I just got this Tempest back from Klaus. Serial number in the 600s, early Odyssey board, and regular transformer, though it has been “wrapped.” I'm guessing all known Odyssey tricks to control hum are in place. Unfortunately, I'm getting transformer hum/buzz from the transformer itself (hum can be heard up to about 5 feet from the unit), hum out of the speakers (same, hum at 5 feet, though it is drowned out when music is playing), and very loud MC phono hum, making this section unusable. The phono hum varies with the volume setting, the others are steady and continuous, regardless of the source.
Tempest is hooked up to a pair of Khartago monos. The power at my outlets is typically around 123VAC and I've measure fluctuating 300-500mv of DC. You would need a good quality meter like a Fluke or an oscilloscope to properly measure the presence of DC in your power. DC is a frequent culprit in transformer hum issues. Pretty much all tricks to troubleshoot hum have been exhausted (there are many, most all were done).
I plugged the preamp into a variac, with the intent of determining if I could reduce the transformer hum by reducing the supply voltage. I have built kit preamps in the past and used power supplies 30% less than the circuit was designed for-- these circuits can perform well with lower power. As expected, transformer hum came down as voltage was dropped to about 90VAC, after which I found the lower voltage beginning to affect sound. So: a method here to reduce transformer hum, reduce the operating voltage to about 90VAC. Makes sense. No ill effects from running at a lower voltage, in fact, would probably prolong life and run everything cooler, though I've never noticed any heat in this Tempest.
Things got more interesting when I switched the unit to phono to see if it had any effect on its hum. As I swept the voltage upward, the hum disappeared at 130VAC! Not reduced, but completely gone. What I've determined, is that there is a “sweet spot” voltage supply for this unit. At the 123VAC directly plugged into the wall, tons of hum—at 130VAC none! Chances are your voltage is below 130VAC and you are getting hum as a result.
So if there was a way to adjust the supply voltage to the unit and hit this sweet spot, one could eliminate the hum. A 130VAC operating voltage is fine with the unit. In fact, a 140VAC supply would put 25VDC onto the board and the caps are rated at 35VDC. Hopefully, you won't need to go that high to hit the sweet spot.
A variac is not a good way to power equipment on a regular basis. Fortunately, there is an alternative that is not only effective, but quite inexpensive. The BK Precision TR-110 was designed just for this purpose. You can switch voltages in 5 VAC increments from 90 to 140VAC. It has the added bonus in that it has an isolation transformer, which will clean up your power, essentially eliminating the DC component of your power supply, if there is any. The unit is rated at 350VAC, the Tempest transformer is 300VAC or less, good there. Forget the $300 Humbuster, an isolation transformer is the tried and true method of cleaning up DC. These TR-110s can be had used for $75-$100 shipped on eBay. They have been discontinued as of April 2011, but there is still new stock out there going for about $250. They are heavy and robust, I think they are used primarily for medical equipment.
Well, how did it work? Glad you asked. I plugged the Tempest into the TR-110 unit and set it on the 130VAC setting and absolutely no phono hum. It also appears that the hum coming from the speakers is gone, probably a result of the isolation transformer filtering out the DC. The unit's transformer, unfortunately, still hums. I can hear it at about 3 feet out, I've placed some heavy books on the unit and it helps, but that hum has not disappeared altogether. The TR-110 hums a little bit also, but it's tucked away behind everything.
I will also note that I moved the Tempest and TR-110 to a Stratos I have in a second system and though it also completely eliminated phono hum there, it did not do away with the transformer nor speaker hums. The Tempest is a very finicky character, indeed.
So, break out the variac if you have one, and confirm what I've stated here. If it works, get yourself a TR-110. You will eliminate phono hum and possibly other types of hum if you have them. If you don't have a variac, wait for additional confirmation from other owners that their units respond like mine. After all, this is only one data point. If it seems to work for others, a TR-110 is in order.
