help! DAC hums, amp goes into protection mode

[PreSapian]

i have a pacific valve modded dac 60 hooked up to sansui au d919 integrated. the dac is connected to it's aux input and all the tone controls are defeated in the amp for the optimum sq. the hum coming from the dac is detectable in the woofers- equal volume at both channels. and it becomes louder at the control of the volume pot. at the near max volume, the hum bottomed out the woofers and the amp went into protection mode. the dac is the source of the hum, im certain, from doing an a/b test with a different dac. what do you think is the cause? sansui 919 is 100v only and the dac has 115v transformers, but both of them are hooked up to 110v downstep transformer that i run from 220v here in china. could this be the reason? how could i go about fixing the hum and also do you think it's safe to keep using the dac until then?

the dac also has a slight volume imbalance where the right channel is a fraction louder- collapsing the soundstage and skewing the imaging to the right.

[Bill Fuss]

You haven't said if it always did this, or if it just started. Anyway, you have a serious problem. I would not use it until it is fixed.

You'll need the schematic, or be experienced with test equipment.

[PreSapian]

its been doing it before i hooked it up to the sansui. but since the sansui is much more sensitive than anything i had used before, the hum became noticeable enough for me to investigate.

do you think it has already negatively affected the amp? this is a vintage AU-D907Limited that i started cherishing from the first moment and has them unobtanium output caps...

anyways, i don't have the equipment or the know how to start the surgery on this dac. i would still like to know the cause of its terminal illness.

[PreSapian]

interesting thing i've discovered just now- the humming is only present when the spdf cable is plugged into the dac. i don't have a coax to try though. the humming is still present when i run a ground from the pc chassis to the amp, and from the dac's. there is also humming from the tube preamp(mingda mc7r) to the dac but the preamp has always hummed with all my other amps to some degree. so now i'm thinking that the hum is an innate trait of tube equipments thats only amplified by the sensitive nature of the amp. also, the amp doesn't hum with my modern yamaha receiver, when its run as a dac/preamp. the hum returns when yamaha is utilized solely as a preamp with the tube dac connected at the other end. what could be the cause? do you think my vintage sansui is just not meant for tubes?

[Irakli]

Seems a ground loop problem for me. Either connections in your system are set the way to cause the loop or it is a bad DAC design.

In any case, start by reading this:
http://www.jensen-transformers.com/a...%20seminar.pdf

It is not easy (but certainly doable by methodic approach), but you need to isolate the loop and break it.

[Bill Fuss]

Originally Posted by PreSapian

interesting thing i've discovered just now- the humming is only present when the spdf cable is plugged into the dac. i don't have a coax to try though. the humming is still present when i run a ground from the pc chassis to the amp, and from the dac's. there is also humming from the tube preamp(mingda mc7r) to the dac but the preamp has always hummed with all my other amps to some degree. so now i'm thinking that the hum is an innate trait of tube equipments thats only amplified by the sensitive nature of the amp. also, the amp doesn't hum with my modern yamaha receiver, when its run as a dac/preamp. the hum returns when yamaha is utilized solely as a preamp with the tube dac connected at the other end. what could be the cause? do you think my vintage sansui is just not meant for tubes?

You are saying the hum disappears when you disconnect the SPDIF cable. Your ground loop must be through a grounded shield in the PC and the Dac. Add a pulse transformer at either location to break the loop.

If it doesn't really disappear when you disconnect the cable then you probably have hidden ground loops or faulty equipment. There is nothing normal about humming equipment.

[Bill Fuss]

You could also switch to optical.

If your PC is 230V you probably have differing ground potentials.

[PreSapian]

i'm sorry. i should've clarified. i am running optical to the dac. this is what makes it puzzling for me- because the optical doesn't have any metal parts, it shouldn't be transmitting any ground issues. perhaps the problem is in the dac after all which only gets turned on when there is a signal being fed whether it be from optical or coax.

[Bill Fuss]

Could be a bad tube or a thousand other things.