Manley VariMu humming

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

jensenmann

Well-known member
Joined
Oct 10, 2004
Messages
2,850
Location
Karlsruhe, Germany
I have an early Manley VariMu on my bench which is humming way too much. In the beginning the hum was acoustically audible and is visible on the FFT. It has three mains transformers and my impression was that all three were humming/vibrating to a degree that it wasn´t usable in a mastering environment from it´s acoustic output alone.

I did the obvious things like recapping and replacing the rectifiers which have suffered from heat. A zener diode in the HV regulator circuit was defective and created a lot of overtones >250 Hz across the entire spectrum. After that step there were still overtones visible on the FFT but hum @ 50, 150 and 250Hz was dominating. The mechanical vibration of those trannies made me suspect and I replaced all carbon comp power resistors which completely erased all overtones >250Hz. What´s left now is the hum at 50, 150 and 250Hz. Since 100Hz isn´t a problem the PSU should be fine now. At this point the mechanical vibration of the mains trannies went down to an usable level.

Tube heaters are regulated. That means the 50Hz hum has to be magnetic interference. By the lack of better ideas I made an mumetal shield from selfadhiesive foil for the output transformers. That helped 6dB @50Hz and 3dB @150 + 250Hz. But there´s still hum in the signal at a too high level.

The last thing I found out is that the power consumption is 90W while Manley´s specs say 80W. But I have no idea what I should check now. I´m out of ideas. If you guys have a hint that´d be highly appreciated.
 
Find a good transformer winder and get him to make you a new tranny. A lot of US transformers have insufficient steel in them and tend to saturate on 50 Hz mains. Maybe put that transformer in an external box. Your expectations for this unit are way above the manufacturers.  And you dont tell us what level these harmonics are at.
 
I was considering a new transformer before I started, but the owner doesn´t want an external box for the new tranny because of the resale value.

At unity gain the 50Hz hum is at -84dBu.
 
radardoug said:
Your expectations for this unit are way above the manufacturers.

Oh well, it seams that I´m 1dB off the spec. After your comment I read the manual on Manley´s homepage where I found the noise spec. I bet I get improved performance if I move the big mains tranny out of the box like they did in later revisions. Thanks for your input, Doug.
 
jensenmann said:
I was considering a new transformer before I started, but the owner doesn´t want an external box for the new tranny because of the resale value.

At unity gain the 50Hz hum is at -84dBu. My Gyraf GX is dead quiet compared to the Manley.
 
Hi Jens
The transformer is what I believe is the problem.
1. Lower the AC input by 10% and see what the spectrum difference is?
2. Use a 60Hz power source and see what the 50Hz vs 60Hz spectrum difference is?
3. If the spectrum difference does not change by a large value, look @ snubbers (RC) across rectifiers.
Duke
 
Audio1Man said:
Hi Jens
The transformer is what I believe is the problem.
1. Lower the AC input by 10% and see what the spectrum difference is?
2. Use a 60Hz power source and see what the 50Hz vs 60Hz spectrum difference is?
3. If the spectrum difference does not change by a large value, look @ snubbers (RC) across rectifiers.
Duke

Good call!
The internal mains voltage switch indicates that this VariMu has been made for 220V mains, not 230V we have here since the early 90s (which mostly is 235-240V in my house).  Reducing the mains voltage from 230V to 220V with a variac helped to lower the dominating 50, 150 and 250Hz by 3dB in the noise spectrum. It´s even audible in the transformer vibration. Above 210V the mains trannies start to vibrate louder. So we have core saturation here which clearly increases stray field.
 
I have had luck curing audible transformer vibration by applying varnish between the bobbin and the core , simply use a brush to apply it and allow gravity to take it down into the gap, ocassionally lams loosen which allows them to vibrate and id say its quite likely if this is the cause you will hear noise down the line in the circuit also . It could also be the cause of the extra dissipation you noted ,as vibrations will cause extra heat to be produced .
 
Here is a plot of a power transformer driven @ 60Hz and then @ 50Hz. The sharp rise in Excitation currents @ the 50Hz is not a good match. The increased Power, Noise, and Distortion products will make the product unusable to the customer.
I used an Audio Precision to drive a Big Power amplifier for this test. I also do the same to test Audio transformers, however the Big Power amplifier is not required as the levels are much lower.
Duke
 

Attachments

  • SAMPLE-TX-PLOT.pdf
    15.6 KB · Views: 16

Latest posts

Back
Top