaded).

The mail rail voltages measured from ground should be ~±40Vdc.

Also the secondary winding should be ~+20Vdc measured from ground.

That 20V feeds an LM217 type regulator which is adjusted to output 18V. My first thought is that's not much headroom for the regulator. If you have a CRO you could look at the output pin (3) of the regulator to see if there is ripple. Alternatively you can put a mulitimeter set to AC on the output to measure the ripple voltage. I would expect to see no more than a few millivolts.