Dynaco ST-150 transformer hum

kdoherty

Member
2020-01-08 4:32 pm
I have a Dynaco ST-150 solid state amp that sounds good except for a persistent low hum coming from the power transformer. This is a mechanical noise that doesn't come through the speakers.

The amp has new electrolytic caps and output transistors but is otherwise stock.

This article (https://www.pooraudiophile.com/2015/03/how-to-fix-dc-offset-and-transformer.html) and similar ones make me think it's a DC offset problem, but I'd like to be sure before I spend the money on a DC offset filter. Other possibilities might be that the transformer itself is bad, or that the level of hum (not very loud, but noticeable within a couple of feet of the amp with no music playing) is normal. I think my ST-70 hums a bit too, but I haven't used it in a while.

Thanks for any advice you can provide.
 
Dynaco ST150 was an EI transformer, produced in house. Transformers were their specialty, before they hired a kit designer. I have 3 of them, none of them have mechanical hum. ST70, PAS2, ST120. I usse the ST120 about 4200 hours a year. Have since 2010.
Drilling 2 corners and putting through a 10-32 rod McMaster-Carr with nuts is not unheard of on EI transformers. Use, washers, elastic stop nuts McMaster-Carr or 2 countertorqued nuts on each end.
 
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kdoherty

Member
2020-01-08 4:32 pm
it would be good to know what is reasonable to expect. if it's operating correctly, should the ST-150 be silent when it's on? or is a certain amount of mechanical noise to be expected? for comparison, my Pass ACA amplifier is absolutely silent when on. If not for the LEDs (and the warm heat sinks) you would not know it's on.