[pingwax]

Hello,

I have an MG HeadDT that I purchased a couple of years ago. I hadn't used it in a while because the tubes went bad, but recently dug it out and bought a matched pair of Tesla/JJ EL84 tubes and a Sovtek 12AX7LPS tube.

I have a brand new pair of Ultrasone HFI-650 DVD headphones that I am listening with. Listening to my Denon receiver at home, with the volume up, but no input, the phones are silent.

Listening at work, the mg headdt hisses, with or without a source connected. Turning the volume up results in a quite severe hissing. With any type of source connected, there is a buzzing sound, and it is more noticable in the left ear. I don't recall having heard this with my sennheiser HD600s before I put this away a year or so ago.

My cube is on the company battery back up system and I have noticed that changing from a UPS on the floor, to the wall outlet (basically linked to a giant UPS) and putting a quality surge protector in the equation also affects the pitch. The worst is when plugged into the UPS on the floor of my cube without the surge protector.

The final detail of this mystery is that my airhead exhibits the same buzzing, but in a much different tone than the MG HeadDT (on battery power, the buzzing disappears, but I can hear a very quiet, but high pitched tone on the airhead

different, less audible, but a bit annoying).

I'm sorry for the book, but does anybody have any thoughts about this?

Edit: I should mention that the buzzing in the left channel is loud enough to hear while listening to music

Thanks in advance.

 

[mkmelt]

The hissing sound is probably external RFI (radio frequency interference) being generated by one or more pieces of computer equipment at your work place. The MG Head may not have adequate RFI shielding to mitigate this noise.

On vintage amplifiers tube shields were usually provided for the phono stage 12AX7 tubes as these were resposible for amplifying the low level signal of a magnetic phono cartridge where RFI noise would be more of a problem.

While you can't put tubes shields on the EL84 tubes (they need to be exposed to the air for adequate airflow and cooling, you can try using a tube shield with the 12AX7 tube. It might reduce the RFI to where it is not audible. Also, a different brand of 12AX7 may not be as sensitive to RFI.

Tube shields come in various types, some snap into a special tube base, Others have an internal spring that slips over the tube leaving a bit of air space around the tube for ventilation. You might have to look around a bit, but these do show up on eBay, also you can try Antigue Electronic Supply here: http://www.tubesandmore.com/
They have tube shields for 7 pin and 9 pin miniature tubes.

 

[go_vtec]

Have you consider using a power conditioner? I found out that plugging in my Dell labtop to a same surge protector caused a buzz (both in Corda and MG OTL32). I've got a line conditioner and problem is only barely noticable using Sony CD3K and no hiss using HD650/HD600. Now, I've got a second line conditioner and plugged my labtop/computer/work-station to the second power supply (to isolate them from my audio equipment even further), problem is now nearly gone--only audiable on CD3K at ear drum-blowing volume (when paused, I'm not stupid).

Also if you have spare time and $$$, tube rolling may help (may be only a little

). I think RCA cleartops/Amperex are very musical (mine uses two 12AU7s) with near zero (=low). Your milage may vary since your are using 12AX7.

 

[pingwax]

Thanks for the suggestions guys

To test whether I was in fact getting interference, I have tried the amp in a few other places. The humming is always there and is definitely worst in the left ear. In some places, I hear only a very quiet hiss in the right channel, but the buzz in the left channel is always there. The problem is also noticeably worse when plugged into my UPS and may be explained by the following info on power conditioning.

Feel free to skip this bit about power conditioning if you want, but I wasn't very familiar with what a power conditioner does and thought it was interesting:

There are basically two kinds: The least expensive kind is basically a surge protector and voltage regulator that provides some interferance filtering. The more desirable kind will also provide these functions, but will also convert the incoming AC signal from whatever wave form it comes in as to a pure sine wave signal at the proper voltage and frequency. Cheap UPSes will output a square wave AC signal, but computer power supplies, which convert from ac to dc are either designed to accomodate this or are not susceptible to the poorly formed signal because of the conversion. However, I have read on several sites that it may negatively impact a lot of audio/video equipment, particularly analog devices. Unfortunately, the boxes that provide this functionality are frequently $1k+, so its a bit out of my price range

. I did read a few very glowing reports on the effect these conditioners had on quality of sound from known equipment so those of you with deep pockets may be interested in investigating this. I may consider picking up a lower end conditioner, which can be had for <$70, but most of the sites I've read seem to indicate that they are less likely to be helpful unless you are experiencing severe power fluctuations (i.e. brown outs). Finally, there are DC to AC converters that appear to be good at producing a pure sine wave (high end UPSes incorporate these), but I actually read a few negative reports regarding the quality of sound coming from known equipment when equipment utilizing this functionality (the main complaint seemed to be making the sound turn "muddy")

Weird if you ask me.

I've noticed that one EL84 tube (lets call it tube A) almost instantly starts glowing when the power comes on, while the other two tubes take about the same amount of time to come on...I wonder if I got a bad tube. I swapped the two EL84 tubes and tube A behaves the same way in either socket, although the buzzing is unaffected and stays in the left channel (I don't know enough to know if that is expected or not).

I'm avoiding NOS tubes for the moment; I want to get this guy working reliably before I start fooling around. I went ahead and ordered a different set of new tubes to try and see if they help. Even if they don't I can hold onto them for future use.

 

[Born2bwire]

The older MG Head models suffer from a problem in that the AC power goes from the back to the power switch in the front. The problem is that the wire goes past the power tube for the right channel, you can often hear this as the buzz. Try popping out the power switch (do this with it unplugged) and putting a few twists in the wire. Also see if you can't move the wire towards the side of the amp away from the tube. This might help reduce the buzzing. You could try replacing the wire with a shielded wire. On my amp I have had it replaced with a coax and it's removed it on all but the most sensitive 12AX7 tubes. That's the other thing, I've found that some tubes somehow pick it up. The Ei 12AX7 and the two Sylvania 5751's that I have both pick up a lowlevel white noise in the right channel but my other tubes do not. The amp is also very sensitive to dirty power. Back in the dorms, when the neighbor would use their microwave, I would hear a pop in the amp from the power flux.

My advice is to fiddle with the power switch, get a decent 12AX7 tube like a GE 5751, and see if you can get it on a better circuit.

 

[pingwax]

Thanks to everybody for their help.

I did find another thread from early 2003 that also discussed the wires from the power switch.

Tonight I removed the old wires and soldered in new wires. Whether it was necessary or not, I twisted them around each other a bit and ran them as far away from anything else that I could. Sadly, I'm not very good at soldering, but I think I more or less got the task accomplished

.

The hiss is gone. This clearly has resolved the problem. My CD3000s require significantly less gain with this amp than any of my other headphones and the hiss was particularly irritating with them, but now I can't hear it at all.

For what it is worth, I could never hear the hiss with my HD600s. I don't know if it was just the open design made it less obvious or if it was a matter of its impedence, but as much as I tried, I couldn't hear it all with these headphones.

Thanks again for the help; hopefully this will be useful information for somebody else some day.